Adding upstream version 110.0.1.upstream/110.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

45 files changed, 20845 insertions, 0 deletions

diff --git a/js/src/builtin/intl/Collator.cpp b/js/src/builtin/intl/Collator.cpp
new file mode 100644
index 0000000000..449de1dc45
--- /dev/null
+++ b/js/src/builtin/intl/Collator.cpp
@@ -0,0 +1,472 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Intl.Collator implementation. */
+
+#include "builtin/intl/Collator.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/intl/Collator.h"
+#include "mozilla/intl/Locale.h"
+#include "mozilla/Span.h"
+
+#include "builtin/Array.h"
+#include "builtin/intl/CommonFunctions.h"
+#include "builtin/intl/FormatBuffer.h"
+#include "builtin/intl/LanguageTag.h"
+#include "builtin/intl/SharedIntlData.h"
+#include "gc/GCContext.h"
+#include "js/PropertySpec.h"
+#include "js/StableStringChars.h"
+#include "js/TypeDecls.h"
+#include "vm/GlobalObject.h"
+#include "vm/JSContext.h"
+#include "vm/PlainObject.h"  // js::PlainObject
+#include "vm/Runtime.h"
+#include "vm/StringType.h"
+#include "vm/WellKnownAtom.h"  // js_*_str
+
+#include "vm/GeckoProfiler-inl.h"
+#include "vm/JSObject-inl.h"
+
+using namespace js;
+
+using JS::AutoStableStringChars;
+
+using js::intl::ReportInternalError;
+using js::intl::SharedIntlData;
+
+const JSClassOps CollatorObject::classOps_ = {
+    nullptr,                   // addProperty
+    nullptr,                   // delProperty
+    nullptr,                   // enumerate
+    nullptr,                   // newEnumerate
+    nullptr,                   // resolve
+    nullptr,                   // mayResolve
+    CollatorObject::finalize,  // finalize
+    nullptr,                   // call
+    nullptr,                   // construct
+    nullptr,                   // trace
+};
+
+const JSClass CollatorObject::class_ = {
+    "Intl.Collator",
+    JSCLASS_HAS_RESERVED_SLOTS(CollatorObject::SLOT_COUNT) |
+        JSCLASS_HAS_CACHED_PROTO(JSProto_Collator) |
+        JSCLASS_FOREGROUND_FINALIZE,
+    &CollatorObject::classOps_, &CollatorObject::classSpec_};
+
+const JSClass& CollatorObject::protoClass_ = PlainObject::class_;
+
+static bool collator_toSource(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  args.rval().setString(cx->names().Collator);
+  return true;
+}
+
+static const JSFunctionSpec collator_static_methods[] = {
+    JS_SELF_HOSTED_FN("supportedLocalesOf", "Intl_Collator_supportedLocalesOf",
+                      1, 0),
+    JS_FS_END};
+
+static const JSFunctionSpec collator_methods[] = {
+    JS_SELF_HOSTED_FN("resolvedOptions", "Intl_Collator_resolvedOptions", 0, 0),
+    JS_FN(js_toSource_str, collator_toSource, 0, 0), JS_FS_END};
+
+static const JSPropertySpec collator_properties[] = {
+    JS_SELF_HOSTED_GET("compare", "$Intl_Collator_compare_get", 0),
+    JS_STRING_SYM_PS(toStringTag, "Intl.Collator", JSPROP_READONLY), JS_PS_END};
+
+static bool Collator(JSContext* cx, unsigned argc, Value* vp);
+
+const ClassSpec CollatorObject::classSpec_ = {
+    GenericCreateConstructor<Collator, 0, gc::AllocKind::FUNCTION>,
+    GenericCreatePrototype<CollatorObject>,
+    collator_static_methods,
+    nullptr,
+    collator_methods,
+    collator_properties,
+    nullptr,
+    ClassSpec::DontDefineConstructor};
+
+/**
+ * 10.1.2 Intl.Collator([ locales [, options]])
+ *
+ * ES2017 Intl draft rev 94045d234762ad107a3d09bb6f7381a65f1a2f9b
+ */
+static bool Collator(JSContext* cx, const CallArgs& args) {
+  AutoJSConstructorProfilerEntry pseudoFrame(cx, "Intl.Collator");
+
+  // Step 1 (Handled by OrdinaryCreateFromConstructor fallback code).
+
+  // Steps 2-5 (Inlined 9.1.14, OrdinaryCreateFromConstructor).
+  RootedObject proto(cx);
+  if (!GetPrototypeFromBuiltinConstructor(cx, args, JSProto_Collator, &proto)) {
+    return false;
+  }
+
+  Rooted<CollatorObject*> collator(
+      cx, NewObjectWithClassProto<CollatorObject>(cx, proto));
+  if (!collator) {
+    return false;
+  }
+
+  HandleValue locales = args.get(0);
+  HandleValue options = args.get(1);
+
+  // Step 6.
+  if (!intl::InitializeObject(cx, collator, cx->names().InitializeCollator,
+                              locales, options)) {
+    return false;
+  }
+
+  args.rval().setObject(*collator);
+  return true;
+}
+
+static bool Collator(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return Collator(cx, args);
+}
+
+bool js::intl_Collator(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 2);
+  MOZ_ASSERT(!args.isConstructing());
+
+  return Collator(cx, args);
+}
+
+void js::CollatorObject::finalize(JS::GCContext* gcx, JSObject* obj) {
+  MOZ_ASSERT(gcx->onMainThread());
+
+  if (mozilla::intl::Collator* coll = obj->as<CollatorObject>().getCollator()) {
+    intl::RemoveICUCellMemory(gcx, obj, CollatorObject::EstimatedMemoryUse);
+    delete coll;
+  }
+}
+
+bool js::intl_availableCollations(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+  MOZ_ASSERT(args[0].isString());
+
+  UniqueChars locale = intl::EncodeLocale(cx, args[0].toString());
+  if (!locale) {
+    return false;
+  }
+  auto keywords =
+      mozilla::intl::Collator::GetBcp47KeywordValuesForLocale(locale.get());
+  if (keywords.isErr()) {
+    ReportInternalError(cx, keywords.unwrapErr());
+    return false;
+  }
+
+  RootedObject collations(cx, NewDenseEmptyArray(cx));
+  if (!collations) {
+    return false;
+  }
+
+  // The first element of the collations array must be |null| per
+  // ES2017 Intl, 10.2.3 Internal Slots.
+  if (!NewbornArrayPush(cx, collations, NullValue())) {
+    return false;
+  }
+
+  for (auto result : keywords.unwrap()) {
+    if (result.isErr()) {
+      ReportInternalError(cx);
+      return false;
+    }
+    mozilla::Span<const char> collation = result.unwrap();
+
+    // Per ECMA-402, 10.2.3, we don't include standard and search:
+    // "The values 'standard' and 'search' must not be used as elements in
+    // any [[sortLocaleData]][locale].co and [[searchLocaleData]][locale].co
+    // array."
+    static constexpr auto standard = mozilla::MakeStringSpan("standard");
+    static constexpr auto search = mozilla::MakeStringSpan("search");
+    if (collation == standard || collation == search) {
+      continue;
+    }
+
+    JSString* jscollation = NewStringCopy<CanGC>(cx, collation);
+    if (!jscollation) {
+      return false;
+    }
+    if (!NewbornArrayPush(cx, collations, StringValue(jscollation))) {
+      return false;
+    }
+  }
+
+  args.rval().setObject(*collations);
+  return true;
+}
+
+/**
+ * Returns a new mozilla::intl::Collator with the locale and collation options
+ * of the given Collator.
+ */
+static mozilla::intl::Collator* NewIntlCollator(
+    JSContext* cx, Handle<CollatorObject*> collator) {
+  RootedValue value(cx);
+
+  RootedObject internals(cx, intl::GetInternalsObject(cx, collator));
+  if (!internals) {
+    return nullptr;
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().locale, &value)) {
+    return nullptr;
+  }
+
+  mozilla::intl::Locale tag;
+  {
+    Rooted<JSLinearString*> locale(cx, value.toString()->ensureLinear(cx));
+    if (!locale) {
+      return nullptr;
+    }
+
+    if (!intl::ParseLocale(cx, locale, tag)) {
+      return nullptr;
+    }
+  }
+
+  using mozilla::intl::Collator;
+
+  Collator::Options options{};
+
+  if (!GetProperty(cx, internals, internals, cx->names().usage, &value)) {
+    return nullptr;
+  }
+
+  enum class Usage { Search, Sort };
+
+  Usage usage;
+  {
+    JSLinearString* str = value.toString()->ensureLinear(cx);
+    if (!str) {
+      return nullptr;
+    }
+
+    if (StringEqualsLiteral(str, "search")) {
+      usage = Usage::Search;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(str, "sort"));
+      usage = Usage::Sort;
+    }
+  }
+
+  JS::RootedVector<intl::UnicodeExtensionKeyword> keywords(cx);
+
+  // ICU expects collation as Unicode locale extensions on locale.
+  if (usage == Usage::Search) {
+    if (!keywords.emplaceBack("co", cx->names().search)) {
+      return nullptr;
+    }
+
+    // Search collations can't select a different collation, so the collation
+    // property is guaranteed to be "default".
+#ifdef DEBUG
+    if (!GetProperty(cx, internals, internals, cx->names().collation, &value)) {
+      return nullptr;
+    }
+
+    JSLinearString* collation = value.toString()->ensureLinear(cx);
+    if (!collation) {
+      return nullptr;
+    }
+
+    MOZ_ASSERT(StringEqualsLiteral(collation, "default"));
+#endif
+  } else {
+    if (!GetProperty(cx, internals, internals, cx->names().collation, &value)) {
+      return nullptr;
+    }
+
+    JSLinearString* collation = value.toString()->ensureLinear(cx);
+    if (!collation) {
+      return nullptr;
+    }
+
+    // Set collation as a Unicode locale extension when it was specified.
+    if (!StringEqualsLiteral(collation, "default")) {
+      if (!keywords.emplaceBack("co", collation)) {
+        return nullptr;
+      }
+    }
+  }
+
+  // |ApplyUnicodeExtensionToTag| applies the new keywords to the front of the
+  // Unicode extension subtag. We're then relying on ICU to follow RFC 6067,
+  // which states that any trailing keywords using the same key should be
+  // ignored.
+  if (!intl::ApplyUnicodeExtensionToTag(cx, tag, keywords)) {
+    return nullptr;
+  }
+
+  intl::FormatBuffer<char> buffer(cx);
+  if (auto result = tag.ToString(buffer); result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return nullptr;
+  }
+
+  UniqueChars locale = buffer.extractStringZ();
+  if (!locale) {
+    return nullptr;
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().sensitivity, &value)) {
+    return nullptr;
+  }
+
+  {
+    JSLinearString* sensitivity = value.toString()->ensureLinear(cx);
+    if (!sensitivity) {
+      return nullptr;
+    }
+    if (StringEqualsLiteral(sensitivity, "base")) {
+      options.sensitivity = Collator::Sensitivity::Base;
+    } else if (StringEqualsLiteral(sensitivity, "accent")) {
+      options.sensitivity = Collator::Sensitivity::Accent;
+    } else if (StringEqualsLiteral(sensitivity, "case")) {
+      options.sensitivity = Collator::Sensitivity::Case;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(sensitivity, "variant"));
+      options.sensitivity = Collator::Sensitivity::Variant;
+    }
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().ignorePunctuation,
+                   &value)) {
+    return nullptr;
+  }
+  options.ignorePunctuation = value.toBoolean();
+
+  if (!GetProperty(cx, internals, internals, cx->names().numeric, &value)) {
+    return nullptr;
+  }
+  if (!value.isUndefined()) {
+    options.numeric = value.toBoolean();
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().caseFirst, &value)) {
+    return nullptr;
+  }
+  if (!value.isUndefined()) {
+    JSLinearString* caseFirst = value.toString()->ensureLinear(cx);
+    if (!caseFirst) {
+      return nullptr;
+    }
+    if (StringEqualsLiteral(caseFirst, "upper")) {
+      options.caseFirst = Collator::CaseFirst::Upper;
+    } else if (StringEqualsLiteral(caseFirst, "lower")) {
+      options.caseFirst = Collator::CaseFirst::Lower;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(caseFirst, "false"));
+      options.caseFirst = Collator::CaseFirst::False;
+    }
+  }
+
+  auto collResult = Collator::TryCreate(locale.get());
+  if (collResult.isErr()) {
+    ReportInternalError(cx, collResult.unwrapErr());
+    return nullptr;
+  }
+  auto coll = collResult.unwrap();
+
+  auto optResult = coll->SetOptions(options);
+  if (optResult.isErr()) {
+    ReportInternalError(cx, optResult.unwrapErr());
+    return nullptr;
+  }
+
+  return coll.release();
+}
+
+static mozilla::intl::Collator* GetOrCreateCollator(
+    JSContext* cx, Handle<CollatorObject*> collator) {
+  // Obtain a cached mozilla::intl::Collator object.
+  mozilla::intl::Collator* coll = collator->getCollator();
+  if (coll) {
+    return coll;
+  }
+
+  coll = NewIntlCollator(cx, collator);
+  if (!coll) {
+    return nullptr;
+  }
+  collator->setCollator(coll);
+
+  intl::AddICUCellMemory(collator, CollatorObject::EstimatedMemoryUse);
+  return coll;
+}
+
+static bool intl_CompareStrings(JSContext* cx, mozilla::intl::Collator* coll,
+                                HandleString str1, HandleString str2,
+                                MutableHandleValue result) {
+  MOZ_ASSERT(str1);
+  MOZ_ASSERT(str2);
+
+  if (str1 == str2) {
+    result.setInt32(0);
+    return true;
+  }
+
+  AutoStableStringChars stableChars1(cx);
+  if (!stableChars1.initTwoByte(cx, str1)) {
+    return false;
+  }
+
+  AutoStableStringChars stableChars2(cx);
+  if (!stableChars2.initTwoByte(cx, str2)) {
+    return false;
+  }
+
+  mozilla::Range<const char16_t> chars1 = stableChars1.twoByteRange();
+  mozilla::Range<const char16_t> chars2 = stableChars2.twoByteRange();
+
+  result.setInt32(coll->CompareStrings(chars1, chars2));
+  return true;
+}
+
+bool js::intl_CompareStrings(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 3);
+  MOZ_ASSERT(args[0].isObject());
+  MOZ_ASSERT(args[1].isString());
+  MOZ_ASSERT(args[2].isString());
+
+  Rooted<CollatorObject*> collator(cx,
+                                   &args[0].toObject().as<CollatorObject>());
+
+  mozilla::intl::Collator* coll = GetOrCreateCollator(cx, collator);
+  if (!coll) {
+    return false;
+  }
+
+  // Use the UCollator to actually compare the strings.
+  RootedString str1(cx, args[1].toString());
+  RootedString str2(cx, args[2].toString());
+  return intl_CompareStrings(cx, coll, str1, str2, args.rval());
+}
+
+bool js::intl_isUpperCaseFirst(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+  MOZ_ASSERT(args[0].isString());
+
+  SharedIntlData& sharedIntlData = cx->runtime()->sharedIntlData.ref();
+
+  RootedString locale(cx, args[0].toString());
+  bool isUpperFirst;
+  if (!sharedIntlData.isUpperCaseFirst(cx, locale, &isUpperFirst)) {
+    return false;
+  }
+
+  args.rval().setBoolean(isUpperFirst);
+  return true;
+}
diff --git a/js/src/builtin/intl/Collator.h b/js/src/builtin/intl/Collator.h
new file mode 100644
index 0000000000..764c838ce8
--- /dev/null
+++ b/js/src/builtin/intl/Collator.h
@@ -0,0 +1,104 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_Collator_h
+#define builtin_intl_Collator_h
+
+#include <stdint.h>
+
+#include "builtin/SelfHostingDefines.h"
+#include "js/Class.h"
+#include "vm/NativeObject.h"
+
+namespace mozilla::intl {
+class Collator;
+}
+
+namespace js {
+
+/******************** Collator ********************/
+
+class CollatorObject : public NativeObject {
+ public:
+  static const JSClass class_;
+  static const JSClass& protoClass_;
+
+  static constexpr uint32_t INTERNALS_SLOT = 0;
+  static constexpr uint32_t INTL_COLLATOR_SLOT = 1;
+  static constexpr uint32_t SLOT_COUNT = 2;
+
+  static_assert(INTERNALS_SLOT == INTL_INTERNALS_OBJECT_SLOT,
+                "INTERNALS_SLOT must match self-hosting define for internals "
+                "object slot");
+
+  // Estimated memory use for UCollator (see IcuMemoryUsage).
+  static constexpr size_t EstimatedMemoryUse = 1128;
+
+  mozilla::intl::Collator* getCollator() const {
+    const auto& slot = getFixedSlot(INTL_COLLATOR_SLOT);
+    if (slot.isUndefined()) {
+      return nullptr;
+    }
+    return static_cast<mozilla::intl::Collator*>(slot.toPrivate());
+  }
+
+  void setCollator(mozilla::intl::Collator* collator) {
+    setFixedSlot(INTL_COLLATOR_SLOT, PrivateValue(collator));
+  }
+
+ private:
+  static const JSClassOps classOps_;
+  static const ClassSpec classSpec_;
+
+  static void finalize(JS::GCContext* gcx, JSObject* obj);
+};
+
+/**
+ * Returns a new instance of the standard built-in Collator constructor.
+ * Self-hosted code cannot cache this constructor (as it does for others in
+ * Utilities.js) because it is initialized after self-hosted code is compiled.
+ *
+ * Usage: collator = intl_Collator(locales, options)
+ */
+[[nodiscard]] extern bool intl_Collator(JSContext* cx, unsigned argc,
+                                        JS::Value* vp);
+
+/**
+ * Returns an array with the collation type identifiers per Unicode
+ * Technical Standard 35, Unicode Locale Data Markup Language, for the
+ * collations supported for the given locale. "standard" and "search" are
+ * excluded.
+ *
+ * Usage: collations = intl_availableCollations(locale)
+ */
+[[nodiscard]] extern bool intl_availableCollations(JSContext* cx, unsigned argc,
+                                                   JS::Value* vp);
+
+/**
+ * Compares x and y (which must be String values), and returns a number less
+ * than 0 if x < y, 0 if x = y, or a number greater than 0 if x > y according
+ * to the sort order for the locale and collation options of the given
+ * Collator.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 10.3.2.
+ *
+ * Usage: result = intl_CompareStrings(collator, x, y)
+ */
+[[nodiscard]] extern bool intl_CompareStrings(JSContext* cx, unsigned argc,
+                                              JS::Value* vp);
+
+/**
+ * Returns true if the given locale sorts upper-case before lower-case
+ * characters.
+ *
+ * Usage: result = intl_isUpperCaseFirst(locale)
+ */
+[[nodiscard]] extern bool intl_isUpperCaseFirst(JSContext* cx, unsigned argc,
+                                                JS::Value* vp);
+
+}  // namespace js
+
+#endif /* builtin_intl_Collator_h */
diff --git a/js/src/builtin/intl/Collator.js b/js/src/builtin/intl/Collator.js
new file mode 100644
index 0000000000..b82f07e3d4
--- /dev/null
+++ b/js/src/builtin/intl/Collator.js
@@ -0,0 +1,468 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Portions Copyright Norbert Lindenberg 2011-2012. */
+
+/**
+ * Compute an internal properties object from |lazyCollatorData|.
+ */
+function resolveCollatorInternals(lazyCollatorData) {
+  assert(IsObject(lazyCollatorData), "lazy data not an object?");
+
+  var internalProps = std_Object_create(null);
+
+  var Collator = collatorInternalProperties;
+
+  // Step 5.
+  internalProps.usage = lazyCollatorData.usage;
+
+  // Steps 6-7.
+  var collatorIsSorting = lazyCollatorData.usage === "sort";
+  var localeData = collatorIsSorting
+    ? Collator.sortLocaleData
+    : Collator.searchLocaleData;
+
+  // Compute effective locale.
+  // Step 16.
+  var relevantExtensionKeys = Collator.relevantExtensionKeys;
+
+  // Step 17.
+  var r = ResolveLocale(
+    "Collator",
+    lazyCollatorData.requestedLocales,
+    lazyCollatorData.opt,
+    relevantExtensionKeys,
+    localeData
+  );
+
+  // Step 18.
+  internalProps.locale = r.locale;
+
+  // Step 19.
+  var collation = r.co;
+
+  // Step 20.
+  if (collation === null) {
+    collation = "default";
+  }
+
+  // Step 21.
+  internalProps.collation = collation;
+
+  // Step 22.
+  internalProps.numeric = r.kn === "true";
+
+  // Step 23.
+  internalProps.caseFirst = r.kf;
+
+  // Compute remaining collation options.
+  // Step 25.
+  var s = lazyCollatorData.rawSensitivity;
+  if (s === undefined) {
+    // In theory the default sensitivity for the "search" collator is
+    // locale dependent; in reality the CLDR/ICU default strength is
+    // always tertiary. Therefore use "variant" as the default value for
+    // both collation modes.
+    s = "variant";
+  }
+
+  // Step 26.
+  internalProps.sensitivity = s;
+
+  // Step 28.
+  internalProps.ignorePunctuation = lazyCollatorData.ignorePunctuation;
+
+  // The caller is responsible for associating |internalProps| with the right
+  // object using |setInternalProperties|.
+  return internalProps;
+}
+
+/**
+ * Returns an object containing the Collator internal properties of |obj|.
+ */
+function getCollatorInternals(obj) {
+  assert(IsObject(obj), "getCollatorInternals called with non-object");
+  assert(
+    intl_GuardToCollator(obj) !== null,
+    "getCollatorInternals called with non-Collator"
+  );
+
+  var internals = getIntlObjectInternals(obj);
+  assert(
+    internals.type === "Collator",
+    "bad type escaped getIntlObjectInternals"
+  );
+
+  // If internal properties have already been computed, use them.
+  var internalProps = maybeInternalProperties(internals);
+  if (internalProps) {
+    return internalProps;
+  }
+
+  // Otherwise it's time to fully create them.
+  internalProps = resolveCollatorInternals(internals.lazyData);
+  setInternalProperties(internals, internalProps);
+  return internalProps;
+}
+
+/**
+ * Initializes an object as a Collator.
+ *
+ * This method is complicated a moderate bit by its implementing initialization
+ * as a *lazy* concept.  Everything that must happen now, does -- but we defer
+ * all the work we can until the object is actually used as a Collator.  This
+ * later work occurs in |resolveCollatorInternals|; steps not noted here occur
+ * there.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 10.1.1.
+ */
+function InitializeCollator(collator, locales, options) {
+  assert(IsObject(collator), "InitializeCollator called with non-object");
+  assert(
+    intl_GuardToCollator(collator) !== null,
+    "InitializeCollator called with non-Collator"
+  );
+
+  // Lazy Collator data has the following structure:
+  //
+  //   {
+  //     requestedLocales: List of locales,
+  //     usage: "sort" / "search",
+  //     opt: // opt object computed in InitializeCollator
+  //       {
+  //         localeMatcher: "lookup" / "best fit",
+  //         co: string matching a Unicode extension type / undefined
+  //         kn: true / false / undefined,
+  //         kf: "upper" / "lower" / "false" / undefined
+  //       }
+  //     rawSensitivity: "base" / "accent" / "case" / "variant" / undefined,
+  //     ignorePunctuation: true / false
+  //   }
+  //
+  // Note that lazy data is only installed as a final step of initialization,
+  // so every Collator lazy data object has *all* these properties, never a
+  // subset of them.
+  var lazyCollatorData = std_Object_create(null);
+
+  // Step 1.
+  var requestedLocales = CanonicalizeLocaleList(locales);
+  lazyCollatorData.requestedLocales = requestedLocales;
+
+  // Steps 2-3.
+  //
+  // If we ever need more speed here at startup, we should try to detect the
+  // case where |options === undefined| and then directly use the default
+  // value for each option.  For now, just keep it simple.
+  if (options === undefined) {
+    options = std_Object_create(null);
+  } else {
+    options = ToObject(options);
+  }
+
+  // Compute options that impact interpretation of locale.
+  // Step 4.
+  var u = GetOption(options, "usage", "string", ["sort", "search"], "sort");
+  lazyCollatorData.usage = u;
+
+  // Step 8.
+  var opt = new_Record();
+  lazyCollatorData.opt = opt;
+
+  // Steps 9-10.
+  var matcher = GetOption(
+    options,
+    "localeMatcher",
+    "string",
+    ["lookup", "best fit"],
+    "best fit"
+  );
+  opt.localeMatcher = matcher;
+
+  // https://github.com/tc39/ecma402/pull/459
+  var collation = GetOption(
+    options,
+    "collation",
+    "string",
+    undefined,
+    undefined
+  );
+  if (collation !== undefined) {
+    collation = intl_ValidateAndCanonicalizeUnicodeExtensionType(
+      collation,
+      "collation",
+      "co"
+    );
+  }
+  opt.co = collation;
+
+  // Steps 11-13.
+  var numericValue = GetOption(
+    options,
+    "numeric",
+    "boolean",
+    undefined,
+    undefined
+  );
+  if (numericValue !== undefined) {
+    numericValue = numericValue ? "true" : "false";
+  }
+  opt.kn = numericValue;
+
+  // Steps 14-15.
+  var caseFirstValue = GetOption(
+    options,
+    "caseFirst",
+    "string",
+    ["upper", "lower", "false"],
+    undefined
+  );
+  opt.kf = caseFirstValue;
+
+  // Compute remaining collation options.
+  // Step 24.
+  var s = GetOption(
+    options,
+    "sensitivity",
+    "string",
+    ["base", "accent", "case", "variant"],
+    undefined
+  );
+  lazyCollatorData.rawSensitivity = s;
+
+  // Step 27.
+  var ip = GetOption(options, "ignorePunctuation", "boolean", undefined, false);
+  lazyCollatorData.ignorePunctuation = ip;
+
+  // Step 29.
+  //
+  // We've done everything that must be done now: mark the lazy data as fully
+  // computed and install it.
+  initializeIntlObject(collator, "Collator", lazyCollatorData);
+}
+
+/**
+ * Returns the subset of the given locale list for which this locale list has a
+ * matching (possibly fallback) locale. Locales appear in the same order in the
+ * returned list as in the input list.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 10.2.2.
+ */
+function Intl_Collator_supportedLocalesOf(locales /*, options*/) {
+  var options = arguments.length > 1 ? arguments[1] : undefined;
+
+  // Step 1.
+  var availableLocales = "Collator";
+
+  // Step 2.
+  var requestedLocales = CanonicalizeLocaleList(locales);
+
+  // Step 3.
+  return SupportedLocales(availableLocales, requestedLocales, options);
+}
+
+/**
+ * Collator internal properties.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.1 and 10.2.3.
+ */
+var collatorInternalProperties = {
+  sortLocaleData: collatorSortLocaleData,
+  searchLocaleData: collatorSearchLocaleData,
+  relevantExtensionKeys: ["co", "kf", "kn"],
+};
+
+/**
+ * Returns the actual locale used when a collator for |locale| is constructed.
+ */
+function collatorActualLocale(locale) {
+  assert(typeof locale === "string", "locale should be string");
+
+  // If |locale| is the default locale (e.g. da-DK), but only supported
+  // through a fallback (da), we need to get the actual locale before we
+  // can call intl_isUpperCaseFirst. Also see intl_BestAvailableLocale.
+  return BestAvailableLocaleIgnoringDefault("Collator", locale);
+}
+
+/**
+ * Returns the default caseFirst values for the given locale. The first
+ * element in the returned array denotes the default value per ES2017 Intl,
+ * 9.1 Internal slots of Service Constructors.
+ */
+function collatorSortCaseFirst(locale) {
+  var actualLocale = collatorActualLocale(locale);
+  if (intl_isUpperCaseFirst(actualLocale)) {
+    return ["upper", "false", "lower"];
+  }
+
+  // Default caseFirst values for all other languages.
+  return ["false", "lower", "upper"];
+}
+
+/**
+ * Returns the default caseFirst value for the given locale.
+ */
+function collatorSortCaseFirstDefault(locale) {
+  var actualLocale = collatorActualLocale(locale);
+  if (intl_isUpperCaseFirst(actualLocale)) {
+    return "upper";
+  }
+
+  // Default caseFirst value for all other languages.
+  return "false";
+}
+
+function collatorSortLocaleData() {
+  /* eslint-disable object-shorthand */
+  return {
+    co: intl_availableCollations,
+    kn: function() {
+      return ["false", "true"];
+    },
+    kf: collatorSortCaseFirst,
+    default: {
+      co: function() {
+        // The first element of the collations array must be |null|
+        // per ES2017 Intl, 10.2.3 Internal Slots.
+        return null;
+      },
+      kn: function() {
+        return "false";
+      },
+      kf: collatorSortCaseFirstDefault,
+    },
+  };
+  /* eslint-enable object-shorthand */
+}
+
+function collatorSearchLocaleData() {
+  /* eslint-disable object-shorthand */
+  return {
+    co: function() {
+      return [null];
+    },
+    kn: function() {
+      return ["false", "true"];
+    },
+    kf: function() {
+      return ["false", "lower", "upper"];
+    },
+    default: {
+      co: function() {
+        return null;
+      },
+      kn: function() {
+        return "false";
+      },
+      kf: function() {
+        return "false";
+      },
+    },
+  };
+  /* eslint-enable object-shorthand */
+}
+
+/**
+ * Create function to be cached and returned by Intl.Collator.prototype.compare.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 10.3.3.1.
+ */
+function createCollatorCompare(collator) {
+  // This function is not inlined in $Intl_Collator_compare_get to avoid
+  // creating a call-object on each call to $Intl_Collator_compare_get.
+  return function(x, y) {
+    // Step 1 (implicit).
+
+    // Step 2.
+    assert(IsObject(collator), "collatorCompareToBind called with non-object");
+    assert(
+      intl_GuardToCollator(collator) !== null,
+      "collatorCompareToBind called with non-Collator"
+    );
+
+    // Steps 3-6
+    var X = ToString(x);
+    var Y = ToString(y);
+
+    // Step 7.
+    return intl_CompareStrings(collator, X, Y);
+  };
+}
+
+/**
+ * Returns a function bound to this Collator that compares x (converted to a
+ * String value) and y (converted to a String value),
+ * and returns a number less than 0 if x < y, 0 if x = y, or a number greater
+ * than 0 if x > y according to the sort order for the locale and collation
+ * options of this Collator object.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 10.3.3.
+ */
+// Uncloned functions with `$` prefix are allocated as extended function
+// to store the original name in `SetCanonicalName`.
+function $Intl_Collator_compare_get() {
+  // Step 1.
+  var collator = this;
+
+  // Steps 2-3.
+  if (
+    !IsObject(collator) ||
+    (collator = intl_GuardToCollator(collator)) === null
+  ) {
+    return callFunction(
+      intl_CallCollatorMethodIfWrapped,
+      this,
+      "$Intl_Collator_compare_get"
+    );
+  }
+
+  var internals = getCollatorInternals(collator);
+
+  // Step 4.
+  if (internals.boundCompare === undefined) {
+    // Steps 4.a-c.
+    internals.boundCompare = createCollatorCompare(collator);
+  }
+
+  // Step 5.
+  return internals.boundCompare;
+}
+SetCanonicalName($Intl_Collator_compare_get, "get compare");
+
+/**
+ * Returns the resolved options for a Collator object.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 10.3.4.
+ */
+function Intl_Collator_resolvedOptions() {
+  // Step 1.
+  var collator = this;
+
+  // Steps 2-3.
+  if (
+    !IsObject(collator) ||
+    (collator = intl_GuardToCollator(collator)) === null
+  ) {
+    return callFunction(
+      intl_CallCollatorMethodIfWrapped,
+      this,
+      "Intl_Collator_resolvedOptions"
+    );
+  }
+
+  var internals = getCollatorInternals(collator);
+
+  // Steps 4-5.
+  var result = {
+    locale: internals.locale,
+    usage: internals.usage,
+    sensitivity: internals.sensitivity,
+    ignorePunctuation: internals.ignorePunctuation,
+    collation: internals.collation,
+    numeric: internals.numeric,
+    caseFirst: internals.caseFirst,
+  };
+
+  // Step 6.
+  return result;
+}
diff --git a/js/src/builtin/intl/CommonFunctions.cpp b/js/src/builtin/intl/CommonFunctions.cpp
new file mode 100644
index 0000000000..86f555baa5
--- /dev/null
+++ b/js/src/builtin/intl/CommonFunctions.cpp
@@ -0,0 +1,149 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Operations used to implement multiple Intl.* classes. */
+
+#include "builtin/intl/CommonFunctions.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/TextUtils.h"
+
+#include <algorithm>
+
+#include "gc/GCEnum.h"
+#include "gc/ZoneAllocator.h"
+#include "js/friend/ErrorMessages.h"  // js::GetErrorMessage, JSMSG_INTERNAL_INTL_ERROR
+#include "js/Value.h"
+#include "vm/JSAtomState.h"
+#include "vm/JSContext.h"
+#include "vm/JSObject.h"
+#include "vm/SelfHosting.h"
+#include "vm/Stack.h"
+#include "vm/StringType.h"
+
+#include "gc/GCContext-inl.h"
+
+bool js::intl::InitializeObject(JSContext* cx, JS::Handle<JSObject*> obj,
+                                JS::Handle<PropertyName*> initializer,
+                                JS::Handle<JS::Value> locales,
+                                JS::Handle<JS::Value> options) {
+  FixedInvokeArgs<3> args(cx);
+
+  args[0].setObject(*obj);
+  args[1].set(locales);
+  args[2].set(options);
+
+  RootedValue ignored(cx);
+  if (!CallSelfHostedFunction(cx, initializer, JS::NullHandleValue, args,
+                              &ignored)) {
+    return false;
+  }
+
+  MOZ_ASSERT(ignored.isUndefined(),
+             "Unexpected return value from non-legacy Intl object initializer");
+  return true;
+}
+
+bool js::intl::LegacyInitializeObject(JSContext* cx, JS::Handle<JSObject*> obj,
+                                      JS::Handle<PropertyName*> initializer,
+                                      JS::Handle<JS::Value> thisValue,
+                                      JS::Handle<JS::Value> locales,
+                                      JS::Handle<JS::Value> options,
+                                      DateTimeFormatOptions dtfOptions,
+                                      JS::MutableHandle<JS::Value> result) {
+  FixedInvokeArgs<5> args(cx);
+
+  args[0].setObject(*obj);
+  args[1].set(thisValue);
+  args[2].set(locales);
+  args[3].set(options);
+  args[4].setBoolean(dtfOptions == DateTimeFormatOptions::EnableMozExtensions);
+
+  if (!CallSelfHostedFunction(cx, initializer, NullHandleValue, args, result)) {
+    return false;
+  }
+
+  MOZ_ASSERT(result.isObject(),
+             "Legacy Intl object initializer must return an object");
+  return true;
+}
+
+JSObject* js::intl::GetInternalsObject(JSContext* cx,
+                                       JS::Handle<JSObject*> obj) {
+  FixedInvokeArgs<1> args(cx);
+
+  args[0].setObject(*obj);
+
+  RootedValue v(cx);
+  if (!js::CallSelfHostedFunction(cx, cx->names().getInternals, NullHandleValue,
+                                  args, &v)) {
+    return nullptr;
+  }
+
+  return &v.toObject();
+}
+
+void js::intl::ReportInternalError(JSContext* cx) {
+  JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                            JSMSG_INTERNAL_INTL_ERROR);
+}
+
+void js::intl::ReportInternalError(JSContext* cx,
+                                   mozilla::intl::ICUError error) {
+  switch (error) {
+    case mozilla::intl::ICUError::OutOfMemory:
+      ReportOutOfMemory(cx);
+      return;
+    case mozilla::intl::ICUError::InternalError:
+      ReportInternalError(cx);
+      return;
+    case mozilla::intl::ICUError::OverflowError:
+      ReportAllocationOverflow(cx);
+      return;
+  }
+  MOZ_CRASH("Unexpected ICU error");
+}
+
+const js::intl::OldStyleLanguageTagMapping
+    js::intl::oldStyleLanguageTagMappings[] = {
+        {"pa-PK", "pa-Arab-PK"}, {"zh-CN", "zh-Hans-CN"},
+        {"zh-HK", "zh-Hant-HK"}, {"zh-SG", "zh-Hans-SG"},
+        {"zh-TW", "zh-Hant-TW"},
+};
+
+js::UniqueChars js::intl::EncodeLocale(JSContext* cx, JSString* locale) {
+  MOZ_ASSERT(locale->length() > 0);
+
+  js::UniqueChars chars = EncodeAscii(cx, locale);
+
+#ifdef DEBUG
+  // Ensure the returned value contains only valid BCP 47 characters.
+  // (Lambdas can't be placed inside MOZ_ASSERT, so move the checks in an
+  // #ifdef block.)
+  if (chars) {
+    auto alnumOrDash = [](char c) {
+      return mozilla::IsAsciiAlphanumeric(c) || c == '-';
+    };
+    MOZ_ASSERT(mozilla::IsAsciiAlpha(chars[0]));
+    MOZ_ASSERT(
+        std::all_of(chars.get(), chars.get() + locale->length(), alnumOrDash));
+  }
+#endif
+
+  return chars;
+}
+
+void js::intl::AddICUCellMemory(JSObject* obj, size_t nbytes) {
+  // Account the (estimated) number of bytes allocated by an ICU object against
+  // the JSObject's zone.
+  AddCellMemory(obj, nbytes, MemoryUse::ICUObject);
+}
+
+void js::intl::RemoveICUCellMemory(JS::GCContext* gcx, JSObject* obj,
+                                   size_t nbytes) {
+  gcx->removeCellMemory(obj, nbytes, MemoryUse::ICUObject);
+}
diff --git a/js/src/builtin/intl/CommonFunctions.h b/js/src/builtin/intl/CommonFunctions.h
new file mode 100644
index 0000000000..3f9681c2b7
--- /dev/null
+++ b/js/src/builtin/intl/CommonFunctions.h
@@ -0,0 +1,103 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_CommonFunctions_h
+#define builtin_intl_CommonFunctions_h
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "js/RootingAPI.h"
+#include "js/Utility.h"
+
+namespace mozilla::intl {
+enum class ICUError : uint8_t;
+}
+
+namespace js {
+
+class PropertyName;
+
+namespace intl {
+
+/**
+ * Initialize a new Intl.* object using the named self-hosted function.
+ */
+extern bool InitializeObject(JSContext* cx, JS::Handle<JSObject*> obj,
+                             JS::Handle<PropertyName*> initializer,
+                             JS::Handle<JS::Value> locales,
+                             JS::Handle<JS::Value> options);
+
+enum class DateTimeFormatOptions {
+  Standard,
+  EnableMozExtensions,
+};
+
+/**
+ * Initialize an existing object as an Intl.* object using the named
+ * self-hosted function.  This is only for a few old Intl.* constructors, for
+ * legacy reasons -- new ones should use the function above instead.
+ */
+extern bool LegacyInitializeObject(JSContext* cx, JS::Handle<JSObject*> obj,
+                                   JS::Handle<PropertyName*> initializer,
+                                   JS::Handle<JS::Value> thisValue,
+                                   JS::Handle<JS::Value> locales,
+                                   JS::Handle<JS::Value> options,
+                                   DateTimeFormatOptions dtfOptions,
+                                   JS::MutableHandle<JS::Value> result);
+
+/**
+ * Returns the object holding the internal properties for obj.
+ */
+extern JSObject* GetInternalsObject(JSContext* cx, JS::Handle<JSObject*> obj);
+
+/** Report an Intl internal error not directly tied to a spec step. */
+extern void ReportInternalError(JSContext* cx);
+
+/** Report an Intl internal error not directly tied to a spec step. */
+extern void ReportInternalError(JSContext* cx, mozilla::intl::ICUError error);
+
+/**
+ * The last-ditch locale is used if none of the available locales satisfies a
+ * request. "en-GB" is used based on the assumptions that English is the most
+ * common second language, that both en-GB and en-US are normally available in
+ * an implementation, and that en-GB is more representative of the English used
+ * in other locales.
+ */
+static inline const char* LastDitchLocale() { return "en-GB"; }
+
+/**
+ * Certain old, commonly-used language tags that lack a script, are expected to
+ * nonetheless imply one. This object maps these old-style tags to modern
+ * equivalents.
+ */
+struct OldStyleLanguageTagMapping {
+  const char* const oldStyle;
+  const char* const modernStyle;
+
+  // Provide a constructor to catch missing initializers in the mappings array.
+  constexpr OldStyleLanguageTagMapping(const char* oldStyle,
+                                       const char* modernStyle)
+      : oldStyle(oldStyle), modernStyle(modernStyle) {}
+};
+
+extern const OldStyleLanguageTagMapping oldStyleLanguageTagMappings[5];
+
+extern JS::UniqueChars EncodeLocale(JSContext* cx, JSString* locale);
+
+// The inline capacity we use for a Vector<char16_t>.  Use this to ensure that
+// our uses of ICU string functions, below and elsewhere, will try to fill the
+// buffer's entire inline capacity before growing it and heap-allocating.
+constexpr size_t INITIAL_CHAR_BUFFER_SIZE = 32;
+
+void AddICUCellMemory(JSObject* obj, size_t nbytes);
+
+void RemoveICUCellMemory(JS::GCContext* gcx, JSObject* obj, size_t nbytes);
+}  // namespace intl
+
+}  // namespace js
+
+#endif /* builtin_intl_CommonFunctions_h */
diff --git a/js/src/builtin/intl/CommonFunctions.js b/js/src/builtin/intl/CommonFunctions.js
new file mode 100644
index 0000000000..c369c49afa
--- /dev/null
+++ b/js/src/builtin/intl/CommonFunctions.js
@@ -0,0 +1,992 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Portions Copyright Norbert Lindenberg 2011-2012. */
+
+#ifdef DEBUG
+#define assertIsValidAndCanonicalLanguageTag(locale, desc) \
+  do { \
+    let canonical = intl_TryValidateAndCanonicalizeLanguageTag(locale); \
+    assert(canonical !== null, \
+           `${desc} is a structurally valid language tag`); \
+    assert(canonical === locale, \
+           `${desc} is a canonicalized language tag`); \
+  } while (false)
+#else
+#define assertIsValidAndCanonicalLanguageTag(locale, desc) ; // Elided assertion.
+#endif
+
+/**
+ * Returns the start index of a "Unicode locale extension sequence", which the
+ * specification defines as: "any substring of a language tag that starts with
+ * a separator '-' and the singleton 'u' and includes the maximum sequence of
+ * following non-singleton subtags and their preceding '-' separators."
+ *
+ * Alternatively, this may be defined as: the components of a language tag that
+ * match the `unicode_locale_extensions` production in UTS 35.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 6.2.1.
+ */
+function startOfUnicodeExtensions(locale) {
+  assert(typeof locale === "string", "locale is a string");
+
+  // Search for "-u-" marking the start of a Unicode extension sequence.
+  var start = callFunction(std_String_indexOf, locale, "-u-");
+  if (start < 0) {
+    return -1;
+  }
+
+  // And search for "-x-" marking the start of any privateuse component to
+  // handle the case when "-u-" was only found within a privateuse subtag.
+  var privateExt = callFunction(std_String_indexOf, locale, "-x-");
+  if (privateExt >= 0 && privateExt < start) {
+    return -1;
+  }
+
+  return start;
+}
+
+/**
+ * Returns the end index of a Unicode locale extension sequence.
+ */
+function endOfUnicodeExtensions(locale, start) {
+  assert(typeof locale === "string", "locale is a string");
+  assert(0 <= start && start < locale.length, "start is an index into locale");
+  assert(
+    Substring(locale, start, 3) === "-u-",
+    "start points to Unicode extension sequence"
+  );
+
+  // Search for the start of the next singleton or privateuse subtag.
+  //
+  // Begin searching after the smallest possible Unicode locale extension
+  // sequence, namely |"-u-" 2alphanum|. End searching once the remaining
+  // characters can't fit the smallest possible singleton or privateuse
+  // subtag, namely |"-x-" alphanum|. Note the reduced end-limit means
+  // indexing inside the loop is always in-range.
+  for (var i = start + 5, end = locale.length - 4; i <= end; i++) {
+    if (locale[i] !== "-") {
+      continue;
+    }
+    if (locale[i + 2] === "-") {
+      return i;
+    }
+
+    // Skip over (i + 1) and (i + 2) because we've just verified they
+    // aren't "-", so the next possible delimiter can only be at (i + 3).
+    i += 2;
+  }
+
+  // If no singleton or privateuse subtag was found, the Unicode extension
+  // sequence extends until the end of the string.
+  return locale.length;
+}
+
+/**
+ * Removes Unicode locale extension sequences from the given language tag.
+ */
+function removeUnicodeExtensions(locale) {
+  assertIsValidAndCanonicalLanguageTag(
+    locale,
+    "locale with possible Unicode extension"
+  );
+
+  var start = startOfUnicodeExtensions(locale);
+  if (start < 0) {
+    return locale;
+  }
+
+  var end = endOfUnicodeExtensions(locale, start);
+
+  var left = Substring(locale, 0, start);
+  var right = Substring(locale, end, locale.length - end);
+  var combined = left + right;
+
+  assertIsValidAndCanonicalLanguageTag(combined, "the recombined locale");
+  assert(
+    startOfUnicodeExtensions(combined) < 0,
+    "recombination failed to remove all Unicode locale extension sequences"
+  );
+
+  return combined;
+}
+
+/**
+ * Returns Unicode locale extension sequences from the given language tag.
+ */
+function getUnicodeExtensions(locale) {
+  assertIsValidAndCanonicalLanguageTag(locale, "locale with Unicode extension");
+
+  var start = startOfUnicodeExtensions(locale);
+  assert(start >= 0, "start of Unicode extension sequence not found");
+  var end = endOfUnicodeExtensions(locale, start);
+
+  return Substring(locale, start, end - start);
+}
+
+/**
+ * Returns true if the input contains only ASCII alphabetical characters.
+ */
+function IsASCIIAlphaString(s) {
+  assert(typeof s === "string", "IsASCIIAlphaString");
+
+  for (var i = 0; i < s.length; i++) {
+    var c = callFunction(std_String_charCodeAt, s, i);
+    if (!((0x41 <= c && c <= 0x5a) || (0x61 <= c && c <= 0x7a))) {
+      return false;
+    }
+  }
+  return true;
+}
+
+var localeCache = {
+  runtimeDefaultLocale: undefined,
+  defaultLocale: undefined,
+};
+
+/**
+ * Returns the BCP 47 language tag for the host environment's current locale.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 6.2.4.
+ */
+function DefaultLocale() {
+  if (intl_IsRuntimeDefaultLocale(localeCache.runtimeDefaultLocale)) {
+    return localeCache.defaultLocale;
+  }
+
+  // If we didn't have a cache hit, compute the candidate default locale.
+  var runtimeDefaultLocale = intl_RuntimeDefaultLocale();
+  var locale = intl_supportedLocaleOrFallback(runtimeDefaultLocale);
+
+  assertIsValidAndCanonicalLanguageTag(locale, "the computed default locale");
+  assert(
+    startOfUnicodeExtensions(locale) < 0,
+    "the computed default locale must not contain a Unicode extension sequence"
+  );
+
+  // Cache the computed locale until the runtime default locale changes.
+  localeCache.defaultLocale = locale;
+  localeCache.runtimeDefaultLocale = runtimeDefaultLocale;
+
+  return locale;
+}
+
+/**
+ * Canonicalizes a locale list.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.1.
+ */
+function CanonicalizeLocaleList(locales) {
+  // Step 1.
+  if (locales === undefined) {
+    return [];
+  }
+
+  // Step 3 (and the remaining steps).
+  var tag = intl_ValidateAndCanonicalizeLanguageTag(locales, false);
+  if (tag !== null) {
+    assert(
+      typeof tag === "string",
+      "intl_ValidateAndCanonicalizeLanguageTag returns a string value"
+    );
+    return [tag];
+  }
+
+  // Step 2.
+  var seen = [];
+
+  // Step 4.
+  var O = ToObject(locales);
+
+  // Step 5.
+  var len = ToLength(O.length);
+
+  // Step 6.
+  var k = 0;
+
+  // Step 7.
+  while (k < len) {
+    // Steps 7.a-c.
+    if (k in O) {
+      // Step 7.c.i.
+      var kValue = O[k];
+
+      // Step 7.c.ii.
+      if (!(typeof kValue === "string" || IsObject(kValue))) {
+        ThrowTypeError(JSMSG_INVALID_LOCALES_ELEMENT);
+      }
+
+      // Steps 7.c.iii-iv.
+      var tag = intl_ValidateAndCanonicalizeLanguageTag(kValue, true);
+      assert(
+        typeof tag === "string",
+        "ValidateAndCanonicalizeLanguageTag returns a string value"
+      );
+
+      // Step 7.c.v.
+      if (callFunction(std_Array_indexOf, seen, tag) === -1) {
+        DefineDataProperty(seen, seen.length, tag);
+      }
+    }
+
+    // Step 7.d.
+    k++;
+  }
+
+  // Step 8.
+  return seen;
+}
+
+/**
+ * Compares a BCP 47 language tag against the locales in availableLocales
+ * and returns the best available match. Uses the fallback
+ * mechanism of RFC 4647, section 3.4.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.2.
+ * Spec: RFC 4647, section 3.4.
+ */
+function BestAvailableLocale(availableLocales, locale) {
+  return intl_BestAvailableLocale(availableLocales, locale, DefaultLocale());
+}
+
+/**
+ * Identical to BestAvailableLocale, but does not consider the default locale
+ * during computation.
+ */
+function BestAvailableLocaleIgnoringDefault(availableLocales, locale) {
+  return intl_BestAvailableLocale(availableLocales, locale, null);
+}
+
+/**
+ * Compares a BCP 47 language priority list against the set of locales in
+ * availableLocales and determines the best available language to meet the
+ * request. Options specified through Unicode extension subsequences are
+ * ignored in the lookup, but information about such subsequences is returned
+ * separately.
+ *
+ * This variant is based on the Lookup algorithm of RFC 4647 section 3.4.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.3.
+ * Spec: RFC 4647, section 3.4.
+ */
+function LookupMatcher(availableLocales, requestedLocales) {
+  // Step 1.
+  var result = new_Record();
+
+  // Step 2.
+  for (var i = 0; i < requestedLocales.length; i++) {
+    var locale = requestedLocales[i];
+
+    // Step 2.a.
+    var noExtensionsLocale = removeUnicodeExtensions(locale);
+
+    // Step 2.b.
+    var availableLocale = BestAvailableLocale(
+      availableLocales,
+      noExtensionsLocale
+    );
+
+    // Step 2.c.
+    if (availableLocale !== undefined) {
+      // Step 2.c.i.
+      result.locale = availableLocale;
+
+      // Step 2.c.ii.
+      if (locale !== noExtensionsLocale) {
+        result.extension = getUnicodeExtensions(locale);
+      }
+
+      // Step 2.c.iii.
+      return result;
+    }
+  }
+
+  // Steps 3-4.
+  result.locale = DefaultLocale();
+
+  // Step 5.
+  return result;
+}
+
+/**
+ * Compares a BCP 47 language priority list against the set of locales in
+ * availableLocales and determines the best available language to meet the
+ * request. Options specified through Unicode extension subsequences are
+ * ignored in the lookup, but information about such subsequences is returned
+ * separately.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.4.
+ */
+function BestFitMatcher(availableLocales, requestedLocales) {
+  // this implementation doesn't have anything better
+  return LookupMatcher(availableLocales, requestedLocales);
+}
+
+/**
+ * Returns the Unicode extension value subtags for the requested key subtag.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.5.
+ */
+function UnicodeExtensionValue(extension, key) {
+  assert(typeof extension === "string", "extension is a string value");
+  assert(
+    callFunction(std_String_startsWith, extension, "-u-") &&
+      getUnicodeExtensions("und" + extension) === extension,
+    "extension is a Unicode extension subtag"
+  );
+  assert(typeof key === "string", "key is a string value");
+
+  // Step 1.
+  assert(key.length === 2, "key is a Unicode extension key subtag");
+
+  // Step 2.
+  var size = extension.length;
+
+  // Step 3.
+  var searchValue = "-" + key + "-";
+
+  // Step 4.
+  var pos = callFunction(std_String_indexOf, extension, searchValue);
+
+  // Step 5.
+  if (pos !== -1) {
+    // Step 5.a.
+    var start = pos + 4;
+
+    // Step 5.b.
+    var end = start;
+
+    // Step 5.c.
+    var k = start;
+
+    // Steps 5.d-e.
+    while (true) {
+      // Step 5.e.i.
+      var e = callFunction(std_String_indexOf, extension, "-", k);
+
+      // Step 5.e.ii.
+      var len = e === -1 ? size - k : e - k;
+
+      // Step 5.e.iii.
+      if (len === 2) {
+        break;
+      }
+
+      // Step 5.e.iv.
+      if (e === -1) {
+        end = size;
+        break;
+      }
+
+      // Step 5.e.v.
+      end = e;
+      k = e + 1;
+    }
+
+    // Step 5.f.
+    return callFunction(String_substring, extension, start, end);
+  }
+
+  // Step 6.
+  searchValue = "-" + key;
+
+  // Steps 7-8.
+  if (callFunction(std_String_endsWith, extension, searchValue)) {
+    return "";
+  }
+
+  // Step 9 (implicit).
+}
+
+/**
+ * Compares a BCP 47 language priority list against availableLocales and
+ * determines the best available language to meet the request. Options specified
+ * through Unicode extension subsequences are negotiated separately, taking the
+ * caller's relevant extensions and locale data as well as client-provided
+ * options into consideration.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.6.
+ */
+function ResolveLocale(
+  availableLocales,
+  requestedLocales,
+  options,
+  relevantExtensionKeys,
+  localeData
+) {
+  // Steps 1-3.
+  var matcher = options.localeMatcher;
+  var r =
+    matcher === "lookup"
+      ? LookupMatcher(availableLocales, requestedLocales)
+      : BestFitMatcher(availableLocales, requestedLocales);
+
+  // Step 4.
+  var foundLocale = r.locale;
+  var extension = r.extension;
+
+  // Step 5.
+  var result = new_Record();
+
+  // Step 6.
+  result.dataLocale = foundLocale;
+
+  // Step 7.
+  var supportedExtension = "-u";
+
+  // In this implementation, localeData is a function, not an object.
+  var localeDataProvider = localeData();
+
+  // Step 8.
+  for (var i = 0; i < relevantExtensionKeys.length; i++) {
+    var key = relevantExtensionKeys[i];
+
+    // Steps 8.a-h (The locale data is only computed when needed).
+    var keyLocaleData = undefined;
+    var value = undefined;
+
+    // Locale tag may override.
+
+    // Step 8.g.
+    var supportedExtensionAddition = "";
+
+    // Step 8.h.
+    if (extension !== undefined) {
+      // Step 8.h.i.
+      var requestedValue = UnicodeExtensionValue(extension, key);
+
+      // Step 8.h.ii.
+      if (requestedValue !== undefined) {
+        // Steps 8.a-d.
+        keyLocaleData = callFunction(
+          localeDataProvider[key],
+          null,
+          foundLocale
+        );
+
+        // Step 8.h.ii.1.
+        if (requestedValue !== "") {
+          // Step 8.h.ii.1.a.
+          if (
+            callFunction(std_Array_indexOf, keyLocaleData, requestedValue) !==
+            -1
+          ) {
+            value = requestedValue;
+            supportedExtensionAddition = "-" + key + "-" + value;
+          }
+        } else {
+          // Step 8.h.ii.2.
+
+          // According to the LDML spec, if there's no type value,
+          // and true is an allowed value, it's used.
+          if (callFunction(std_Array_indexOf, keyLocaleData, "true") !== -1) {
+            value = "true";
+            supportedExtensionAddition = "-" + key;
+          }
+        }
+      }
+    }
+
+    // Options override all.
+
+    // Step 8.i.i.
+    var optionsValue = options[key];
+
+    // Step 8.i.ii.
+    assert(
+      typeof optionsValue === "string" ||
+        optionsValue === undefined ||
+        optionsValue === null,
+      "unexpected type for options value"
+    );
+
+    // Steps 8.i, 8.i.iii.1.
+    if (optionsValue !== undefined && optionsValue !== value) {
+      // Steps 8.a-d.
+      if (keyLocaleData === undefined) {
+        keyLocaleData = callFunction(
+          localeDataProvider[key],
+          null,
+          foundLocale
+        );
+      }
+
+      // Step 8.i.iii.
+      if (callFunction(std_Array_indexOf, keyLocaleData, optionsValue) !== -1) {
+        value = optionsValue;
+        supportedExtensionAddition = "";
+      }
+    }
+
+    // Locale data provides default value.
+    if (value === undefined) {
+      // Steps 8.a-f.
+      value =
+        keyLocaleData === undefined
+          ? callFunction(localeDataProvider.default[key], null, foundLocale)
+          : keyLocaleData[0];
+    }
+
+    // Step 8.j.
+    assert(
+      typeof value === "string" || value === null,
+      "unexpected locale data value"
+    );
+    result[key] = value;
+
+    // Step 8.k.
+    supportedExtension += supportedExtensionAddition;
+  }
+
+  // Step 9.
+  if (supportedExtension.length > 2) {
+    foundLocale = addUnicodeExtension(foundLocale, supportedExtension);
+  }
+
+  // Step 10.
+  result.locale = foundLocale;
+
+  // Step 11.
+  return result;
+}
+
+/**
+ * Adds a Unicode extension subtag to a locale.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.6.
+ */
+function addUnicodeExtension(locale, extension) {
+  assert(typeof locale === "string", "locale is a string value");
+  assert(
+    !callFunction(std_String_startsWith, locale, "x-"),
+    "unexpected privateuse-only locale"
+  );
+  assert(
+    startOfUnicodeExtensions(locale) < 0,
+    "Unicode extension subtag already present in locale"
+  );
+
+  assert(typeof extension === "string", "extension is a string value");
+  assert(
+    callFunction(std_String_startsWith, extension, "-u-") &&
+      getUnicodeExtensions("und" + extension) === extension,
+    "extension is a Unicode extension subtag"
+  );
+
+  // Step 9.a.
+  var privateIndex = callFunction(std_String_indexOf, locale, "-x-");
+
+  // Steps 9.b-c.
+  if (privateIndex === -1) {
+    locale += extension;
+  } else {
+    var preExtension = callFunction(String_substring, locale, 0, privateIndex);
+    var postExtension = callFunction(String_substring, locale, privateIndex);
+    locale = preExtension + extension + postExtension;
+  }
+
+  // Steps 9.d-e (Step 9.e is not required in this implementation, because we don't canonicalize
+  // Unicode extension subtags).
+  assertIsValidAndCanonicalLanguageTag(locale, "locale after concatenation");
+
+  return locale;
+}
+
+/**
+ * Returns the subset of requestedLocales for which availableLocales has a
+ * matching (possibly fallback) locale. Locales appear in the same order in the
+ * returned list as in the input list.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.7.
+ */
+function LookupSupportedLocales(availableLocales, requestedLocales) {
+  // Step 1.
+  var subset = [];
+
+  // Step 2.
+  for (var i = 0; i < requestedLocales.length; i++) {
+    var locale = requestedLocales[i];
+
+    // Step 2.a.
+    var noExtensionsLocale = removeUnicodeExtensions(locale);
+
+    // Step 2.b.
+    var availableLocale = BestAvailableLocale(
+      availableLocales,
+      noExtensionsLocale
+    );
+
+    // Step 2.c.
+    if (availableLocale !== undefined) {
+      DefineDataProperty(subset, subset.length, locale);
+    }
+  }
+
+  // Step 3.
+  return subset;
+}
+
+/**
+ * Returns the subset of requestedLocales for which availableLocales has a
+ * matching (possibly fallback) locale. Locales appear in the same order in the
+ * returned list as in the input list.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.8.
+ */
+function BestFitSupportedLocales(availableLocales, requestedLocales) {
+  // don't have anything better
+  return LookupSupportedLocales(availableLocales, requestedLocales);
+}
+
+/**
+ * Returns the subset of requestedLocales for which availableLocales has a
+ * matching (possibly fallback) locale. Locales appear in the same order in the
+ * returned list as in the input list.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.9.
+ */
+function SupportedLocales(availableLocales, requestedLocales, options) {
+  // Step 1.
+  var matcher;
+  if (options !== undefined) {
+    // Step 1.a.
+    options = ToObject(options);
+
+    // Step 1.b
+    matcher = options.localeMatcher;
+    if (matcher !== undefined) {
+      matcher = ToString(matcher);
+      if (matcher !== "lookup" && matcher !== "best fit") {
+        ThrowRangeError(JSMSG_INVALID_LOCALE_MATCHER, matcher);
+      }
+    }
+  }
+
+  // Steps 2-5.
+  return matcher === undefined || matcher === "best fit"
+    ? BestFitSupportedLocales(availableLocales, requestedLocales)
+    : LookupSupportedLocales(availableLocales, requestedLocales);
+}
+
+/**
+ * Extracts a property value from the provided options object, converts it to
+ * the required type, checks whether it is one of a list of allowed values,
+ * and fills in a fallback value if necessary.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.10.
+ */
+function GetOption(options, property, type, values, fallback) {
+  // Step 1.
+  var value = options[property];
+
+  // Step 2.
+  if (value !== undefined) {
+    // Steps 2.a-c.
+    if (type === "boolean") {
+      value = ToBoolean(value);
+    } else if (type === "string") {
+      value = ToString(value);
+    } else {
+      assert(false, "GetOption");
+    }
+
+    // Step 2.d.
+    if (
+      values !== undefined &&
+      callFunction(std_Array_indexOf, values, value) === -1
+    ) {
+      ThrowRangeError(JSMSG_INVALID_OPTION_VALUE, property, `"${value}"`);
+    }
+
+    // Step 2.e.
+    return value;
+  }
+
+  // Step 3.
+  return fallback;
+}
+
+/**
+ * Extracts a property value from the provided options object, converts it to
+ * a boolean or string, checks whether it is one of a list of allowed values,
+ * and fills in a fallback value if necessary.
+ */
+function GetStringOrBooleanOption(
+  options,
+  property,
+  values,
+  trueValue,
+  falsyValue,
+  fallback
+) {
+  assert(IsObject(values), "GetStringOrBooleanOption");
+
+  // Step 1.
+  var value = options[property];
+
+  // Step 2.
+  if (value === undefined) {
+    return fallback;
+  }
+
+  // Step 3.
+  if (value === true) {
+    return trueValue;
+  }
+
+  // Steps 4-5.
+  if (!value) {
+    return falsyValue;
+  }
+
+  // Step 6.
+  value = ToString(value);
+
+  // Step 7.
+  if (callFunction(std_Array_indexOf, values, value) === -1) {
+    return fallback;
+  }
+
+  // Step 8.
+  return value;
+}
+
+/**
+ * The abstract operation DefaultNumberOption converts value to a Number value,
+ * checks whether it is in the allowed range, and fills in a fallback value if
+ * necessary.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.11.
+ */
+function DefaultNumberOption(value, minimum, maximum, fallback) {
+  assert(
+    typeof minimum === "number" && (minimum | 0) === minimum,
+    "DefaultNumberOption"
+  );
+  assert(
+    typeof maximum === "number" && (maximum | 0) === maximum,
+    "DefaultNumberOption"
+  );
+  assert(
+    fallback === undefined ||
+      (typeof fallback === "number" && (fallback | 0) === fallback),
+    "DefaultNumberOption"
+  );
+  assert(
+    fallback === undefined || (minimum <= fallback && fallback <= maximum),
+    "DefaultNumberOption"
+  );
+
+  // Step 1.
+  if (value === undefined) {
+    return fallback;
+  }
+
+  // Step 2.
+  value = ToNumber(value);
+
+  // Step 3.
+  if (Number_isNaN(value) || value < minimum || value > maximum) {
+    ThrowRangeError(JSMSG_INVALID_DIGITS_VALUE, value);
+  }
+
+  // Step 4.
+  // Apply bitwise-or to convert -0 to +0 per ES2017, 5.2 and to ensure the
+  // result is an int32 value.
+  return std_Math_floor(value) | 0;
+}
+
+/**
+ * Extracts a property value from the provided options object, converts it to a
+ * Number value, checks whether it is in the allowed range, and fills in a
+ * fallback value if necessary.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.12.
+ */
+function GetNumberOption(options, property, minimum, maximum, fallback) {
+  // Steps 1-2.
+  return DefaultNumberOption(options[property], minimum, maximum, fallback);
+}
+
+// Symbols in the self-hosting compartment can't be cloned, use a separate
+// object to hold the actual symbol value.
+// TODO: Can we add support to clone symbols?
+var intlFallbackSymbolHolder = { value: undefined };
+
+/**
+ * The [[FallbackSymbol]] symbol of the %Intl% intrinsic object.
+ *
+ * This symbol is used to implement the legacy constructor semantics for
+ * Intl.DateTimeFormat and Intl.NumberFormat.
+ */
+function intlFallbackSymbol() {
+  var fallbackSymbol = intlFallbackSymbolHolder.value;
+  if (!fallbackSymbol) {
+    let Symbol = GetBuiltinConstructor("Symbol");
+    fallbackSymbol = Symbol("IntlLegacyConstructedSymbol");
+    intlFallbackSymbolHolder.value = fallbackSymbol;
+  }
+  return fallbackSymbol;
+}
+
+/**
+ * Initializes the INTL_INTERNALS_OBJECT_SLOT of the given object.
+ */
+function initializeIntlObject(obj, type, lazyData) {
+  assert(IsObject(obj), "Non-object passed to initializeIntlObject");
+  assert(
+    (type === "Collator" && intl_GuardToCollator(obj) !== null) ||
+      (type === "DateTimeFormat" && intl_GuardToDateTimeFormat(obj) !== null) ||
+      (type === "DisplayNames" && intl_GuardToDisplayNames(obj) !== null) ||
+      (type === "ListFormat" && intl_GuardToListFormat(obj) !== null) ||
+      (type === "NumberFormat" && intl_GuardToNumberFormat(obj) !== null) ||
+      (type === "PluralRules" && intl_GuardToPluralRules(obj) !== null) ||
+      (type === "RelativeTimeFormat" &&
+        intl_GuardToRelativeTimeFormat(obj) !== null),
+    "type must match the object's class"
+  );
+  assert(IsObject(lazyData), "non-object lazy data");
+
+  // The meaning of an internals object for an object |obj| is as follows.
+  //
+  // The .type property indicates the type of Intl object that |obj| is. It
+  // must be one of:
+  // - Collator
+  // - DateTimeFormat
+  // - DisplayNames
+  // - ListFormat
+  // - NumberFormat
+  // - PluralRules
+  // - RelativeTimeFormat
+  //
+  // The .lazyData property stores information needed to compute -- without
+  // observable side effects -- the actual internal Intl properties of
+  // |obj|.  If it is non-null, then the actual internal properties haven't
+  // been computed, and .lazyData must be processed by
+  // |setInternalProperties| before internal Intl property values are
+  // available.  If it is null, then the .internalProps property contains an
+  // object whose properties are the internal Intl properties of |obj|.
+
+  var internals = std_Object_create(null);
+  internals.type = type;
+  internals.lazyData = lazyData;
+  internals.internalProps = null;
+
+  assert(
+    UnsafeGetReservedSlot(obj, INTL_INTERNALS_OBJECT_SLOT) === undefined,
+    "Internal slot already initialized?"
+  );
+  UnsafeSetReservedSlot(obj, INTL_INTERNALS_OBJECT_SLOT, internals);
+}
+
+/**
+ * Set the internal properties object for an |internals| object previously
+ * associated with lazy data.
+ */
+function setInternalProperties(internals, internalProps) {
+  assert(IsObject(internals.lazyData), "lazy data must exist already");
+  assert(IsObject(internalProps), "internalProps argument should be an object");
+
+  // Set in reverse order so that the .lazyData nulling is a barrier.
+  internals.internalProps = internalProps;
+  internals.lazyData = null;
+}
+
+/**
+ * Get the existing internal properties out of a non-newborn |internals|, or
+ * null if none have been computed.
+ */
+function maybeInternalProperties(internals) {
+  assert(IsObject(internals), "non-object passed to maybeInternalProperties");
+  var lazyData = internals.lazyData;
+  if (lazyData) {
+    return null;
+  }
+  assert(
+    IsObject(internals.internalProps),
+    "missing lazy data and computed internals"
+  );
+  return internals.internalProps;
+}
+
+/**
+ * Return |obj|'s internals object (*not* the object holding its internal
+ * properties!), with structure specified above.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 10.3.
+ * Spec: ECMAScript Internationalization API Specification, 11.3.
+ * Spec: ECMAScript Internationalization API Specification, 12.3.
+ */
+function getIntlObjectInternals(obj) {
+  assert(IsObject(obj), "getIntlObjectInternals called with non-Object");
+  assert(
+    intl_GuardToCollator(obj) !== null ||
+      intl_GuardToDateTimeFormat(obj) !== null ||
+      intl_GuardToDisplayNames(obj) !== null ||
+      intl_GuardToListFormat(obj) !== null ||
+      intl_GuardToNumberFormat(obj) !== null ||
+      intl_GuardToPluralRules(obj) !== null ||
+      intl_GuardToRelativeTimeFormat(obj) !== null,
+    "getIntlObjectInternals called with non-Intl object"
+  );
+
+  var internals = UnsafeGetReservedSlot(obj, INTL_INTERNALS_OBJECT_SLOT);
+
+  assert(IsObject(internals), "internals not an object");
+  assert(hasOwn("type", internals), "missing type");
+  assert(
+    (internals.type === "Collator" && intl_GuardToCollator(obj) !== null) ||
+      (internals.type === "DateTimeFormat" &&
+        intl_GuardToDateTimeFormat(obj) !== null) ||
+      (internals.type === "DisplayNames" &&
+        intl_GuardToDisplayNames(obj) !== null) ||
+      (internals.type === "ListFormat" &&
+        intl_GuardToListFormat(obj) !== null) ||
+      (internals.type === "NumberFormat" &&
+        intl_GuardToNumberFormat(obj) !== null) ||
+      (internals.type === "PluralRules" &&
+        intl_GuardToPluralRules(obj) !== null) ||
+      (internals.type === "RelativeTimeFormat" &&
+        intl_GuardToRelativeTimeFormat(obj) !== null),
+    "type must match the object's class"
+  );
+  assert(hasOwn("lazyData", internals), "missing lazyData");
+  assert(hasOwn("internalProps", internals), "missing internalProps");
+
+  return internals;
+}
+
+/**
+ * Get the internal properties of known-Intl object |obj|.  For use only by
+ * C++ code that knows what it's doing!
+ */
+function getInternals(obj) {
+  var internals = getIntlObjectInternals(obj);
+
+  // If internal properties have already been computed, use them.
+  var internalProps = maybeInternalProperties(internals);
+  if (internalProps) {
+    return internalProps;
+  }
+
+  // Otherwise it's time to fully create them.
+  var type = internals.type;
+  if (type === "Collator") {
+    internalProps = resolveCollatorInternals(internals.lazyData);
+  } else if (type === "DateTimeFormat") {
+    internalProps = resolveDateTimeFormatInternals(internals.lazyData);
+  } else if (type === "DisplayNames") {
+    internalProps = resolveDisplayNamesInternals(internals.lazyData);
+  } else if (type === "ListFormat") {
+    internalProps = resolveListFormatInternals(internals.lazyData);
+  } else if (type === "NumberFormat") {
+    internalProps = resolveNumberFormatInternals(internals.lazyData);
+  } else if (type === "PluralRules") {
+    internalProps = resolvePluralRulesInternals(internals.lazyData);
+  } else {
+    internalProps = resolveRelativeTimeFormatInternals(internals.lazyData);
+  }
+  setInternalProperties(internals, internalProps);
+  return internalProps;
+}
diff --git a/js/src/builtin/intl/CurrencyDataGenerated.js b/js/src/builtin/intl/CurrencyDataGenerated.js
new file mode 100644
index 0000000000..b2fed80bd6
--- /dev/null
+++ b/js/src/builtin/intl/CurrencyDataGenerated.js
@@ -0,0 +1,78 @@
+// Generated by make_intl_data.py. DO NOT EDIT.
+// Version: 2022-04-01
+
+/**
+ * Mapping from currency codes to the number of decimal digits used for them.
+ * Default is 2 digits.
+ *
+ * Spec: ISO 4217 Currency and Funds Code List.
+ * http://www.currency-iso.org/en/home/tables/table-a1.html
+ */
+var currencyDigits = {
+  // Bahraini Dinar (BAHRAIN)
+  BHD: 3,
+  // Burundi Franc (BURUNDI)
+  BIF: 0,
+  // Unidad de Fomento (CHILE)
+  CLF: 4,
+  // Chilean Peso (CHILE)
+  CLP: 0,
+  // Djibouti Franc (DJIBOUTI)
+  DJF: 0,
+  // Guinean Franc (GUINEA)
+  GNF: 0,
+  // Iraqi Dinar (IRAQ)
+  IQD: 3,
+  // Iceland Krona (ICELAND)
+  ISK: 0,
+  // Jordanian Dinar (JORDAN)
+  JOD: 3,
+  // Yen (JAPAN)
+  JPY: 0,
+  // Comorian Franc  (COMOROS (THE))
+  KMF: 0,
+  // Won (KOREA (THE REPUBLIC OF))
+  KRW: 0,
+  // Kuwaiti Dinar (KUWAIT)
+  KWD: 3,
+  // Libyan Dinar (LIBYA)
+  LYD: 3,
+  // Rial Omani (OMAN)
+  OMR: 3,
+  // Guarani (PARAGUAY)
+  PYG: 0,
+  // Rwanda Franc (RWANDA)
+  RWF: 0,
+  // Tunisian Dinar (TUNISIA)
+  TND: 3,
+  // Uganda Shilling (UGANDA)
+  UGX: 0,
+  // Uruguay Peso en Unidades Indexadas (UI) (URUGUAY)
+  UYI: 0,
+  // Unidad Previsional (URUGUAY)
+  UYW: 4,
+  // Dong (VIET NAM)
+  VND: 0,
+  // Vatu (VANUATU)
+  VUV: 0,
+  // CFA Franc BEAC (CAMEROON)
+  // CFA Franc BEAC (CENTRAL AFRICAN REPUBLIC (THE))
+  // CFA Franc BEAC (CHAD)
+  // CFA Franc BEAC (CONGO (THE))
+  // CFA Franc BEAC (EQUATORIAL GUINEA)
+  // CFA Franc BEAC (GABON)
+  XAF: 0,
+  // CFA Franc BCEAO (BENIN)
+  // CFA Franc BCEAO (BURKINA FASO)
+  // CFA Franc BCEAO (CÔTE D'IVOIRE)
+  // CFA Franc BCEAO (GUINEA-BISSAU)
+  // CFA Franc BCEAO (MALI)
+  // CFA Franc BCEAO (NIGER (THE))
+  // CFA Franc BCEAO (SENEGAL)
+  // CFA Franc BCEAO (TOGO)
+  XOF: 0,
+  // CFP Franc (FRENCH POLYNESIA)
+  // CFP Franc (NEW CALEDONIA)
+  // CFP Franc (WALLIS AND FUTUNA)
+  XPF: 0,
+};
diff --git a/js/src/builtin/intl/DateTimeFormat.cpp b/js/src/builtin/intl/DateTimeFormat.cpp
new file mode 100644
index 0000000000..bb5f259b9a
--- /dev/null
+++ b/js/src/builtin/intl/DateTimeFormat.cpp
@@ -0,0 +1,1580 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Intl.DateTimeFormat implementation. */
+
+#include "builtin/intl/DateTimeFormat.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/intl/Calendar.h"
+#include "mozilla/intl/DateIntervalFormat.h"
+#include "mozilla/intl/DateTimeFormat.h"
+#include "mozilla/intl/DateTimePart.h"
+#include "mozilla/intl/Locale.h"
+#include "mozilla/intl/TimeZone.h"
+#include "mozilla/Range.h"
+#include "mozilla/Span.h"
+
+#include "builtin/Array.h"
+#include "builtin/intl/CommonFunctions.h"
+#include "builtin/intl/FormatBuffer.h"
+#include "builtin/intl/LanguageTag.h"
+#include "builtin/intl/SharedIntlData.h"
+#include "gc/GCContext.h"
+#include "js/Date.h"
+#include "js/experimental/Intl.h"     // JS::AddMozDateTimeFormatConstructor
+#include "js/friend/ErrorMessages.h"  // js::GetErrorMessage, JSMSG_*
+#include "js/GCAPI.h"
+#include "js/PropertyAndElement.h"  // JS_DefineFunctions, JS_DefineProperties
+#include "js/PropertySpec.h"
+#include "js/StableStringChars.h"
+#include "vm/DateTime.h"
+#include "vm/GlobalObject.h"
+#include "vm/JSContext.h"
+#include "vm/PlainObject.h"  // js::PlainObject
+#include "vm/Runtime.h"
+#include "vm/WellKnownAtom.h"  // js_*_str
+
+#include "vm/GeckoProfiler-inl.h"
+#include "vm/JSObject-inl.h"
+#include "vm/NativeObject-inl.h"
+
+using namespace js;
+
+using JS::AutoStableStringChars;
+using JS::ClippedTime;
+using JS::TimeClip;
+
+using js::intl::DateTimeFormatOptions;
+using js::intl::FormatBuffer;
+using js::intl::INITIAL_CHAR_BUFFER_SIZE;
+using js::intl::SharedIntlData;
+
+const JSClassOps DateTimeFormatObject::classOps_ = {
+    nullptr,                         // addProperty
+    nullptr,                         // delProperty
+    nullptr,                         // enumerate
+    nullptr,                         // newEnumerate
+    nullptr,                         // resolve
+    nullptr,                         // mayResolve
+    DateTimeFormatObject::finalize,  // finalize
+    nullptr,                         // call
+    nullptr,                         // construct
+    nullptr,                         // trace
+};
+
+const JSClass DateTimeFormatObject::class_ = {
+    "Intl.DateTimeFormat",
+    JSCLASS_HAS_RESERVED_SLOTS(DateTimeFormatObject::SLOT_COUNT) |
+        JSCLASS_HAS_CACHED_PROTO(JSProto_DateTimeFormat) |
+        JSCLASS_FOREGROUND_FINALIZE,
+    &DateTimeFormatObject::classOps_, &DateTimeFormatObject::classSpec_};
+
+const JSClass& DateTimeFormatObject::protoClass_ = PlainObject::class_;
+
+static bool dateTimeFormat_toSource(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  args.rval().setString(cx->names().DateTimeFormat);
+  return true;
+}
+
+static const JSFunctionSpec dateTimeFormat_static_methods[] = {
+    JS_SELF_HOSTED_FN("supportedLocalesOf",
+                      "Intl_DateTimeFormat_supportedLocalesOf", 1, 0),
+    JS_FS_END};
+
+static const JSFunctionSpec dateTimeFormat_methods[] = {
+    JS_SELF_HOSTED_FN("resolvedOptions", "Intl_DateTimeFormat_resolvedOptions",
+                      0, 0),
+    JS_SELF_HOSTED_FN("formatToParts", "Intl_DateTimeFormat_formatToParts", 1,
+                      0),
+    JS_SELF_HOSTED_FN("formatRange", "Intl_DateTimeFormat_formatRange", 2, 0),
+    JS_SELF_HOSTED_FN("formatRangeToParts",
+                      "Intl_DateTimeFormat_formatRangeToParts", 2, 0),
+    JS_FN(js_toSource_str, dateTimeFormat_toSource, 0, 0),
+    JS_FS_END};
+
+static const JSPropertySpec dateTimeFormat_properties[] = {
+    JS_SELF_HOSTED_GET("format", "$Intl_DateTimeFormat_format_get", 0),
+    JS_STRING_SYM_PS(toStringTag, "Intl.DateTimeFormat", JSPROP_READONLY),
+    JS_PS_END};
+
+static bool DateTimeFormat(JSContext* cx, unsigned argc, Value* vp);
+
+const ClassSpec DateTimeFormatObject::classSpec_ = {
+    GenericCreateConstructor<DateTimeFormat, 0, gc::AllocKind::FUNCTION>,
+    GenericCreatePrototype<DateTimeFormatObject>,
+    dateTimeFormat_static_methods,
+    nullptr,
+    dateTimeFormat_methods,
+    dateTimeFormat_properties,
+    nullptr,
+    ClassSpec::DontDefineConstructor};
+
+/**
+ * 12.2.1 Intl.DateTimeFormat([ locales [, options]])
+ *
+ * ES2017 Intl draft rev 94045d234762ad107a3d09bb6f7381a65f1a2f9b
+ */
+static bool DateTimeFormat(JSContext* cx, const CallArgs& args, bool construct,
+                           DateTimeFormatOptions dtfOptions) {
+  AutoJSConstructorProfilerEntry pseudoFrame(cx, "Intl.DateTimeFormat");
+
+  // Step 1 (Handled by OrdinaryCreateFromConstructor fallback code).
+
+  // Step 2 (Inlined 9.1.14, OrdinaryCreateFromConstructor).
+  JSProtoKey protoKey = dtfOptions == DateTimeFormatOptions::Standard
+                            ? JSProto_DateTimeFormat
+                            : JSProto_Null;
+  RootedObject proto(cx);
+  if (!GetPrototypeFromBuiltinConstructor(cx, args, protoKey, &proto)) {
+    return false;
+  }
+
+  Rooted<DateTimeFormatObject*> dateTimeFormat(cx);
+  dateTimeFormat = NewObjectWithClassProto<DateTimeFormatObject>(cx, proto);
+  if (!dateTimeFormat) {
+    return false;
+  }
+
+  RootedValue thisValue(
+      cx, construct ? ObjectValue(*dateTimeFormat) : args.thisv());
+  HandleValue locales = args.get(0);
+  HandleValue options = args.get(1);
+
+  // Step 3.
+  return intl::LegacyInitializeObject(
+      cx, dateTimeFormat, cx->names().InitializeDateTimeFormat, thisValue,
+      locales, options, dtfOptions, args.rval());
+}
+
+static bool DateTimeFormat(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return DateTimeFormat(cx, args, args.isConstructing(),
+                        DateTimeFormatOptions::Standard);
+}
+
+static bool MozDateTimeFormat(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+
+  // Don't allow to call mozIntl.DateTimeFormat as a function. That way we
+  // don't need to worry how to handle the legacy initialization semantics
+  // when applied on mozIntl.DateTimeFormat.
+  if (!ThrowIfNotConstructing(cx, args, "mozIntl.DateTimeFormat")) {
+    return false;
+  }
+
+  return DateTimeFormat(cx, args, true,
+                        DateTimeFormatOptions::EnableMozExtensions);
+}
+
+bool js::intl_DateTimeFormat(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 2);
+  MOZ_ASSERT(!args.isConstructing());
+  // intl_DateTimeFormat is an intrinsic for self-hosted JavaScript, so it
+  // cannot be used with "new", but it still has to be treated as a
+  // constructor.
+  return DateTimeFormat(cx, args, true, DateTimeFormatOptions::Standard);
+}
+
+void js::DateTimeFormatObject::finalize(JS::GCContext* gcx, JSObject* obj) {
+  MOZ_ASSERT(gcx->onMainThread());
+
+  auto* dateTimeFormat = &obj->as<DateTimeFormatObject>();
+  mozilla::intl::DateTimeFormat* df = dateTimeFormat->getDateFormat();
+  mozilla::intl::DateIntervalFormat* dif =
+      dateTimeFormat->getDateIntervalFormat();
+
+  if (df) {
+    intl::RemoveICUCellMemory(
+        gcx, obj, DateTimeFormatObject::UDateFormatEstimatedMemoryUse);
+
+    delete df;
+  }
+
+  if (dif) {
+    intl::RemoveICUCellMemory(
+        gcx, obj, DateTimeFormatObject::UDateIntervalFormatEstimatedMemoryUse);
+
+    delete dif;
+  }
+}
+
+bool JS::AddMozDateTimeFormatConstructor(JSContext* cx,
+                                         JS::Handle<JSObject*> intl) {
+  RootedObject ctor(
+      cx, GlobalObject::createConstructor(cx, MozDateTimeFormat,
+                                          cx->names().DateTimeFormat, 0));
+  if (!ctor) {
+    return false;
+  }
+
+  RootedObject proto(
+      cx, GlobalObject::createBlankPrototype<PlainObject>(cx, cx->global()));
+  if (!proto) {
+    return false;
+  }
+
+  if (!LinkConstructorAndPrototype(cx, ctor, proto)) {
+    return false;
+  }
+
+  // 12.3.2
+  if (!JS_DefineFunctions(cx, ctor, dateTimeFormat_static_methods)) {
+    return false;
+  }
+
+  // 12.4.4 and 12.4.5
+  if (!JS_DefineFunctions(cx, proto, dateTimeFormat_methods)) {
+    return false;
+  }
+
+  // 12.4.2 and 12.4.3
+  if (!JS_DefineProperties(cx, proto, dateTimeFormat_properties)) {
+    return false;
+  }
+
+  RootedValue ctorValue(cx, ObjectValue(*ctor));
+  return DefineDataProperty(cx, intl, cx->names().DateTimeFormat, ctorValue, 0);
+}
+
+static bool DefaultCalendar(JSContext* cx, const UniqueChars& locale,
+                            MutableHandleValue rval) {
+  auto calendar = mozilla::intl::Calendar::TryCreate(locale.get());
+  if (calendar.isErr()) {
+    intl::ReportInternalError(cx, calendar.unwrapErr());
+    return false;
+  }
+
+  auto type = calendar.unwrap()->GetBcp47Type();
+  if (type.isErr()) {
+    intl::ReportInternalError(cx, type.unwrapErr());
+    return false;
+  }
+
+  JSString* str = NewStringCopy<CanGC>(cx, type.unwrap());
+  if (!str) {
+    return false;
+  }
+
+  rval.setString(str);
+  return true;
+}
+
+bool js::intl_availableCalendars(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+  MOZ_ASSERT(args[0].isString());
+
+  UniqueChars locale = intl::EncodeLocale(cx, args[0].toString());
+  if (!locale) {
+    return false;
+  }
+
+  RootedObject calendars(cx, NewDenseEmptyArray(cx));
+  if (!calendars) {
+    return false;
+  }
+
+  // We need the default calendar for the locale as the first result.
+  RootedValue defaultCalendar(cx);
+  if (!DefaultCalendar(cx, locale, &defaultCalendar)) {
+    return false;
+  }
+
+  if (!NewbornArrayPush(cx, calendars, defaultCalendar)) {
+    return false;
+  }
+
+  // Now get the calendars that "would make a difference", i.e., not the
+  // default.
+  auto keywords =
+      mozilla::intl::Calendar::GetBcp47KeywordValuesForLocale(locale.get());
+  if (keywords.isErr()) {
+    intl::ReportInternalError(cx, keywords.unwrapErr());
+    return false;
+  }
+
+  for (auto keyword : keywords.unwrap()) {
+    if (keyword.isErr()) {
+      intl::ReportInternalError(cx);
+      return false;
+    }
+
+    JSString* jscalendar = NewStringCopy<CanGC>(cx, keyword.unwrap());
+    if (!jscalendar) {
+      return false;
+    }
+    if (!NewbornArrayPush(cx, calendars, StringValue(jscalendar))) {
+      return false;
+    }
+  }
+
+  args.rval().setObject(*calendars);
+  return true;
+}
+
+bool js::intl_defaultCalendar(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+  MOZ_ASSERT(args[0].isString());
+
+  UniqueChars locale = intl::EncodeLocale(cx, args[0].toString());
+  if (!locale) {
+    return false;
+  }
+
+  return DefaultCalendar(cx, locale, args.rval());
+}
+
+bool js::intl_IsValidTimeZoneName(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+  MOZ_ASSERT(args[0].isString());
+
+  SharedIntlData& sharedIntlData = cx->runtime()->sharedIntlData.ref();
+
+  RootedString timeZone(cx, args[0].toString());
+  Rooted<JSAtom*> validatedTimeZone(cx);
+  if (!sharedIntlData.validateTimeZoneName(cx, timeZone, &validatedTimeZone)) {
+    return false;
+  }
+
+  if (validatedTimeZone) {
+    cx->markAtom(validatedTimeZone);
+    args.rval().setString(validatedTimeZone);
+  } else {
+    args.rval().setNull();
+  }
+
+  return true;
+}
+
+bool js::intl_canonicalizeTimeZone(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+  MOZ_ASSERT(args[0].isString());
+
+  SharedIntlData& sharedIntlData = cx->runtime()->sharedIntlData.ref();
+
+  // Some time zone names are canonicalized differently by ICU -- handle
+  // those first:
+  RootedString timeZone(cx, args[0].toString());
+  Rooted<JSAtom*> ianaTimeZone(cx);
+  if (!sharedIntlData.tryCanonicalizeTimeZoneConsistentWithIANA(
+          cx, timeZone, &ianaTimeZone)) {
+    return false;
+  }
+
+  if (ianaTimeZone) {
+    cx->markAtom(ianaTimeZone);
+    args.rval().setString(ianaTimeZone);
+    return true;
+  }
+
+  AutoStableStringChars stableChars(cx);
+  if (!stableChars.initTwoByte(cx, timeZone)) {
+    return false;
+  }
+
+  FormatBuffer<char16_t, intl::INITIAL_CHAR_BUFFER_SIZE> canonicalTimeZone(cx);
+  auto result = mozilla::intl::TimeZone::GetCanonicalTimeZoneID(
+      stableChars.twoByteRange(), canonicalTimeZone);
+  if (result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return false;
+  }
+
+  JSString* str = canonicalTimeZone.toString(cx);
+  if (!str) {
+    return false;
+  }
+
+  args.rval().setString(str);
+  return true;
+}
+
+bool js::intl_defaultTimeZone(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 0);
+
+  // The current default might be stale, because JS::ResetTimeZone() doesn't
+  // immediately update ICU's default time zone. So perform an update if
+  // needed.
+  js::ResyncICUDefaultTimeZone();
+
+  FormatBuffer<char16_t, intl::INITIAL_CHAR_BUFFER_SIZE> timeZone(cx);
+  auto result = mozilla::intl::TimeZone::GetDefaultTimeZone(timeZone);
+  if (result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return false;
+  }
+
+  JSString* str = timeZone.toString(cx);
+  if (!str) {
+    return false;
+  }
+
+  args.rval().setString(str);
+  return true;
+}
+
+bool js::intl_defaultTimeZoneOffset(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 0);
+
+  auto timeZone = mozilla::intl::TimeZone::TryCreate();
+  if (timeZone.isErr()) {
+    intl::ReportInternalError(cx, timeZone.unwrapErr());
+    return false;
+  }
+
+  auto offset = timeZone.unwrap()->GetRawOffsetMs();
+  if (offset.isErr()) {
+    intl::ReportInternalError(cx, offset.unwrapErr());
+    return false;
+  }
+
+  args.rval().setInt32(offset.unwrap());
+  return true;
+}
+
+bool js::intl_isDefaultTimeZone(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+  MOZ_ASSERT(args[0].isString() || args[0].isUndefined());
+
+  // |undefined| is the default value when the Intl runtime caches haven't
+  // yet been initialized. Handle it the same way as a cache miss.
+  if (args[0].isUndefined()) {
+    args.rval().setBoolean(false);
+    return true;
+  }
+
+  // The current default might be stale, because JS::ResetTimeZone() doesn't
+  // immediately update ICU's default time zone. So perform an update if
+  // needed.
+  js::ResyncICUDefaultTimeZone();
+
+  FormatBuffer<char16_t, intl::INITIAL_CHAR_BUFFER_SIZE> chars(cx);
+  auto result = mozilla::intl::TimeZone::GetDefaultTimeZone(chars);
+  if (result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return false;
+  }
+
+  JSLinearString* str = args[0].toString()->ensureLinear(cx);
+  if (!str) {
+    return false;
+  }
+
+  bool equals;
+  if (str->length() == chars.length()) {
+    JS::AutoCheckCannotGC nogc;
+    equals =
+        str->hasLatin1Chars()
+            ? EqualChars(str->latin1Chars(nogc), chars.data(), str->length())
+            : EqualChars(str->twoByteChars(nogc), chars.data(), str->length());
+  } else {
+    equals = false;
+  }
+
+  args.rval().setBoolean(equals);
+  return true;
+}
+
+enum class HourCycle {
+  // 12 hour cycle, from 0 to 11.
+  H11,
+
+  // 12 hour cycle, from 1 to 12.
+  H12,
+
+  // 24 hour cycle, from 0 to 23.
+  H23,
+
+  // 24 hour cycle, from 1 to 24.
+  H24
+};
+
+static UniqueChars DateTimeFormatLocale(
+    JSContext* cx, HandleObject internals,
+    mozilla::Maybe<mozilla::intl::DateTimeFormat::HourCycle> hourCycle =
+        mozilla::Nothing()) {
+  RootedValue value(cx);
+  if (!GetProperty(cx, internals, internals, cx->names().locale, &value)) {
+    return nullptr;
+  }
+
+  // ICU expects calendar, numberingSystem, and hourCycle as Unicode locale
+  // extensions on locale.
+
+  mozilla::intl::Locale tag;
+  {
+    Rooted<JSLinearString*> locale(cx, value.toString()->ensureLinear(cx));
+    if (!locale) {
+      return nullptr;
+    }
+
+    if (!intl::ParseLocale(cx, locale, tag)) {
+      return nullptr;
+    }
+  }
+
+  JS::RootedVector<intl::UnicodeExtensionKeyword> keywords(cx);
+
+  if (!GetProperty(cx, internals, internals, cx->names().calendar, &value)) {
+    return nullptr;
+  }
+
+  {
+    JSLinearString* calendar = value.toString()->ensureLinear(cx);
+    if (!calendar) {
+      return nullptr;
+    }
+
+    if (!keywords.emplaceBack("ca", calendar)) {
+      return nullptr;
+    }
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().numberingSystem,
+                   &value)) {
+    return nullptr;
+  }
+
+  {
+    JSLinearString* numberingSystem = value.toString()->ensureLinear(cx);
+    if (!numberingSystem) {
+      return nullptr;
+    }
+
+    if (!keywords.emplaceBack("nu", numberingSystem)) {
+      return nullptr;
+    }
+  }
+
+  if (hourCycle) {
+    JSAtom* hourCycleStr;
+    switch (*hourCycle) {
+      case mozilla::intl::DateTimeFormat::HourCycle::H11:
+        hourCycleStr = cx->names().h11;
+        break;
+      case mozilla::intl::DateTimeFormat::HourCycle::H12:
+        hourCycleStr = cx->names().h12;
+        break;
+      case mozilla::intl::DateTimeFormat::HourCycle::H23:
+        hourCycleStr = cx->names().h23;
+        break;
+      case mozilla::intl::DateTimeFormat::HourCycle::H24:
+        hourCycleStr = cx->names().h24;
+        break;
+    }
+
+    if (!keywords.emplaceBack("hc", hourCycleStr)) {
+      return nullptr;
+    }
+  }
+
+  // |ApplyUnicodeExtensionToTag| applies the new keywords to the front of
+  // the Unicode extension subtag. We're then relying on ICU to follow RFC
+  // 6067, which states that any trailing keywords using the same key
+  // should be ignored.
+  if (!intl::ApplyUnicodeExtensionToTag(cx, tag, keywords)) {
+    return nullptr;
+  }
+
+  FormatBuffer<char> buffer(cx);
+  if (auto result = tag.ToString(buffer); result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return nullptr;
+  }
+  return buffer.extractStringZ();
+}
+
+static bool AssignTextComponent(
+    JSContext* cx, HandleObject internals, Handle<PropertyName*> property,
+    mozilla::Maybe<mozilla::intl::DateTimeFormat::Text>* text) {
+  RootedValue value(cx);
+  if (!GetProperty(cx, internals, internals, property, &value)) {
+    return false;
+  }
+
+  if (value.isString()) {
+    JSLinearString* string = value.toString()->ensureLinear(cx);
+    if (!string) {
+      return false;
+    }
+    if (StringEqualsLiteral(string, "narrow")) {
+      *text = mozilla::Some(mozilla::intl::DateTimeFormat::Text::Narrow);
+    } else if (StringEqualsLiteral(string, "short")) {
+      *text = mozilla::Some(mozilla::intl::DateTimeFormat::Text::Short);
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(string, "long"));
+      *text = mozilla::Some(mozilla::intl::DateTimeFormat::Text::Long);
+    }
+  } else {
+    MOZ_ASSERT(value.isUndefined());
+  }
+
+  return true;
+}
+
+static bool AssignNumericComponent(
+    JSContext* cx, HandleObject internals, Handle<PropertyName*> property,
+    mozilla::Maybe<mozilla::intl::DateTimeFormat::Numeric>* numeric) {
+  RootedValue value(cx);
+  if (!GetProperty(cx, internals, internals, property, &value)) {
+    return false;
+  }
+
+  if (value.isString()) {
+    JSLinearString* string = value.toString()->ensureLinear(cx);
+    if (!string) {
+      return false;
+    }
+    if (StringEqualsLiteral(string, "numeric")) {
+      *numeric = mozilla::Some(mozilla::intl::DateTimeFormat::Numeric::Numeric);
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(string, "2-digit"));
+      *numeric =
+          mozilla::Some(mozilla::intl::DateTimeFormat::Numeric::TwoDigit);
+    }
+  } else {
+    MOZ_ASSERT(value.isUndefined());
+  }
+
+  return true;
+}
+
+static bool AssignMonthComponent(
+    JSContext* cx, HandleObject internals, Handle<PropertyName*> property,
+    mozilla::Maybe<mozilla::intl::DateTimeFormat::Month>* month) {
+  RootedValue value(cx);
+  if (!GetProperty(cx, internals, internals, property, &value)) {
+    return false;
+  }
+
+  if (value.isString()) {
+    JSLinearString* string = value.toString()->ensureLinear(cx);
+    if (!string) {
+      return false;
+    }
+    if (StringEqualsLiteral(string, "numeric")) {
+      *month = mozilla::Some(mozilla::intl::DateTimeFormat::Month::Numeric);
+    } else if (StringEqualsLiteral(string, "2-digit")) {
+      *month = mozilla::Some(mozilla::intl::DateTimeFormat::Month::TwoDigit);
+    } else if (StringEqualsLiteral(string, "long")) {
+      *month = mozilla::Some(mozilla::intl::DateTimeFormat::Month::Long);
+    } else if (StringEqualsLiteral(string, "short")) {
+      *month = mozilla::Some(mozilla::intl::DateTimeFormat::Month::Short);
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(string, "narrow"));
+      *month = mozilla::Some(mozilla::intl::DateTimeFormat::Month::Narrow);
+    }
+  } else {
+    MOZ_ASSERT(value.isUndefined());
+  }
+
+  return true;
+}
+
+static bool AssignTimeZoneNameComponent(
+    JSContext* cx, HandleObject internals, Handle<PropertyName*> property,
+    mozilla::Maybe<mozilla::intl::DateTimeFormat::TimeZoneName>* tzName) {
+  RootedValue value(cx);
+  if (!GetProperty(cx, internals, internals, property, &value)) {
+    return false;
+  }
+
+  if (value.isString()) {
+    JSLinearString* string = value.toString()->ensureLinear(cx);
+    if (!string) {
+      return false;
+    }
+    if (StringEqualsLiteral(string, "long")) {
+      *tzName =
+          mozilla::Some(mozilla::intl::DateTimeFormat::TimeZoneName::Long);
+    } else if (StringEqualsLiteral(string, "short")) {
+      *tzName =
+          mozilla::Some(mozilla::intl::DateTimeFormat::TimeZoneName::Short);
+    } else if (StringEqualsLiteral(string, "shortOffset")) {
+      *tzName = mozilla::Some(
+          mozilla::intl::DateTimeFormat::TimeZoneName::ShortOffset);
+    } else if (StringEqualsLiteral(string, "longOffset")) {
+      *tzName = mozilla::Some(
+          mozilla::intl::DateTimeFormat::TimeZoneName::LongOffset);
+    } else if (StringEqualsLiteral(string, "shortGeneric")) {
+      *tzName = mozilla::Some(
+          mozilla::intl::DateTimeFormat::TimeZoneName::ShortGeneric);
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(string, "longGeneric"));
+      *tzName = mozilla::Some(
+          mozilla::intl::DateTimeFormat::TimeZoneName::LongGeneric);
+    }
+  } else {
+    MOZ_ASSERT(value.isUndefined());
+  }
+
+  return true;
+}
+
+static bool AssignHourCycleComponent(
+    JSContext* cx, HandleObject internals, Handle<PropertyName*> property,
+    mozilla::Maybe<mozilla::intl::DateTimeFormat::HourCycle>* hourCycle) {
+  RootedValue value(cx);
+  if (!GetProperty(cx, internals, internals, property, &value)) {
+    return false;
+  }
+
+  if (value.isString()) {
+    JSLinearString* string = value.toString()->ensureLinear(cx);
+    if (!string) {
+      return false;
+    }
+    if (StringEqualsLiteral(string, "h11")) {
+      *hourCycle = mozilla::Some(mozilla::intl::DateTimeFormat::HourCycle::H11);
+    } else if (StringEqualsLiteral(string, "h12")) {
+      *hourCycle = mozilla::Some(mozilla::intl::DateTimeFormat::HourCycle::H12);
+    } else if (StringEqualsLiteral(string, "h23")) {
+      *hourCycle = mozilla::Some(mozilla::intl::DateTimeFormat::HourCycle::H23);
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(string, "h24"));
+      *hourCycle = mozilla::Some(mozilla::intl::DateTimeFormat::HourCycle::H24);
+    }
+  } else {
+    MOZ_ASSERT(value.isUndefined());
+  }
+
+  return true;
+}
+
+static bool AssignHour12Component(JSContext* cx, HandleObject internals,
+                                  mozilla::Maybe<bool>* hour12) {
+  RootedValue value(cx);
+  if (!GetProperty(cx, internals, internals, cx->names().hour12, &value)) {
+    return false;
+  }
+  if (value.isBoolean()) {
+    *hour12 = mozilla::Some(value.toBoolean());
+  } else {
+    MOZ_ASSERT(value.isUndefined());
+  }
+
+  return true;
+}
+
+static bool AssignDateTimeLength(
+    JSContext* cx, HandleObject internals, Handle<PropertyName*> property,
+    mozilla::Maybe<mozilla::intl::DateTimeFormat::Style>* style) {
+  RootedValue value(cx);
+  if (!GetProperty(cx, internals, internals, property, &value)) {
+    return false;
+  }
+
+  if (value.isString()) {
+    JSLinearString* string = value.toString()->ensureLinear(cx);
+    if (!string) {
+      return false;
+    }
+    if (StringEqualsLiteral(string, "full")) {
+      *style = mozilla::Some(mozilla::intl::DateTimeFormat::Style::Full);
+    } else if (StringEqualsLiteral(string, "long")) {
+      *style = mozilla::Some(mozilla::intl::DateTimeFormat::Style::Long);
+    } else if (StringEqualsLiteral(string, "medium")) {
+      *style = mozilla::Some(mozilla::intl::DateTimeFormat::Style::Medium);
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(string, "short"));
+      *style = mozilla::Some(mozilla::intl::DateTimeFormat::Style::Short);
+    }
+  } else {
+    MOZ_ASSERT(value.isUndefined());
+  }
+
+  return true;
+}
+
+/**
+ * Returns a new mozilla::intl::DateTimeFormat with the locale and date-time
+ * formatting options of the given DateTimeFormat.
+ */
+static mozilla::intl::DateTimeFormat* NewDateTimeFormat(
+    JSContext* cx, Handle<DateTimeFormatObject*> dateTimeFormat) {
+  RootedValue value(cx);
+
+  RootedObject internals(cx, intl::GetInternalsObject(cx, dateTimeFormat));
+  if (!internals) {
+    return nullptr;
+  }
+
+  UniqueChars locale = DateTimeFormatLocale(cx, internals);
+  if (!locale) {
+    return nullptr;
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().timeZone, &value)) {
+    return nullptr;
+  }
+
+  AutoStableStringChars timeZone(cx);
+  if (!timeZone.initTwoByte(cx, value.toString())) {
+    return nullptr;
+  }
+
+  mozilla::Range<const char16_t> timeZoneChars = timeZone.twoByteRange();
+
+  if (!GetProperty(cx, internals, internals, cx->names().pattern, &value)) {
+    return nullptr;
+  }
+  bool hasPattern = value.isString();
+
+  if (!GetProperty(cx, internals, internals, cx->names().timeStyle, &value)) {
+    return nullptr;
+  }
+  bool hasStyle = value.isString();
+  if (!hasStyle) {
+    if (!GetProperty(cx, internals, internals, cx->names().dateStyle, &value)) {
+      return nullptr;
+    }
+    hasStyle = value.isString();
+  }
+
+  mozilla::UniquePtr<mozilla::intl::DateTimeFormat> df = nullptr;
+  if (hasPattern) {
+    // This is a DateTimeFormat defined by a pattern option. This is internal
+    // to Mozilla, and not part of the ECMA-402 API.
+    if (!GetProperty(cx, internals, internals, cx->names().pattern, &value)) {
+      return nullptr;
+    }
+
+    AutoStableStringChars pattern(cx);
+    if (!pattern.initTwoByte(cx, value.toString())) {
+      return nullptr;
+    }
+
+    auto dfResult = mozilla::intl::DateTimeFormat::TryCreateFromPattern(
+        mozilla::MakeStringSpan(locale.get()), pattern.twoByteRange(),
+        mozilla::Some(timeZoneChars));
+    if (dfResult.isErr()) {
+      intl::ReportInternalError(cx, dfResult.unwrapErr());
+      return nullptr;
+    }
+
+    df = dfResult.unwrap();
+  } else if (hasStyle) {
+    // This is a DateTimeFormat defined by a time style or date style.
+    mozilla::intl::DateTimeFormat::StyleBag style;
+    if (!AssignDateTimeLength(cx, internals, cx->names().timeStyle,
+                              &style.time)) {
+      return nullptr;
+    }
+    if (!AssignDateTimeLength(cx, internals, cx->names().dateStyle,
+                              &style.date)) {
+      return nullptr;
+    }
+    if (!AssignHourCycleComponent(cx, internals, cx->names().hourCycle,
+                                  &style.hourCycle)) {
+      return nullptr;
+    }
+
+    if (!AssignHour12Component(cx, internals, &style.hour12)) {
+      return nullptr;
+    }
+
+    SharedIntlData& sharedIntlData = cx->runtime()->sharedIntlData.ref();
+
+    mozilla::intl::DateTimePatternGenerator* gen =
+        sharedIntlData.getDateTimePatternGenerator(cx, locale.get());
+    if (!gen) {
+      return nullptr;
+    }
+    auto dfResult = mozilla::intl::DateTimeFormat::TryCreateFromStyle(
+        mozilla::MakeStringSpan(locale.get()), style, gen,
+        mozilla::Some(timeZoneChars));
+    if (dfResult.isErr()) {
+      intl::ReportInternalError(cx, dfResult.unwrapErr());
+      return nullptr;
+    }
+    df = dfResult.unwrap();
+  } else {
+    // This is a DateTimeFormat defined by a components bag.
+    mozilla::intl::DateTimeFormat::ComponentsBag bag;
+
+    if (!AssignTextComponent(cx, internals, cx->names().era, &bag.era)) {
+      return nullptr;
+    }
+    if (!AssignNumericComponent(cx, internals, cx->names().year, &bag.year)) {
+      return nullptr;
+    }
+    if (!AssignMonthComponent(cx, internals, cx->names().month, &bag.month)) {
+      return nullptr;
+    }
+    if (!AssignNumericComponent(cx, internals, cx->names().day, &bag.day)) {
+      return nullptr;
+    }
+    if (!AssignTextComponent(cx, internals, cx->names().weekday,
+                             &bag.weekday)) {
+      return nullptr;
+    }
+    if (!AssignNumericComponent(cx, internals, cx->names().hour, &bag.hour)) {
+      return nullptr;
+    }
+    if (!AssignNumericComponent(cx, internals, cx->names().minute,
+                                &bag.minute)) {
+      return nullptr;
+    }
+    if (!AssignNumericComponent(cx, internals, cx->names().second,
+                                &bag.second)) {
+      return nullptr;
+    }
+    if (!AssignTimeZoneNameComponent(cx, internals, cx->names().timeZoneName,
+                                     &bag.timeZoneName)) {
+      return nullptr;
+    }
+    if (!AssignHourCycleComponent(cx, internals, cx->names().hourCycle,
+                                  &bag.hourCycle)) {
+      return nullptr;
+    }
+    if (!AssignTextComponent(cx, internals, cx->names().dayPeriod,
+                             &bag.dayPeriod)) {
+      return nullptr;
+    }
+    if (!AssignHour12Component(cx, internals, &bag.hour12)) {
+      return nullptr;
+    }
+
+    if (!GetProperty(cx, internals, internals,
+                     cx->names().fractionalSecondDigits, &value)) {
+      return nullptr;
+    }
+    if (value.isInt32()) {
+      bag.fractionalSecondDigits = mozilla::Some(value.toInt32());
+    } else {
+      MOZ_ASSERT(value.isUndefined());
+    }
+
+    SharedIntlData& sharedIntlData = cx->runtime()->sharedIntlData.ref();
+    auto* dtpg = sharedIntlData.getDateTimePatternGenerator(cx, locale.get());
+    if (!dtpg) {
+      return nullptr;
+    }
+
+    auto dfResult = mozilla::intl::DateTimeFormat::TryCreateFromComponents(
+        mozilla::MakeStringSpan(locale.get()), bag, dtpg,
+        mozilla::Some(timeZoneChars));
+    if (dfResult.isErr()) {
+      intl::ReportInternalError(cx, dfResult.unwrapErr());
+      return nullptr;
+    }
+    df = dfResult.unwrap();
+  }
+
+  // ECMAScript requires the Gregorian calendar to be used from the beginning
+  // of ECMAScript time.
+  df->SetStartTimeIfGregorian(StartOfTime);
+
+  return df.release();
+}
+
+static mozilla::intl::DateTimeFormat* GetOrCreateDateTimeFormat(
+    JSContext* cx, Handle<DateTimeFormatObject*> dateTimeFormat) {
+  // Obtain a cached mozilla::intl::DateTimeFormat object.
+  mozilla::intl::DateTimeFormat* df = dateTimeFormat->getDateFormat();
+  if (df) {
+    return df;
+  }
+
+  df = NewDateTimeFormat(cx, dateTimeFormat);
+  if (!df) {
+    return nullptr;
+  }
+  dateTimeFormat->setDateFormat(df);
+
+  intl::AddICUCellMemory(dateTimeFormat,
+                         DateTimeFormatObject::UDateFormatEstimatedMemoryUse);
+  return df;
+}
+
+template <typename T>
+static bool SetResolvedProperty(JSContext* cx, HandleObject resolved,
+                                Handle<PropertyName*> name,
+                                mozilla::Maybe<T> intlProp) {
+  if (!intlProp) {
+    return true;
+  }
+  JSString* str = NewStringCopyZ<CanGC>(
+      cx, mozilla::intl::DateTimeFormat::ToString(*intlProp));
+  if (!str) {
+    return false;
+  }
+  RootedValue value(cx, StringValue(str));
+  return DefineDataProperty(cx, resolved, name, value);
+}
+
+bool js::intl_resolveDateTimeFormatComponents(JSContext* cx, unsigned argc,
+                                              Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 3);
+  MOZ_ASSERT(args[0].isObject());
+  MOZ_ASSERT(args[1].isObject());
+  MOZ_ASSERT(args[2].isBoolean());
+
+  Rooted<DateTimeFormatObject*> dateTimeFormat(cx);
+  dateTimeFormat = &args[0].toObject().as<DateTimeFormatObject>();
+
+  RootedObject resolved(cx, &args[1].toObject());
+
+  bool includeDateTimeFields = args[2].toBoolean();
+
+  mozilla::intl::DateTimeFormat* df =
+      GetOrCreateDateTimeFormat(cx, dateTimeFormat);
+  if (!df) {
+    return false;
+  }
+
+  auto result = df->ResolveComponents();
+  if (result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return false;
+  }
+
+  mozilla::intl::DateTimeFormat::ComponentsBag components = result.unwrap();
+
+  // Map the resolved mozilla::intl::DateTimeFormat::ComponentsBag to the
+  // options object as returned by DateTimeFormat.prototype.resolvedOptions.
+  //
+  // Resolved options must match the ordering as defined in:
+  // https://tc39.es/ecma402/#sec-intl.datetimeformat.prototype.resolvedoptions
+
+  if (!SetResolvedProperty(cx, resolved, cx->names().hourCycle,
+                           components.hourCycle)) {
+    return false;
+  }
+
+  if (components.hour12) {
+    RootedValue value(cx, BooleanValue(*components.hour12));
+    if (!DefineDataProperty(cx, resolved, cx->names().hour12, value)) {
+      return false;
+    }
+  }
+
+  if (!includeDateTimeFields) {
+    args.rval().setUndefined();
+    // Do not include date time fields.
+    return true;
+  }
+
+  if (!SetResolvedProperty(cx, resolved, cx->names().weekday,
+                           components.weekday)) {
+    return false;
+  }
+  if (!SetResolvedProperty(cx, resolved, cx->names().era, components.era)) {
+    return false;
+  }
+  if (!SetResolvedProperty(cx, resolved, cx->names().year, components.year)) {
+    return false;
+  }
+  if (!SetResolvedProperty(cx, resolved, cx->names().month, components.month)) {
+    return false;
+  }
+  if (!SetResolvedProperty(cx, resolved, cx->names().day, components.day)) {
+    return false;
+  }
+  if (!SetResolvedProperty(cx, resolved, cx->names().dayPeriod,
+                           components.dayPeriod)) {
+    return false;
+  }
+  if (!SetResolvedProperty(cx, resolved, cx->names().hour, components.hour)) {
+    return false;
+  }
+  if (!SetResolvedProperty(cx, resolved, cx->names().minute,
+                           components.minute)) {
+    return false;
+  }
+  if (!SetResolvedProperty(cx, resolved, cx->names().second,
+                           components.second)) {
+    return false;
+  }
+  if (!SetResolvedProperty(cx, resolved, cx->names().timeZoneName,
+                           components.timeZoneName)) {
+    return false;
+  }
+
+  if (components.fractionalSecondDigits) {
+    RootedValue value(cx, Int32Value(*components.fractionalSecondDigits));
+    if (!DefineDataProperty(cx, resolved, cx->names().fractionalSecondDigits,
+                            value)) {
+      return false;
+    }
+  }
+
+  args.rval().setUndefined();
+  return true;
+}
+
+static bool intl_FormatDateTime(JSContext* cx,
+                                const mozilla::intl::DateTimeFormat* df,
+                                ClippedTime x, MutableHandleValue result) {
+  MOZ_ASSERT(x.isValid());
+
+  FormatBuffer<char16_t, INITIAL_CHAR_BUFFER_SIZE> buffer(cx);
+  auto dfResult = df->TryFormat(x.toDouble(), buffer);
+  if (dfResult.isErr()) {
+    intl::ReportInternalError(cx, dfResult.unwrapErr());
+    return false;
+  }
+
+  JSString* str = buffer.toString(cx);
+  if (!str) {
+    return false;
+  }
+
+  result.setString(str);
+  return true;
+}
+
+using FieldType = js::ImmutableTenuredPtr<PropertyName*> JSAtomState::*;
+
+static FieldType GetFieldTypeForPartType(mozilla::intl::DateTimePartType type) {
+  switch (type) {
+    case mozilla::intl::DateTimePartType::Literal:
+      return &JSAtomState::literal;
+    case mozilla::intl::DateTimePartType::Era:
+      return &JSAtomState::era;
+    case mozilla::intl::DateTimePartType::Year:
+      return &JSAtomState::year;
+    case mozilla::intl::DateTimePartType::YearName:
+      return &JSAtomState::yearName;
+    case mozilla::intl::DateTimePartType::RelatedYear:
+      return &JSAtomState::relatedYear;
+    case mozilla::intl::DateTimePartType::Month:
+      return &JSAtomState::month;
+    case mozilla::intl::DateTimePartType::Day:
+      return &JSAtomState::day;
+    case mozilla::intl::DateTimePartType::Hour:
+      return &JSAtomState::hour;
+    case mozilla::intl::DateTimePartType::Minute:
+      return &JSAtomState::minute;
+    case mozilla::intl::DateTimePartType::Second:
+      return &JSAtomState::second;
+    case mozilla::intl::DateTimePartType::Weekday:
+      return &JSAtomState::weekday;
+    case mozilla::intl::DateTimePartType::DayPeriod:
+      return &JSAtomState::dayPeriod;
+    case mozilla::intl::DateTimePartType::TimeZoneName:
+      return &JSAtomState::timeZoneName;
+    case mozilla::intl::DateTimePartType::FractionalSecondDigits:
+      return &JSAtomState::fractionalSecond;
+    case mozilla::intl::DateTimePartType::Unknown:
+      return &JSAtomState::unknown;
+  }
+
+  MOZ_CRASH(
+      "unenumerated, undocumented format field returned "
+      "by iterator");
+}
+
+static FieldType GetFieldTypeForPartSource(
+    mozilla::intl::DateTimePartSource source) {
+  switch (source) {
+    case mozilla::intl::DateTimePartSource::Shared:
+      return &JSAtomState::shared;
+    case mozilla::intl::DateTimePartSource::StartRange:
+      return &JSAtomState::startRange;
+    case mozilla::intl::DateTimePartSource::EndRange:
+      return &JSAtomState::endRange;
+  }
+
+  MOZ_CRASH(
+      "unenumerated, undocumented format field returned "
+      "by iterator");
+}
+
+// A helper function to create an ArrayObject from DateTimePart objects.
+// When hasNoSource is true, we don't need to create the ||Source|| property for
+// the DateTimePart object.
+static bool CreateDateTimePartArray(
+    JSContext* cx, mozilla::Span<const char16_t> formattedSpan,
+    bool hasNoSource, const mozilla::intl::DateTimePartVector& parts,
+    MutableHandleValue result) {
+  RootedString overallResult(cx, NewStringCopy<CanGC>(cx, formattedSpan));
+  if (!overallResult) {
+    return false;
+  }
+
+  Rooted<ArrayObject*> partsArray(
+      cx, NewDenseFullyAllocatedArray(cx, parts.length()));
+  if (!partsArray) {
+    return false;
+  }
+  partsArray->ensureDenseInitializedLength(0, parts.length());
+
+  if (overallResult->length() == 0) {
+    // An empty string contains no parts, so avoid extra work below.
+    result.setObject(*partsArray);
+    return true;
+  }
+
+  RootedObject singlePart(cx);
+  RootedValue val(cx);
+
+  size_t index = 0;
+  size_t beginIndex = 0;
+  for (const mozilla::intl::DateTimePart& part : parts) {
+    singlePart = NewPlainObject(cx);
+    if (!singlePart) {
+      return false;
+    }
+
+    FieldType type = GetFieldTypeForPartType(part.mType);
+    val = StringValue(cx->names().*type);
+    if (!DefineDataProperty(cx, singlePart, cx->names().type, val)) {
+      return false;
+    }
+
+    MOZ_ASSERT(part.mEndIndex > beginIndex);
+    JSLinearString* partStr = NewDependentString(cx, overallResult, beginIndex,
+                                                 part.mEndIndex - beginIndex);
+    if (!partStr) {
+      return false;
+    }
+    val = StringValue(partStr);
+    if (!DefineDataProperty(cx, singlePart, cx->names().value, val)) {
+      return false;
+    }
+
+    if (!hasNoSource) {
+      FieldType source = GetFieldTypeForPartSource(part.mSource);
+      val = StringValue(cx->names().*source);
+      if (!DefineDataProperty(cx, singlePart, cx->names().source, val)) {
+        return false;
+      }
+    }
+
+    beginIndex = part.mEndIndex;
+    partsArray->initDenseElement(index++, ObjectValue(*singlePart));
+  }
+
+  MOZ_ASSERT(index == parts.length());
+  MOZ_ASSERT(beginIndex == formattedSpan.size());
+  result.setObject(*partsArray);
+  return true;
+}
+
+static bool intl_FormatToPartsDateTime(JSContext* cx,
+                                       const mozilla::intl::DateTimeFormat* df,
+                                       ClippedTime x, bool hasNoSource,
+                                       MutableHandleValue result) {
+  MOZ_ASSERT(x.isValid());
+
+  FormatBuffer<char16_t, intl::INITIAL_CHAR_BUFFER_SIZE> buffer(cx);
+  mozilla::intl::DateTimePartVector parts;
+  auto r = df->TryFormatToParts(x.toDouble(), buffer, parts);
+  if (r.isErr()) {
+    intl::ReportInternalError(cx, r.unwrapErr());
+    return false;
+  }
+
+  return CreateDateTimePartArray(cx, buffer, hasNoSource, parts, result);
+}
+
+bool js::intl_FormatDateTime(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 3);
+  MOZ_ASSERT(args[0].isObject());
+  MOZ_ASSERT(args[1].isNumber());
+  MOZ_ASSERT(args[2].isBoolean());
+
+  Rooted<DateTimeFormatObject*> dateTimeFormat(cx);
+  dateTimeFormat = &args[0].toObject().as<DateTimeFormatObject>();
+
+  bool formatToParts = args[2].toBoolean();
+
+  ClippedTime x = TimeClip(args[1].toNumber());
+  if (!x.isValid()) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_DATE_NOT_FINITE, "DateTimeFormat",
+                              formatToParts ? "formatToParts" : "format");
+    return false;
+  }
+
+  mozilla::intl::DateTimeFormat* df =
+      GetOrCreateDateTimeFormat(cx, dateTimeFormat);
+  if (!df) {
+    return false;
+  }
+
+  // Use the DateTimeFormat to actually format the time stamp.
+  return formatToParts ? intl_FormatToPartsDateTime(
+                             cx, df, x, /* hasNoSource */ true, args.rval())
+                       : intl_FormatDateTime(cx, df, x, args.rval());
+}
+
+/**
+ * Returns a new DateIntervalFormat with the locale and date-time formatting
+ * options of the given DateTimeFormat.
+ */
+static mozilla::intl::DateIntervalFormat* NewDateIntervalFormat(
+    JSContext* cx, Handle<DateTimeFormatObject*> dateTimeFormat,
+    mozilla::intl::DateTimeFormat& mozDtf) {
+  RootedValue value(cx);
+  RootedObject internals(cx, intl::GetInternalsObject(cx, dateTimeFormat));
+  if (!internals) {
+    return nullptr;
+  }
+
+  FormatBuffer<char16_t, intl::INITIAL_CHAR_BUFFER_SIZE> pattern(cx);
+  auto result = mozDtf.GetPattern(pattern);
+  if (result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return nullptr;
+  }
+
+  // Determine the hour cycle used in the resolved pattern.
+  mozilla::Maybe<mozilla::intl::DateTimeFormat::HourCycle> hcPattern =
+      mozilla::intl::DateTimeFormat::HourCycleFromPattern(pattern);
+
+  UniqueChars locale = DateTimeFormatLocale(cx, internals, hcPattern);
+  if (!locale) {
+    return nullptr;
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().timeZone, &value)) {
+    return nullptr;
+  }
+
+  AutoStableStringChars timeZone(cx);
+  if (!timeZone.initTwoByte(cx, value.toString())) {
+    return nullptr;
+  }
+  mozilla::Span<const char16_t> timeZoneChars = timeZone.twoByteRange();
+
+  FormatBuffer<char16_t, intl::INITIAL_CHAR_BUFFER_SIZE> skeleton(cx);
+  auto skelResult = mozDtf.GetOriginalSkeleton(skeleton);
+  if (skelResult.isErr()) {
+    intl::ReportInternalError(cx, skelResult.unwrapErr());
+    return nullptr;
+  }
+
+  auto dif = mozilla::intl::DateIntervalFormat::TryCreate(
+      mozilla::MakeStringSpan(locale.get()), skeleton, timeZoneChars);
+
+  if (dif.isErr()) {
+    js::intl::ReportInternalError(cx, dif.unwrapErr());
+    return nullptr;
+  }
+
+  return dif.unwrap().release();
+}
+
+static mozilla::intl::DateIntervalFormat* GetOrCreateDateIntervalFormat(
+    JSContext* cx, Handle<DateTimeFormatObject*> dateTimeFormat,
+    mozilla::intl::DateTimeFormat& mozDtf) {
+  // Obtain a cached DateIntervalFormat object.
+  mozilla::intl::DateIntervalFormat* dif =
+      dateTimeFormat->getDateIntervalFormat();
+  if (dif) {
+    return dif;
+  }
+
+  dif = NewDateIntervalFormat(cx, dateTimeFormat, mozDtf);
+  if (!dif) {
+    return nullptr;
+  }
+  dateTimeFormat->setDateIntervalFormat(dif);
+
+  intl::AddICUCellMemory(
+      dateTimeFormat,
+      DateTimeFormatObject::UDateIntervalFormatEstimatedMemoryUse);
+  return dif;
+}
+
+/**
+ * PartitionDateTimeRangePattern ( dateTimeFormat, x, y )
+ */
+static bool PartitionDateTimeRangePattern(
+    JSContext* cx, const mozilla::intl::DateTimeFormat* df,
+    const mozilla::intl::DateIntervalFormat* dif,
+    mozilla::intl::AutoFormattedDateInterval& formatted, ClippedTime x,
+    ClippedTime y, bool* equal) {
+  MOZ_ASSERT(x.isValid());
+  MOZ_ASSERT(y.isValid());
+
+  // We can't access the calendar used by UDateIntervalFormat to change it to a
+  // proleptic Gregorian calendar. Instead we need to call a different formatter
+  // function which accepts UCalendar instead of UDate.
+  // But creating new UCalendar objects for each call is slow, so when we can
+  // ensure that the input dates are later than the Gregorian change date,
+  // directly call the formatter functions taking UDate.
+
+  // The Gregorian change date "1582-10-15T00:00:00.000Z".
+  constexpr double GregorianChangeDate = -12219292800000.0;
+
+  // Add a full day to account for time zone offsets.
+  constexpr double GregorianChangeDatePlusOneDay =
+      GregorianChangeDate + msPerDay;
+
+  mozilla::intl::ICUResult result = Ok();
+  if (x.toDouble() < GregorianChangeDatePlusOneDay ||
+      y.toDouble() < GregorianChangeDatePlusOneDay) {
+    // Create calendar objects for the start and end date by cloning the date
+    // formatter calendar. The date formatter calendar already has the correct
+    // time zone set and was changed to use a proleptic Gregorian calendar.
+    auto startCal = df->CloneCalendar(x.toDouble());
+    if (startCal.isErr()) {
+      intl::ReportInternalError(cx, startCal.unwrapErr());
+      return false;
+    }
+
+    auto endCal = df->CloneCalendar(y.toDouble());
+    if (endCal.isErr()) {
+      intl::ReportInternalError(cx, endCal.unwrapErr());
+      return false;
+    }
+
+    result = dif->TryFormatCalendar(*startCal.unwrap(), *endCal.unwrap(),
+                                    formatted, equal);
+  } else {
+    // The common fast path which doesn't require creating calendar objects.
+    result =
+        dif->TryFormatDateTime(x.toDouble(), y.toDouble(), formatted, equal);
+  }
+
+  if (result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return false;
+  }
+
+  return true;
+}
+
+/**
+ * FormatDateTimeRange( dateTimeFormat, x, y )
+ */
+static bool FormatDateTimeRange(JSContext* cx,
+                                const mozilla::intl::DateTimeFormat* df,
+                                const mozilla::intl::DateIntervalFormat* dif,
+                                ClippedTime x, ClippedTime y,
+                                MutableHandleValue result) {
+  mozilla::intl::AutoFormattedDateInterval formatted;
+  if (!formatted.IsValid()) {
+    intl::ReportInternalError(cx, formatted.GetError());
+    return false;
+  }
+
+  bool equal;
+  if (!PartitionDateTimeRangePattern(cx, df, dif, formatted, x, y, &equal)) {
+    return false;
+  }
+
+  // PartitionDateTimeRangePattern, step 12.
+  if (equal) {
+    return intl_FormatDateTime(cx, df, x, result);
+  }
+
+  auto spanResult = formatted.ToSpan();
+  if (spanResult.isErr()) {
+    intl::ReportInternalError(cx, spanResult.unwrapErr());
+    return false;
+  }
+  JSString* resultStr = NewStringCopy<CanGC>(cx, spanResult.unwrap());
+  if (!resultStr) {
+    return false;
+  }
+
+  result.setString(resultStr);
+  return true;
+}
+
+/**
+ * FormatDateTimeRangeToParts ( dateTimeFormat, x, y )
+ */
+static bool FormatDateTimeRangeToParts(
+    JSContext* cx, const mozilla::intl::DateTimeFormat* df,
+    const mozilla::intl::DateIntervalFormat* dif, ClippedTime x, ClippedTime y,
+    MutableHandleValue result) {
+  mozilla::intl::AutoFormattedDateInterval formatted;
+  if (!formatted.IsValid()) {
+    intl::ReportInternalError(cx, formatted.GetError());
+    return false;
+  }
+
+  bool equal;
+  if (!PartitionDateTimeRangePattern(cx, df, dif, formatted, x, y, &equal)) {
+    return false;
+  }
+
+  // PartitionDateTimeRangePattern, step 12.
+  if (equal) {
+    return intl_FormatToPartsDateTime(cx, df, x, /* hasNoSource */ false,
+                                      result);
+  }
+
+  mozilla::intl::DateTimePartVector parts;
+  auto r = dif->TryFormattedToParts(formatted, parts);
+  if (r.isErr()) {
+    intl::ReportInternalError(cx, r.unwrapErr());
+    return false;
+  }
+
+  auto spanResult = formatted.ToSpan();
+  if (spanResult.isErr()) {
+    intl::ReportInternalError(cx, spanResult.unwrapErr());
+    return false;
+  }
+  return CreateDateTimePartArray(cx, spanResult.unwrap(),
+                                 /* hasNoSource */ false, parts, result);
+}
+
+bool js::intl_FormatDateTimeRange(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 4);
+  MOZ_ASSERT(args[0].isObject());
+  MOZ_ASSERT(args[1].isNumber());
+  MOZ_ASSERT(args[2].isNumber());
+  MOZ_ASSERT(args[3].isBoolean());
+
+  Rooted<DateTimeFormatObject*> dateTimeFormat(cx);
+  dateTimeFormat = &args[0].toObject().as<DateTimeFormatObject>();
+
+  bool formatToParts = args[3].toBoolean();
+
+  // PartitionDateTimeRangePattern, steps 1-2.
+  ClippedTime x = TimeClip(args[1].toNumber());
+  if (!x.isValid()) {
+    JS_ReportErrorNumberASCII(
+        cx, GetErrorMessage, nullptr, JSMSG_DATE_NOT_FINITE, "DateTimeFormat",
+        formatToParts ? "formatRangeToParts" : "formatRange");
+    return false;
+  }
+
+  // PartitionDateTimeRangePattern, steps 3-4.
+  ClippedTime y = TimeClip(args[2].toNumber());
+  if (!y.isValid()) {
+    JS_ReportErrorNumberASCII(
+        cx, GetErrorMessage, nullptr, JSMSG_DATE_NOT_FINITE, "DateTimeFormat",
+        formatToParts ? "formatRangeToParts" : "formatRange");
+    return false;
+  }
+
+  mozilla::intl::DateTimeFormat* df =
+      GetOrCreateDateTimeFormat(cx, dateTimeFormat);
+  if (!df) {
+    return false;
+  }
+
+  mozilla::intl::DateIntervalFormat* dif =
+      GetOrCreateDateIntervalFormat(cx, dateTimeFormat, *df);
+  if (!dif) {
+    return false;
+  }
+
+  // Use the DateIntervalFormat to actually format the time range.
+  return formatToParts
+             ? FormatDateTimeRangeToParts(cx, df, dif, x, y, args.rval())
+             : FormatDateTimeRange(cx, df, dif, x, y, args.rval());
+}
diff --git a/js/src/builtin/intl/DateTimeFormat.h b/js/src/builtin/intl/DateTimeFormat.h
new file mode 100644
index 0000000000..cd93af56c6
--- /dev/null
+++ b/js/src/builtin/intl/DateTimeFormat.h
@@ -0,0 +1,188 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_DateTimeFormat_h
+#define builtin_intl_DateTimeFormat_h
+
+#include "builtin/SelfHostingDefines.h"
+#include "js/Class.h"
+#include "vm/NativeObject.h"
+
+namespace mozilla::intl {
+class DateTimeFormat;
+class DateIntervalFormat;
+}  // namespace mozilla::intl
+
+namespace js {
+
+class DateTimeFormatObject : public NativeObject {
+ public:
+  static const JSClass class_;
+  static const JSClass& protoClass_;
+
+  static constexpr uint32_t INTERNALS_SLOT = 0;
+  static constexpr uint32_t DATE_FORMAT_SLOT = 1;
+  static constexpr uint32_t DATE_INTERVAL_FORMAT_SLOT = 2;
+  static constexpr uint32_t SLOT_COUNT = 3;
+
+  static_assert(INTERNALS_SLOT == INTL_INTERNALS_OBJECT_SLOT,
+                "INTERNALS_SLOT must match self-hosting define for internals "
+                "object slot");
+
+  // Estimated memory use for UDateFormat (see IcuMemoryUsage).
+  static constexpr size_t UDateFormatEstimatedMemoryUse = 105402;
+
+  // Estimated memory use for UDateIntervalFormat (see IcuMemoryUsage).
+  static constexpr size_t UDateIntervalFormatEstimatedMemoryUse = 133064;
+
+  mozilla::intl::DateTimeFormat* getDateFormat() const {
+    const auto& slot = getFixedSlot(DATE_FORMAT_SLOT);
+    if (slot.isUndefined()) {
+      return nullptr;
+    }
+    return static_cast<mozilla::intl::DateTimeFormat*>(slot.toPrivate());
+  }
+
+  void setDateFormat(mozilla::intl::DateTimeFormat* dateFormat) {
+    setFixedSlot(DATE_FORMAT_SLOT, PrivateValue(dateFormat));
+  }
+
+  mozilla::intl::DateIntervalFormat* getDateIntervalFormat() const {
+    const auto& slot = getFixedSlot(DATE_INTERVAL_FORMAT_SLOT);
+    if (slot.isUndefined()) {
+      return nullptr;
+    }
+    return static_cast<mozilla::intl::DateIntervalFormat*>(slot.toPrivate());
+  }
+
+  void setDateIntervalFormat(
+      mozilla::intl::DateIntervalFormat* dateIntervalFormat) {
+    setFixedSlot(DATE_INTERVAL_FORMAT_SLOT, PrivateValue(dateIntervalFormat));
+  }
+
+ private:
+  static const JSClassOps classOps_;
+  static const ClassSpec classSpec_;
+
+  static void finalize(JS::GCContext* gcx, JSObject* obj);
+};
+
+/**
+ * Returns a new instance of the standard built-in DateTimeFormat constructor.
+ * Self-hosted code cannot cache this constructor (as it does for others in
+ * Utilities.js) because it is initialized after self-hosted code is compiled.
+ *
+ * Usage: dateTimeFormat = intl_DateTimeFormat(locales, options)
+ */
+[[nodiscard]] extern bool intl_DateTimeFormat(JSContext* cx, unsigned argc,
+                                              JS::Value* vp);
+
+/**
+ * Returns an array with the calendar type identifiers per Unicode
+ * Technical Standard 35, Unicode Locale Data Markup Language, for the
+ * supported calendars for the given locale. The default calendar is
+ * element 0.
+ *
+ * Usage: calendars = intl_availableCalendars(locale)
+ */
+[[nodiscard]] extern bool intl_availableCalendars(JSContext* cx, unsigned argc,
+                                                  JS::Value* vp);
+
+/**
+ * Returns the calendar type identifier per Unicode Technical Standard 35,
+ * Unicode Locale Data Markup Language, for the default calendar for the given
+ * locale.
+ *
+ * Usage: calendar = intl_defaultCalendar(locale)
+ */
+[[nodiscard]] extern bool intl_defaultCalendar(JSContext* cx, unsigned argc,
+                                               JS::Value* vp);
+
+/**
+ * 6.4.1 IsValidTimeZoneName ( timeZone )
+ *
+ * Verifies that the given string is a valid time zone name. If it is a valid
+ * time zone name, its IANA time zone name is returned. Otherwise returns null.
+ *
+ * ES2017 Intl draft rev 4a23f407336d382ed5e3471200c690c9b020b5f3
+ *
+ * Usage: ianaTimeZone = intl_IsValidTimeZoneName(timeZone)
+ */
+[[nodiscard]] extern bool intl_IsValidTimeZoneName(JSContext* cx, unsigned argc,
+                                                   JS::Value* vp);
+
+/**
+ * Return the canonicalized time zone name. Canonicalization resolves link
+ * names to their target time zones.
+ *
+ * Usage: ianaTimeZone = intl_canonicalizeTimeZone(timeZone)
+ */
+[[nodiscard]] extern bool intl_canonicalizeTimeZone(JSContext* cx,
+                                                    unsigned argc,
+                                                    JS::Value* vp);
+
+/**
+ * Return the default time zone name. The time zone name is not canonicalized.
+ *
+ * Usage: icuDefaultTimeZone = intl_defaultTimeZone()
+ */
+[[nodiscard]] extern bool intl_defaultTimeZone(JSContext* cx, unsigned argc,
+                                               JS::Value* vp);
+
+/**
+ * Return the raw offset from GMT in milliseconds for the default time zone.
+ *
+ * Usage: defaultTimeZoneOffset = intl_defaultTimeZoneOffset()
+ */
+[[nodiscard]] extern bool intl_defaultTimeZoneOffset(JSContext* cx,
+                                                     unsigned argc,
+                                                     JS::Value* vp);
+
+/**
+ * Return true if the given string is the default time zone as returned by
+ * intl_defaultTimeZone(). Otherwise return false.
+ *
+ * Usage: isIcuDefaultTimeZone = intl_isDefaultTimeZone(icuDefaultTimeZone)
+ */
+[[nodiscard]] extern bool intl_isDefaultTimeZone(JSContext* cx, unsigned argc,
+                                                 JS::Value* vp);
+
+/**
+ * Returns a String value representing x (which must be a Number value)
+ * according to the effective locale and the formatting options of the
+ * given DateTimeFormat.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 12.3.2.
+ *
+ * Usage: formatted = intl_FormatDateTime(dateTimeFormat, x, formatToParts)
+ */
+[[nodiscard]] extern bool intl_FormatDateTime(JSContext* cx, unsigned argc,
+                                              JS::Value* vp);
+
+/**
+ * Returns a String value representing the range between x and y (which both
+ * must be Number values) according to the effective locale and the formatting
+ * options of the given DateTimeFormat.
+ *
+ * Spec: Intl.DateTimeFormat.prototype.formatRange proposal
+ *
+ * Usage: formatted = intl_FormatDateTimeRange(dateTimeFmt, x, y, formatToParts)
+ */
+[[nodiscard]] extern bool intl_FormatDateTimeRange(JSContext* cx, unsigned argc,
+                                                   JS::Value* vp);
+
+/**
+ * Extracts the resolved components from a DateTimeFormat and applies them to
+ * the object for resolved components.
+ *
+ * Usage: intl_resolveDateTimeFormatComponents(dateTimeFormat, resolved)
+ */
+[[nodiscard]] extern bool intl_resolveDateTimeFormatComponents(JSContext* cx,
+                                                               unsigned argc,
+                                                               JS::Value* vp);
+}  // namespace js
+
+#endif /* builtin_intl_DateTimeFormat_h */
diff --git a/js/src/builtin/intl/DateTimeFormat.js b/js/src/builtin/intl/DateTimeFormat.js
new file mode 100644
index 0000000000..91a22fe5e4
--- /dev/null
+++ b/js/src/builtin/intl/DateTimeFormat.js
@@ -0,0 +1,1008 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Portions Copyright Norbert Lindenberg 2011-2012. */
+
+/**
+ * Compute an internal properties object from |lazyDateTimeFormatData|.
+ */
+function resolveDateTimeFormatInternals(lazyDateTimeFormatData) {
+  assert(IsObject(lazyDateTimeFormatData), "lazy data not an object?");
+
+  // Lazy DateTimeFormat data has the following structure:
+  //
+  //   {
+  //     requestedLocales: List of locales,
+  //
+  //     localeOpt: // *first* opt computed in InitializeDateTimeFormat
+  //       {
+  //         localeMatcher: "lookup" / "best fit",
+  //
+  //         ca: string matching a Unicode extension type, // optional
+  //
+  //         nu: string matching a Unicode extension type, // optional
+  //
+  //         hc: "h11" / "h12" / "h23" / "h24", // optional
+  //       }
+  //
+  //     timeZone: IANA time zone name,
+  //
+  //     formatOpt: // *second* opt computed in InitializeDateTimeFormat
+  //       {
+  //         // all the properties/values listed in Table 3
+  //         // (weekday, era, year, month, day, &c.)
+  //
+  //         hour12: true / false,  // optional
+  //       }
+  //
+  //     formatMatcher: "basic" / "best fit",
+  //
+  //     dateStyle: "full" / "long" / "medium" / "short" / undefined,
+  //
+  //     timeStyle: "full" / "long" / "medium" / "short" / undefined,
+  //
+  //     patternOption:
+  //       String representing LDML Date Format pattern or undefined
+  //   }
+  //
+  // Note that lazy data is only installed as a final step of initialization,
+  // so every DateTimeFormat lazy data object has *all* these properties,
+  // never a subset of them.
+
+  var internalProps = std_Object_create(null);
+
+  var DateTimeFormat = dateTimeFormatInternalProperties;
+
+  // Compute effective locale.
+
+  // Step 10.
+  var localeData = DateTimeFormat.localeData;
+
+  // Step 11.
+  var r = ResolveLocale(
+    "DateTimeFormat",
+    lazyDateTimeFormatData.requestedLocales,
+    lazyDateTimeFormatData.localeOpt,
+    DateTimeFormat.relevantExtensionKeys,
+    localeData
+  );
+
+  // Steps 12-13, 15.
+  internalProps.locale = r.locale;
+  internalProps.calendar = r.ca;
+  internalProps.numberingSystem = r.nu;
+
+  // Step 20.
+  internalProps.timeZone = lazyDateTimeFormatData.timeZone;
+
+  // Step 21.
+  var formatOpt = lazyDateTimeFormatData.formatOpt;
+
+  // Step 14.
+  // Copy the hourCycle setting, if present, to the format options. But
+  // only do this if no hour12 option is present, because the latter takes
+  // precedence over hourCycle.
+  if (r.hc !== null && formatOpt.hour12 === undefined) {
+    formatOpt.hourCycle = r.hc;
+  }
+
+  // Steps 26-31, more or less - see comment after this function.
+  if (lazyDateTimeFormatData.patternOption !== undefined) {
+    internalProps.pattern = lazyDateTimeFormatData.patternOption;
+  } else if (
+    lazyDateTimeFormatData.dateStyle !== undefined ||
+    lazyDateTimeFormatData.timeStyle !== undefined
+  ) {
+    internalProps.hourCycle = formatOpt.hourCycle;
+    internalProps.hour12 = formatOpt.hour12;
+    internalProps.dateStyle = lazyDateTimeFormatData.dateStyle;
+    internalProps.timeStyle = lazyDateTimeFormatData.timeStyle;
+  } else {
+    internalProps.hourCycle = formatOpt.hourCycle;
+    internalProps.hour12 = formatOpt.hour12;
+    internalProps.weekday = formatOpt.weekday;
+    internalProps.era = formatOpt.era;
+    internalProps.year = formatOpt.year;
+    internalProps.month = formatOpt.month;
+    internalProps.day = formatOpt.day;
+    internalProps.dayPeriod = formatOpt.dayPeriod;
+    internalProps.hour = formatOpt.hour;
+    internalProps.minute = formatOpt.minute;
+    internalProps.second = formatOpt.second;
+    internalProps.fractionalSecondDigits = formatOpt.fractionalSecondDigits;
+    internalProps.timeZoneName = formatOpt.timeZoneName;
+  }
+
+  // The caller is responsible for associating |internalProps| with the right
+  // object using |setInternalProperties|.
+  return internalProps;
+}
+
+/**
+ * Returns an object containing the DateTimeFormat internal properties of |obj|.
+ */
+function getDateTimeFormatInternals(obj) {
+  assert(IsObject(obj), "getDateTimeFormatInternals called with non-object");
+  assert(
+    intl_GuardToDateTimeFormat(obj) !== null,
+    "getDateTimeFormatInternals called with non-DateTimeFormat"
+  );
+
+  var internals = getIntlObjectInternals(obj);
+  assert(
+    internals.type === "DateTimeFormat",
+    "bad type escaped getIntlObjectInternals"
+  );
+
+  // If internal properties have already been computed, use them.
+  var internalProps = maybeInternalProperties(internals);
+  if (internalProps) {
+    return internalProps;
+  }
+
+  // Otherwise it's time to fully create them.
+  internalProps = resolveDateTimeFormatInternals(internals.lazyData);
+  setInternalProperties(internals, internalProps);
+  return internalProps;
+}
+
+/**
+ * 12.1.10 UnwrapDateTimeFormat( dtf )
+ */
+function UnwrapDateTimeFormat(dtf) {
+  // Steps 2 and 4 (error handling moved to caller).
+  if (
+    IsObject(dtf) &&
+    intl_GuardToDateTimeFormat(dtf) === null &&
+    !intl_IsWrappedDateTimeFormat(dtf) &&
+    callFunction(
+      std_Object_isPrototypeOf,
+      GetBuiltinPrototype("DateTimeFormat"),
+      dtf
+    )
+  ) {
+    dtf = dtf[intlFallbackSymbol()];
+  }
+  return dtf;
+}
+
+/**
+ * 6.4.2 CanonicalizeTimeZoneName ( timeZone )
+ *
+ * Canonicalizes the given IANA time zone name.
+ *
+ * ES2017 Intl draft rev 4a23f407336d382ed5e3471200c690c9b020b5f3
+ */
+function CanonicalizeTimeZoneName(timeZone) {
+  assert(typeof timeZone === "string", "CanonicalizeTimeZoneName");
+
+  // Step 1. (Not applicable, the input is already a valid IANA time zone.)
+  assert(timeZone !== "Etc/Unknown", "Invalid time zone");
+  assert(
+    timeZone === intl_IsValidTimeZoneName(timeZone),
+    "Time zone name not normalized"
+  );
+
+  // Step 2.
+  var ianaTimeZone = intl_canonicalizeTimeZone(timeZone);
+  assert(ianaTimeZone !== "Etc/Unknown", "Invalid canonical time zone");
+  assert(
+    ianaTimeZone === intl_IsValidTimeZoneName(ianaTimeZone),
+    "Unsupported canonical time zone"
+  );
+
+  // Step 3.
+  if (ianaTimeZone === "Etc/UTC" || ianaTimeZone === "Etc/GMT") {
+    ianaTimeZone = "UTC";
+  }
+
+  // Step 4.
+  return ianaTimeZone;
+}
+
+var timeZoneCache = {
+  icuDefaultTimeZone: undefined,
+  defaultTimeZone: undefined,
+};
+
+/**
+ * 6.4.3 DefaultTimeZone ()
+ *
+ * Returns the IANA time zone name for the host environment's current time zone.
+ *
+ * ES2017 Intl draft rev 4a23f407336d382ed5e3471200c690c9b020b5f3
+ */
+function DefaultTimeZone() {
+  if (intl_isDefaultTimeZone(timeZoneCache.icuDefaultTimeZone)) {
+    return timeZoneCache.defaultTimeZone;
+  }
+
+  // Verify that the current ICU time zone is a valid ECMA-402 time zone.
+  var icuDefaultTimeZone = intl_defaultTimeZone();
+  var timeZone = intl_IsValidTimeZoneName(icuDefaultTimeZone);
+  if (timeZone === null) {
+    // Before defaulting to "UTC", try to represent the default time zone
+    // using the Etc/GMT + offset format. This format only accepts full
+    // hour offsets.
+    const msPerHour = 60 * 60 * 1000;
+    var offset = intl_defaultTimeZoneOffset();
+    assert(
+      offset === (offset | 0),
+      "milliseconds offset shouldn't be able to exceed int32_t range"
+    );
+    var offsetHours = offset / msPerHour;
+    var offsetHoursFraction = offset % msPerHour;
+    if (offsetHoursFraction === 0) {
+      // Etc/GMT + offset uses POSIX-style signs, i.e. a positive offset
+      // means a location west of GMT.
+      timeZone =
+        "Etc/GMT" + (offsetHours < 0 ? "+" : "-") + std_Math_abs(offsetHours);
+
+      // Check if the fallback is valid.
+      timeZone = intl_IsValidTimeZoneName(timeZone);
+    }
+
+    // Fallback to "UTC" if everything else fails.
+    if (timeZone === null) {
+      timeZone = "UTC";
+    }
+  }
+
+  // Canonicalize the ICU time zone, e.g. change Etc/UTC to UTC.
+  var defaultTimeZone = CanonicalizeTimeZoneName(timeZone);
+
+  timeZoneCache.defaultTimeZone = defaultTimeZone;
+  timeZoneCache.icuDefaultTimeZone = icuDefaultTimeZone;
+
+  return defaultTimeZone;
+}
+
+/**
+ * Initializes an object as a DateTimeFormat.
+ *
+ * This method is complicated a moderate bit by its implementing initialization
+ * as a *lazy* concept.  Everything that must happen now, does -- but we defer
+ * all the work we can until the object is actually used as a DateTimeFormat.
+ * This later work occurs in |resolveDateTimeFormatInternals|; steps not noted
+ * here occur there.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 12.1.1.
+ */
+function InitializeDateTimeFormat(
+  dateTimeFormat,
+  thisValue,
+  locales,
+  options,
+  mozExtensions
+) {
+  assert(
+    IsObject(dateTimeFormat),
+    "InitializeDateTimeFormat called with non-Object"
+  );
+  assert(
+    intl_GuardToDateTimeFormat(dateTimeFormat) !== null,
+    "InitializeDateTimeFormat called with non-DateTimeFormat"
+  );
+
+  // Lazy DateTimeFormat data has the following structure:
+  //
+  //   {
+  //     requestedLocales: List of locales,
+  //
+  //     localeOpt: // *first* opt computed in InitializeDateTimeFormat
+  //       {
+  //         localeMatcher: "lookup" / "best fit",
+  //
+  //         ca: string matching a Unicode extension type, // optional
+  //
+  //         nu: string matching a Unicode extension type, // optional
+  //
+  //         hc: "h11" / "h12" / "h23" / "h24", // optional
+  //       }
+  //
+  //     timeZone: IANA time zone name,
+  //
+  //     formatOpt: // *second* opt computed in InitializeDateTimeFormat
+  //       {
+  //         // all the properties/values listed in Table 3
+  //         // (weekday, era, year, month, day, &c.)
+  //
+  //         hour12: true / false,  // optional
+  //       }
+  //
+  //     formatMatcher: "basic" / "best fit",
+  //   }
+  //
+  // Note that lazy data is only installed as a final step of initialization,
+  // so every DateTimeFormat lazy data object has *all* these properties,
+  // never a subset of them.
+  var lazyDateTimeFormatData = std_Object_create(null);
+
+  // Step 1.
+  var requestedLocales = CanonicalizeLocaleList(locales);
+  lazyDateTimeFormatData.requestedLocales = requestedLocales;
+
+  // Step 2.
+  options = ToDateTimeOptions(options, "any", "date");
+
+  // Compute options that impact interpretation of locale.
+  // Step 3.
+  var localeOpt = new_Record();
+  lazyDateTimeFormatData.localeOpt = localeOpt;
+
+  // Steps 4-5.
+  var localeMatcher = GetOption(
+    options,
+    "localeMatcher",
+    "string",
+    ["lookup", "best fit"],
+    "best fit"
+  );
+  localeOpt.localeMatcher = localeMatcher;
+
+  var calendar = GetOption(options, "calendar", "string", undefined, undefined);
+
+  if (calendar !== undefined) {
+    calendar = intl_ValidateAndCanonicalizeUnicodeExtensionType(
+      calendar,
+      "calendar",
+      "ca"
+    );
+  }
+
+  localeOpt.ca = calendar;
+
+  var numberingSystem = GetOption(
+    options,
+    "numberingSystem",
+    "string",
+    undefined,
+    undefined
+  );
+
+  if (numberingSystem !== undefined) {
+    numberingSystem = intl_ValidateAndCanonicalizeUnicodeExtensionType(
+      numberingSystem,
+      "numberingSystem",
+      "nu"
+    );
+  }
+
+  localeOpt.nu = numberingSystem;
+
+  // Step 6.
+  var hr12 = GetOption(options, "hour12", "boolean", undefined, undefined);
+
+  // Step 7.
+  var hc = GetOption(
+    options,
+    "hourCycle",
+    "string",
+    ["h11", "h12", "h23", "h24"],
+    undefined
+  );
+
+  // Step 8.
+  if (hr12 !== undefined) {
+    // The "hourCycle" option is ignored if "hr12" is also present.
+    hc = null;
+  }
+
+  // Step 9.
+  localeOpt.hc = hc;
+
+  // Steps 10-16 (see resolveDateTimeFormatInternals).
+
+  // Steps 17-20.
+  var tz = options.timeZone;
+  if (tz !== undefined) {
+    // Step 18.a.
+    tz = ToString(tz);
+
+    // Step 18.b.
+    var timeZone = intl_IsValidTimeZoneName(tz);
+    if (timeZone === null) {
+      ThrowRangeError(JSMSG_INVALID_TIME_ZONE, tz);
+    }
+
+    // Step 18.c.
+    tz = CanonicalizeTimeZoneName(timeZone);
+  } else {
+    // Step 19.
+    tz = DefaultTimeZone();
+  }
+  lazyDateTimeFormatData.timeZone = tz;
+
+  // Step 21.
+  var formatOpt = new_Record();
+  lazyDateTimeFormatData.formatOpt = formatOpt;
+
+  if (mozExtensions) {
+    let pattern = GetOption(options, "pattern", "string", undefined, undefined);
+    lazyDateTimeFormatData.patternOption = pattern;
+  }
+
+  // Step 22.
+  // 12.1, Table 5: Components of date and time formats.
+  formatOpt.weekday = GetOption(
+    options,
+    "weekday",
+    "string",
+    ["narrow", "short", "long"],
+    undefined
+  );
+  formatOpt.era = GetOption(
+    options,
+    "era",
+    "string",
+    ["narrow", "short", "long"],
+    undefined
+  );
+  formatOpt.year = GetOption(
+    options,
+    "year",
+    "string",
+    ["2-digit", "numeric"],
+    undefined
+  );
+  formatOpt.month = GetOption(
+    options,
+    "month",
+    "string",
+    ["2-digit", "numeric", "narrow", "short", "long"],
+    undefined
+  );
+  formatOpt.day = GetOption(
+    options,
+    "day",
+    "string",
+    ["2-digit", "numeric"],
+    undefined
+  );
+  formatOpt.dayPeriod = GetOption(
+    options,
+    "dayPeriod",
+    "string",
+    ["narrow", "short", "long"],
+    undefined
+  );
+  formatOpt.hour = GetOption(
+    options,
+    "hour",
+    "string",
+    ["2-digit", "numeric"],
+    undefined
+  );
+  formatOpt.minute = GetOption(
+    options,
+    "minute",
+    "string",
+    ["2-digit", "numeric"],
+    undefined
+  );
+  formatOpt.second = GetOption(
+    options,
+    "second",
+    "string",
+    ["2-digit", "numeric"],
+    undefined
+  );
+  formatOpt.fractionalSecondDigits = GetNumberOption(
+    options,
+    "fractionalSecondDigits",
+    1,
+    3,
+    undefined
+  );
+  formatOpt.timeZoneName = GetOption(
+    options,
+    "timeZoneName",
+    "string",
+    [
+      "short",
+      "long",
+      "shortOffset",
+      "longOffset",
+      "shortGeneric",
+      "longGeneric",
+    ],
+    undefined
+  );
+
+  // Steps 23-24 provided by ICU - see comment after this function.
+
+  // Step 25.
+  //
+  // For some reason (ICU not exposing enough interface?) we drop the
+  // requested format matcher on the floor after this.  In any case, even if
+  // doing so is justified, we have to do this work here in case it triggers
+  // getters or similar. (bug 852837)
+  var formatMatcher = GetOption(
+    options,
+    "formatMatcher",
+    "string",
+    ["basic", "best fit"],
+    "best fit"
+  );
+  void formatMatcher;
+
+  // "DateTimeFormat dateStyle & timeStyle" propsal
+  // https://github.com/tc39/proposal-intl-datetime-style
+  var dateStyle = GetOption(
+    options,
+    "dateStyle",
+    "string",
+    ["full", "long", "medium", "short"],
+    undefined
+  );
+  lazyDateTimeFormatData.dateStyle = dateStyle;
+
+  var timeStyle = GetOption(
+    options,
+    "timeStyle",
+    "string",
+    ["full", "long", "medium", "short"],
+    undefined
+  );
+  lazyDateTimeFormatData.timeStyle = timeStyle;
+
+  if (dateStyle !== undefined || timeStyle !== undefined) {
+    var optionsList = [
+      "weekday",
+      "era",
+      "year",
+      "month",
+      "day",
+      "dayPeriod",
+      "hour",
+      "minute",
+      "second",
+      "fractionalSecondDigits",
+      "timeZoneName",
+    ];
+
+    for (var i = 0; i < optionsList.length; i++) {
+      var option = optionsList[i];
+      if (formatOpt[option] !== undefined) {
+        ThrowTypeError(
+          JSMSG_INVALID_DATETIME_OPTION,
+          option,
+          dateStyle !== undefined ? "dateStyle" : "timeStyle"
+        );
+      }
+    }
+  }
+
+  // Steps 26-28 provided by ICU, more or less - see comment after this function.
+
+  // Steps 29-30.
+  // Pass hr12 on to ICU.
+  if (hr12 !== undefined) {
+    formatOpt.hour12 = hr12;
+  }
+
+  // Step 32.
+  //
+  // We've done everything that must be done now: mark the lazy data as fully
+  // computed and install it.
+  initializeIntlObject(
+    dateTimeFormat,
+    "DateTimeFormat",
+    lazyDateTimeFormatData
+  );
+
+  // 12.2.1, steps 4-5.
+  if (
+    dateTimeFormat !== thisValue &&
+    callFunction(
+      std_Object_isPrototypeOf,
+      GetBuiltinPrototype("DateTimeFormat"),
+      thisValue
+    )
+  ) {
+    DefineDataProperty(
+      thisValue,
+      intlFallbackSymbol(),
+      dateTimeFormat,
+      ATTR_NONENUMERABLE | ATTR_NONCONFIGURABLE | ATTR_NONWRITABLE
+    );
+
+    return thisValue;
+  }
+
+  // 12.2.1, step 6.
+  return dateTimeFormat;
+}
+
+/**
+ * Returns a new options object that includes the provided options (if any)
+ * and fills in default components if required components are not defined.
+ * Required can be "date", "time", or "any".
+ * Defaults can be "date", "time", or "all".
+ *
+ * Spec: ECMAScript Internationalization API Specification, 12.1.1.
+ */
+function ToDateTimeOptions(options, required, defaults) {
+  assert(typeof required === "string", "ToDateTimeOptions");
+  assert(typeof defaults === "string", "ToDateTimeOptions");
+
+  // Steps 1-2.
+  if (options === undefined) {
+    options = null;
+  } else {
+    options = ToObject(options);
+  }
+  options = std_Object_create(options);
+
+  // Step 3.
+  var needDefaults = true;
+
+  // Step 4.
+  if (required === "date" || required === "any") {
+    if (options.weekday !== undefined) {
+      needDefaults = false;
+    }
+    if (options.year !== undefined) {
+      needDefaults = false;
+    }
+    if (options.month !== undefined) {
+      needDefaults = false;
+    }
+    if (options.day !== undefined) {
+      needDefaults = false;
+    }
+  }
+
+  // Step 5.
+  if (required === "time" || required === "any") {
+    if (options.dayPeriod !== undefined) {
+      needDefaults = false;
+    }
+    if (options.hour !== undefined) {
+      needDefaults = false;
+    }
+    if (options.minute !== undefined) {
+      needDefaults = false;
+    }
+    if (options.second !== undefined) {
+      needDefaults = false;
+    }
+    if (options.fractionalSecondDigits !== undefined) {
+      needDefaults = false;
+    }
+  }
+
+  // "DateTimeFormat dateStyle & timeStyle" propsal
+  // https://github.com/tc39/proposal-intl-datetime-style
+  var dateStyle = options.dateStyle;
+  var timeStyle = options.timeStyle;
+
+  if (dateStyle !== undefined || timeStyle !== undefined) {
+    needDefaults = false;
+  }
+
+  if (required === "date" && timeStyle !== undefined) {
+    ThrowTypeError(
+      JSMSG_INVALID_DATETIME_STYLE,
+      "timeStyle",
+      "toLocaleDateString"
+    );
+  }
+
+  if (required === "time" && dateStyle !== undefined) {
+    ThrowTypeError(
+      JSMSG_INVALID_DATETIME_STYLE,
+      "dateStyle",
+      "toLocaleTimeString"
+    );
+  }
+
+  // Step 6.
+  if (needDefaults && (defaults === "date" || defaults === "all")) {
+    // The specification says to call [[DefineOwnProperty]] with false for
+    // the Throw parameter, while Object.defineProperty uses true. For the
+    // calls here, the difference doesn't matter because we're adding
+    // properties to a new object.
+    DefineDataProperty(options, "year", "numeric");
+    DefineDataProperty(options, "month", "numeric");
+    DefineDataProperty(options, "day", "numeric");
+  }
+
+  // Step 7.
+  if (needDefaults && (defaults === "time" || defaults === "all")) {
+    // See comment for step 7.
+    DefineDataProperty(options, "hour", "numeric");
+    DefineDataProperty(options, "minute", "numeric");
+    DefineDataProperty(options, "second", "numeric");
+  }
+
+  // Step 8.
+  return options;
+}
+
+/**
+ * Returns the subset of the given locale list for which this locale list has a
+ * matching (possibly fallback) locale. Locales appear in the same order in the
+ * returned list as in the input list.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 12.3.2.
+ */
+function Intl_DateTimeFormat_supportedLocalesOf(locales /*, options*/) {
+  var options = arguments.length > 1 ? arguments[1] : undefined;
+
+  // Step 1.
+  var availableLocales = "DateTimeFormat";
+
+  // Step 2.
+  var requestedLocales = CanonicalizeLocaleList(locales);
+
+  // Step 3.
+  return SupportedLocales(availableLocales, requestedLocales, options);
+}
+
+/**
+ * DateTimeFormat internal properties.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.1 and 12.3.3.
+ */
+var dateTimeFormatInternalProperties = {
+  localeData: dateTimeFormatLocaleData,
+  relevantExtensionKeys: ["ca", "hc", "nu"],
+};
+
+function dateTimeFormatLocaleData() {
+  return {
+    ca: intl_availableCalendars,
+    nu: getNumberingSystems,
+    hc: () => {
+      return [null, "h11", "h12", "h23", "h24"];
+    },
+    default: {
+      ca: intl_defaultCalendar,
+      nu: intl_numberingSystem,
+      hc: () => {
+        return null;
+      },
+    },
+  };
+}
+
+/**
+ * Create function to be cached and returned by Intl.DateTimeFormat.prototype.format.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 12.1.5.
+ */
+function createDateTimeFormatFormat(dtf) {
+  // This function is not inlined in $Intl_DateTimeFormat_format_get to avoid
+  // creating a call-object on each call to $Intl_DateTimeFormat_format_get.
+  return function(date) {
+    // Step 1 (implicit).
+
+    // Step 2.
+    assert(IsObject(dtf), "dateTimeFormatFormatToBind called with non-Object");
+    assert(
+      intl_GuardToDateTimeFormat(dtf) !== null,
+      "dateTimeFormatFormatToBind called with non-DateTimeFormat"
+    );
+
+    // Steps 3-4.
+    var x = date === undefined ? std_Date_now() : ToNumber(date);
+
+    // Step 5.
+    return intl_FormatDateTime(dtf, x, /* formatToParts = */ false);
+  };
+}
+
+/**
+ * Returns a function bound to this DateTimeFormat that returns a String value
+ * representing the result of calling ToNumber(date) according to the
+ * effective locale and the formatting options of this DateTimeFormat.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 12.4.3.
+ */
+// Uncloned functions with `$` prefix are allocated as extended function
+// to store the original name in `SetCanonicalName`.
+function $Intl_DateTimeFormat_format_get() {
+  // Steps 1-3.
+  var thisArg = UnwrapDateTimeFormat(this);
+  var dtf = thisArg;
+  if (!IsObject(dtf) || (dtf = intl_GuardToDateTimeFormat(dtf)) === null) {
+    return callFunction(
+      intl_CallDateTimeFormatMethodIfWrapped,
+      thisArg,
+      "$Intl_DateTimeFormat_format_get"
+    );
+  }
+
+  var internals = getDateTimeFormatInternals(dtf);
+
+  // Step 4.
+  if (internals.boundFormat === undefined) {
+    // Steps 4.a-c.
+    internals.boundFormat = createDateTimeFormatFormat(dtf);
+  }
+
+  // Step 5.
+  return internals.boundFormat;
+}
+SetCanonicalName($Intl_DateTimeFormat_format_get, "get format");
+
+/**
+ * Intl.DateTimeFormat.prototype.formatToParts ( date )
+ *
+ * Spec: ECMAScript Internationalization API Specification, 12.4.4.
+ */
+function Intl_DateTimeFormat_formatToParts(date) {
+  // Step 1.
+  var dtf = this;
+
+  // Steps 2-3.
+  if (!IsObject(dtf) || (dtf = intl_GuardToDateTimeFormat(dtf)) === null) {
+    return callFunction(
+      intl_CallDateTimeFormatMethodIfWrapped,
+      this,
+      date,
+      "Intl_DateTimeFormat_formatToParts"
+    );
+  }
+
+  // Steps 4-5.
+  var x = date === undefined ? std_Date_now() : ToNumber(date);
+
+  // Ensure the DateTimeFormat internals are resolved.
+  getDateTimeFormatInternals(dtf);
+
+  // Step 6.
+  return intl_FormatDateTime(dtf, x, /* formatToParts = */ true);
+}
+
+/**
+ * Intl.DateTimeFormat.prototype.formatRange ( startDate , endDate )
+ *
+ * Spec: Intl.DateTimeFormat.prototype.formatRange proposal
+ */
+function Intl_DateTimeFormat_formatRange(startDate, endDate) {
+  // Step 1.
+  var dtf = this;
+
+  // Step 2.
+  if (!IsObject(dtf) || (dtf = intl_GuardToDateTimeFormat(dtf)) === null) {
+    return callFunction(
+      intl_CallDateTimeFormatMethodIfWrapped,
+      this,
+      startDate,
+      endDate,
+      "Intl_DateTimeFormat_formatRange"
+    );
+  }
+
+  // Step 3.
+  if (startDate === undefined || endDate === undefined) {
+    ThrowTypeError(
+      JSMSG_UNDEFINED_DATE,
+      startDate === undefined ? "start" : "end",
+      "formatRange"
+    );
+  }
+
+  // Step 4.
+  var x = ToNumber(startDate);
+
+  // Step 5.
+  var y = ToNumber(endDate);
+
+  // Ensure the DateTimeFormat internals are resolved.
+  getDateTimeFormatInternals(dtf);
+
+  // Step 6.
+  return intl_FormatDateTimeRange(dtf, x, y, /* formatToParts = */ false);
+}
+
+/**
+ * Intl.DateTimeFormat.prototype.formatRangeToParts ( startDate , endDate )
+ *
+ * Spec: Intl.DateTimeFormat.prototype.formatRange proposal
+ */
+function Intl_DateTimeFormat_formatRangeToParts(startDate, endDate) {
+  // Step 1.
+  var dtf = this;
+
+  // Step 2.
+  if (!IsObject(dtf) || (dtf = intl_GuardToDateTimeFormat(dtf)) === null) {
+    return callFunction(
+      intl_CallDateTimeFormatMethodIfWrapped,
+      this,
+      startDate,
+      endDate,
+      "Intl_DateTimeFormat_formatRangeToParts"
+    );
+  }
+
+  // Step 3.
+  if (startDate === undefined || endDate === undefined) {
+    ThrowTypeError(
+      JSMSG_UNDEFINED_DATE,
+      startDate === undefined ? "start" : "end",
+      "formatRangeToParts"
+    );
+  }
+
+  // Step 4.
+  var x = ToNumber(startDate);
+
+  // Step 5.
+  var y = ToNumber(endDate);
+
+  // Ensure the DateTimeFormat internals are resolved.
+  getDateTimeFormatInternals(dtf);
+
+  // Step 6.
+  return intl_FormatDateTimeRange(dtf, x, y, /* formatToParts = */ true);
+}
+
+/**
+ * Returns the resolved options for a DateTimeFormat object.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 12.4.5.
+ */
+function Intl_DateTimeFormat_resolvedOptions() {
+  // Steps 1-3.
+  var thisArg = UnwrapDateTimeFormat(this);
+  var dtf = thisArg;
+  if (!IsObject(dtf) || (dtf = intl_GuardToDateTimeFormat(dtf)) === null) {
+    return callFunction(
+      intl_CallDateTimeFormatMethodIfWrapped,
+      thisArg,
+      "Intl_DateTimeFormat_resolvedOptions"
+    );
+  }
+
+  // Ensure the internals are resolved.
+  var internals = getDateTimeFormatInternals(dtf);
+
+  // Steps 4-5.
+  var result = {
+    locale: internals.locale,
+    calendar: internals.calendar,
+    numberingSystem: internals.numberingSystem,
+    timeZone: internals.timeZone,
+  };
+
+  if (internals.pattern !== undefined) {
+    // The raw pattern option is only internal to Mozilla, and not part of the
+    // ECMA-402 API.
+    DefineDataProperty(result, "pattern", internals.pattern);
+  }
+
+  var hasDateStyle = internals.dateStyle !== undefined;
+  var hasTimeStyle = internals.timeStyle !== undefined;
+
+  if (hasDateStyle || hasTimeStyle) {
+    if (hasTimeStyle) {
+      // timeStyle (unlike dateStyle) requires resolving the pattern to
+      // ensure "hourCycle" and "hour12" properties are added to |result|.
+      intl_resolveDateTimeFormatComponents(
+        dtf,
+        result,
+        /* includeDateTimeFields = */ false
+      );
+    }
+    if (hasDateStyle) {
+      DefineDataProperty(result, "dateStyle", internals.dateStyle);
+    }
+    if (hasTimeStyle) {
+      DefineDataProperty(result, "timeStyle", internals.timeStyle);
+    }
+  } else {
+    // Components bag or a (Mozilla-only) raw pattern.
+    intl_resolveDateTimeFormatComponents(
+      dtf,
+      result,
+      /* includeDateTimeFields = */ true
+    );
+  }
+
+  // Step 6.
+  return result;
+}
diff --git a/js/src/builtin/intl/DecimalNumber.cpp b/js/src/builtin/intl/DecimalNumber.cpp
new file mode 100644
index 0000000000..ef83edbb87
--- /dev/null
+++ b/js/src/builtin/intl/DecimalNumber.cpp
@@ -0,0 +1,263 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "builtin/intl/DecimalNumber.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/TextUtils.h"
+
+#include "js/GCAPI.h"
+#include "util/Text.h"
+#include "util/Unicode.h"
+#include "vm/StringType.h"
+
+int32_t js::intl::DecimalNumber::compareTo(const DecimalNumber& other) const {
+  // Can't compare if the exponent is too large.
+  MOZ_ASSERT(!exponentTooLarge());
+  MOZ_ASSERT(!other.exponentTooLarge());
+
+  // If the signs don't match, the negative number is smaller.
+  if (isNegative() != other.isNegative()) {
+    return isNegative() ? -1 : 1;
+  }
+
+  // Next handle the case when one of the numbers is zero.
+  if (isZero()) {
+    return other.isZero() ? 0 : other.isNegative() ? 1 : -1;
+  }
+  if (other.isZero()) {
+    return isNegative() ? -1 : 1;
+  }
+
+  // If the exponent is different, the number with the smaller exponent is
+  // smaller in total, unless the numbers are negative.
+  if (exponent() != other.exponent()) {
+    return (exponent() < other.exponent() ? -1 : 1) * (isNegative() ? -1 : 1);
+  }
+
+  class Significand {
+    const DecimalNumber& decimal_;
+    size_t index_;
+
+   public:
+    explicit Significand(const DecimalNumber& decimal) : decimal_(decimal) {
+      index_ = decimal.significandStart_;
+    }
+
+    int32_t next() {
+      // Any remaining digits in the significand are implicit zeros.
+      if (index_ >= decimal_.significandEnd_) {
+        return 0;
+      }
+
+      char ch = decimal_.charAt(index_++);
+
+      // Skip over the decimal point.
+      if (ch == '.') {
+        if (index_ >= decimal_.significandEnd_) {
+          return 0;
+        }
+        ch = decimal_.charAt(index_++);
+      }
+
+      MOZ_ASSERT(mozilla::IsAsciiDigit(ch));
+      return AsciiDigitToNumber(ch);
+    }
+  };
+
+  // Both numbers have the same sign, neither of them is zero, and they have the
+  // same exponent. Next compare the significand digit by digit until we find
+  // the first difference.
+
+  Significand s1(*this);
+  Significand s2(other);
+  for (int32_t e = std::abs(exponent()); e >= 0; e--) {
+    int32_t x = s1.next();
+    int32_t y = s2.next();
+    if (int32_t r = x - y) {
+      return r * (isNegative() ? -1 : 1);
+    }
+  }
+
+  // No different significand digit was found, so the numbers are equal.
+  return 0;
+}
+
+mozilla::Maybe<js::intl::DecimalNumber> js::intl::DecimalNumber::from(
+    JSLinearString* str, JS::AutoCheckCannotGC& nogc) {
+  return str->hasLatin1Chars() ? from<Latin1Char>(str->latin1Range(nogc))
+                               : from<char16_t>(str->twoByteRange(nogc));
+}
+
+template <typename CharT>
+mozilla::Maybe<js::intl::DecimalNumber> js::intl::DecimalNumber::from(
+    mozilla::Span<const CharT> chars) {
+  // This algorithm matches a subset of the `StringNumericLiteral` grammar
+  // production of ECMAScript. In particular, we do *not* allow:
+  // - NonDecimalIntegerLiteral (eg. "0x10")
+  // - NumericLiteralSeparator (eg. "123_456")
+  // - Infinity (eg. "-Infinity")
+
+  DecimalNumber number(chars);
+
+  // Skip over leading whitespace.
+  size_t i = 0;
+  while (i < chars.size() && unicode::IsSpace(chars[i])) {
+    i++;
+  }
+
+  // The number is only whitespace, treat as zero.
+  if (i == chars.size()) {
+    number.zero_ = true;
+    return mozilla::Some(number);
+  }
+
+  // Read the optional sign.
+  if (auto ch = chars[i]; ch == '-' || ch == '+') {
+    i++;
+    number.negative_ = ch == '-';
+
+    if (i == chars.size()) {
+      return mozilla::Nothing();
+    }
+  }
+
+  // Must start with either a digit or the decimal point.
+  size_t startInteger = i;
+  size_t endInteger = i;
+  if (auto ch = chars[i]; mozilla::IsAsciiDigit(ch)) {
+    // Skip over leading zeros.
+    while (i < chars.size() && chars[i] == '0') {
+      i++;
+    }
+
+    // Read the integer part.
+    startInteger = i;
+    while (i < chars.size() && mozilla::IsAsciiDigit(chars[i])) {
+      i++;
+    }
+    endInteger = i;
+  } else if (ch == '.') {
+    // There must be a digit when the number starts with the decimal point.
+    if (i + 1 == chars.size() || !mozilla::IsAsciiDigit(chars[i + 1])) {
+      return mozilla::Nothing();
+    }
+  } else {
+    return mozilla::Nothing();
+  }
+
+  // Read the fractional part.
+  size_t startFraction = i;
+  size_t endFraction = i;
+  if (i < chars.size() && chars[i] == '.') {
+    i++;
+
+    startFraction = i;
+    while (i < chars.size() && mozilla::IsAsciiDigit(chars[i])) {
+      i++;
+    }
+    endFraction = i;
+
+    // Ignore trailing zeros in the fractional part.
+    while (startFraction <= endFraction && chars[endFraction - 1] == '0') {
+      endFraction--;
+    }
+  }
+
+  // Read the exponent.
+  if (i < chars.size() && (chars[i] == 'e' || chars[i] == 'E')) {
+    i++;
+
+    if (i == chars.size()) {
+      return mozilla::Nothing();
+    }
+
+    int32_t exponentSign = 1;
+    if (auto ch = chars[i]; ch == '-' || ch == '+') {
+      i++;
+      exponentSign = ch == '-' ? -1 : +1;
+
+      if (i == chars.size()) {
+        return mozilla::Nothing();
+      }
+    }
+
+    if (!mozilla::IsAsciiDigit(chars[i])) {
+      return mozilla::Nothing();
+    }
+
+    mozilla::CheckedInt32 exp = 0;
+    while (i < chars.size() && mozilla::IsAsciiDigit(chars[i])) {
+      exp *= 10;
+      exp += AsciiDigitToNumber(chars[i]);
+
+      i++;
+    }
+
+    // Check for exponent overflow.
+    if (exp.isValid()) {
+      number.exponent_ = exp.value() * exponentSign;
+    } else {
+      number.exponentTooLarge_ = true;
+    }
+  }
+
+  // Skip over trailing whitespace.
+  while (i < chars.size() && unicode::IsSpace(chars[i])) {
+    i++;
+  }
+
+  // The complete string must have been parsed.
+  if (i != chars.size()) {
+    return mozilla::Nothing();
+  }
+
+  if (startInteger < endInteger) {
+    // We have a non-zero integer part.
+
+    mozilla::CheckedInt32 integerExponent = number.exponent_;
+    integerExponent += size_t(endInteger - startInteger);
+
+    if (integerExponent.isValid()) {
+      number.exponent_ = integerExponent.value();
+    } else {
+      number.exponent_ = 0;
+      number.exponentTooLarge_ = true;
+    }
+
+    number.significandStart_ = startInteger;
+    number.significandEnd_ = endFraction;
+  } else if (startFraction < endFraction) {
+    // We have a non-zero fractional part.
+
+    // Skip over leading zeros
+    size_t i = startFraction;
+    while (i < endFraction && chars[i] == '0') {
+      i++;
+    }
+
+    mozilla::CheckedInt32 fractionExponent = number.exponent_;
+    fractionExponent -= size_t(i - startFraction);
+
+    if (fractionExponent.isValid() && fractionExponent.value() != INT32_MIN) {
+      number.exponent_ = fractionExponent.value();
+    } else {
+      number.exponent_ = 0;
+      number.exponentTooLarge_ = true;
+    }
+
+    number.significandStart_ = i;
+    number.significandEnd_ = endFraction;
+  } else {
+    // The number is zero, clear the error flag if it was set.
+    number.zero_ = true;
+    number.exponent_ = 0;
+    number.exponentTooLarge_ = false;
+  }
+
+  return mozilla::Some(number);
+}
diff --git a/js/src/builtin/intl/DecimalNumber.h b/js/src/builtin/intl/DecimalNumber.h
new file mode 100644
index 0000000000..2373ae0f7f
--- /dev/null
+++ b/js/src/builtin/intl/DecimalNumber.h
@@ -0,0 +1,117 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_DecimalNumber_h
+#define builtin_intl_DecimalNumber_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Span.h"
+#include "mozilla/Variant.h"
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "jstypes.h"
+
+#include "js/TypeDecls.h"
+
+class JSLinearString;
+
+namespace JS {
+class JS_PUBLIC_API AutoCheckCannotGC;
+}
+
+namespace js::intl {
+
+/**
+ * Representation of a decimal number in normalized form.
+ *
+ * Examples of normalized forms:
+ * - "123" is normalized to "0.123e3".
+ * - "0.01e-4" is normalized to "0.1e-5".
+ * - "12.3" is normalized to "0.123e2".
+ *
+ * Note: Internally we leave the decimal point where it lies to avoid copying
+ * the string, but otherwise ignore it once we calculate the normalized
+ * exponent.
+ *
+ * TODO: Remove unused capabilities once there's a concrete PR for
+ * <https://github.com/tc39/proposal-intl-numberformat-v3/issues/98>.
+ */
+class MOZ_STACK_CLASS DecimalNumber final {
+  using Latin1String = mozilla::Span<const JS::Latin1Char>;
+  using TwoByteString = mozilla::Span<const char16_t>;
+
+  mozilla::Variant<Latin1String, TwoByteString> string_;
+
+  char charAt(size_t i) const {
+    if (string_.is<Latin1String>()) {
+      return static_cast<char>(string_.as<Latin1String>()[i]);
+    }
+    return static_cast<char>(string_.as<TwoByteString>()[i]);
+  }
+
+  // Decimal exponent. Valid range is (INT32_MIN, INT_MAX32].
+  int32_t exponent_ = 0;
+
+  // Start and end position of the significand.
+  size_t significandStart_ = 0;
+  size_t significandEnd_ = 0;
+
+  // Flag if the number is zero.
+  bool zero_ = false;
+
+  // Flag for negative numbers.
+  bool negative_ = false;
+
+  // Error flag when the exponent is too large.
+  bool exponentTooLarge_ = false;
+
+  template <typename CharT>
+  explicit DecimalNumber(mozilla::Span<const CharT> string) : string_(string) {}
+
+ public:
+  /** Return true if this decimal is zero. */
+  bool isZero() const { return zero_; }
+
+  /** Return true if this decimal is negative. */
+  bool isNegative() const { return negative_; }
+
+  /** Return true if the exponent is too large. */
+  bool exponentTooLarge() const { return exponentTooLarge_; }
+
+  /** Return the exponent of this decimal. */
+  int32_t exponent() const { return exponent_; }
+
+  // Exposed for testing.
+  size_t significandStart() const { return significandStart_; }
+  size_t significandEnd() const { return significandEnd_; }
+
+  /**
+   * Compare this decimal to another decimal. Returns a negative value if this
+   * decimal is smaller; zero if this decimal is equal; or a positive value if
+   * this decimal is larger than the input.
+   */
+  int32_t compareTo(const DecimalNumber& other) const;
+
+  /**
+   * Create a decimal number from the input. Returns |mozilla::Nothing| if the
+   * input can't be parsed.
+   */
+  template <typename CharT>
+  static mozilla::Maybe<DecimalNumber> from(mozilla::Span<const CharT> chars);
+
+  /**
+   * Create a decimal number from the input. Returns |mozilla::Nothing| if the
+   * input can't be parsed.
+   */
+  static mozilla::Maybe<DecimalNumber> from(JSLinearString* str,
+                                            JS::AutoCheckCannotGC& nogc);
+};
+}  // namespace js::intl
+
+#endif /* builtin_intl_DecimalNumber_h */
diff --git a/js/src/builtin/intl/DisplayNames.cpp b/js/src/builtin/intl/DisplayNames.cpp
new file mode 100644
index 0000000000..2ddffe0365
--- /dev/null
+++ b/js/src/builtin/intl/DisplayNames.cpp
@@ -0,0 +1,551 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Intl.DisplayNames implementation. */
+
+#include "builtin/intl/DisplayNames.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/intl/DisplayNames.h"
+#include "mozilla/PodOperations.h"
+#include "mozilla/Span.h"
+
+#include <algorithm>
+
+#include "jsnum.h"
+#include "jspubtd.h"
+
+#include "builtin/intl/CommonFunctions.h"
+#include "builtin/intl/FormatBuffer.h"
+#include "gc/AllocKind.h"
+#include "gc/GCContext.h"
+#include "js/CallArgs.h"
+#include "js/Class.h"
+#include "js/experimental/Intl.h"     // JS::AddMozDisplayNamesConstructor
+#include "js/friend/ErrorMessages.h"  // js::GetErrorMessage, JSMSG_*
+#include "js/PropertyAndElement.h"    // JS_DefineFunctions, JS_DefineProperties
+#include "js/PropertyDescriptor.h"
+#include "js/PropertySpec.h"
+#include "js/RootingAPI.h"
+#include "js/TypeDecls.h"
+#include "js/Utility.h"
+#include "vm/GlobalObject.h"
+#include "vm/JSContext.h"
+#include "vm/JSObject.h"
+#include "vm/Printer.h"
+#include "vm/Runtime.h"
+#include "vm/SelfHosting.h"
+#include "vm/Stack.h"
+#include "vm/StringType.h"
+#include "vm/WellKnownAtom.h"  // js_*_str
+
+#include "vm/JSObject-inl.h"
+#include "vm/NativeObject-inl.h"
+
+using namespace js;
+
+const JSClassOps DisplayNamesObject::classOps_ = {nullptr, /* addProperty */
+                                                  nullptr, /* delProperty */
+                                                  nullptr, /* enumerate */
+                                                  nullptr, /* newEnumerate */
+                                                  nullptr, /* resolve */
+                                                  nullptr, /* mayResolve */
+                                                  DisplayNamesObject::finalize};
+
+const JSClass DisplayNamesObject::class_ = {
+    "Intl.DisplayNames",
+    JSCLASS_HAS_RESERVED_SLOTS(DisplayNamesObject::SLOT_COUNT) |
+        JSCLASS_HAS_CACHED_PROTO(JSProto_DisplayNames) |
+        JSCLASS_FOREGROUND_FINALIZE,
+    &DisplayNamesObject::classOps_, &DisplayNamesObject::classSpec_};
+
+const JSClass& DisplayNamesObject::protoClass_ = PlainObject::class_;
+
+static bool displayNames_toSource(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  args.rval().setString(cx->names().DisplayNames);
+  return true;
+}
+
+static const JSFunctionSpec displayNames_static_methods[] = {
+    JS_SELF_HOSTED_FN("supportedLocalesOf",
+                      "Intl_DisplayNames_supportedLocalesOf", 1, 0),
+    JS_FS_END};
+
+static const JSFunctionSpec displayNames_methods[] = {
+    JS_SELF_HOSTED_FN("of", "Intl_DisplayNames_of", 1, 0),
+    JS_SELF_HOSTED_FN("resolvedOptions", "Intl_DisplayNames_resolvedOptions", 0,
+                      0),
+    JS_FN(js_toSource_str, displayNames_toSource, 0, 0), JS_FS_END};
+
+static const JSPropertySpec displayNames_properties[] = {
+    JS_STRING_SYM_PS(toStringTag, "Intl.DisplayNames", JSPROP_READONLY),
+    JS_PS_END};
+
+static bool DisplayNames(JSContext* cx, unsigned argc, Value* vp);
+
+const ClassSpec DisplayNamesObject::classSpec_ = {
+    GenericCreateConstructor<DisplayNames, 2, gc::AllocKind::FUNCTION>,
+    GenericCreatePrototype<DisplayNamesObject>,
+    displayNames_static_methods,
+    nullptr,
+    displayNames_methods,
+    displayNames_properties,
+    nullptr,
+    ClassSpec::DontDefineConstructor};
+
+enum class DisplayNamesOptions {
+  Standard,
+
+  // Calendar display names are no longer available with the current spec
+  // proposal text, but may be re-enabled in the future. For our internal use
+  // we still need to have them present, so use a feature guard for now.
+  EnableMozExtensions,
+};
+
+/**
+ * Initialize a new Intl.DisplayNames object using the named self-hosted
+ * function.
+ */
+static bool InitializeDisplayNamesObject(JSContext* cx, HandleObject obj,
+                                         Handle<PropertyName*> initializer,
+                                         HandleValue locales,
+                                         HandleValue options,
+                                         DisplayNamesOptions dnoptions) {
+  FixedInvokeArgs<4> args(cx);
+
+  args[0].setObject(*obj);
+  args[1].set(locales);
+  args[2].set(options);
+  args[3].setBoolean(dnoptions == DisplayNamesOptions::EnableMozExtensions);
+
+  RootedValue ignored(cx);
+  if (!CallSelfHostedFunction(cx, initializer, NullHandleValue, args,
+                              &ignored)) {
+    return false;
+  }
+
+  MOZ_ASSERT(ignored.isUndefined(),
+             "Unexpected return value from non-legacy Intl object initializer");
+  return true;
+}
+
+/**
+ * Intl.DisplayNames ([ locales [ , options ]])
+ */
+static bool DisplayNames(JSContext* cx, const CallArgs& args,
+                         DisplayNamesOptions dnoptions) {
+  // Step 1.
+  if (!ThrowIfNotConstructing(cx, args, "Intl.DisplayNames")) {
+    return false;
+  }
+
+  // Step 2 (Inlined 9.1.14, OrdinaryCreateFromConstructor).
+  RootedObject proto(cx);
+  if (dnoptions == DisplayNamesOptions::Standard) {
+    if (!GetPrototypeFromBuiltinConstructor(cx, args, JSProto_DisplayNames,
+                                            &proto)) {
+      return false;
+    }
+  } else {
+    RootedObject newTarget(cx, &args.newTarget().toObject());
+    if (!GetPrototypeFromConstructor(cx, newTarget, JSProto_Null, &proto)) {
+      return false;
+    }
+  }
+
+  Rooted<DisplayNamesObject*> displayNames(cx);
+  displayNames = NewObjectWithClassProto<DisplayNamesObject>(cx, proto);
+  if (!displayNames) {
+    return false;
+  }
+
+  HandleValue locales = args.get(0);
+  HandleValue options = args.get(1);
+
+  // Steps 3-26.
+  if (!InitializeDisplayNamesObject(cx, displayNames,
+                                    cx->names().InitializeDisplayNames, locales,
+                                    options, dnoptions)) {
+    return false;
+  }
+
+  // Step 27.
+  args.rval().setObject(*displayNames);
+  return true;
+}
+
+static bool DisplayNames(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return DisplayNames(cx, args, DisplayNamesOptions::Standard);
+}
+
+static bool MozDisplayNames(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return DisplayNames(cx, args, DisplayNamesOptions::EnableMozExtensions);
+}
+
+void js::DisplayNamesObject::finalize(JS::GCContext* gcx, JSObject* obj) {
+  MOZ_ASSERT(gcx->onMainThread());
+
+  if (mozilla::intl::DisplayNames* displayNames =
+          obj->as<DisplayNamesObject>().getDisplayNames()) {
+    intl::RemoveICUCellMemory(gcx, obj, DisplayNamesObject::EstimatedMemoryUse);
+    delete displayNames;
+  }
+}
+
+bool JS::AddMozDisplayNamesConstructor(JSContext* cx, HandleObject intl) {
+  RootedObject ctor(cx, GlobalObject::createConstructor(
+                            cx, MozDisplayNames, cx->names().DisplayNames, 2));
+  if (!ctor) {
+    return false;
+  }
+
+  RootedObject proto(
+      cx, GlobalObject::createBlankPrototype<PlainObject>(cx, cx->global()));
+  if (!proto) {
+    return false;
+  }
+
+  if (!LinkConstructorAndPrototype(cx, ctor, proto)) {
+    return false;
+  }
+
+  if (!JS_DefineFunctions(cx, ctor, displayNames_static_methods)) {
+    return false;
+  }
+
+  if (!JS_DefineFunctions(cx, proto, displayNames_methods)) {
+    return false;
+  }
+
+  if (!JS_DefineProperties(cx, proto, displayNames_properties)) {
+    return false;
+  }
+
+  RootedValue ctorValue(cx, ObjectValue(*ctor));
+  return DefineDataProperty(cx, intl, cx->names().DisplayNames, ctorValue, 0);
+}
+
+static mozilla::intl::DisplayNames* NewDisplayNames(
+    JSContext* cx, const char* locale,
+    mozilla::intl::DisplayNames::Options& options) {
+  auto result = mozilla::intl::DisplayNames::TryCreate(locale, options);
+  if (result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return nullptr;
+  }
+  return result.unwrap().release();
+}
+
+static mozilla::intl::DisplayNames* GetOrCreateDisplayNames(
+    JSContext* cx, Handle<DisplayNamesObject*> displayNames, const char* locale,
+    mozilla::intl::DisplayNames::Options& options) {
+  // Obtain a cached mozilla::intl::DisplayNames object.
+  mozilla::intl::DisplayNames* dn = displayNames->getDisplayNames();
+  if (!dn) {
+    dn = NewDisplayNames(cx, locale, options);
+    if (!dn) {
+      return nullptr;
+    }
+    displayNames->setDisplayNames(dn);
+
+    intl::AddICUCellMemory(displayNames,
+                           DisplayNamesObject::EstimatedMemoryUse);
+  }
+  return dn;
+}
+
+static void ReportInvalidOptionError(JSContext* cx, HandleString type,
+                                     HandleString option) {
+  if (UniqueChars optionStr = QuoteString(cx, option, '"')) {
+    if (UniqueChars typeStr = QuoteString(cx, type)) {
+      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                JSMSG_INVALID_OPTION_VALUE, typeStr.get(),
+                                optionStr.get());
+    }
+  }
+}
+
+static void ReportInvalidOptionError(JSContext* cx, const char* type,
+                                     HandleString option) {
+  if (UniqueChars str = QuoteString(cx, option, '"')) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_INVALID_OPTION_VALUE, type, str.get());
+  }
+}
+
+static void ReportInvalidOptionError(JSContext* cx, const char* type,
+                                     double option) {
+  ToCStringBuf cbuf;
+  const char* str = NumberToCString(&cbuf, option);
+  MOZ_ASSERT(str);
+  JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                            JSMSG_INVALID_DIGITS_VALUE, str);
+}
+
+/**
+ * intl_ComputeDisplayName(displayNames, locale, calendar, style,
+ *                         languageDisplay, fallback, type, code)
+ */
+bool js::intl_ComputeDisplayName(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 8);
+
+  Rooted<DisplayNamesObject*> displayNames(
+      cx, &args[0].toObject().as<DisplayNamesObject>());
+
+  UniqueChars locale = intl::EncodeLocale(cx, args[1].toString());
+  if (!locale) {
+    return false;
+  }
+
+  Rooted<JSLinearString*> calendar(cx, args[2].toString()->ensureLinear(cx));
+  if (!calendar) {
+    return false;
+  }
+
+  Rooted<JSLinearString*> code(cx, args[7].toString()->ensureLinear(cx));
+  if (!code) {
+    return false;
+  }
+
+  mozilla::intl::DisplayNames::Style style;
+  {
+    JSLinearString* styleStr = args[3].toString()->ensureLinear(cx);
+    if (!styleStr) {
+      return false;
+    }
+
+    if (StringEqualsLiteral(styleStr, "long")) {
+      style = mozilla::intl::DisplayNames::Style::Long;
+    } else if (StringEqualsLiteral(styleStr, "short")) {
+      style = mozilla::intl::DisplayNames::Style::Short;
+    } else if (StringEqualsLiteral(styleStr, "narrow")) {
+      style = mozilla::intl::DisplayNames::Style::Narrow;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(styleStr, "abbreviated"));
+      style = mozilla::intl::DisplayNames::Style::Abbreviated;
+    }
+  }
+
+  mozilla::intl::DisplayNames::LanguageDisplay languageDisplay;
+  {
+    JSLinearString* language = args[4].toString()->ensureLinear(cx);
+    if (!language) {
+      return false;
+    }
+
+    if (StringEqualsLiteral(language, "dialect")) {
+      languageDisplay = mozilla::intl::DisplayNames::LanguageDisplay::Dialect;
+    } else {
+      MOZ_ASSERT(language->empty() ||
+                 StringEqualsLiteral(language, "standard"));
+      languageDisplay = mozilla::intl::DisplayNames::LanguageDisplay::Standard;
+    }
+  }
+
+  mozilla::intl::DisplayNames::Fallback fallback;
+  {
+    JSLinearString* fallbackStr = args[5].toString()->ensureLinear(cx);
+    if (!fallbackStr) {
+      return false;
+    }
+
+    if (StringEqualsLiteral(fallbackStr, "none")) {
+      fallback = mozilla::intl::DisplayNames::Fallback::None;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(fallbackStr, "code"));
+      fallback = mozilla::intl::DisplayNames::Fallback::Code;
+    }
+  }
+
+  Rooted<JSLinearString*> type(cx, args[6].toString()->ensureLinear(cx));
+  if (!type) {
+    return false;
+  }
+
+  mozilla::intl::DisplayNames::Options options{
+      style,
+      languageDisplay,
+  };
+
+  // If a calendar exists, set it as an option.
+  JS::UniqueChars calendarChars = nullptr;
+  if (!calendar->empty()) {
+    calendarChars = JS_EncodeStringToUTF8(cx, calendar);
+    if (!calendarChars) {
+      return false;
+    }
+  }
+
+  mozilla::intl::DisplayNames* dn =
+      GetOrCreateDisplayNames(cx, displayNames, locale.get(), options);
+  if (!dn) {
+    return false;
+  }
+
+  // The "code" is usually a small ASCII string, so try to avoid an allocation
+  // by copying it to the stack. Unfortunately we can't pass a string span of
+  // the JSString directly to the unified DisplayNames API, as the
+  // intl::FormatBuffer will be written to. This writing can trigger a GC and
+  // invalidate the span, creating a nogc rooting hazard.
+  JS::UniqueChars utf8 = nullptr;
+  unsigned char ascii[32];
+  mozilla::Span<const char> codeSpan = nullptr;
+  if (code->length() < 32 && code->hasLatin1Chars() && StringIsAscii(code)) {
+    JS::AutoCheckCannotGC nogc;
+    mozilla::PodCopy(ascii, code->latin1Chars(nogc), code->length());
+    codeSpan =
+        mozilla::Span(reinterpret_cast<const char*>(ascii), code->length());
+  } else {
+    utf8 = JS_EncodeStringToUTF8(cx, code);
+    if (!utf8) {
+      return false;
+    }
+    codeSpan = mozilla::MakeStringSpan(utf8.get());
+  }
+
+  intl::FormatBuffer<char16_t, intl::INITIAL_CHAR_BUFFER_SIZE> buffer(cx);
+  mozilla::Result<mozilla::Ok, mozilla::intl::DisplayNamesError> result =
+      mozilla::Ok{};
+
+  if (StringEqualsLiteral(type, "language")) {
+    result = dn->GetLanguage(buffer, codeSpan, fallback);
+  } else if (StringEqualsLiteral(type, "script")) {
+    result = dn->GetScript(buffer, codeSpan, fallback);
+  } else if (StringEqualsLiteral(type, "region")) {
+    result = dn->GetRegion(buffer, codeSpan, fallback);
+  } else if (StringEqualsLiteral(type, "currency")) {
+    result = dn->GetCurrency(buffer, codeSpan, fallback);
+  } else if (StringEqualsLiteral(type, "calendar")) {
+    result = dn->GetCalendar(buffer, codeSpan, fallback);
+  } else if (StringEqualsLiteral(type, "weekday")) {
+    double d = LinearStringToNumber(code);
+    if (!IsInteger(d) || d < 1 || d > 7) {
+      ReportInvalidOptionError(cx, "weekday", d);
+      return false;
+    }
+    result =
+        dn->GetWeekday(buffer, static_cast<mozilla::intl::Weekday>(d),
+                       mozilla::MakeStringSpan(calendarChars.get()), fallback);
+  } else if (StringEqualsLiteral(type, "month")) {
+    double d = LinearStringToNumber(code);
+    if (!IsInteger(d) || d < 1 || d > 13) {
+      ReportInvalidOptionError(cx, "month", d);
+      return false;
+    }
+
+    result =
+        dn->GetMonth(buffer, static_cast<mozilla::intl::Month>(d),
+                     mozilla::MakeStringSpan(calendarChars.get()), fallback);
+
+  } else if (StringEqualsLiteral(type, "quarter")) {
+    double d = LinearStringToNumber(code);
+
+    // Inlined implementation of `IsValidQuarterCode ( quarter )`.
+    if (!IsInteger(d) || d < 1 || d > 4) {
+      ReportInvalidOptionError(cx, "quarter", d);
+      return false;
+    }
+
+    result =
+        dn->GetQuarter(buffer, static_cast<mozilla::intl::Quarter>(d),
+                       mozilla::MakeStringSpan(calendarChars.get()), fallback);
+
+  } else if (StringEqualsLiteral(type, "dayPeriod")) {
+    mozilla::intl::DayPeriod dayPeriod;
+    if (StringEqualsLiteral(code, "am")) {
+      dayPeriod = mozilla::intl::DayPeriod::AM;
+    } else if (StringEqualsLiteral(code, "pm")) {
+      dayPeriod = mozilla::intl::DayPeriod::PM;
+    } else {
+      ReportInvalidOptionError(cx, "dayPeriod", code);
+      return false;
+    }
+    result = dn->GetDayPeriod(buffer, dayPeriod,
+                              mozilla::MakeStringSpan(calendarChars.get()),
+                              fallback);
+
+  } else {
+    MOZ_ASSERT(StringEqualsLiteral(type, "dateTimeField"));
+    mozilla::intl::DateTimeField field;
+    if (StringEqualsLiteral(code, "era")) {
+      field = mozilla::intl::DateTimeField::Era;
+    } else if (StringEqualsLiteral(code, "year")) {
+      field = mozilla::intl::DateTimeField::Year;
+    } else if (StringEqualsLiteral(code, "quarter")) {
+      field = mozilla::intl::DateTimeField::Quarter;
+    } else if (StringEqualsLiteral(code, "month")) {
+      field = mozilla::intl::DateTimeField::Month;
+    } else if (StringEqualsLiteral(code, "weekOfYear")) {
+      field = mozilla::intl::DateTimeField::WeekOfYear;
+    } else if (StringEqualsLiteral(code, "weekday")) {
+      field = mozilla::intl::DateTimeField::Weekday;
+    } else if (StringEqualsLiteral(code, "day")) {
+      field = mozilla::intl::DateTimeField::Day;
+    } else if (StringEqualsLiteral(code, "dayPeriod")) {
+      field = mozilla::intl::DateTimeField::DayPeriod;
+    } else if (StringEqualsLiteral(code, "hour")) {
+      field = mozilla::intl::DateTimeField::Hour;
+    } else if (StringEqualsLiteral(code, "minute")) {
+      field = mozilla::intl::DateTimeField::Minute;
+    } else if (StringEqualsLiteral(code, "second")) {
+      field = mozilla::intl::DateTimeField::Second;
+    } else if (StringEqualsLiteral(code, "timeZoneName")) {
+      field = mozilla::intl::DateTimeField::TimeZoneName;
+    } else {
+      ReportInvalidOptionError(cx, "dateTimeField", code);
+      return false;
+    }
+
+    intl::SharedIntlData& sharedIntlData = cx->runtime()->sharedIntlData.ref();
+    mozilla::intl::DateTimePatternGenerator* dtpgen =
+        sharedIntlData.getDateTimePatternGenerator(cx, locale.get());
+    if (!dtpgen) {
+      return false;
+    }
+
+    result = dn->GetDateTimeField(buffer, field, *dtpgen, fallback);
+  }
+
+  if (result.isErr()) {
+    switch (result.unwrapErr()) {
+      case mozilla::intl::DisplayNamesError::InternalError:
+        intl::ReportInternalError(cx);
+        break;
+      case mozilla::intl::DisplayNamesError::OutOfMemory:
+        ReportOutOfMemory(cx);
+        break;
+      case mozilla::intl::DisplayNamesError::InvalidOption:
+        ReportInvalidOptionError(cx, type, code);
+        break;
+      case mozilla::intl::DisplayNamesError::DuplicateVariantSubtag:
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_DUPLICATE_VARIANT_SUBTAG);
+        break;
+      case mozilla::intl::DisplayNamesError::InvalidLanguageTag:
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_INVALID_LANGUAGE_TAG);
+        break;
+    }
+    return false;
+  }
+
+  JSString* str = buffer.toString(cx);
+  if (!str) {
+    return false;
+  }
+
+  if (str->empty()) {
+    args.rval().setUndefined();
+  } else {
+    args.rval().setString(str);
+  }
+
+  return true;
+}
diff --git a/js/src/builtin/intl/DisplayNames.h b/js/src/builtin/intl/DisplayNames.h
new file mode 100644
index 0000000000..9fd6c63a62
--- /dev/null
+++ b/js/src/builtin/intl/DisplayNames.h
@@ -0,0 +1,79 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_DisplayNames_h
+#define builtin_intl_DisplayNames_h
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "jstypes.h"
+#include "NamespaceImports.h"
+
+#include "builtin/SelfHostingDefines.h"
+#include "js/Class.h"  // JSClass, JSClassOps, js::ClassSpec
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "vm/NativeObject.h"
+
+struct JS_PUBLIC_API JSContext;
+
+namespace mozilla::intl {
+class DisplayNames;
+}
+
+namespace js {
+struct ClassSpec;
+
+class DisplayNamesObject : public NativeObject {
+ public:
+  static const JSClass class_;
+  static const JSClass& protoClass_;
+
+  static constexpr uint32_t INTERNALS_SLOT = 0;
+  static constexpr uint32_t LOCALE_DISPLAY_NAMES_SLOT = 1;
+  static constexpr uint32_t SLOT_COUNT = 3;
+
+  static_assert(INTERNALS_SLOT == INTL_INTERNALS_OBJECT_SLOT,
+                "INTERNALS_SLOT must match self-hosting define for internals "
+                "object slot");
+
+  // Estimated memory use for ULocaleDisplayNames (see IcuMemoryUsage).
+  static constexpr size_t EstimatedMemoryUse = 1238;
+
+  mozilla::intl::DisplayNames* getDisplayNames() const {
+    const auto& slot = getFixedSlot(LOCALE_DISPLAY_NAMES_SLOT);
+    if (slot.isUndefined()) {
+      return nullptr;
+    }
+    return static_cast<mozilla::intl::DisplayNames*>(slot.toPrivate());
+  }
+
+  void setDisplayNames(mozilla::intl::DisplayNames* displayNames) {
+    setFixedSlot(LOCALE_DISPLAY_NAMES_SLOT, PrivateValue(displayNames));
+  }
+
+ private:
+  static const JSClassOps classOps_;
+  static const ClassSpec classSpec_;
+
+  static void finalize(JS::GCContext* gcx, JSObject* obj);
+};
+
+/**
+ * Return the display name for the requested code or undefined if no applicable
+ * display name was found.
+ *
+ * Usage: result = intl_ComputeDisplayName(displayNames, locale, calendar,
+ *                                         style, languageDisplay, fallback,
+ *                                         type, code)
+ */
+[[nodiscard]] extern bool intl_ComputeDisplayName(JSContext* cx, unsigned argc,
+                                                  Value* vp);
+
+}  // namespace js
+
+#endif /* builtin_intl_DisplayNames_h */
diff --git a/js/src/builtin/intl/DisplayNames.js b/js/src/builtin/intl/DisplayNames.js
new file mode 100644
index 0000000000..5eb699fc89
--- /dev/null
+++ b/js/src/builtin/intl/DisplayNames.js
@@ -0,0 +1,418 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * Intl.DisplayNames internal properties.
+ */
+function displayNamesLocaleData() {
+  // Intl.DisplayNames doesn't support any extension keys.
+  return {};
+}
+var displayNamesInternalProperties = {
+  localeData: displayNamesLocaleData,
+  relevantExtensionKeys: [],
+};
+
+function mozDisplayNamesLocaleData() {
+  return {
+    ca: intl_availableCalendars,
+    default: {
+      ca: intl_defaultCalendar,
+    },
+  };
+}
+var mozDisplayNamesInternalProperties = {
+  localeData: mozDisplayNamesLocaleData,
+  relevantExtensionKeys: ["ca"],
+};
+
+/**
+ * Intl.DisplayNames ( [ locales [ , options ] ] )
+ *
+ * Compute an internal properties object from |lazyDisplayNamesData|.
+ */
+function resolveDisplayNamesInternals(lazyDisplayNamesData) {
+  assert(IsObject(lazyDisplayNamesData), "lazy data not an object?");
+
+  var internalProps = std_Object_create(null);
+
+  var mozExtensions = lazyDisplayNamesData.mozExtensions;
+
+  var DisplayNames = mozExtensions
+    ? mozDisplayNamesInternalProperties
+    : displayNamesInternalProperties;
+
+  // Compute effective locale.
+
+  // Step 7.
+  var localeData = DisplayNames.localeData;
+
+  // Step 10.
+  var r = ResolveLocale(
+    "DisplayNames",
+    lazyDisplayNamesData.requestedLocales,
+    lazyDisplayNamesData.opt,
+    DisplayNames.relevantExtensionKeys,
+    localeData
+  );
+
+  // Step 12.
+  internalProps.style = lazyDisplayNamesData.style;
+
+  // Step 14.
+  var type = lazyDisplayNamesData.type;
+  internalProps.type = type;
+
+  // Step 16.
+  internalProps.fallback = lazyDisplayNamesData.fallback;
+
+  // Step 17.
+  internalProps.locale = r.locale;
+
+  // Step 25.
+  if (type === "language") {
+    internalProps.languageDisplay = lazyDisplayNamesData.languageDisplay;
+  }
+
+  if (mozExtensions) {
+    internalProps.calendar = r.ca;
+  }
+
+  // The caller is responsible for associating |internalProps| with the right
+  // object using |setInternalProperties|.
+  return internalProps;
+}
+
+/**
+ * Returns an object containing the DisplayNames internal properties of |obj|.
+ */
+function getDisplayNamesInternals(obj) {
+  assert(IsObject(obj), "getDisplayNamesInternals called with non-object");
+  assert(
+    intl_GuardToDisplayNames(obj) !== null,
+    "getDisplayNamesInternals called with non-DisplayNames"
+  );
+
+  var internals = getIntlObjectInternals(obj);
+  assert(
+    internals.type === "DisplayNames",
+    "bad type escaped getIntlObjectInternals"
+  );
+
+  // If internal properties have already been computed, use them.
+  var internalProps = maybeInternalProperties(internals);
+  if (internalProps) {
+    return internalProps;
+  }
+
+  // Otherwise it's time to fully create them.
+  internalProps = resolveDisplayNamesInternals(internals.lazyData);
+  setInternalProperties(internals, internalProps);
+  return internalProps;
+}
+
+/**
+ * Intl.DisplayNames ( [ locales [ , options ] ] )
+ *
+ * Initializes an object as a DisplayNames.
+ *
+ * This method is complicated a moderate bit by its implementing initialization
+ * as a *lazy* concept.  Everything that must happen now, does -- but we defer
+ * all the work we can until the object is actually used as a DisplayNames.
+ * This later work occurs in |resolveDisplayNamesInternals|; steps not noted
+ * here occur there.
+ */
+function InitializeDisplayNames(displayNames, locales, options, mozExtensions) {
+  assert(
+    IsObject(displayNames),
+    "InitializeDisplayNames called with non-object"
+  );
+  assert(
+    intl_GuardToDisplayNames(displayNames) !== null,
+    "InitializeDisplayNames called with non-DisplayNames"
+  );
+
+  // Lazy DisplayNames data has the following structure:
+  //
+  //   {
+  //     requestedLocales: List of locales,
+  //
+  //     opt: // opt object computed in InitializeDisplayNames
+  //       {
+  //         localeMatcher: "lookup" / "best fit",
+  //
+  //         ca: string matching a Unicode extension type, // optional
+  //       }
+  //
+  //     localeMatcher: "lookup" / "best fit",
+  //
+  //     style: "narrow" / "short" / "abbreviated" / "long",
+  //
+  //     type: "language" / "region" / "script" / "currency" / "weekday" /
+  //           "month" / "quarter" / "dayPeriod" / "dateTimeField"
+  //
+  //     fallback: "code" / "none",
+  //
+  //     // field present only if type === "language":
+  //     languageDisplay: "dialect" / "standard",
+  //
+  //     mozExtensions: true / false,
+  //   }
+  //
+  // Note that lazy data is only installed as a final step of initialization,
+  // so every DisplayNames lazy data object has *all* these properties, never a
+  // subset of them.
+  var lazyDisplayNamesData = std_Object_create(null);
+
+  // Step 3.
+  var requestedLocales = CanonicalizeLocaleList(locales);
+  lazyDisplayNamesData.requestedLocales = requestedLocales;
+
+  // Step 4.
+  if (!IsObject(options)) {
+    ThrowTypeError(
+      JSMSG_OBJECT_REQUIRED,
+      options === null ? "null" : typeof options
+    );
+  }
+
+  // Step 5.
+  var opt = new_Record();
+  lazyDisplayNamesData.opt = opt;
+  lazyDisplayNamesData.mozExtensions = mozExtensions;
+
+  // Steps 7-8.
+  var matcher = GetOption(
+    options,
+    "localeMatcher",
+    "string",
+    ["lookup", "best fit"],
+    "best fit"
+  );
+  opt.localeMatcher = matcher;
+
+  if (mozExtensions) {
+    var calendar = GetOption(
+      options,
+      "calendar",
+      "string",
+      undefined,
+      undefined
+    );
+
+    if (calendar !== undefined) {
+      calendar = intl_ValidateAndCanonicalizeUnicodeExtensionType(
+        calendar,
+        "calendar",
+        "ca"
+      );
+    }
+
+    opt.ca = calendar;
+  }
+
+  // Step 10.
+  var style;
+  if (mozExtensions) {
+    style = GetOption(
+      options,
+      "style",
+      "string",
+      ["narrow", "short", "abbreviated", "long"],
+      "long"
+    );
+  } else {
+    style = GetOption(
+      options,
+      "style",
+      "string",
+      ["narrow", "short", "long"],
+      "long"
+    );
+  }
+
+  // Step 11.
+  lazyDisplayNamesData.style = style;
+
+  // Step 12.
+  var type;
+  if (mozExtensions) {
+    type = GetOption(
+      options,
+      "type",
+      "string",
+      [
+        "language",
+        "region",
+        "script",
+        "currency",
+        "calendar",
+        "dateTimeField",
+        "weekday",
+        "month",
+        "quarter",
+        "dayPeriod",
+      ],
+      undefined
+    );
+  } else {
+    type = GetOption(
+      options,
+      "type",
+      "string",
+      ["language", "region", "script", "currency", "calendar", "dateTimeField"],
+      undefined
+    );
+  }
+
+  // Step 13.
+  if (type === undefined) {
+    ThrowTypeError(JSMSG_UNDEFINED_TYPE);
+  }
+
+  // Step 14.
+  lazyDisplayNamesData.type = type;
+
+  // Step 15.
+  var fallback = GetOption(
+    options,
+    "fallback",
+    "string",
+    ["code", "none"],
+    "code"
+  );
+
+  // Step 16.
+  lazyDisplayNamesData.fallback = fallback;
+
+  // Step 24.
+  var languageDisplay = GetOption(
+    options,
+    "languageDisplay",
+    "string",
+    ["dialect", "standard"],
+    "dialect"
+  );
+
+  // Step 25.
+  if (type === "language") {
+    lazyDisplayNamesData.languageDisplay = languageDisplay;
+  }
+
+  // We've done everything that must be done now: mark the lazy data as fully
+  // computed and install it.
+  initializeIntlObject(displayNames, "DisplayNames", lazyDisplayNamesData);
+}
+
+/**
+ * Returns the subset of the given locale list for which this locale list has a
+ * matching (possibly fallback) locale. Locales appear in the same order in the
+ * returned list as in the input list.
+ */
+function Intl_DisplayNames_supportedLocalesOf(locales /*, options*/) {
+  var options = arguments.length > 1 ? arguments[1] : undefined;
+
+  // Step 1.
+  var availableLocales = "DisplayNames";
+
+  // Step 2.
+  var requestedLocales = CanonicalizeLocaleList(locales);
+
+  // Step 3.
+  return SupportedLocales(availableLocales, requestedLocales, options);
+}
+
+/**
+ * Returns the resolved options for a DisplayNames object.
+ */
+function Intl_DisplayNames_of(code) {
+  // Step 1.
+  var displayNames = this;
+
+  // Steps 2-3.
+  if (
+    !IsObject(displayNames) ||
+    (displayNames = intl_GuardToDisplayNames(displayNames)) === null
+  ) {
+    return callFunction(
+      intl_CallDisplayNamesMethodIfWrapped,
+      this,
+      "Intl_DisplayNames_of"
+    );
+  }
+
+  code = ToString(code);
+
+  var internals = getDisplayNamesInternals(displayNames);
+
+  // Unpack the internals object to avoid a slow runtime to selfhosted JS call
+  // in |intl_ComputeDisplayName()|.
+  var {
+    locale,
+    calendar = "",
+    style,
+    type,
+    languageDisplay = "",
+    fallback,
+  } = internals;
+
+  // Steps 5-10.
+  return intl_ComputeDisplayName(
+    displayNames,
+    locale,
+    calendar,
+    style,
+    languageDisplay,
+    fallback,
+    type,
+    code
+  );
+}
+
+/**
+ * Returns the resolved options for a DisplayNames object.
+ */
+function Intl_DisplayNames_resolvedOptions() {
+  // Step 1.
+  var displayNames = this;
+
+  // Steps 2-3.
+  if (
+    !IsObject(displayNames) ||
+    (displayNames = intl_GuardToDisplayNames(displayNames)) === null
+  ) {
+    return callFunction(
+      intl_CallDisplayNamesMethodIfWrapped,
+      this,
+      "Intl_DisplayNames_resolvedOptions"
+    );
+  }
+
+  var internals = getDisplayNamesInternals(displayNames);
+
+  // Steps 4-5.
+  var options = {
+    locale: internals.locale,
+    style: internals.style,
+    type: internals.type,
+    fallback: internals.fallback,
+  };
+
+  // languageDisplay is only present for language display names.
+  assert(
+    hasOwn("languageDisplay", internals) === (internals.type === "language"),
+    "languageDisplay is present iff type is 'language'"
+  );
+
+  if (hasOwn("languageDisplay", internals)) {
+    DefineDataProperty(options, "languageDisplay", internals.languageDisplay);
+  }
+
+  if (hasOwn("calendar", internals)) {
+    DefineDataProperty(options, "calendar", internals.calendar);
+  }
+
+  // Step 6.
+  return options;
+}
diff --git a/js/src/builtin/intl/FormatBuffer.h b/js/src/builtin/intl/FormatBuffer.h
new file mode 100644
index 0000000000..73d450a546
--- /dev/null
+++ b/js/src/builtin/intl/FormatBuffer.h
@@ -0,0 +1,155 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_FormatBuffer_h
+#define builtin_intl_FormatBuffer_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Span.h"
+#include "mozilla/TextUtils.h"
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "gc/Allocator.h"
+#include "js/AllocPolicy.h"
+#include "js/CharacterEncoding.h"
+#include "js/TypeDecls.h"
+#include "js/UniquePtr.h"
+#include "js/Vector.h"
+#include "vm/StringType.h"
+
+namespace js::intl {
+
+/**
+ * A buffer for formatting unified intl data.
+ */
+template <typename CharT, size_t MinInlineCapacity = 0,
+          class AllocPolicy = TempAllocPolicy>
+class FormatBuffer {
+ public:
+  using CharType = CharT;
+
+  // Allow move constructors, but not copy constructors, as this class owns a
+  // js::Vector.
+  FormatBuffer(FormatBuffer&& other) noexcept = default;
+  FormatBuffer& operator=(FormatBuffer&& other) noexcept = default;
+
+  explicit FormatBuffer(AllocPolicy aP = AllocPolicy())
+      : buffer_(std::move(aP)) {
+    // The initial capacity matches the requested minimum inline capacity, as
+    // long as it doesn't exceed |Vector::kMaxInlineBytes / sizeof(CharT)|. If
+    // this assertion should ever fail, either reduce |MinInlineCapacity| or
+    // make the FormatBuffer initialization fallible.
+    MOZ_ASSERT(buffer_.capacity() == MinInlineCapacity);
+    if constexpr (MinInlineCapacity > 0) {
+      // Ensure the full capacity is marked as reserved.
+      //
+      // Reserving the minimum inline capacity can never fail, even when
+      // simulating OOM.
+      MOZ_ALWAYS_TRUE(buffer_.reserve(MinInlineCapacity));
+    }
+  }
+
+  // Implicitly convert to a Span.
+  operator mozilla::Span<CharType>() { return buffer_; }
+  operator mozilla::Span<const CharType>() const { return buffer_; }
+
+  /**
+   * Ensures the buffer has enough space to accommodate |size| elements.
+   */
+  [[nodiscard]] bool reserve(size_t size) {
+    // Call |reserve| a second time to ensure its full capacity is marked as
+    // reserved.
+    return buffer_.reserve(size) && buffer_.reserve(buffer_.capacity());
+  }
+
+  /**
+   * Returns the raw data inside the buffer.
+   */
+  CharType* data() { return buffer_.begin(); }
+
+  /**
+   * Returns the count of elements written into the buffer.
+   */
+  size_t length() const { return buffer_.length(); }
+
+  /**
+   * Returns the buffer's overall capacity.
+   */
+  size_t capacity() const { return buffer_.capacity(); }
+
+  /**
+   * Resizes the buffer to the given amount of written elements.
+   */
+  void written(size_t amount) {
+    MOZ_ASSERT(amount <= buffer_.capacity());
+    // This sets |buffer_|'s internal size so that it matches how much was
+    // written. This is necessary because the write happens across FFI
+    // boundaries.
+    size_t curLength = length();
+    if (amount > curLength) {
+      buffer_.infallibleGrowByUninitialized(amount - curLength);
+    } else {
+      buffer_.shrinkBy(curLength - amount);
+    }
+  }
+
+  /**
+   * Copies the buffer's data to a JSString.
+   *
+   * TODO(#1715842) - This should be more explicit on needing to handle OOM
+   * errors. In this case it returns a nullptr that must be checked, but it may
+   * not be obvious.
+   */
+  JSLinearString* toString(JSContext* cx) const {
+    if constexpr (std::is_same_v<CharT, uint8_t> ||
+                  std::is_same_v<CharT, unsigned char> ||
+                  std::is_same_v<CharT, char>) {
+      // Handle the UTF-8 encoding case.
+      return NewStringCopyUTF8N(
+          cx, JS::UTF8Chars(buffer_.begin(), buffer_.length()));
+    } else {
+      // Handle the UTF-16 encoding case.
+      static_assert(std::is_same_v<CharT, char16_t>);
+      return NewStringCopyN<CanGC>(cx, buffer_.begin(), buffer_.length());
+    }
+  }
+
+  /**
+   * Copies the buffer's data to a JSString. The buffer must contain only
+   * ASCII characters.
+   */
+  JSLinearString* toAsciiString(JSContext* cx) const {
+    static_assert(std::is_same_v<CharT, char>);
+
+    MOZ_ASSERT(mozilla::IsAscii(buffer_));
+    return NewStringCopyN<CanGC>(cx, buffer_.begin(), buffer_.length());
+  }
+
+  /**
+   * Extract this buffer's content as a null-terminated string.
+   */
+  UniquePtr<CharType[], JS::FreePolicy> extractStringZ() {
+    // Adding the NUL character on an already null-terminated string is likely
+    // an error. If there's ever a valid use case which triggers this assertion,
+    // we should change the below code to only conditionally add '\0'.
+    MOZ_ASSERT_IF(!buffer_.empty(), buffer_.end()[-1] != '\0');
+
+    if (!buffer_.append('\0')) {
+      return nullptr;
+    }
+    return UniquePtr<CharType[], JS::FreePolicy>(
+        buffer_.extractOrCopyRawBuffer());
+  }
+
+ private:
+  js::Vector<CharT, MinInlineCapacity, AllocPolicy> buffer_;
+};
+
+}  // namespace js::intl
+
+#endif /* builtin_intl_FormatBuffer_h */
diff --git a/js/src/builtin/intl/IcuMemoryUsage.java b/js/src/builtin/intl/IcuMemoryUsage.java
new file mode 100644
index 0000000000..15e4f1fd38
--- /dev/null
+++ b/js/src/builtin/intl/IcuMemoryUsage.java
@@ -0,0 +1,260 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import java.io.*;
+import java.nio.charset.StandardCharsets;
+import java.util.*;
+import java.util.regex.*;
+import java.util.stream.Collectors;
+
+/**
+ * Java program to estimate the memory usage of ICU objects (bug 1585536).
+ *
+ * It computes for each Intl constructor the amount of allocated memory. We're
+ * currently using the maximum memory ("max" in the output) to estimate the
+ * memory consumption of ICU objects.
+ *
+ * Insert before {@code JS_InitWithFailureDiagnostic} in "js.cpp":
+ * 
+ * <pre>
+ * <code>
+ * JS_SetICUMemoryFunctions(
+ *     [](const void*, size_t size) {
+ *       void* ptr = malloc(size);
+ *       if (ptr) {
+ *         printf("  alloc: %p -> %zu\n", ptr, size);
+ *       }
+ *       return ptr;
+ *     },
+ *     [](const void*, void* p, size_t size) {
+ *       void* ptr = realloc(p, size);
+ *       if (p) {
+ *         printf("  realloc: %p -> %p -> %zu\n", p, ptr, size);
+ *       } else {
+ *         printf("  alloc: %p -> %zu\n", ptr, size);
+ *       }
+ *       return ptr;
+ *     },
+ *     [](const void*, void* p) {
+ *       if (p) {
+ *         printf("  free: %p\n", p);
+ *       }
+ *       free(p);
+ *     });
+ * </code>
+ * </pre>
+ * 
+ * Run this script with:
+ * {@code java --enable-preview --source=14 IcuMemoryUsage.java $MOZ_JS_SHELL}.
+ */
+@SuppressWarnings("preview")
+public class IcuMemoryUsage {
+    private enum Phase {
+        None, Create, Init, Destroy, Collect, Quit
+    }
+
+    private static final class Memory {
+        private Phase phase = Phase.None;
+        private HashMap<Long, Map.Entry<Phase, Long>> allocations = new HashMap<>();
+        private HashSet<Long> freed = new HashSet<>();
+        private HashMap<Long, Map.Entry<Phase, Long>> completeAllocations = new HashMap<>();
+        private int allocCount = 0;
+        private ArrayList<Long> allocSizes = new ArrayList<>();
+
+        void transition(Phase nextPhase) {
+            assert phase.ordinal() + 1 == nextPhase.ordinal() || (phase == Phase.Collect && nextPhase == Phase.Create);
+            phase = nextPhase;
+
+            // Create a clean slate when starting a new create cycle or before termination.
+            if (phase == Phase.Create || phase == Phase.Quit) {
+                transferAllocations();
+            }
+
+            // Only measure the allocation size when creating the second object with the
+            // same locale.
+            if (phase == Phase.Collect && ++allocCount % 2 == 0) {
+                long size = allocations.values().stream().map(Map.Entry::getValue).reduce(0L, (a, c) -> a + c);
+                allocSizes.add(size);
+            }
+        }
+
+        void transferAllocations() {
+            completeAllocations.putAll(allocations);
+            completeAllocations.keySet().removeAll(freed);
+            allocations.clear();
+            freed.clear();
+        }
+
+        void alloc(long ptr, long size) {
+            allocations.put(ptr, Map.entry(phase, size));
+        }
+
+        void realloc(long oldPtr, long newPtr, long size) {
+            free(oldPtr);
+            allocations.put(newPtr, Map.entry(phase, size));
+        }
+
+        void free(long ptr) {
+            if (allocations.remove(ptr) == null) {
+                freed.add(ptr);
+            }
+        }
+
+        LongSummaryStatistics statistics() {
+            return allocSizes.stream().collect(Collectors.summarizingLong(Long::valueOf));
+        }
+
+        double percentile(double p) {
+            var size = allocSizes.size();
+            return allocSizes.stream().sorted().skip((long) ((size - 1) * p)).limit(2 - size % 2)
+                    .mapToDouble(Long::doubleValue).average().getAsDouble();
+        }
+
+        long persistent() {
+            return completeAllocations.values().stream().map(Map.Entry::getValue).reduce(0L, (a, c) -> a + c);
+        }
+    }
+
+    private static long parseSize(Matcher m, int group) {
+        return Long.parseLong(m.group(group), 10);
+    }
+
+    private static long parsePointer(Matcher m, int group) {
+        return Long.parseLong(m.group(group), 16);
+    }
+
+    private static void measure(String exec, String constructor, String description, String initializer) throws IOException {
+        var locales = Arrays.stream(Locale.getAvailableLocales()).map(Locale::toLanguageTag).sorted()
+                .collect(Collectors.toUnmodifiableList());
+
+        var pb = new ProcessBuilder(exec, "--file=-", "--", constructor, initializer,
+                locales.stream().collect(Collectors.joining(",")));
+        var process = pb.start();
+
+        try (var writer = new BufferedWriter(
+                new OutputStreamWriter(process.getOutputStream(), StandardCharsets.UTF_8))) {
+            writer.write(sourceCode);
+            writer.flush();
+        }
+
+        var memory = new Memory();
+
+        try (var reader = new BufferedReader(new InputStreamReader(process.getInputStream()))) {
+            var reAlloc = Pattern.compile("\\s+alloc: 0x(\\p{XDigit}+) -> (\\p{Digit}+)");
+            var reRealloc = Pattern.compile("\\s+realloc: 0x(\\p{XDigit}+) -> 0x(\\p{XDigit}+) -> (\\p{Digit}+)");
+            var reFree = Pattern.compile("\\s+free: 0x(\\p{XDigit}+)");
+
+            String line;
+            while ((line = reader.readLine()) != null) {
+                Matcher m;
+                if ((m = reAlloc.matcher(line)).matches()) {
+                    var ptr = parsePointer(m, 1);
+                    var size = parseSize(m, 2);
+                    memory.alloc(ptr, size);
+                } else if ((m = reRealloc.matcher(line)).matches()) {
+                    var oldPtr = parsePointer(m, 1);
+                    var newPtr = parsePointer(m, 2);
+                    var size = parseSize(m, 3);
+                    memory.realloc(oldPtr, newPtr, size);
+                } else if ((m = reFree.matcher(line)).matches()) {
+                    var ptr = parsePointer(m, 1);
+                    memory.free(ptr);
+                } else {
+                    memory.transition(Phase.valueOf(line));
+                }
+            }
+        }
+
+        try (var errorReader = new BufferedReader(new InputStreamReader(process.getErrorStream()))) {
+            String line;
+            while ((line = errorReader.readLine()) != null) {
+                System.err.println(line);
+            }
+        }
+
+        var stats = memory.statistics();
+
+        System.out.printf("%s%n", description);
+        System.out.printf("  max: %d%n", stats.getMax());
+        System.out.printf("  min: %d%n", stats.getMin());
+        System.out.printf("  avg: %.0f%n", stats.getAverage());
+        System.out.printf("  50p: %.0f%n", memory.percentile(0.50));
+        System.out.printf("  75p: %.0f%n", memory.percentile(0.75));
+        System.out.printf("  85p: %.0f%n", memory.percentile(0.85));
+        System.out.printf("  95p: %.0f%n", memory.percentile(0.95));
+        System.out.printf("  99p: %.0f%n", memory.percentile(0.99));
+        System.out.printf("  mem: %d%n", memory.persistent());
+
+        memory.transferAllocations();
+        assert memory.persistent() == 0 : String.format("Leaked %d bytes", memory.persistent());
+    }
+
+    public static void main(String[] args) throws IOException {
+        if (args.length == 0) {
+            throw new RuntimeException("The first argument must point to the SpiderMonkey shell executable");
+        }
+
+        record Entry (String constructor, String description, String initializer) {
+            public static Entry of(String constructor, String description, String initializer) {
+                return new Entry(constructor, description, initializer);
+            }
+
+            public static Entry of(String constructor, String initializer) {
+                return new Entry(constructor, constructor, initializer);
+            }
+        }
+
+        var objects = new ArrayList<Entry>();
+        objects.add(Entry.of("Collator", "o.compare('a', 'b')"));
+        objects.add(Entry.of("DateTimeFormat", "DateTimeFormat (UDateFormat)", "o.format(0)"));
+        objects.add(Entry.of("DateTimeFormat", "DateTimeFormat (UDateFormat+UDateIntervalFormat)",
+                             "o.formatRange(0, 24*60*60*1000)"));
+        objects.add(Entry.of("DisplayNames", "o.of('en')"));
+        objects.add(Entry.of("ListFormat", "o.format(['a', 'b'])"));
+        objects.add(Entry.of("NumberFormat", "o.format(0)"));
+        objects.add(Entry.of("NumberFormat", "NumberFormat (UNumberRangeFormatter)",
+                             "o.formatRange(0, 1000)"));
+        objects.add(Entry.of("PluralRules", "o.select(0)"));
+        objects.add(Entry.of("RelativeTimeFormat", "o.format(0, 'hour')"));
+
+        for (var entry : objects) {
+            measure(args[0], entry.constructor, entry.description, entry.initializer);
+        }
+    }
+
+    private static final String sourceCode = """
+const constructorName = scriptArgs[0];
+const initializer = Function("o", scriptArgs[1]);
+const locales = scriptArgs[2].split(",");
+
+const extras = {};
+addIntlExtras(extras);
+
+for (let i = 0; i < locales.length; ++i) {
+  // Loop twice in case the first time we create an object with a new locale
+  // allocates additional memory when loading the locale data.
+  for (let j = 0; j < 2; ++j) {
+    let constructor = Intl[constructorName];
+    let options = undefined;
+    if (constructor === Intl.DisplayNames) {
+      options = {type: "language"};
+    }
+
+    print("Create");
+    let obj = new constructor(locales[i], options);
+
+    print("Init");
+    initializer(obj);
+
+    print("Destroy");
+    gc();
+    gc();
+    print("Collect");
+  }
+}
+
+print("Quit");
+quit();
+""";
+}
diff --git a/js/src/builtin/intl/IntlObject.cpp b/js/src/builtin/intl/IntlObject.cpp
new file mode 100644
index 0000000000..299964d07b
--- /dev/null
+++ b/js/src/builtin/intl/IntlObject.cpp
@@ -0,0 +1,910 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Implementation of the Intl object and its non-constructor properties. */
+
+#include "builtin/intl/IntlObject.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/intl/Calendar.h"
+#include "mozilla/intl/Collator.h"
+#include "mozilla/intl/Currency.h"
+#include "mozilla/intl/Locale.h"
+#include "mozilla/intl/MeasureUnitGenerated.h"
+#include "mozilla/intl/TimeZone.h"
+
+#include <algorithm>
+#include <array>
+#include <cstring>
+#include <iterator>
+#include <string_view>
+
+#include "builtin/Array.h"
+#include "builtin/intl/CommonFunctions.h"
+#include "builtin/intl/FormatBuffer.h"
+#include "builtin/intl/NumberingSystemsGenerated.h"
+#include "builtin/intl/SharedIntlData.h"
+#include "builtin/intl/StringAsciiChars.h"
+#include "ds/Sort.h"
+#include "js/Class.h"
+#include "js/friend/ErrorMessages.h"  // js::GetErrorMessage, JSMSG_*
+#include "js/GCAPI.h"
+#include "js/GCVector.h"
+#include "js/PropertySpec.h"
+#include "js/Result.h"
+#include "js/StableStringChars.h"
+#include "vm/GlobalObject.h"
+#include "vm/JSAtom.h"
+#include "vm/JSContext.h"
+#include "vm/PlainObject.h"  // js::PlainObject
+#include "vm/StringType.h"
+#include "vm/WellKnownAtom.h"  // js_*_str
+
+#include "vm/JSObject-inl.h"
+#include "vm/NativeObject-inl.h"
+
+using namespace js;
+
+/******************** Intl ********************/
+
+bool js::intl_GetCalendarInfo(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+
+  UniqueChars locale = intl::EncodeLocale(cx, args[0].toString());
+  if (!locale) {
+    return false;
+  }
+
+  auto result = mozilla::intl::Calendar::TryCreate(locale.get());
+  if (result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return false;
+  }
+  auto calendar = result.unwrap();
+
+  RootedObject info(cx, NewPlainObject(cx));
+  if (!info) {
+    return false;
+  }
+
+  RootedValue v(cx);
+
+  v.setInt32(static_cast<int32_t>(calendar->GetFirstDayOfWeek()));
+  if (!DefineDataProperty(cx, info, cx->names().firstDayOfWeek, v)) {
+    return false;
+  }
+
+  v.setInt32(calendar->GetMinimalDaysInFirstWeek());
+  if (!DefineDataProperty(cx, info, cx->names().minDays, v)) {
+    return false;
+  }
+
+  Rooted<ArrayObject*> weekendArray(cx, NewDenseEmptyArray(cx));
+  if (!weekendArray) {
+    return false;
+  }
+
+  auto weekend = calendar->GetWeekend();
+  if (weekend.isErr()) {
+    intl::ReportInternalError(cx, weekend.unwrapErr());
+    return false;
+  }
+
+  for (auto day : weekend.unwrap()) {
+    if (!NewbornArrayPush(cx, weekendArray,
+                          Int32Value(static_cast<int32_t>(day)))) {
+      return false;
+    }
+  }
+
+  v.setObject(*weekendArray);
+  if (!DefineDataProperty(cx, info, cx->names().weekend, v)) {
+    return false;
+  }
+
+  args.rval().setObject(*info);
+  return true;
+}
+
+static void ReportBadKey(JSContext* cx, JSString* key) {
+  if (UniqueChars chars = QuoteString(cx, key, '"')) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_INVALID_KEY,
+                              chars.get());
+  }
+}
+
+static bool SameOrParentLocale(JSLinearString* locale,
+                               JSLinearString* otherLocale) {
+  // Return true if |locale| is the same locale as |otherLocale|.
+  if (locale->length() == otherLocale->length()) {
+    return EqualStrings(locale, otherLocale);
+  }
+
+  // Also return true if |locale| is the parent locale of |otherLocale|.
+  if (locale->length() < otherLocale->length()) {
+    return HasSubstringAt(otherLocale, locale, 0) &&
+           otherLocale->latin1OrTwoByteChar(locale->length()) == '-';
+  }
+
+  return false;
+}
+
+using SupportedLocaleKind = js::intl::SharedIntlData::SupportedLocaleKind;
+
+// 9.2.2 BestAvailableLocale ( availableLocales, locale )
+static JS::Result<JSLinearString*> BestAvailableLocale(
+    JSContext* cx, SupportedLocaleKind kind, Handle<JSLinearString*> locale,
+    Handle<JSLinearString*> defaultLocale) {
+  // In the spec, [[availableLocales]] is formally a list of all available
+  // locales. But in our implementation, it's an *incomplete* list, not
+  // necessarily including the default locale (and all locales implied by it,
+  // e.g. "de" implied by "de-CH"), if that locale isn't in every
+  // [[availableLocales]] list (because that locale is supported through
+  // fallback, e.g. "de-CH" supported through "de").
+  //
+  // If we're considering the default locale, augment the spec loop with
+  // additional checks to also test whether the current prefix is a prefix of
+  // the default locale.
+
+  intl::SharedIntlData& sharedIntlData = cx->runtime()->sharedIntlData.ref();
+
+  auto findLast = [](const auto* chars, size_t length) {
+    auto rbegin = std::make_reverse_iterator(chars + length);
+    auto rend = std::make_reverse_iterator(chars);
+    auto p = std::find(rbegin, rend, '-');
+
+    // |dist(chars, p.base())| is equal to |dist(p, rend)|, pick whichever you
+    // find easier to reason about when using reserve iterators.
+    ptrdiff_t r = std::distance(chars, p.base());
+    MOZ_ASSERT(r == std::distance(p, rend));
+
+    // But always subtract one to convert from the reverse iterator result to
+    // the correspoding forward iterator value, because reserve iterators point
+    // to one element past the forward iterator value.
+    return r - 1;
+  };
+
+  // Step 1.
+  Rooted<JSLinearString*> candidate(cx, locale);
+
+  // Step 2.
+  while (true) {
+    // Step 2.a.
+    bool supported = false;
+    if (!sharedIntlData.isSupportedLocale(cx, kind, candidate, &supported)) {
+      return cx->alreadyReportedError();
+    }
+    if (supported) {
+      return candidate.get();
+    }
+
+    if (defaultLocale && SameOrParentLocale(candidate, defaultLocale)) {
+      return candidate.get();
+    }
+
+    // Step 2.b.
+    ptrdiff_t pos;
+    if (candidate->hasLatin1Chars()) {
+      JS::AutoCheckCannotGC nogc;
+      pos = findLast(candidate->latin1Chars(nogc), candidate->length());
+    } else {
+      JS::AutoCheckCannotGC nogc;
+      pos = findLast(candidate->twoByteChars(nogc), candidate->length());
+    }
+
+    if (pos < 0) {
+      return nullptr;
+    }
+
+    // Step 2.c.
+    size_t length = size_t(pos);
+    if (length >= 2 && candidate->latin1OrTwoByteChar(length - 2) == '-') {
+      length -= 2;
+    }
+
+    // Step 2.d.
+    candidate = NewDependentString(cx, candidate, 0, length);
+    if (!candidate) {
+      return cx->alreadyReportedError();
+    }
+  }
+}
+
+// 9.2.2 BestAvailableLocale ( availableLocales, locale )
+//
+// Carries an additional third argument in our implementation to provide the
+// default locale. See the doc-comment in the header file.
+bool js::intl_BestAvailableLocale(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 3);
+
+  SupportedLocaleKind kind;
+  {
+    JSLinearString* typeStr = args[0].toString()->ensureLinear(cx);
+    if (!typeStr) {
+      return false;
+    }
+
+    if (StringEqualsLiteral(typeStr, "Collator")) {
+      kind = SupportedLocaleKind::Collator;
+    } else if (StringEqualsLiteral(typeStr, "DateTimeFormat")) {
+      kind = SupportedLocaleKind::DateTimeFormat;
+    } else if (StringEqualsLiteral(typeStr, "DisplayNames")) {
+      kind = SupportedLocaleKind::DisplayNames;
+    } else if (StringEqualsLiteral(typeStr, "ListFormat")) {
+      kind = SupportedLocaleKind::ListFormat;
+    } else if (StringEqualsLiteral(typeStr, "NumberFormat")) {
+      kind = SupportedLocaleKind::NumberFormat;
+    } else if (StringEqualsLiteral(typeStr, "PluralRules")) {
+      kind = SupportedLocaleKind::PluralRules;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(typeStr, "RelativeTimeFormat"));
+      kind = SupportedLocaleKind::RelativeTimeFormat;
+    }
+  }
+
+  Rooted<JSLinearString*> locale(cx, args[1].toString()->ensureLinear(cx));
+  if (!locale) {
+    return false;
+  }
+
+#ifdef DEBUG
+  {
+    MOZ_ASSERT(StringIsAscii(locale), "language tags are ASCII-only");
+
+    // |locale| is a structurally valid language tag.
+    mozilla::intl::Locale tag;
+
+    using ParserError = mozilla::intl::LocaleParser::ParserError;
+    mozilla::Result<mozilla::Ok, ParserError> parse_result = Ok();
+    {
+      intl::StringAsciiChars chars(locale);
+      if (!chars.init(cx)) {
+        return false;
+      }
+
+      parse_result = mozilla::intl::LocaleParser::TryParse(chars, tag);
+    }
+
+    if (parse_result.isErr()) {
+      MOZ_ASSERT(parse_result.unwrapErr() == ParserError::OutOfMemory,
+                 "locale is a structurally valid language tag");
+
+      intl::ReportInternalError(cx);
+      return false;
+    }
+
+    MOZ_ASSERT(!tag.GetUnicodeExtension(),
+               "locale must contain no Unicode extensions");
+
+    if (auto result = tag.Canonicalize(); result.isErr()) {
+      MOZ_ASSERT(
+          result.unwrapErr() !=
+          mozilla::intl::Locale::CanonicalizationError::DuplicateVariant);
+      intl::ReportInternalError(cx);
+      return false;
+    }
+
+    intl::FormatBuffer<char, intl::INITIAL_CHAR_BUFFER_SIZE> buffer(cx);
+    if (auto result = tag.ToString(buffer); result.isErr()) {
+      intl::ReportInternalError(cx, result.unwrapErr());
+      return false;
+    }
+
+    JSLinearString* tagStr = buffer.toString(cx);
+    if (!tagStr) {
+      return false;
+    }
+
+    MOZ_ASSERT(EqualStrings(locale, tagStr),
+               "locale is a canonicalized language tag");
+  }
+#endif
+
+  MOZ_ASSERT(args[2].isNull() || args[2].isString());
+
+  Rooted<JSLinearString*> defaultLocale(cx);
+  if (args[2].isString()) {
+    defaultLocale = args[2].toString()->ensureLinear(cx);
+    if (!defaultLocale) {
+      return false;
+    }
+  }
+
+  JSString* result;
+  JS_TRY_VAR_OR_RETURN_FALSE(
+      cx, result, BestAvailableLocale(cx, kind, locale, defaultLocale));
+
+  if (result) {
+    args.rval().setString(result);
+  } else {
+    args.rval().setUndefined();
+  }
+  return true;
+}
+
+bool js::intl_supportedLocaleOrFallback(JSContext* cx, unsigned argc,
+                                        Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+
+  Rooted<JSLinearString*> locale(cx, args[0].toString()->ensureLinear(cx));
+  if (!locale) {
+    return false;
+  }
+
+  mozilla::intl::Locale tag;
+  bool canParseLocale = false;
+  if (StringIsAscii(locale)) {
+    intl::StringAsciiChars chars(locale);
+    if (!chars.init(cx)) {
+      return false;
+    }
+
+    // Tell the analysis the |tag.canonicalize()| method can't GC.
+    JS::AutoSuppressGCAnalysis nogc;
+
+    canParseLocale = mozilla::intl::LocaleParser::TryParse(chars, tag).isOk() &&
+                     tag.Canonicalize().isOk();
+  }
+
+  Rooted<JSLinearString*> candidate(cx);
+  if (!canParseLocale) {
+    candidate = NewStringCopyZ<CanGC>(cx, intl::LastDitchLocale());
+    if (!candidate) {
+      return false;
+    }
+  } else {
+    // The default locale must be in [[AvailableLocales]], and that list must
+    // not contain any locales with Unicode extension sequences, so remove any
+    // present in the candidate.
+    tag.ClearUnicodeExtension();
+
+    intl::FormatBuffer<char, intl::INITIAL_CHAR_BUFFER_SIZE> buffer(cx);
+    if (auto result = tag.ToString(buffer); result.isErr()) {
+      intl::ReportInternalError(cx, result.unwrapErr());
+      return false;
+    }
+
+    candidate = buffer.toAsciiString(cx);
+    if (!candidate) {
+      return false;
+    }
+
+    // Certain old-style language tags lack a script code, but in current
+    // usage they *would* include a script code. Map these over to modern
+    // forms.
+    for (const auto& mapping : js::intl::oldStyleLanguageTagMappings) {
+      const char* oldStyle = mapping.oldStyle;
+      const char* modernStyle = mapping.modernStyle;
+
+      if (StringEqualsAscii(candidate, oldStyle)) {
+        candidate = NewStringCopyZ<CanGC>(cx, modernStyle);
+        if (!candidate) {
+          return false;
+        }
+        break;
+      }
+    }
+  }
+
+  // 9.1 Internal slots of Service Constructors
+  //
+  // - [[AvailableLocales]] is a List [...]. The list must include the value
+  //   returned by the DefaultLocale abstract operation (6.2.4), [...].
+  //
+  // That implies we must ignore any candidate which isn't supported by all
+  // Intl service constructors.
+
+  Rooted<JSLinearString*> supportedCollator(cx);
+  JS_TRY_VAR_OR_RETURN_FALSE(
+      cx, supportedCollator,
+      BestAvailableLocale(cx, SupportedLocaleKind::Collator, candidate,
+                          nullptr));
+
+  Rooted<JSLinearString*> supportedDateTimeFormat(cx);
+  JS_TRY_VAR_OR_RETURN_FALSE(
+      cx, supportedDateTimeFormat,
+      BestAvailableLocale(cx, SupportedLocaleKind::DateTimeFormat, candidate,
+                          nullptr));
+
+#ifdef DEBUG
+  // Note: We don't test the supported locales of the remaining Intl service
+  // constructors, because the set of supported locales is exactly equal to
+  // the set of supported locales of Intl.DateTimeFormat.
+  for (auto kind :
+       {SupportedLocaleKind::DisplayNames, SupportedLocaleKind::ListFormat,
+        SupportedLocaleKind::NumberFormat, SupportedLocaleKind::PluralRules,
+        SupportedLocaleKind::RelativeTimeFormat}) {
+    JSLinearString* supported;
+    JS_TRY_VAR_OR_RETURN_FALSE(
+        cx, supported, BestAvailableLocale(cx, kind, candidate, nullptr));
+
+    MOZ_ASSERT(!!supported == !!supportedDateTimeFormat);
+    MOZ_ASSERT_IF(supported, EqualStrings(supported, supportedDateTimeFormat));
+  }
+#endif
+
+  // Accept the candidate locale if it is supported by all Intl service
+  // constructors.
+  if (supportedCollator && supportedDateTimeFormat) {
+    // Use the actually supported locale instead of the candidate locale. For
+    // example when the candidate locale "en-US-posix" is supported through
+    // "en-US", use "en-US" as the default locale.
+    //
+    // Also prefer the supported locale with more subtags. For example when
+    // requesting "de-CH" and Intl.DateTimeFormat supports "de-CH", but
+    // Intl.Collator only "de", still return "de-CH" as the result.
+    if (SameOrParentLocale(supportedCollator, supportedDateTimeFormat)) {
+      candidate = supportedDateTimeFormat;
+    } else {
+      candidate = supportedCollator;
+    }
+  } else {
+    candidate = NewStringCopyZ<CanGC>(cx, intl::LastDitchLocale());
+    if (!candidate) {
+      return false;
+    }
+  }
+
+  args.rval().setString(candidate);
+  return true;
+}
+
+using StringList = GCVector<JSLinearString*>;
+
+/**
+ * Create a sorted array from a list of strings.
+ */
+static ArrayObject* CreateArrayFromList(JSContext* cx,
+                                        MutableHandle<StringList> list) {
+  // Reserve scratch space for MergeSort().
+  size_t initialLength = list.length();
+  if (!list.growBy(initialLength)) {
+    return nullptr;
+  }
+
+  // Sort all strings in alphabetical order.
+  MOZ_ALWAYS_TRUE(
+      MergeSort(list.begin(), initialLength, list.begin() + initialLength,
+                [](const auto* a, const auto* b, bool* lessOrEqual) {
+                  *lessOrEqual = CompareStrings(a, b) <= 0;
+                  return true;
+                }));
+
+  // Ensure we don't add duplicate entries to the array.
+  auto* end = std::unique(
+      list.begin(), list.begin() + initialLength,
+      [](const auto* a, const auto* b) { return EqualStrings(a, b); });
+
+  // std::unique leaves the elements after |end| with an unspecified value, so
+  // remove them first. And also delete the elements in the scratch space.
+  list.shrinkBy(std::distance(end, list.end()));
+
+  // And finally copy the strings into the result array.
+  auto* array = NewDenseFullyAllocatedArray(cx, list.length());
+  if (!array) {
+    return nullptr;
+  }
+  array->setDenseInitializedLength(list.length());
+
+  for (size_t i = 0; i < list.length(); ++i) {
+    array->initDenseElement(i, StringValue(list[i]));
+  }
+
+  return array;
+}
+
+/**
+ * Create an array from a sorted list of strings.
+ */
+template <size_t N>
+static ArrayObject* CreateArrayFromSortedList(
+    JSContext* cx, const std::array<const char*, N>& list) {
+  // Ensure the list is sorted and doesn't contain duplicates.
+#ifdef DEBUG
+  // See bug 1583449 for why the lambda can't be in the MOZ_ASSERT.
+  auto isLargerThanOrEqual = [](const auto& a, const auto& b) {
+    return std::strcmp(a, b) >= 0;
+  };
+#endif
+  MOZ_ASSERT(std::adjacent_find(std::begin(list), std::end(list),
+                                isLargerThanOrEqual) == std::end(list));
+
+  size_t length = std::size(list);
+
+  Rooted<ArrayObject*> array(cx, NewDenseFullyAllocatedArray(cx, length));
+  if (!array) {
+    return nullptr;
+  }
+  array->ensureDenseInitializedLength(0, length);
+
+  for (size_t i = 0; i < length; ++i) {
+    auto* str = NewStringCopyZ<CanGC>(cx, list[i]);
+    if (!str) {
+      return nullptr;
+    }
+    array->initDenseElement(i, StringValue(str));
+  }
+  return array;
+}
+
+/**
+ * Create an array from an intl::Enumeration.
+ */
+template <const auto& unsupported, class Enumeration>
+static bool EnumerationIntoList(JSContext* cx, Enumeration values,
+                                MutableHandle<StringList> list) {
+  for (auto value : values) {
+    if (value.isErr()) {
+      intl::ReportInternalError(cx);
+      return false;
+    }
+    auto span = value.unwrap();
+
+    // Skip over known, unsupported values.
+    std::string_view sv(span.data(), span.size());
+    if (std::any_of(std::begin(unsupported), std::end(unsupported),
+                    [sv](const auto& e) { return sv == e; })) {
+      continue;
+    }
+
+    auto* string = NewStringCopy<CanGC>(cx, span);
+    if (!string) {
+      return false;
+    }
+    if (!list.append(string)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+/**
+ * Returns the list of calendar types which mustn't be returned by
+ * |Intl.supportedValuesOf()|.
+ */
+static constexpr auto UnsupportedCalendars() {
+  // No calendar values are currently unsupported.
+  return std::array<const char*, 0>{};
+}
+
+// Defined outside of the function to workaround bugs in GCC<9.
+// Also see <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85589>.
+static constexpr auto UnsupportedCalendarsArray = UnsupportedCalendars();
+
+/**
+ * AvailableCalendars ( )
+ */
+static ArrayObject* AvailableCalendars(JSContext* cx) {
+  Rooted<StringList> list(cx, StringList(cx));
+
+  {
+    // Hazard analysis complains that the mozilla::Result destructor calls a
+    // GC function, which is unsound when returning an unrooted value. Work
+    // around this issue by restricting the lifetime of |keywords| to a
+    // separate block.
+    auto keywords = mozilla::intl::Calendar::GetBcp47KeywordValuesForLocale("");
+    if (keywords.isErr()) {
+      intl::ReportInternalError(cx, keywords.unwrapErr());
+      return nullptr;
+    }
+
+    static constexpr auto& unsupported = UnsupportedCalendarsArray;
+
+    if (!EnumerationIntoList<unsupported>(cx, keywords.unwrap(), &list)) {
+      return nullptr;
+    }
+  }
+
+  return CreateArrayFromList(cx, &list);
+}
+
+/**
+ * Returns the list of collation types which mustn't be returned by
+ * |Intl.supportedValuesOf()|.
+ */
+static constexpr auto UnsupportedCollations() {
+  return std::array{
+      "search",
+      "standard",
+  };
+}
+
+// Defined outside of the function to workaround bugs in GCC<9.
+// Also see <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85589>.
+static constexpr auto UnsupportedCollationsArray = UnsupportedCollations();
+
+/**
+ * AvailableCollations ( )
+ */
+static ArrayObject* AvailableCollations(JSContext* cx) {
+  Rooted<StringList> list(cx, StringList(cx));
+
+  {
+    // Hazard analysis complains that the mozilla::Result destructor calls a
+    // GC function, which is unsound when returning an unrooted value. Work
+    // around this issue by restricting the lifetime of |keywords| to a
+    // separate block.
+    auto keywords = mozilla::intl::Collator::GetBcp47KeywordValues();
+    if (keywords.isErr()) {
+      intl::ReportInternalError(cx, keywords.unwrapErr());
+      return nullptr;
+    }
+
+    static constexpr auto& unsupported = UnsupportedCollationsArray;
+
+    if (!EnumerationIntoList<unsupported>(cx, keywords.unwrap(), &list)) {
+      return nullptr;
+    }
+  }
+
+  return CreateArrayFromList(cx, &list);
+}
+
+/**
+ * Returns a list of known, unsupported currencies which are returned by
+ * |Currency::GetISOCurrencies()|.
+ */
+static constexpr auto UnsupportedCurrencies() {
+  // "MVP" is also marked with "questionable, remove?" in ucurr.cpp, but only
+  // this single currency code isn't supported by |Intl.DisplayNames| and
+  // therefore must be excluded by |Intl.supportedValuesOf|.
+  return std::array{
+      "LSM",  // https://unicode-org.atlassian.net/browse/ICU-21687
+  };
+}
+
+/**
+ * Return a list of known, missing currencies which aren't returned by
+ * |Currency::GetISOCurrencies()|.
+ */
+static constexpr auto MissingCurrencies() {
+  return std::array{
+      "SLE",  // https://unicode-org.atlassian.net/browse/ICU-21989
+      "VED",  // https://unicode-org.atlassian.net/browse/ICU-21989
+  };
+}
+
+// Defined outside of the function to workaround bugs in GCC<9.
+// Also see <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85589>.
+static constexpr auto UnsupportedCurrenciesArray = UnsupportedCurrencies();
+static constexpr auto MissingCurrenciesArray = MissingCurrencies();
+
+/**
+ * AvailableCurrencies ( )
+ */
+static ArrayObject* AvailableCurrencies(JSContext* cx) {
+  Rooted<StringList> list(cx, StringList(cx));
+
+  {
+    // Hazard analysis complains that the mozilla::Result destructor calls a
+    // GC function, which is unsound when returning an unrooted value. Work
+    // around this issue by restricting the lifetime of |currencies| to a
+    // separate block.
+    auto currencies = mozilla::intl::Currency::GetISOCurrencies();
+    if (currencies.isErr()) {
+      intl::ReportInternalError(cx, currencies.unwrapErr());
+      return nullptr;
+    }
+
+    static constexpr auto& unsupported = UnsupportedCurrenciesArray;
+
+    if (!EnumerationIntoList<unsupported>(cx, currencies.unwrap(), &list)) {
+      return nullptr;
+    }
+  }
+
+  // Add known missing values.
+  for (const char* value : MissingCurrenciesArray) {
+    auto* string = NewStringCopyZ<CanGC>(cx, value);
+    if (!string) {
+      return nullptr;
+    }
+    if (!list.append(string)) {
+      return nullptr;
+    }
+  }
+
+  return CreateArrayFromList(cx, &list);
+}
+
+/**
+ * AvailableNumberingSystems ( )
+ */
+static ArrayObject* AvailableNumberingSystems(JSContext* cx) {
+  static constexpr std::array numberingSystems = {
+      NUMBERING_SYSTEMS_WITH_SIMPLE_DIGIT_MAPPINGS};
+
+  return CreateArrayFromSortedList(cx, numberingSystems);
+}
+
+/**
+ * AvailableTimeZones ( )
+ */
+static ArrayObject* AvailableTimeZones(JSContext* cx) {
+  // Unsorted list of canonical time zone names, possibly containing
+  // duplicates.
+  Rooted<StringList> timeZones(cx, StringList(cx));
+
+  intl::SharedIntlData& sharedIntlData = cx->runtime()->sharedIntlData.ref();
+  auto iterResult = sharedIntlData.availableTimeZonesIteration(cx);
+  if (iterResult.isErr()) {
+    return nullptr;
+  }
+  auto iter = iterResult.unwrap();
+
+  Rooted<JSAtom*> validatedTimeZone(cx);
+  Rooted<JSAtom*> ianaTimeZone(cx);
+  for (; !iter.done(); iter.next()) {
+    validatedTimeZone = iter.get();
+
+    // Canonicalize the time zone before adding it to the result array.
+
+    // Some time zone names are canonicalized differently by ICU -- handle
+    // those first.
+    ianaTimeZone.set(nullptr);
+    if (!sharedIntlData.tryCanonicalizeTimeZoneConsistentWithIANA(
+            cx, validatedTimeZone, &ianaTimeZone)) {
+      return nullptr;
+    }
+
+    JSLinearString* timeZone;
+    if (ianaTimeZone) {
+      cx->markAtom(ianaTimeZone);
+
+      timeZone = ianaTimeZone;
+    } else {
+      // Call into ICU to canonicalize the time zone.
+
+      JS::AutoStableStringChars stableChars(cx);
+      if (!stableChars.initTwoByte(cx, validatedTimeZone)) {
+        return nullptr;
+      }
+
+      intl::FormatBuffer<char16_t, intl::INITIAL_CHAR_BUFFER_SIZE>
+          canonicalTimeZone(cx);
+      auto result = mozilla::intl::TimeZone::GetCanonicalTimeZoneID(
+          stableChars.twoByteRange(), canonicalTimeZone);
+      if (result.isErr()) {
+        intl::ReportInternalError(cx, result.unwrapErr());
+        return nullptr;
+      }
+
+      timeZone = canonicalTimeZone.toString(cx);
+      if (!timeZone) {
+        return nullptr;
+      }
+
+      // Canonicalize both to "UTC" per CanonicalizeTimeZoneName().
+      if (StringEqualsLiteral(timeZone, "Etc/UTC") ||
+          StringEqualsLiteral(timeZone, "Etc/GMT")) {
+        timeZone = cx->names().UTC;
+      }
+    }
+
+    if (!timeZones.append(timeZone)) {
+      return nullptr;
+    }
+  }
+
+  return CreateArrayFromList(cx, &timeZones);
+}
+
+template <size_t N>
+constexpr auto MeasurementUnitNames(
+    const mozilla::intl::SimpleMeasureUnit (&units)[N]) {
+  std::array<const char*, N> array = {};
+  for (size_t i = 0; i < N; ++i) {
+    array[i] = units[i].name;
+  }
+  return array;
+}
+
+/**
+ * AvailableUnits ( )
+ */
+static ArrayObject* AvailableUnits(JSContext* cx) {
+  static constexpr auto simpleMeasureUnitNames =
+      MeasurementUnitNames(mozilla::intl::simpleMeasureUnits);
+
+  return CreateArrayFromSortedList(cx, simpleMeasureUnitNames);
+}
+
+bool js::intl_SupportedValuesOf(JSContext* cx, unsigned argc, JS::Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+  MOZ_ASSERT(args[0].isString());
+
+  JSLinearString* key = args[0].toString()->ensureLinear(cx);
+  if (!key) {
+    return false;
+  }
+
+  ArrayObject* list;
+  if (StringEqualsLiteral(key, "calendar")) {
+    list = AvailableCalendars(cx);
+  } else if (StringEqualsLiteral(key, "collation")) {
+    list = AvailableCollations(cx);
+  } else if (StringEqualsLiteral(key, "currency")) {
+    list = AvailableCurrencies(cx);
+  } else if (StringEqualsLiteral(key, "numberingSystem")) {
+    list = AvailableNumberingSystems(cx);
+  } else if (StringEqualsLiteral(key, "timeZone")) {
+    list = AvailableTimeZones(cx);
+  } else if (StringEqualsLiteral(key, "unit")) {
+    list = AvailableUnits(cx);
+  } else {
+    ReportBadKey(cx, key);
+    return false;
+  }
+  if (!list) {
+    return false;
+  }
+
+  args.rval().setObject(*list);
+  return true;
+}
+
+static bool intl_toSource(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  args.rval().setString(cx->names().Intl);
+  return true;
+}
+
+static const JSFunctionSpec intl_static_methods[] = {
+    JS_FN(js_toSource_str, intl_toSource, 0, 0),
+    JS_SELF_HOSTED_FN("getCanonicalLocales", "Intl_getCanonicalLocales", 1, 0),
+    JS_SELF_HOSTED_FN("supportedValuesOf", "Intl_supportedValuesOf", 1, 0),
+    JS_FS_END};
+
+static const JSPropertySpec intl_static_properties[] = {
+    JS_STRING_SYM_PS(toStringTag, "Intl", JSPROP_READONLY), JS_PS_END};
+
+static JSObject* CreateIntlObject(JSContext* cx, JSProtoKey key) {
+  RootedObject proto(cx, &cx->global()->getObjectPrototype());
+
+  // The |Intl| object is just a plain object with some "static" function
+  // properties and some constructor properties.
+  return NewTenuredObjectWithGivenProto(cx, &IntlClass, proto);
+}
+
+/**
+ * Initializes the Intl Object and its standard built-in properties.
+ * Spec: ECMAScript Internationalization API Specification, 8.0, 8.1
+ */
+static bool IntlClassFinish(JSContext* cx, HandleObject intl,
+                            HandleObject proto) {
+  // Add the constructor properties.
+  RootedId ctorId(cx);
+  RootedValue ctorValue(cx);
+  for (const auto& protoKey :
+       {JSProto_Collator, JSProto_DateTimeFormat, JSProto_DisplayNames,
+        JSProto_ListFormat, JSProto_Locale, JSProto_NumberFormat,
+        JSProto_PluralRules, JSProto_RelativeTimeFormat}) {
+    JSObject* ctor = GlobalObject::getOrCreateConstructor(cx, protoKey);
+    if (!ctor) {
+      return false;
+    }
+
+    ctorId = NameToId(ClassName(protoKey, cx));
+    ctorValue.setObject(*ctor);
+    if (!DefineDataProperty(cx, intl, ctorId, ctorValue, 0)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+static const ClassSpec IntlClassSpec = {
+    CreateIntlObject, nullptr, intl_static_methods, intl_static_properties,
+    nullptr,          nullptr, IntlClassFinish};
+
+const JSClass js::IntlClass = {"Intl", JSCLASS_HAS_CACHED_PROTO(JSProto_Intl),
+                               JS_NULL_CLASS_OPS, &IntlClassSpec};
diff --git a/js/src/builtin/intl/IntlObject.h b/js/src/builtin/intl/IntlObject.h
new file mode 100644
index 0000000000..5b79f74e92
--- /dev/null
+++ b/js/src/builtin/intl/IntlObject.h
@@ -0,0 +1,82 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_IntlObject_h
+#define builtin_intl_IntlObject_h
+
+#include "js/TypeDecls.h"
+
+namespace js {
+
+extern const JSClass IntlClass;
+
+/**
+ * Returns a plain object with calendar information for a single valid locale
+ * (callers must perform this validation).  The object will have these
+ * properties:
+ *
+ *   firstDayOfWeek
+ *     an integer in the range 1=Monday to 7=Sunday indicating the day
+ *     considered the first day of the week in calendars, e.g. 7 for en-US,
+ *     1 for en-GB, 7 for bn-IN
+ *   minDays
+ *     an integer in the range of 1 to 7 indicating the minimum number
+ *     of days required in the first week of the year, e.g. 1 for en-US,
+ *     4 for de
+ *   weekend
+ *     an array with values in the range 1=Monday to 7=Sunday indicating the
+ *     days of the week considered as part of the weekend, e.g. [6, 7] for en-US
+ *     and en-GB, [7] for bn-IN (note that "weekend" is *not* necessarily two
+ *     days)
+ *
+ * NOTE: "calendar" and "locale" properties are *not* added to the object.
+ */
+[[nodiscard]] extern bool intl_GetCalendarInfo(JSContext* cx, unsigned argc,
+                                               JS::Value* vp);
+
+/**
+ * Compares a BCP 47 language tag against the locales in availableLocales and
+ * returns the best available match -- or |undefined| if no match was found.
+ * Uses the fallback mechanism of RFC 4647, section 3.4.
+ *
+ * The set of available locales consulted doesn't necessarily include the
+ * default locale or any generalized forms of it (e.g. "de" is a more-general
+ * form of "de-CH"). If you want to be sure to consider the default local and
+ * its generalized forms (you usually will), pass the default locale as the
+ * value of |defaultOrNull|; otherwise pass null.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.2.2.
+ * Spec: RFC 4647, section 3.4.
+ *
+ * Usage: result = intl_BestAvailableLocale("Collator", locale, defaultOrNull)
+ */
+[[nodiscard]] extern bool intl_BestAvailableLocale(JSContext* cx, unsigned argc,
+                                                   JS::Value* vp);
+
+/**
+ * Return the supported locale for the input locale if ICU supports that locale
+ * (perhaps via fallback, e.g. supporting "de-CH" through "de" support implied
+ * by a "de-DE" locale). Otherwise uses the last-ditch locale.
+ *
+ * Usage: result = intl_supportedLocaleOrFallback(locale)
+ */
+[[nodiscard]] extern bool intl_supportedLocaleOrFallback(JSContext* cx,
+                                                         unsigned argc,
+                                                         JS::Value* vp);
+
+/**
+ * Returns the list of supported values for the given key. Throws a RangeError
+ * if the key isn't one of {"calendar", "collation", "currency",
+ * "numberingSystem", "timeZone", "unit"}.
+ *
+ * Usage: list = intl_SupportedValuesOf(key)
+ */
+[[nodiscard]] extern bool intl_SupportedValuesOf(JSContext* cx, unsigned argc,
+                                                 JS::Value* vp);
+
+}  // namespace js
+
+#endif /* builtin_intl_IntlObject_h */
diff --git a/js/src/builtin/intl/IntlObject.js b/js/src/builtin/intl/IntlObject.js
new file mode 100644
index 0000000000..95b158bb27
--- /dev/null
+++ b/js/src/builtin/intl/IntlObject.js
@@ -0,0 +1,81 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * 8.2.1 Intl.getCanonicalLocales ( locales )
+ *
+ * ES2017 Intl draft rev 947aa9a0c853422824a0c9510d8f09be3eb416b9
+ */
+function Intl_getCanonicalLocales(locales) {
+  // Steps 1-2.
+  return CanonicalizeLocaleList(locales);
+}
+
+/**
+ * Intl.supportedValuesOf ( key )
+ */
+function Intl_supportedValuesOf(key) {
+  // Step 1.
+  key = ToString(key);
+
+  // Steps 2-9.
+  return intl_SupportedValuesOf(key);
+}
+
+/**
+ * This function is a custom function in the style of the standard Intl.*
+ * functions, that isn't part of any spec or proposal yet.
+ *
+ * Returns an object with the following properties:
+ *   locale:
+ *     The actual resolved locale.
+ *
+ *   calendar:
+ *     The default calendar of the resolved locale.
+ *
+ *   firstDayOfWeek:
+ *     The first day of the week for the resolved locale.
+ *
+ *   minDays:
+ *     The minimum number of days in a week for the resolved locale.
+ *
+ *   weekend:
+ *     The days of the week considered as the weekend for the resolved locale.
+ *
+ * Days are encoded as integers in the range 1=Monday to 7=Sunday.
+ */
+function Intl_getCalendarInfo(locales) {
+  // 1. Let requestLocales be ? CanonicalizeLocaleList(locales).
+  const requestedLocales = CanonicalizeLocaleList(locales);
+
+  const DateTimeFormat = dateTimeFormatInternalProperties;
+
+  // 2. Let localeData be %DateTimeFormat%.[[localeData]].
+  const localeData = DateTimeFormat.localeData;
+
+  // 3. Let localeOpt be a new Record.
+  const localeOpt = new_Record();
+
+  // 4. Set localeOpt.[[localeMatcher]] to "best fit".
+  localeOpt.localeMatcher = "best fit";
+
+  // 5. Let r be ResolveLocale(%DateTimeFormat%.[[availableLocales]],
+  //    requestedLocales, localeOpt,
+  //    %DateTimeFormat%.[[relevantExtensionKeys]], localeData).
+  const r = ResolveLocale(
+    "DateTimeFormat",
+    requestedLocales,
+    localeOpt,
+    DateTimeFormat.relevantExtensionKeys,
+    localeData
+  );
+
+  // 6. Let result be GetCalendarInfo(r.[[locale]]).
+  const result = intl_GetCalendarInfo(r.locale);
+  DefineDataProperty(result, "calendar", r.ca);
+  DefineDataProperty(result, "locale", r.locale);
+
+  // 7. Return result.
+  return result;
+}
diff --git a/js/src/builtin/intl/LanguageTag.cpp b/js/src/builtin/intl/LanguageTag.cpp
new file mode 100644
index 0000000000..3372f5d99a
--- /dev/null
+++ b/js/src/builtin/intl/LanguageTag.cpp
@@ -0,0 +1,193 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "builtin/intl/LanguageTag.h"
+
+#include "mozilla/intl/Locale.h"
+#include "mozilla/Span.h"
+
+#include "builtin/intl/StringAsciiChars.h"
+#include "gc/Tracer.h"
+#include "vm/JSContext.h"
+
+namespace js {
+namespace intl {
+
+[[nodiscard]] bool ParseLocale(JSContext* cx, Handle<JSLinearString*> str,
+                               mozilla::intl::Locale& result) {
+  if (StringIsAscii(str)) {
+    intl::StringAsciiChars chars(str);
+    if (!chars.init(cx)) {
+      return false;
+    }
+
+    if (mozilla::intl::LocaleParser::TryParse(chars, result).isOk()) {
+      return true;
+    }
+  }
+
+  if (UniqueChars localeChars = QuoteString(cx, str, '"')) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_INVALID_LANGUAGE_TAG, localeChars.get());
+  }
+  return false;
+}
+
+bool ParseStandaloneLanguageTag(Handle<JSLinearString*> str,
+                                mozilla::intl::LanguageSubtag& result) {
+  // Tell the analysis the |IsStructurallyValidLanguageTag| function can't GC.
+  JS::AutoSuppressGCAnalysis nogc;
+
+  if (str->hasLatin1Chars()) {
+    if (!mozilla::intl::IsStructurallyValidLanguageTag<Latin1Char>(
+            str->latin1Range(nogc))) {
+      return false;
+    }
+    result.Set<Latin1Char>(str->latin1Range(nogc));
+  } else {
+    if (!mozilla::intl::IsStructurallyValidLanguageTag<char16_t>(
+            str->twoByteRange(nogc))) {
+      return false;
+    }
+    result.Set<char16_t>(str->twoByteRange(nogc));
+  }
+  return true;
+}
+
+bool ParseStandaloneScriptTag(Handle<JSLinearString*> str,
+                              mozilla::intl::ScriptSubtag& result) {
+  // Tell the analysis the |IsStructurallyValidScriptTag| function can't GC.
+  JS::AutoSuppressGCAnalysis nogc;
+
+  if (str->hasLatin1Chars()) {
+    if (!mozilla::intl::IsStructurallyValidScriptTag<Latin1Char>(
+            str->latin1Range(nogc))) {
+      return false;
+    }
+    result.Set<Latin1Char>(str->latin1Range(nogc));
+  } else {
+    if (!mozilla::intl::IsStructurallyValidScriptTag<char16_t>(
+            str->twoByteRange(nogc))) {
+      return false;
+    }
+    result.Set<char16_t>(str->twoByteRange(nogc));
+  }
+  return true;
+}
+
+bool ParseStandaloneRegionTag(Handle<JSLinearString*> str,
+                              mozilla::intl::RegionSubtag& result) {
+  // Tell the analysis the |IsStructurallyValidRegionTag| function can't GC.
+  JS::AutoSuppressGCAnalysis nogc;
+
+  if (str->hasLatin1Chars()) {
+    if (!mozilla::intl::IsStructurallyValidRegionTag<Latin1Char>(
+            str->latin1Range(nogc))) {
+      return false;
+    }
+    result.Set<Latin1Char>(str->latin1Range(nogc));
+  } else {
+    if (!mozilla::intl::IsStructurallyValidRegionTag<char16_t>(
+            str->twoByteRange(nogc))) {
+      return false;
+    }
+    result.Set<char16_t>(str->twoByteRange(nogc));
+  }
+  return true;
+}
+
+template <typename CharT>
+static bool IsAsciiLowercaseAlpha(mozilla::Span<const CharT> span) {
+  // Tell the analysis the |std::all_of| function can't GC.
+  JS::AutoSuppressGCAnalysis nogc;
+
+  const CharT* ptr = span.data();
+  size_t length = span.size();
+  return std::all_of(ptr, ptr + length, mozilla::IsAsciiLowercaseAlpha<CharT>);
+}
+
+static bool IsAsciiLowercaseAlpha(JSLinearString* str) {
+  JS::AutoCheckCannotGC nogc;
+  if (str->hasLatin1Chars()) {
+    return IsAsciiLowercaseAlpha<Latin1Char>(str->latin1Range(nogc));
+  }
+  return IsAsciiLowercaseAlpha<char16_t>(str->twoByteRange(nogc));
+}
+
+template <typename CharT>
+static bool IsAsciiAlpha(mozilla::Span<const CharT> span) {
+  // Tell the analysis the |std::all_of| function can't GC.
+  JS::AutoSuppressGCAnalysis nogc;
+
+  const CharT* ptr = span.data();
+  size_t length = span.size();
+  return std::all_of(ptr, ptr + length, mozilla::IsAsciiAlpha<CharT>);
+}
+
+static bool IsAsciiAlpha(JSLinearString* str) {
+  JS::AutoCheckCannotGC nogc;
+  if (str->hasLatin1Chars()) {
+    return IsAsciiAlpha<Latin1Char>(str->latin1Range(nogc));
+  }
+  return IsAsciiAlpha<char16_t>(str->twoByteRange(nogc));
+}
+
+JS::Result<JSString*> ParseStandaloneISO639LanguageTag(
+    JSContext* cx, Handle<JSLinearString*> str) {
+  // ISO-639 language codes contain either two or three characters.
+  size_t length = str->length();
+  if (length != 2 && length != 3) {
+    return nullptr;
+  }
+
+  // We can directly the return the input below if it's in the correct case.
+  bool isLowerCase = IsAsciiLowercaseAlpha(str);
+  if (!isLowerCase) {
+    // Must be an ASCII alpha string.
+    if (!IsAsciiAlpha(str)) {
+      return nullptr;
+    }
+  }
+
+  mozilla::intl::LanguageSubtag languageTag;
+  if (str->hasLatin1Chars()) {
+    JS::AutoCheckCannotGC nogc;
+    languageTag.Set<Latin1Char>(str->latin1Range(nogc));
+  } else {
+    JS::AutoCheckCannotGC nogc;
+    languageTag.Set<char16_t>(str->twoByteRange(nogc));
+  }
+
+  if (!isLowerCase) {
+    // The language subtag is canonicalized to lower case.
+    languageTag.ToLowerCase();
+  }
+
+  // Reject the input if the canonical tag contains more than just a single
+  // language subtag.
+  if (mozilla::intl::Locale::ComplexLanguageMapping(languageTag)) {
+    return nullptr;
+  }
+
+  // Take care to replace deprecated subtags with their preferred values.
+  JSString* result;
+  if (mozilla::intl::Locale::LanguageMapping(languageTag) || !isLowerCase) {
+    result = NewStringCopy<CanGC>(cx, languageTag.Span());
+  } else {
+    result = str;
+  }
+  if (!result) {
+    return cx->alreadyReportedOOM();
+  }
+  return result;
+}
+
+void js::intl::UnicodeExtensionKeyword::trace(JSTracer* trc) {
+  TraceRoot(trc, &type_, "UnicodeExtensionKeyword::type");
+}
+
+}  // namespace intl
+}  // namespace js
diff --git a/js/src/builtin/intl/LanguageTag.h b/js/src/builtin/intl/LanguageTag.h
new file mode 100644
index 0000000000..e896411e19
--- /dev/null
+++ b/js/src/builtin/intl/LanguageTag.h
@@ -0,0 +1,91 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Structured representation of Unicode locale IDs used with Intl functions. */
+
+#ifndef builtin_intl_LanguageTag_h
+#define builtin_intl_LanguageTag_h
+
+#include "mozilla/intl/Locale.h"
+#include "mozilla/Span.h"
+
+#include "js/Result.h"
+#include "js/RootingAPI.h"
+
+struct JS_PUBLIC_API JSContext;
+class JSLinearString;
+class JS_PUBLIC_API JSString;
+class JS_PUBLIC_API JSTracer;
+
+namespace js {
+
+namespace intl {
+
+/**
+ * Parse a string Unicode BCP 47 locale identifier. If successful, store in
+ * |result| and return true. Otherwise return false.
+ */
+[[nodiscard]] bool ParseLocale(JSContext* cx, JS::Handle<JSLinearString*> str,
+                               mozilla::intl::Locale& result);
+
+/**
+ * Parse a string as a standalone |language| tag. If |str| is a standalone
+ * language tag, store it in |result| and return true. Otherwise return false.
+ */
+[[nodiscard]] bool ParseStandaloneLanguageTag(
+    JS::Handle<JSLinearString*> str, mozilla::intl::LanguageSubtag& result);
+
+/**
+ * Parse a string as a standalone |script| tag. If |str| is a standalone script
+ * tag, store it in |result| and return true. Otherwise return false.
+ */
+[[nodiscard]] bool ParseStandaloneScriptTag(
+    JS::Handle<JSLinearString*> str, mozilla::intl::ScriptSubtag& result);
+
+/**
+ * Parse a string as a standalone |region| tag. If |str| is a standalone region
+ * tag, store it in |result| and return true. Otherwise return false.
+ */
+[[nodiscard]] bool ParseStandaloneRegionTag(
+    JS::Handle<JSLinearString*> str, mozilla::intl::RegionSubtag& result);
+
+/**
+ * Parse a string as an ISO-639 language code. Return |nullptr| in the result if
+ * the input could not be parsed or the canonical form of the resulting language
+ * tag contains more than a single language subtag.
+ */
+JS::Result<JSString*> ParseStandaloneISO639LanguageTag(
+    JSContext* cx, JS::Handle<JSLinearString*> str);
+
+class UnicodeExtensionKeyword final {
+  char key_[mozilla::intl::LanguageTagLimits::UnicodeKeyLength];
+  JSLinearString* type_;
+
+ public:
+  using UnicodeKey =
+      const char (&)[mozilla::intl::LanguageTagLimits::UnicodeKeyLength + 1];
+  using UnicodeKeySpan =
+      mozilla::Span<const char,
+                    mozilla::intl::LanguageTagLimits::UnicodeKeyLength>;
+
+  UnicodeExtensionKeyword(UnicodeKey key, JSLinearString* type)
+      : key_{key[0], key[1]}, type_(type) {}
+
+  UnicodeKeySpan key() const { return {key_, sizeof(key_)}; }
+  JSLinearString* type() const { return type_; }
+
+  void trace(JSTracer* trc);
+};
+
+[[nodiscard]] extern bool ApplyUnicodeExtensionToTag(
+    JSContext* cx, mozilla::intl::Locale& tag,
+    JS::HandleVector<UnicodeExtensionKeyword> keywords);
+
+}  // namespace intl
+
+}  // namespace js
+
+#endif /* builtin_intl_LanguageTag_h */
diff --git a/js/src/builtin/intl/ListFormat.cpp b/js/src/builtin/intl/ListFormat.cpp
new file mode 100644
index 0000000000..7d11d5acd6
--- /dev/null
+++ b/js/src/builtin/intl/ListFormat.cpp
@@ -0,0 +1,373 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "builtin/intl/ListFormat.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/intl/ListFormat.h"
+
+#include <stddef.h>
+
+#include "builtin/Array.h"
+#include "builtin/intl/CommonFunctions.h"
+#include "builtin/intl/FormatBuffer.h"
+#include "gc/GCContext.h"
+#include "js/Utility.h"
+#include "js/Vector.h"
+#include "vm/JSContext.h"
+#include "vm/PlainObject.h"  // js::PlainObject
+#include "vm/StringType.h"
+#include "vm/WellKnownAtom.h"  // js_*_str
+
+#include "vm/JSObject-inl.h"
+#include "vm/NativeObject-inl.h"
+#include "vm/ObjectOperations-inl.h"
+
+using namespace js;
+
+const JSClassOps ListFormatObject::classOps_ = {
+    nullptr,                     // addProperty
+    nullptr,                     // delProperty
+    nullptr,                     // enumerate
+    nullptr,                     // newEnumerate
+    nullptr,                     // resolve
+    nullptr,                     // mayResolve
+    ListFormatObject::finalize,  // finalize
+    nullptr,                     // call
+    nullptr,                     // construct
+    nullptr,                     // trace
+};
+const JSClass ListFormatObject::class_ = {
+    "Intl.ListFormat",
+    JSCLASS_HAS_RESERVED_SLOTS(ListFormatObject::SLOT_COUNT) |
+        JSCLASS_HAS_CACHED_PROTO(JSProto_ListFormat) |
+        JSCLASS_FOREGROUND_FINALIZE,
+    &ListFormatObject::classOps_, &ListFormatObject::classSpec_};
+
+const JSClass& ListFormatObject::protoClass_ = PlainObject::class_;
+
+static bool listFormat_toSource(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  args.rval().setString(cx->names().ListFormat);
+  return true;
+}
+
+static const JSFunctionSpec listFormat_static_methods[] = {
+    JS_SELF_HOSTED_FN("supportedLocalesOf",
+                      "Intl_ListFormat_supportedLocalesOf", 1, 0),
+    JS_FS_END};
+
+static const JSFunctionSpec listFormat_methods[] = {
+    JS_SELF_HOSTED_FN("resolvedOptions", "Intl_ListFormat_resolvedOptions", 0,
+                      0),
+    JS_SELF_HOSTED_FN("format", "Intl_ListFormat_format", 1, 0),
+    JS_SELF_HOSTED_FN("formatToParts", "Intl_ListFormat_formatToParts", 1, 0),
+    JS_FN(js_toSource_str, listFormat_toSource, 0, 0), JS_FS_END};
+
+static const JSPropertySpec listFormat_properties[] = {
+    JS_STRING_SYM_PS(toStringTag, "Intl.ListFormat", JSPROP_READONLY),
+    JS_PS_END};
+
+static bool ListFormat(JSContext* cx, unsigned argc, Value* vp);
+
+const ClassSpec ListFormatObject::classSpec_ = {
+    GenericCreateConstructor<ListFormat, 0, gc::AllocKind::FUNCTION>,
+    GenericCreatePrototype<ListFormatObject>,
+    listFormat_static_methods,
+    nullptr,
+    listFormat_methods,
+    listFormat_properties,
+    nullptr,
+    ClassSpec::DontDefineConstructor};
+
+/**
+ * Intl.ListFormat([ locales [, options]])
+ */
+static bool ListFormat(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+
+  // Step 1.
+  if (!ThrowIfNotConstructing(cx, args, "Intl.ListFormat")) {
+    return false;
+  }
+
+  // Step 2 (Inlined 9.1.14, OrdinaryCreateFromConstructor).
+  RootedObject proto(cx);
+  if (!GetPrototypeFromBuiltinConstructor(cx, args, JSProto_ListFormat,
+                                          &proto)) {
+    return false;
+  }
+
+  Rooted<ListFormatObject*> listFormat(
+      cx, NewObjectWithClassProto<ListFormatObject>(cx, proto));
+  if (!listFormat) {
+    return false;
+  }
+
+  HandleValue locales = args.get(0);
+  HandleValue options = args.get(1);
+
+  // Step 3.
+  if (!intl::InitializeObject(cx, listFormat, cx->names().InitializeListFormat,
+                              locales, options)) {
+    return false;
+  }
+
+  args.rval().setObject(*listFormat);
+  return true;
+}
+
+void js::ListFormatObject::finalize(JS::GCContext* gcx, JSObject* obj) {
+  MOZ_ASSERT(gcx->onMainThread());
+
+  mozilla::intl::ListFormat* lf =
+      obj->as<ListFormatObject>().getListFormatSlot();
+  if (lf) {
+    intl::RemoveICUCellMemory(gcx, obj, ListFormatObject::EstimatedMemoryUse);
+    delete lf;
+  }
+}
+
+/**
+ * Returns a new ListFormat with the locale and list formatting options
+ * of the given ListFormat.
+ */
+static mozilla::intl::ListFormat* NewListFormat(
+    JSContext* cx, Handle<ListFormatObject*> listFormat) {
+  RootedObject internals(cx, intl::GetInternalsObject(cx, listFormat));
+  if (!internals) {
+    return nullptr;
+  }
+
+  RootedValue value(cx);
+
+  if (!GetProperty(cx, internals, internals, cx->names().locale, &value)) {
+    return nullptr;
+  }
+  UniqueChars locale = intl::EncodeLocale(cx, value.toString());
+  if (!locale) {
+    return nullptr;
+  }
+
+  mozilla::intl::ListFormat::Options options;
+
+  using ListFormatType = mozilla::intl::ListFormat::Type;
+  if (!GetProperty(cx, internals, internals, cx->names().type, &value)) {
+    return nullptr;
+  }
+  {
+    JSLinearString* strType = value.toString()->ensureLinear(cx);
+    if (!strType) {
+      return nullptr;
+    }
+
+    if (StringEqualsLiteral(strType, "conjunction")) {
+      options.mType = ListFormatType::Conjunction;
+    } else if (StringEqualsLiteral(strType, "disjunction")) {
+      options.mType = ListFormatType::Disjunction;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(strType, "unit"));
+      options.mType = ListFormatType::Unit;
+    }
+  }
+
+  using ListFormatStyle = mozilla::intl::ListFormat::Style;
+  if (!GetProperty(cx, internals, internals, cx->names().style, &value)) {
+    return nullptr;
+  }
+  {
+    JSLinearString* strStyle = value.toString()->ensureLinear(cx);
+    if (!strStyle) {
+      return nullptr;
+    }
+
+    if (StringEqualsLiteral(strStyle, "long")) {
+      options.mStyle = ListFormatStyle::Long;
+    } else if (StringEqualsLiteral(strStyle, "short")) {
+      options.mStyle = ListFormatStyle::Short;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(strStyle, "narrow"));
+      options.mStyle = ListFormatStyle::Narrow;
+    }
+  }
+
+  auto result = mozilla::intl::ListFormat::TryCreate(
+      mozilla::MakeStringSpan(locale.get()), options);
+
+  if (result.isOk()) {
+    return result.unwrap().release();
+  }
+
+  js::intl::ReportInternalError(cx, result.unwrapErr());
+  return nullptr;
+}
+
+static mozilla::intl::ListFormat* GetOrCreateListFormat(
+    JSContext* cx, Handle<ListFormatObject*> listFormat) {
+  // Obtain a cached mozilla::intl::ListFormat object.
+  mozilla::intl::ListFormat* lf = listFormat->getListFormatSlot();
+  if (lf) {
+    return lf;
+  }
+
+  lf = NewListFormat(cx, listFormat);
+  if (!lf) {
+    return nullptr;
+  }
+  listFormat->setListFormatSlot(lf);
+
+  intl::AddICUCellMemory(listFormat, ListFormatObject::EstimatedMemoryUse);
+  return lf;
+}
+
+/**
+ * FormatList ( listFormat, list )
+ */
+static bool FormatList(JSContext* cx, mozilla::intl::ListFormat* lf,
+                       const mozilla::intl::ListFormat::StringList& list,
+                       MutableHandleValue result) {
+  intl::FormatBuffer<char16_t, intl::INITIAL_CHAR_BUFFER_SIZE> formatBuffer(cx);
+  auto formatResult = lf->Format(list, formatBuffer);
+  if (formatResult.isErr()) {
+    js::intl::ReportInternalError(cx, formatResult.unwrapErr());
+    return false;
+  }
+
+  JSString* str = formatBuffer.toString(cx);
+  if (!str) {
+    return false;
+  }
+  result.setString(str);
+  return true;
+}
+
+/**
+ * FormatListToParts ( listFormat, list )
+ */
+static bool FormatListToParts(JSContext* cx, mozilla::intl::ListFormat* lf,
+                              const mozilla::intl::ListFormat::StringList& list,
+                              MutableHandleValue result) {
+  intl::FormatBuffer<char16_t, intl::INITIAL_CHAR_BUFFER_SIZE> buffer(cx);
+  mozilla::intl::ListFormat::PartVector parts;
+  auto formatResult = lf->FormatToParts(list, buffer, parts);
+  if (formatResult.isErr()) {
+    intl::ReportInternalError(cx, formatResult.unwrapErr());
+    return false;
+  }
+
+  RootedString overallResult(cx, buffer.toString(cx));
+  if (!overallResult) {
+    return false;
+  }
+
+  Rooted<ArrayObject*> partsArray(
+      cx, NewDenseFullyAllocatedArray(cx, parts.length()));
+  if (!partsArray) {
+    return false;
+  }
+  partsArray->ensureDenseInitializedLength(0, parts.length());
+
+  RootedObject singlePart(cx);
+  RootedValue val(cx);
+
+  size_t index = 0;
+  size_t beginIndex = 0;
+  for (const mozilla::intl::ListFormat::Part& part : parts) {
+    singlePart = NewPlainObject(cx);
+    if (!singlePart) {
+      return false;
+    }
+
+    if (part.first == mozilla::intl::ListFormat::PartType::Element) {
+      val = StringValue(cx->names().element);
+    } else {
+      val = StringValue(cx->names().literal);
+    }
+
+    if (!DefineDataProperty(cx, singlePart, cx->names().type, val)) {
+      return false;
+    }
+
+    // There could be an empty string so the endIndex coule be equal to
+    // beginIndex.
+    MOZ_ASSERT(part.second >= beginIndex);
+    JSLinearString* partStr = NewDependentString(cx, overallResult, beginIndex,
+                                                 part.second - beginIndex);
+    if (!partStr) {
+      return false;
+    }
+    val = StringValue(partStr);
+    if (!DefineDataProperty(cx, singlePart, cx->names().value, val)) {
+      return false;
+    }
+
+    beginIndex = part.second;
+    partsArray->initDenseElement(index++, ObjectValue(*singlePart));
+  }
+
+  MOZ_ASSERT(index == parts.length());
+  MOZ_ASSERT(beginIndex == buffer.length());
+  result.setObject(*partsArray);
+
+  return true;
+}
+
+bool js::intl_FormatList(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 3);
+
+  Rooted<ListFormatObject*> listFormat(
+      cx, &args[0].toObject().as<ListFormatObject>());
+
+  bool formatToParts = args[2].toBoolean();
+
+  mozilla::intl::ListFormat* lf = GetOrCreateListFormat(cx, listFormat);
+  if (!lf) {
+    return false;
+  }
+
+  // Collect all strings and their lengths.
+  //
+  // 'strings' takes the ownership of those strings, and 'list' will be passed
+  // to mozilla::intl::ListFormat as a Span.
+  Vector<UniqueTwoByteChars, mozilla::intl::DEFAULT_LIST_LENGTH> strings(cx);
+  mozilla::intl::ListFormat::StringList list;
+
+  Rooted<ArrayObject*> listObj(cx, &args[1].toObject().as<ArrayObject>());
+  RootedValue value(cx);
+  uint32_t listLen = listObj->length();
+  for (uint32_t i = 0; i < listLen; i++) {
+    if (!GetElement(cx, listObj, listObj, i, &value)) {
+      return false;
+    }
+
+    JSLinearString* linear = value.toString()->ensureLinear(cx);
+    if (!linear) {
+      return false;
+    }
+
+    size_t linearLength = linear->length();
+
+    UniqueTwoByteChars chars = cx->make_pod_array<char16_t>(linearLength);
+    if (!chars) {
+      return false;
+    }
+    CopyChars(chars.get(), *linear);
+
+    if (!strings.append(std::move(chars))) {
+      return false;
+    }
+
+    if (!list.emplaceBack(strings[i].get(), linearLength)) {
+      return false;
+    }
+  }
+
+  if (formatToParts) {
+    return FormatListToParts(cx, lf, list, args.rval());
+  }
+  return FormatList(cx, lf, list, args.rval());
+}
diff --git a/js/src/builtin/intl/ListFormat.h b/js/src/builtin/intl/ListFormat.h
new file mode 100644
index 0000000000..da0daa711b
--- /dev/null
+++ b/js/src/builtin/intl/ListFormat.h
@@ -0,0 +1,69 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_ListFormat_h
+#define builtin_intl_ListFormat_h
+
+#include <stdint.h>
+
+#include "builtin/SelfHostingDefines.h"
+#include "js/Class.h"
+#include "js/TypeDecls.h"
+#include "vm/NativeObject.h"
+
+namespace mozilla::intl {
+class ListFormat;
+}  // namespace mozilla::intl
+
+namespace js {
+
+class ListFormatObject : public NativeObject {
+ public:
+  static const JSClass class_;
+  static const JSClass& protoClass_;
+
+  static constexpr uint32_t INTERNALS_SLOT = 0;
+  static constexpr uint32_t LIST_FORMAT_SLOT = 1;
+  static constexpr uint32_t SLOT_COUNT = 2;
+
+  static_assert(INTERNALS_SLOT == INTL_INTERNALS_OBJECT_SLOT,
+                "INTERNALS_SLOT must match self-hosting define for internals "
+                "object slot");
+
+  // Estimated memory use for UListFormatter (see IcuMemoryUsage).
+  static constexpr size_t EstimatedMemoryUse = 24;
+
+  mozilla::intl::ListFormat* getListFormatSlot() const {
+    const auto& slot = getFixedSlot(LIST_FORMAT_SLOT);
+    if (slot.isUndefined()) {
+      return nullptr;
+    }
+    return static_cast<mozilla::intl::ListFormat*>(slot.toPrivate());
+  }
+
+  void setListFormatSlot(mozilla::intl::ListFormat* format) {
+    setFixedSlot(LIST_FORMAT_SLOT, PrivateValue(format));
+  }
+
+ private:
+  static const JSClassOps classOps_;
+  static const ClassSpec classSpec_;
+
+  static void finalize(JS::GCContext* gcx, JSObject* obj);
+};
+
+/**
+ * Returns a string representing the array of string values |list| according to
+ * the effective locale and the formatting options of the given ListFormat.
+ *
+ * Usage: formatted = intl_FormatList(listFormat, list, formatToParts)
+ */
+[[nodiscard]] extern bool intl_FormatList(JSContext* cx, unsigned argc,
+                                          Value* vp);
+
+}  // namespace js
+
+#endif /* builtin_intl_ListFormat_h */
diff --git a/js/src/builtin/intl/ListFormat.js b/js/src/builtin/intl/ListFormat.js
new file mode 100644
index 0000000000..59ac29b944
--- /dev/null
+++ b/js/src/builtin/intl/ListFormat.js
@@ -0,0 +1,330 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * ListFormat internal properties.
+ */
+function listFormatLocaleData() {
+  // ListFormat don't support any extension keys.
+  return {};
+}
+var listFormatInternalProperties = {
+  localeData: listFormatLocaleData,
+  relevantExtensionKeys: [],
+};
+
+/**
+ * Intl.ListFormat ( [ locales [ , options ] ] )
+ *
+ * Compute an internal properties object from |lazyListFormatData|.
+ */
+function resolveListFormatInternals(lazyListFormatData) {
+  assert(IsObject(lazyListFormatData), "lazy data not an object?");
+
+  var internalProps = std_Object_create(null);
+
+  var ListFormat = listFormatInternalProperties;
+
+  // Compute effective locale.
+
+  // Step 9.
+  var localeData = ListFormat.localeData;
+
+  // Step 10.
+  var r = ResolveLocale(
+    "ListFormat",
+    lazyListFormatData.requestedLocales,
+    lazyListFormatData.opt,
+    ListFormat.relevantExtensionKeys,
+    localeData
+  );
+
+  // Step 11.
+  internalProps.locale = r.locale;
+
+  // Step 13.
+  internalProps.type = lazyListFormatData.type;
+
+  // Step 15.
+  internalProps.style = lazyListFormatData.style;
+
+  // Steps 16-23 (not applicable in our implementation).
+
+  // The caller is responsible for associating |internalProps| with the right
+  // object using |setInternalProperties|.
+  return internalProps;
+}
+
+/**
+ * Returns an object containing the ListFormat internal properties of |obj|.
+ */
+function getListFormatInternals(obj) {
+  assert(IsObject(obj), "getListFormatInternals called with non-object");
+  assert(
+    intl_GuardToListFormat(obj) !== null,
+    "getListFormatInternals called with non-ListFormat"
+  );
+
+  var internals = getIntlObjectInternals(obj);
+  assert(
+    internals.type === "ListFormat",
+    "bad type escaped getIntlObjectInternals"
+  );
+
+  // If internal properties have already been computed, use them.
+  var internalProps = maybeInternalProperties(internals);
+  if (internalProps) {
+    return internalProps;
+  }
+
+  // Otherwise it's time to fully create them.
+  internalProps = resolveListFormatInternals(internals.lazyData);
+  setInternalProperties(internals, internalProps);
+  return internalProps;
+}
+
+/**
+ * Intl.ListFormat ( [ locales [ , options ] ] )
+ *
+ * Initializes an object as a ListFormat.
+ *
+ * This method is complicated a moderate bit by its implementing initialization
+ * as a *lazy* concept.  Everything that must happen now, does -- but we defer
+ * all the work we can until the object is actually used as a ListFormat.
+ * This later work occurs in |resolveListFormatInternals|; steps not noted
+ * here occur there.
+ */
+function InitializeListFormat(listFormat, locales, options) {
+  assert(IsObject(listFormat), "InitializeListFormat called with non-object");
+  assert(
+    intl_GuardToListFormat(listFormat) !== null,
+    "InitializeListFormat called with non-ListFormat"
+  );
+
+  // Lazy ListFormat data has the following structure:
+  //
+  //   {
+  //     requestedLocales: List of locales,
+  //     type: "conjunction" / "disjunction" / "unit",
+  //     style: "long" / "short" / "narrow",
+  //
+  //     opt: // opt object computed in InitializeListFormat
+  //       {
+  //         localeMatcher: "lookup" / "best fit",
+  //       }
+  //   }
+  //
+  // Note that lazy data is only installed as a final step of initialization,
+  // so every ListFormat lazy data object has *all* these properties, never a
+  // subset of them.
+  var lazyListFormatData = std_Object_create(null);
+
+  // Step 3.
+  var requestedLocales = CanonicalizeLocaleList(locales);
+  lazyListFormatData.requestedLocales = requestedLocales;
+
+  // Steps 4-5.
+  if (options === undefined) {
+    options = std_Object_create(null);
+  } else if (!IsObject(options)) {
+    ThrowTypeError(
+      JSMSG_OBJECT_REQUIRED,
+      options === null ? "null" : typeof options
+    );
+  }
+
+  // Step 6.
+  var opt = new_Record();
+  lazyListFormatData.opt = opt;
+
+  // Steps 7-8.
+  let matcher = GetOption(
+    options,
+    "localeMatcher",
+    "string",
+    ["lookup", "best fit"],
+    "best fit"
+  );
+  opt.localeMatcher = matcher;
+
+  // Compute formatting options.
+
+  // Steps 12-13.
+  var type = GetOption(
+    options,
+    "type",
+    "string",
+    ["conjunction", "disjunction", "unit"],
+    "conjunction"
+  );
+  lazyListFormatData.type = type;
+
+  // Steps 14-15.
+  var style = GetOption(
+    options,
+    "style",
+    "string",
+    ["long", "short", "narrow"],
+    "long"
+  );
+  lazyListFormatData.style = style;
+
+  // We've done everything that must be done now: mark the lazy data as fully
+  // computed and install it.
+  initializeIntlObject(listFormat, "ListFormat", lazyListFormatData);
+}
+
+/**
+ * Returns the subset of the given locale list for which this locale list has a
+ * matching (possibly fallback) locale. Locales appear in the same order in the
+ * returned list as in the input list.
+ */
+function Intl_ListFormat_supportedLocalesOf(locales /*, options*/) {
+  var options = arguments.length > 1 ? arguments[1] : undefined;
+
+  // Step 1.
+  var availableLocales = "ListFormat";
+
+  // Step 2.
+  var requestedLocales = CanonicalizeLocaleList(locales);
+
+  // Step 3.
+  return SupportedLocales(availableLocales, requestedLocales, options);
+}
+
+/**
+ * StringListFromIterable ( iterable )
+ */
+function StringListFromIterable(iterable, methodName) {
+  // Step 1.
+  if (iterable === undefined) {
+    return [];
+  }
+
+  // Step 3.
+  var list = [];
+
+  // Steps 2, 4-5.
+  for (var element of allowContentIter(iterable)) {
+    // Step 5.b.ii.
+    if (typeof element !== "string") {
+      ThrowTypeError(
+        JSMSG_NOT_EXPECTED_TYPE,
+        methodName,
+        "string",
+        typeof element
+      );
+    }
+
+    // Step 5.b.iii.
+    DefineDataProperty(list, list.length, element);
+  }
+
+  // Step 6.
+  return list;
+}
+
+/**
+ * Intl.ListFormat.prototype.format ( list )
+ */
+function Intl_ListFormat_format(list) {
+  // Step 1.
+  var listFormat = this;
+
+  // Steps 2-3.
+  if (
+    !IsObject(listFormat) ||
+    (listFormat = intl_GuardToListFormat(listFormat)) === null
+  ) {
+    return callFunction(
+      intl_CallListFormatMethodIfWrapped,
+      this,
+      list,
+      "Intl_ListFormat_format"
+    );
+  }
+
+  // Step 4.
+  var stringList = StringListFromIterable(list, "format");
+
+  // We can directly return if |stringList| contains less than two elements.
+  if (stringList.length < 2) {
+    return stringList.length === 0 ? "" : stringList[0];
+  }
+
+  // Ensure the ListFormat internals are resolved.
+  getListFormatInternals(listFormat);
+
+  // Step 5.
+  return intl_FormatList(listFormat, stringList, /* formatToParts = */ false);
+}
+
+/**
+ * Intl.ListFormat.prototype.formatToParts ( list )
+ */
+function Intl_ListFormat_formatToParts(list) {
+  // Step 1.
+  var listFormat = this;
+
+  // Steps 2-3.
+  if (
+    !IsObject(listFormat) ||
+    (listFormat = intl_GuardToListFormat(listFormat)) === null
+  ) {
+    return callFunction(
+      intl_CallListFormatMethodIfWrapped,
+      this,
+      list,
+      "Intl_ListFormat_formatToParts"
+    );
+  }
+
+  // Step 4.
+  var stringList = StringListFromIterable(list, "formatToParts");
+
+  // We can directly return if |stringList| contains less than two elements.
+  if (stringList.length < 2) {
+    return stringList.length === 0
+      ? []
+      : [{ type: "element", value: stringList[0] }];
+  }
+
+  // Ensure the ListFormat internals are resolved.
+  getListFormatInternals(listFormat);
+
+  // Step 5.
+  return intl_FormatList(listFormat, stringList, /* formatToParts = */ true);
+}
+
+/**
+ * Returns the resolved options for a ListFormat object.
+ */
+function Intl_ListFormat_resolvedOptions() {
+  // Step 1.
+  var listFormat = this;
+
+  // Steps 2-3.
+  if (
+    !IsObject(listFormat) ||
+    (listFormat = intl_GuardToListFormat(listFormat)) === null
+  ) {
+    return callFunction(
+      intl_CallListFormatMethodIfWrapped,
+      this,
+      "Intl_ListFormat_resolvedOptions"
+    );
+  }
+
+  var internals = getListFormatInternals(listFormat);
+
+  // Steps 4-5.
+  var result = {
+    locale: internals.locale,
+    type: internals.type,
+    style: internals.style,
+  };
+
+  // Step 6.
+  return result;
+}
diff --git a/js/src/builtin/intl/Locale.cpp b/js/src/builtin/intl/Locale.cpp
new file mode 100644
index 0000000000..e5e0e7b6ae
--- /dev/null
+++ b/js/src/builtin/intl/Locale.cpp
@@ -0,0 +1,1517 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Intl.Locale implementation. */
+
+#include "builtin/intl/Locale.h"
+
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/intl/Locale.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Span.h"
+#include "mozilla/TextUtils.h"
+
+#include <algorithm>
+#include <string>
+#include <string.h>
+#include <utility>
+
+#include "builtin/Boolean.h"
+#include "builtin/intl/CommonFunctions.h"
+#include "builtin/intl/FormatBuffer.h"
+#include "builtin/intl/LanguageTag.h"
+#include "builtin/intl/StringAsciiChars.h"
+#include "builtin/String.h"
+#include "js/Conversions.h"
+#include "js/friend/ErrorMessages.h"  // js::GetErrorMessage, JSMSG_*
+#include "js/TypeDecls.h"
+#include "js/Wrapper.h"
+#include "vm/Compartment.h"
+#include "vm/GlobalObject.h"
+#include "vm/JSContext.h"
+#include "vm/PlainObject.h"  // js::PlainObject
+#include "vm/Printer.h"
+#include "vm/StringType.h"
+#include "vm/WellKnownAtom.h"  // js_*_str
+
+#include "vm/JSObject-inl.h"
+#include "vm/NativeObject-inl.h"
+
+using namespace js;
+using namespace mozilla::intl::LanguageTagLimits;
+
+const JSClass LocaleObject::class_ = {
+    "Intl.Locale",
+    JSCLASS_HAS_RESERVED_SLOTS(LocaleObject::SLOT_COUNT) |
+        JSCLASS_HAS_CACHED_PROTO(JSProto_Locale),
+    JS_NULL_CLASS_OPS, &LocaleObject::classSpec_};
+
+const JSClass& LocaleObject::protoClass_ = PlainObject::class_;
+
+static inline bool IsLocale(HandleValue v) {
+  return v.isObject() && v.toObject().is<LocaleObject>();
+}
+
+// Return the length of the base-name subtags.
+static size_t BaseNameLength(const mozilla::intl::Locale& tag) {
+  size_t baseNameLength = tag.Language().Length();
+  if (tag.Script().Present()) {
+    baseNameLength += 1 + tag.Script().Length();
+  }
+  if (tag.Region().Present()) {
+    baseNameLength += 1 + tag.Region().Length();
+  }
+  for (const auto& variant : tag.Variants()) {
+    baseNameLength += 1 + variant.size();
+  }
+  return baseNameLength;
+}
+
+struct IndexAndLength {
+  size_t index;
+  size_t length;
+
+  IndexAndLength(size_t index, size_t length) : index(index), length(length){};
+
+  template <typename T>
+  mozilla::Span<const T> spanOf(const T* ptr) const {
+    return {ptr + index, length};
+  }
+};
+
+// Compute the Unicode extension's index and length in the extension subtag.
+static mozilla::Maybe<IndexAndLength> UnicodeExtensionPosition(
+    const mozilla::intl::Locale& tag) {
+  size_t index = 0;
+  for (const auto& extension : tag.Extensions()) {
+    MOZ_ASSERT(!mozilla::IsAsciiUppercaseAlpha(extension[0]),
+               "extensions are case normalized to lowercase");
+
+    size_t extensionLength = extension.size();
+    if (extension[0] == 'u') {
+      return mozilla::Some(IndexAndLength{index, extensionLength});
+    }
+
+    // Add +1 to skip over the preceding separator.
+    index += 1 + extensionLength;
+  }
+  return mozilla::Nothing();
+}
+
+static LocaleObject* CreateLocaleObject(JSContext* cx, HandleObject prototype,
+                                        const mozilla::intl::Locale& tag) {
+  intl::FormatBuffer<char, intl::INITIAL_CHAR_BUFFER_SIZE> buffer(cx);
+  if (auto result = tag.ToString(buffer); result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return nullptr;
+  }
+
+  RootedString tagStr(cx, buffer.toAsciiString(cx));
+  if (!tagStr) {
+    return nullptr;
+  }
+
+  size_t baseNameLength = BaseNameLength(tag);
+
+  RootedString baseName(cx, NewDependentString(cx, tagStr, 0, baseNameLength));
+  if (!baseName) {
+    return nullptr;
+  }
+
+  RootedValue unicodeExtension(cx, UndefinedValue());
+  if (auto result = UnicodeExtensionPosition(tag)) {
+    JSString* str = NewDependentString(
+        cx, tagStr, baseNameLength + 1 + result->index, result->length);
+    if (!str) {
+      return nullptr;
+    }
+
+    unicodeExtension.setString(str);
+  }
+
+  auto* locale = NewObjectWithClassProto<LocaleObject>(cx, prototype);
+  if (!locale) {
+    return nullptr;
+  }
+
+  locale->setFixedSlot(LocaleObject::LANGUAGE_TAG_SLOT, StringValue(tagStr));
+  locale->setFixedSlot(LocaleObject::BASENAME_SLOT, StringValue(baseName));
+  locale->setFixedSlot(LocaleObject::UNICODE_EXTENSION_SLOT, unicodeExtension);
+
+  return locale;
+}
+
+static inline bool IsValidUnicodeExtensionValue(JSContext* cx,
+                                                JSLinearString* linear,
+                                                bool* isValid) {
+  if (linear->length() == 0) {
+    *isValid = false;
+    return true;
+  }
+
+  if (!StringIsAscii(linear)) {
+    *isValid = false;
+    return true;
+  }
+
+  intl::StringAsciiChars chars(linear);
+  if (!chars.init(cx)) {
+    return false;
+  }
+
+  *isValid =
+      mozilla::intl::LocaleParser::CanParseUnicodeExtensionType(chars).isOk();
+  return true;
+}
+
+/** Iterate through (sep keyword) in a valid, lowercased Unicode extension. */
+template <typename CharT>
+class SepKeywordIterator {
+  const CharT* iter_;
+  const CharT* const end_;
+
+ public:
+  SepKeywordIterator(const CharT* unicodeExtensionBegin,
+                     const CharT* unicodeExtensionEnd)
+      : iter_(unicodeExtensionBegin), end_(unicodeExtensionEnd) {}
+
+  /**
+   * Return (sep keyword) in the Unicode locale extension from begin to end.
+   * The first call after all (sep keyword) are consumed returns |nullptr|; no
+   * further calls are allowed.
+   */
+  const CharT* next() {
+    MOZ_ASSERT(iter_ != nullptr,
+               "can't call next() once it's returned nullptr");
+
+    constexpr size_t SepKeyLength = 1 + UnicodeKeyLength;  // "-co"/"-nu"/etc.
+
+    MOZ_ASSERT(iter_ + SepKeyLength <= end_,
+               "overall Unicode locale extension or non-leading subtags must "
+               "be at least key-sized");
+
+    MOZ_ASSERT((iter_[0] == 'u' && iter_[1] == '-') || iter_[0] == '-');
+
+    while (true) {
+      // Skip past '-' so |std::char_traits::find| makes progress. Skipping
+      // 'u' is harmless -- skip or not, |find| returns the first '-'.
+      iter_++;
+
+      // Find the next separator.
+      iter_ = std::char_traits<CharT>::find(
+          iter_, mozilla::PointerRangeSize(iter_, end_), CharT('-'));
+      if (!iter_) {
+        return nullptr;
+      }
+
+      MOZ_ASSERT(iter_ + SepKeyLength <= end_,
+                 "non-leading subtags in a Unicode locale extension are all "
+                 "at least as long as a key");
+
+      if (iter_ + SepKeyLength == end_ ||  // key is terminal subtag
+          iter_[SepKeyLength] == '-') {    // key is followed by more subtags
+        break;
+      }
+    }
+
+    MOZ_ASSERT(iter_[0] == '-');
+    MOZ_ASSERT(mozilla::IsAsciiLowercaseAlpha(iter_[1]) ||
+               mozilla::IsAsciiDigit(iter_[1]));
+    MOZ_ASSERT(mozilla::IsAsciiLowercaseAlpha(iter_[2]));
+    MOZ_ASSERT_IF(iter_ + SepKeyLength < end_, iter_[SepKeyLength] == '-');
+    return iter_;
+  }
+};
+
+/**
+ * 9.2.10 GetOption ( options, property, type, values, fallback )
+ *
+ * If the requested property is present and not-undefined, set the result string
+ * to |ToString(value)|. Otherwise set the result string to nullptr.
+ */
+static bool GetStringOption(JSContext* cx, HandleObject options,
+                            Handle<PropertyName*> name,
+                            MutableHandle<JSLinearString*> string) {
+  // Step 1.
+  RootedValue option(cx);
+  if (!GetProperty(cx, options, options, name, &option)) {
+    return false;
+  }
+
+  // Step 2.
+  JSLinearString* linear = nullptr;
+  if (!option.isUndefined()) {
+    // Steps 2.a-b, 2.d (not applicable).
+
+    // Steps 2.c, 2.e.
+    JSString* str = ToString(cx, option);
+    if (!str) {
+      return false;
+    }
+    linear = str->ensureLinear(cx);
+    if (!linear) {
+      return false;
+    }
+  }
+
+  // Step 3.
+  string.set(linear);
+  return true;
+}
+
+/**
+ * 9.2.10 GetOption ( options, property, type, values, fallback )
+ *
+ * If the requested property is present and not-undefined, set the result string
+ * to |ToString(ToBoolean(value))|. Otherwise set the result string to nullptr.
+ */
+static bool GetBooleanOption(JSContext* cx, HandleObject options,
+                             Handle<PropertyName*> name,
+                             MutableHandle<JSLinearString*> string) {
+  // Step 1.
+  RootedValue option(cx);
+  if (!GetProperty(cx, options, options, name, &option)) {
+    return false;
+  }
+
+  // Step 2.
+  JSLinearString* linear = nullptr;
+  if (!option.isUndefined()) {
+    // Steps 2.a, 2.c-d (not applicable).
+
+    // Steps 2.c, 2.e.
+    linear = BooleanToString(cx, ToBoolean(option));
+  }
+
+  // Step 3.
+  string.set(linear);
+  return true;
+}
+
+/**
+ * ApplyOptionsToTag ( tag, options )
+ */
+static bool ApplyOptionsToTag(JSContext* cx, mozilla::intl::Locale& tag,
+                              HandleObject options) {
+  // Steps 1-2 (Already performed in caller).
+
+  Rooted<JSLinearString*> option(cx);
+
+  // Step 3.
+  if (!GetStringOption(cx, options, cx->names().language, &option)) {
+    return false;
+  }
+
+  // Step 4.
+  mozilla::intl::LanguageSubtag language;
+  if (option && !intl::ParseStandaloneLanguageTag(option, language)) {
+    if (UniqueChars str = QuoteString(cx, option, '"')) {
+      JS_ReportErrorNumberASCII(cx, js::GetErrorMessage, nullptr,
+                                JSMSG_INVALID_OPTION_VALUE, "language",
+                                str.get());
+    }
+    return false;
+  }
+
+  // Step 5.
+  if (!GetStringOption(cx, options, cx->names().script, &option)) {
+    return false;
+  }
+
+  // Step 6.
+  mozilla::intl::ScriptSubtag script;
+  if (option && !intl::ParseStandaloneScriptTag(option, script)) {
+    if (UniqueChars str = QuoteString(cx, option, '"')) {
+      JS_ReportErrorNumberASCII(cx, js::GetErrorMessage, nullptr,
+                                JSMSG_INVALID_OPTION_VALUE, "script",
+                                str.get());
+    }
+    return false;
+  }
+
+  // Step 7.
+  if (!GetStringOption(cx, options, cx->names().region, &option)) {
+    return false;
+  }
+
+  // Step 8.
+  mozilla::intl::RegionSubtag region;
+  if (option && !intl::ParseStandaloneRegionTag(option, region)) {
+    if (UniqueChars str = QuoteString(cx, option, '"')) {
+      JS_ReportErrorNumberASCII(cx, js::GetErrorMessage, nullptr,
+                                JSMSG_INVALID_OPTION_VALUE, "region",
+                                str.get());
+    }
+    return false;
+  }
+
+  // Step 9 (Already performed in caller).
+
+  // Skip steps 10-13 when no subtags were modified.
+  if (language.Present() || script.Present() || region.Present()) {
+    // Step 10.
+    if (language.Present()) {
+      tag.SetLanguage(language);
+    }
+
+    // Step 11.
+    if (script.Present()) {
+      tag.SetScript(script);
+    }
+
+    // Step 12.
+    if (region.Present()) {
+      tag.SetRegion(region);
+    }
+
+    // Step 13.
+    // Optimized to only canonicalize the base-name subtags. All other
+    // canonicalization steps will happen later.
+    auto result = tag.CanonicalizeBaseName();
+    if (result.isErr()) {
+      if (result.unwrapErr() ==
+          mozilla::intl::Locale::CanonicalizationError::DuplicateVariant) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_DUPLICATE_VARIANT_SUBTAG);
+      } else {
+        intl::ReportInternalError(cx);
+      }
+      return false;
+    }
+  }
+
+  return true;
+}
+
+/**
+ * ApplyUnicodeExtensionToTag( tag, options, relevantExtensionKeys )
+ */
+bool js::intl::ApplyUnicodeExtensionToTag(
+    JSContext* cx, mozilla::intl::Locale& tag,
+    JS::HandleVector<intl::UnicodeExtensionKeyword> keywords) {
+  // If no Unicode extensions were present in the options object, we can skip
+  // everything below and directly return.
+  if (keywords.length() == 0) {
+    return true;
+  }
+
+  Vector<char, 32> newExtension(cx);
+  if (!newExtension.append('u')) {
+    return false;
+  }
+
+  // Check if there's an existing Unicode extension subtag.
+
+  const char* unicodeExtensionEnd = nullptr;
+  const char* unicodeExtensionKeywords = nullptr;
+  if (auto unicodeExtension = tag.GetUnicodeExtension()) {
+    const char* unicodeExtensionBegin = unicodeExtension->data();
+    unicodeExtensionEnd = unicodeExtensionBegin + unicodeExtension->size();
+
+    SepKeywordIterator<char> iter(unicodeExtensionBegin, unicodeExtensionEnd);
+
+    // Find the start of the first keyword.
+    unicodeExtensionKeywords = iter.next();
+
+    // Copy any attributes present before the first keyword.
+    const char* attributesEnd = unicodeExtensionKeywords
+                                    ? unicodeExtensionKeywords
+                                    : unicodeExtensionEnd;
+    if (!newExtension.append(unicodeExtensionBegin + 1, attributesEnd)) {
+      return false;
+    }
+  }
+
+  // Append the new keywords before any existing keywords. That way any previous
+  // keyword with the same key is detected as a duplicate when canonicalizing
+  // the Unicode extension subtag and gets discarded.
+
+  for (const auto& keyword : keywords) {
+    UnicodeExtensionKeyword::UnicodeKeySpan key = keyword.key();
+    if (!newExtension.append('-')) {
+      return false;
+    }
+    if (!newExtension.append(key.data(), key.size())) {
+      return false;
+    }
+    if (!newExtension.append('-')) {
+      return false;
+    }
+
+    JS::AutoCheckCannotGC nogc;
+    JSLinearString* type = keyword.type();
+    if (type->hasLatin1Chars()) {
+      if (!newExtension.append(type->latin1Chars(nogc), type->length())) {
+        return false;
+      }
+    } else {
+      if (!newExtension.append(type->twoByteChars(nogc), type->length())) {
+        return false;
+      }
+    }
+  }
+
+  // Append the remaining keywords from the previous Unicode extension subtag.
+  if (unicodeExtensionKeywords) {
+    if (!newExtension.append(unicodeExtensionKeywords, unicodeExtensionEnd)) {
+      return false;
+    }
+  }
+
+  if (auto res = tag.SetUnicodeExtension(newExtension); res.isErr()) {
+    intl::ReportInternalError(cx, res.unwrapErr());
+    return false;
+  }
+
+  return true;
+}
+
+static JS::Result<JSString*> LanguageTagFromMaybeWrappedLocale(JSContext* cx,
+                                                               JSObject* obj) {
+  if (obj->is<LocaleObject>()) {
+    return obj->as<LocaleObject>().languageTag();
+  }
+
+  JSObject* unwrapped = CheckedUnwrapStatic(obj);
+  if (!unwrapped) {
+    ReportAccessDenied(cx);
+    return cx->alreadyReportedError();
+  }
+
+  if (!unwrapped->is<LocaleObject>()) {
+    return nullptr;
+  }
+
+  RootedString tagStr(cx, unwrapped->as<LocaleObject>().languageTag());
+  if (!cx->compartment()->wrap(cx, &tagStr)) {
+    return cx->alreadyReportedError();
+  }
+  return tagStr.get();
+}
+
+/**
+ * Intl.Locale( tag[, options] )
+ */
+static bool Locale(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+
+  // Step 1.
+  if (!ThrowIfNotConstructing(cx, args, "Intl.Locale")) {
+    return false;
+  }
+
+  // Steps 2-6 (Inlined 9.1.14, OrdinaryCreateFromConstructor).
+  RootedObject proto(cx);
+  if (!GetPrototypeFromBuiltinConstructor(cx, args, JSProto_Locale, &proto)) {
+    return false;
+  }
+
+  // Steps 7-9.
+  HandleValue tagValue = args.get(0);
+  JSString* tagStr;
+  if (tagValue.isObject()) {
+    JS_TRY_VAR_OR_RETURN_FALSE(
+        cx, tagStr,
+        LanguageTagFromMaybeWrappedLocale(cx, &tagValue.toObject()));
+    if (!tagStr) {
+      tagStr = ToString(cx, tagValue);
+      if (!tagStr) {
+        return false;
+      }
+    }
+  } else if (tagValue.isString()) {
+    tagStr = tagValue.toString();
+  } else {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_INVALID_LOCALES_ELEMENT);
+    return false;
+  }
+
+  Rooted<JSLinearString*> tagLinearStr(cx, tagStr->ensureLinear(cx));
+  if (!tagLinearStr) {
+    return false;
+  }
+
+  // Steps 10-11.
+  RootedObject options(cx);
+  if (args.hasDefined(1)) {
+    options = ToObject(cx, args[1]);
+    if (!options) {
+      return false;
+    }
+  }
+
+  // ApplyOptionsToTag, steps 2 and 9.
+  mozilla::intl::Locale tag;
+  if (!intl::ParseLocale(cx, tagLinearStr, tag)) {
+    return false;
+  }
+
+  if (auto result = tag.CanonicalizeBaseName(); result.isErr()) {
+    if (result.unwrapErr() ==
+        mozilla::intl::Locale::CanonicalizationError::DuplicateVariant) {
+      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                JSMSG_DUPLICATE_VARIANT_SUBTAG);
+    } else {
+      intl::ReportInternalError(cx);
+    }
+    return false;
+  }
+
+  if (options) {
+    // Step 12.
+    if (!ApplyOptionsToTag(cx, tag, options)) {
+      return false;
+    }
+
+    // Step 13.
+    JS::RootedVector<intl::UnicodeExtensionKeyword> keywords(cx);
+
+    // Step 14.
+    Rooted<JSLinearString*> calendar(cx);
+    if (!GetStringOption(cx, options, cx->names().calendar, &calendar)) {
+      return false;
+    }
+
+    // Steps 15-16.
+    if (calendar) {
+      bool isValid;
+      if (!IsValidUnicodeExtensionValue(cx, calendar, &isValid)) {
+        return false;
+      }
+
+      if (!isValid) {
+        if (UniqueChars str = QuoteString(cx, calendar, '"')) {
+          JS_ReportErrorNumberASCII(cx, js::GetErrorMessage, nullptr,
+                                    JSMSG_INVALID_OPTION_VALUE, "calendar",
+                                    str.get());
+        }
+        return false;
+      }
+
+      if (!keywords.emplaceBack("ca", calendar)) {
+        return false;
+      }
+    }
+
+    // Step 17.
+    Rooted<JSLinearString*> collation(cx);
+    if (!GetStringOption(cx, options, cx->names().collation, &collation)) {
+      return false;
+    }
+
+    // Steps 18-19.
+    if (collation) {
+      bool isValid;
+      if (!IsValidUnicodeExtensionValue(cx, collation, &isValid)) {
+        return false;
+      }
+
+      if (!isValid) {
+        if (UniqueChars str = QuoteString(cx, collation, '"')) {
+          JS_ReportErrorNumberASCII(cx, js::GetErrorMessage, nullptr,
+                                    JSMSG_INVALID_OPTION_VALUE, "collation",
+                                    str.get());
+        }
+        return false;
+      }
+
+      if (!keywords.emplaceBack("co", collation)) {
+        return false;
+      }
+    }
+
+    // Step 20 (without validation).
+    Rooted<JSLinearString*> hourCycle(cx);
+    if (!GetStringOption(cx, options, cx->names().hourCycle, &hourCycle)) {
+      return false;
+    }
+
+    // Steps 20-21.
+    if (hourCycle) {
+      if (!StringEqualsLiteral(hourCycle, "h11") &&
+          !StringEqualsLiteral(hourCycle, "h12") &&
+          !StringEqualsLiteral(hourCycle, "h23") &&
+          !StringEqualsLiteral(hourCycle, "h24")) {
+        if (UniqueChars str = QuoteString(cx, hourCycle, '"')) {
+          JS_ReportErrorNumberASCII(cx, js::GetErrorMessage, nullptr,
+                                    JSMSG_INVALID_OPTION_VALUE, "hourCycle",
+                                    str.get());
+        }
+        return false;
+      }
+
+      if (!keywords.emplaceBack("hc", hourCycle)) {
+        return false;
+      }
+    }
+
+    // Step 22 (without validation).
+    Rooted<JSLinearString*> caseFirst(cx);
+    if (!GetStringOption(cx, options, cx->names().caseFirst, &caseFirst)) {
+      return false;
+    }
+
+    // Steps 22-23.
+    if (caseFirst) {
+      if (!StringEqualsLiteral(caseFirst, "upper") &&
+          !StringEqualsLiteral(caseFirst, "lower") &&
+          !StringEqualsLiteral(caseFirst, "false")) {
+        if (UniqueChars str = QuoteString(cx, caseFirst, '"')) {
+          JS_ReportErrorNumberASCII(cx, js::GetErrorMessage, nullptr,
+                                    JSMSG_INVALID_OPTION_VALUE, "caseFirst",
+                                    str.get());
+        }
+        return false;
+      }
+
+      if (!keywords.emplaceBack("kf", caseFirst)) {
+        return false;
+      }
+    }
+
+    // Steps 24-25.
+    Rooted<JSLinearString*> numeric(cx);
+    if (!GetBooleanOption(cx, options, cx->names().numeric, &numeric)) {
+      return false;
+    }
+
+    // Step 26.
+    if (numeric) {
+      if (!keywords.emplaceBack("kn", numeric)) {
+        return false;
+      }
+    }
+
+    // Step 27.
+    Rooted<JSLinearString*> numberingSystem(cx);
+    if (!GetStringOption(cx, options, cx->names().numberingSystem,
+                         &numberingSystem)) {
+      return false;
+    }
+
+    // Steps 28-29.
+    if (numberingSystem) {
+      bool isValid;
+      if (!IsValidUnicodeExtensionValue(cx, numberingSystem, &isValid)) {
+        return false;
+      }
+      if (!isValid) {
+        if (UniqueChars str = QuoteString(cx, numberingSystem, '"')) {
+          JS_ReportErrorNumberASCII(cx, js::GetErrorMessage, nullptr,
+                                    JSMSG_INVALID_OPTION_VALUE,
+                                    "numberingSystem", str.get());
+        }
+        return false;
+      }
+
+      if (!keywords.emplaceBack("nu", numberingSystem)) {
+        return false;
+      }
+    }
+
+    // Step 30.
+    if (!ApplyUnicodeExtensionToTag(cx, tag, keywords)) {
+      return false;
+    }
+  }
+
+  // ApplyOptionsToTag, steps 9 and 13.
+  // ApplyUnicodeExtensionToTag, step 9.
+  if (auto result = tag.CanonicalizeExtensions(); result.isErr()) {
+    if (result.unwrapErr() ==
+        mozilla::intl::Locale::CanonicalizationError::DuplicateVariant) {
+      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                JSMSG_DUPLICATE_VARIANT_SUBTAG);
+    } else {
+      intl::ReportInternalError(cx);
+    }
+    return false;
+  }
+
+  // Steps 6, 31-37.
+  JSObject* obj = CreateLocaleObject(cx, proto, tag);
+  if (!obj) {
+    return false;
+  }
+
+  // Step 38.
+  args.rval().setObject(*obj);
+  return true;
+}
+
+using UnicodeKey = const char (&)[UnicodeKeyLength + 1];
+
+// Returns the tuple [index, length] of the `type` in the `keyword` in Unicode
+// locale extension |extension| that has |key| as its `key`. If `keyword` lacks
+// a type, the returned |index| will be where `type` would have been, and
+// |length| will be set to zero.
+template <typename CharT>
+static mozilla::Maybe<IndexAndLength> FindUnicodeExtensionType(
+    const CharT* extension, size_t length, UnicodeKey key) {
+  MOZ_ASSERT(extension[0] == 'u');
+  MOZ_ASSERT(extension[1] == '-');
+
+  const CharT* end = extension + length;
+
+  SepKeywordIterator<CharT> iter(extension, end);
+
+  // Search all keywords until a match was found.
+  const CharT* beginKey;
+  while (true) {
+    beginKey = iter.next();
+    if (!beginKey) {
+      return mozilla::Nothing();
+    }
+
+    // Add +1 to skip over the separator preceding the keyword.
+    MOZ_ASSERT(beginKey[0] == '-');
+    beginKey++;
+
+    // Exit the loop on the first match.
+    if (std::equal(beginKey, beginKey + UnicodeKeyLength, key)) {
+      break;
+    }
+  }
+
+  // Skip over the key.
+  const CharT* beginType = beginKey + UnicodeKeyLength;
+
+  // Find the start of the next keyword.
+  const CharT* endType = iter.next();
+
+  // No further keyword present, the current keyword ends the Unicode extension.
+  if (!endType) {
+    endType = end;
+  }
+
+  // If the keyword has a type, skip over the separator preceding the type.
+  if (beginType != endType) {
+    MOZ_ASSERT(beginType[0] == '-');
+    beginType++;
+  }
+  return mozilla::Some(IndexAndLength{size_t(beginType - extension),
+                                      size_t(endType - beginType)});
+}
+
+static inline auto FindUnicodeExtensionType(JSLinearString* unicodeExtension,
+                                            UnicodeKey key) {
+  JS::AutoCheckCannotGC nogc;
+  return unicodeExtension->hasLatin1Chars()
+             ? FindUnicodeExtensionType(unicodeExtension->latin1Chars(nogc),
+                                        unicodeExtension->length(), key)
+             : FindUnicodeExtensionType(unicodeExtension->twoByteChars(nogc),
+                                        unicodeExtension->length(), key);
+}
+
+// Return the sequence of types for the Unicode extension keyword specified by
+// key or undefined when the keyword isn't present.
+static bool GetUnicodeExtension(JSContext* cx, LocaleObject* locale,
+                                UnicodeKey key, MutableHandleValue value) {
+  // Return undefined when no Unicode extension subtag is present.
+  const Value& unicodeExtensionValue = locale->unicodeExtension();
+  if (unicodeExtensionValue.isUndefined()) {
+    value.setUndefined();
+    return true;
+  }
+
+  JSLinearString* unicodeExtension =
+      unicodeExtensionValue.toString()->ensureLinear(cx);
+  if (!unicodeExtension) {
+    return false;
+  }
+
+  // Find the type of the requested key in the Unicode extension subtag.
+  auto result = FindUnicodeExtensionType(unicodeExtension, key);
+
+  // Return undefined if the requested key isn't present in the extension.
+  if (!result) {
+    value.setUndefined();
+    return true;
+  }
+
+  size_t index = result->index;
+  size_t length = result->length;
+
+  // Otherwise return the type value of the found keyword.
+  JSString* str = NewDependentString(cx, unicodeExtension, index, length);
+  if (!str) {
+    return false;
+  }
+  value.setString(str);
+  return true;
+}
+
+struct BaseNamePartsResult {
+  IndexAndLength language;
+  mozilla::Maybe<IndexAndLength> script;
+  mozilla::Maybe<IndexAndLength> region;
+};
+
+// Returns [language-length, script-index, region-index, region-length].
+template <typename CharT>
+static BaseNamePartsResult BaseNameParts(const CharT* baseName, size_t length) {
+  size_t languageLength;
+  size_t scriptIndex = 0;
+  size_t regionIndex = 0;
+  size_t regionLength = 0;
+
+  // Search the first separator to find the end of the language subtag.
+  if (const CharT* sep = std::char_traits<CharT>::find(baseName, length, '-')) {
+    languageLength = sep - baseName;
+
+    // Add +1 to skip over the separator character.
+    size_t nextSubtag = languageLength + 1;
+
+    // Script subtags are always four characters long, but take care for a four
+    // character long variant subtag. These start with a digit.
+    if ((nextSubtag + ScriptLength == length ||
+         (nextSubtag + ScriptLength < length &&
+          baseName[nextSubtag + ScriptLength] == '-')) &&
+        mozilla::IsAsciiAlpha(baseName[nextSubtag])) {
+      scriptIndex = nextSubtag;
+      nextSubtag = scriptIndex + ScriptLength + 1;
+    }
+
+    // Region subtags can be either two or three characters long.
+    if (nextSubtag < length) {
+      for (size_t rlen : {AlphaRegionLength, DigitRegionLength}) {
+        MOZ_ASSERT(nextSubtag + rlen <= length);
+        if (nextSubtag + rlen == length || baseName[nextSubtag + rlen] == '-') {
+          regionIndex = nextSubtag;
+          regionLength = rlen;
+          break;
+        }
+      }
+    }
+  } else {
+    // No separator found, the base-name consists of just a language subtag.
+    languageLength = length;
+  }
+
+  // Tell the analysis the |IsStructurallyValid*Tag| functions can't GC.
+  JS::AutoSuppressGCAnalysis nogc;
+
+  IndexAndLength language{0, languageLength};
+  MOZ_ASSERT(
+      mozilla::intl::IsStructurallyValidLanguageTag(language.spanOf(baseName)));
+
+  mozilla::Maybe<IndexAndLength> script{};
+  if (scriptIndex) {
+    script.emplace(scriptIndex, ScriptLength);
+    MOZ_ASSERT(
+        mozilla::intl::IsStructurallyValidScriptTag(script->spanOf(baseName)));
+  }
+
+  mozilla::Maybe<IndexAndLength> region{};
+  if (regionIndex) {
+    region.emplace(regionIndex, regionLength);
+    MOZ_ASSERT(
+        mozilla::intl::IsStructurallyValidRegionTag(region->spanOf(baseName)));
+  }
+
+  return {language, script, region};
+}
+
+static inline auto BaseNameParts(JSLinearString* baseName) {
+  JS::AutoCheckCannotGC nogc;
+  return baseName->hasLatin1Chars()
+             ? BaseNameParts(baseName->latin1Chars(nogc), baseName->length())
+             : BaseNameParts(baseName->twoByteChars(nogc), baseName->length());
+}
+
+// Intl.Locale.prototype.maximize ()
+static bool Locale_maximize(JSContext* cx, const CallArgs& args) {
+  MOZ_ASSERT(IsLocale(args.thisv()));
+
+  // Step 3.
+  auto* locale = &args.thisv().toObject().as<LocaleObject>();
+  Rooted<JSLinearString*> tagStr(cx, locale->languageTag()->ensureLinear(cx));
+  if (!tagStr) {
+    return false;
+  }
+
+  mozilla::intl::Locale tag;
+  if (!intl::ParseLocale(cx, tagStr, tag)) {
+    return false;
+  }
+
+  if (auto result = tag.AddLikelySubtags(); result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return false;
+  }
+
+  // Step 4.
+  auto* result = CreateLocaleObject(cx, nullptr, tag);
+  if (!result) {
+    return false;
+  }
+  args.rval().setObject(*result);
+  return true;
+}
+
+// Intl.Locale.prototype.maximize ()
+static bool Locale_maximize(JSContext* cx, unsigned argc, Value* vp) {
+  // Steps 1-2.
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return CallNonGenericMethod<IsLocale, Locale_maximize>(cx, args);
+}
+
+// Intl.Locale.prototype.minimize ()
+static bool Locale_minimize(JSContext* cx, const CallArgs& args) {
+  MOZ_ASSERT(IsLocale(args.thisv()));
+
+  // Step 3.
+  auto* locale = &args.thisv().toObject().as<LocaleObject>();
+  Rooted<JSLinearString*> tagStr(cx, locale->languageTag()->ensureLinear(cx));
+  if (!tagStr) {
+    return false;
+  }
+
+  mozilla::intl::Locale tag;
+  if (!intl::ParseLocale(cx, tagStr, tag)) {
+    return false;
+  }
+
+  if (auto result = tag.RemoveLikelySubtags(); result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return false;
+  }
+
+  // Step 4.
+  auto* result = CreateLocaleObject(cx, nullptr, tag);
+  if (!result) {
+    return false;
+  }
+  args.rval().setObject(*result);
+  return true;
+}
+
+// Intl.Locale.prototype.minimize ()
+static bool Locale_minimize(JSContext* cx, unsigned argc, Value* vp) {
+  // Steps 1-2.
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return CallNonGenericMethod<IsLocale, Locale_minimize>(cx, args);
+}
+
+// Intl.Locale.prototype.toString ()
+static bool Locale_toString(JSContext* cx, const CallArgs& args) {
+  MOZ_ASSERT(IsLocale(args.thisv()));
+
+  // Step 3.
+  auto* locale = &args.thisv().toObject().as<LocaleObject>();
+  args.rval().setString(locale->languageTag());
+  return true;
+}
+
+// Intl.Locale.prototype.toString ()
+static bool Locale_toString(JSContext* cx, unsigned argc, Value* vp) {
+  // Steps 1-2.
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return CallNonGenericMethod<IsLocale, Locale_toString>(cx, args);
+}
+
+// get Intl.Locale.prototype.baseName
+static bool Locale_baseName(JSContext* cx, const CallArgs& args) {
+  MOZ_ASSERT(IsLocale(args.thisv()));
+
+  // Steps 3-4.
+  auto* locale = &args.thisv().toObject().as<LocaleObject>();
+  args.rval().setString(locale->baseName());
+  return true;
+}
+
+// get Intl.Locale.prototype.baseName
+static bool Locale_baseName(JSContext* cx, unsigned argc, Value* vp) {
+  // Steps 1-2.
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return CallNonGenericMethod<IsLocale, Locale_baseName>(cx, args);
+}
+
+// get Intl.Locale.prototype.calendar
+static bool Locale_calendar(JSContext* cx, const CallArgs& args) {
+  MOZ_ASSERT(IsLocale(args.thisv()));
+
+  // Step 3.
+  auto* locale = &args.thisv().toObject().as<LocaleObject>();
+  return GetUnicodeExtension(cx, locale, "ca", args.rval());
+}
+
+// get Intl.Locale.prototype.calendar
+static bool Locale_calendar(JSContext* cx, unsigned argc, Value* vp) {
+  // Steps 1-2.
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return CallNonGenericMethod<IsLocale, Locale_calendar>(cx, args);
+}
+
+// get Intl.Locale.prototype.caseFirst
+static bool Locale_caseFirst(JSContext* cx, const CallArgs& args) {
+  MOZ_ASSERT(IsLocale(args.thisv()));
+
+  // Step 3.
+  auto* locale = &args.thisv().toObject().as<LocaleObject>();
+  return GetUnicodeExtension(cx, locale, "kf", args.rval());
+}
+
+// get Intl.Locale.prototype.caseFirst
+static bool Locale_caseFirst(JSContext* cx, unsigned argc, Value* vp) {
+  // Steps 1-2.
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return CallNonGenericMethod<IsLocale, Locale_caseFirst>(cx, args);
+}
+
+// get Intl.Locale.prototype.collation
+static bool Locale_collation(JSContext* cx, const CallArgs& args) {
+  MOZ_ASSERT(IsLocale(args.thisv()));
+
+  // Step 3.
+  auto* locale = &args.thisv().toObject().as<LocaleObject>();
+  return GetUnicodeExtension(cx, locale, "co", args.rval());
+}
+
+// get Intl.Locale.prototype.collation
+static bool Locale_collation(JSContext* cx, unsigned argc, Value* vp) {
+  // Steps 1-2.
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return CallNonGenericMethod<IsLocale, Locale_collation>(cx, args);
+}
+
+// get Intl.Locale.prototype.hourCycle
+static bool Locale_hourCycle(JSContext* cx, const CallArgs& args) {
+  MOZ_ASSERT(IsLocale(args.thisv()));
+
+  // Step 3.
+  auto* locale = &args.thisv().toObject().as<LocaleObject>();
+  return GetUnicodeExtension(cx, locale, "hc", args.rval());
+}
+
+// get Intl.Locale.prototype.hourCycle
+static bool Locale_hourCycle(JSContext* cx, unsigned argc, Value* vp) {
+  // Steps 1-2.
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return CallNonGenericMethod<IsLocale, Locale_hourCycle>(cx, args);
+}
+
+// get Intl.Locale.prototype.numeric
+static bool Locale_numeric(JSContext* cx, const CallArgs& args) {
+  MOZ_ASSERT(IsLocale(args.thisv()));
+
+  // Step 3.
+  auto* locale = &args.thisv().toObject().as<LocaleObject>();
+  RootedValue value(cx);
+  if (!GetUnicodeExtension(cx, locale, "kn", &value)) {
+    return false;
+  }
+
+  // Compare against the empty string per Intl.Locale, step 36.a. The Unicode
+  // extension is already canonicalized, so we don't need to compare against
+  // "true" at this point.
+  MOZ_ASSERT(value.isUndefined() || value.isString());
+  MOZ_ASSERT_IF(value.isString(),
+                !StringEqualsLiteral(&value.toString()->asLinear(), "true"));
+
+  args.rval().setBoolean(value.isString() && value.toString()->empty());
+  return true;
+}
+
+// get Intl.Locale.prototype.numeric
+static bool Locale_numeric(JSContext* cx, unsigned argc, Value* vp) {
+  // Steps 1-2.
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return CallNonGenericMethod<IsLocale, Locale_numeric>(cx, args);
+}
+
+// get Intl.Locale.prototype.numberingSystem
+static bool Intl_Locale_numberingSystem(JSContext* cx, const CallArgs& args) {
+  MOZ_ASSERT(IsLocale(args.thisv()));
+
+  // Step 3.
+  auto* locale = &args.thisv().toObject().as<LocaleObject>();
+  return GetUnicodeExtension(cx, locale, "nu", args.rval());
+}
+
+// get Intl.Locale.prototype.numberingSystem
+static bool Locale_numberingSystem(JSContext* cx, unsigned argc, Value* vp) {
+  // Steps 1-2.
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return CallNonGenericMethod<IsLocale, Intl_Locale_numberingSystem>(cx, args);
+}
+
+// get Intl.Locale.prototype.language
+static bool Locale_language(JSContext* cx, const CallArgs& args) {
+  MOZ_ASSERT(IsLocale(args.thisv()));
+
+  // Step 3.
+  auto* locale = &args.thisv().toObject().as<LocaleObject>();
+  JSLinearString* baseName = locale->baseName()->ensureLinear(cx);
+  if (!baseName) {
+    return false;
+  }
+
+  // Step 4 (Unnecessary assertion).
+
+  auto language = BaseNameParts(baseName).language;
+
+  size_t index = language.index;
+  size_t length = language.length;
+
+  // Step 5.
+  JSString* str = NewDependentString(cx, baseName, index, length);
+  if (!str) {
+    return false;
+  }
+
+  args.rval().setString(str);
+  return true;
+}
+
+// get Intl.Locale.prototype.language
+static bool Locale_language(JSContext* cx, unsigned argc, Value* vp) {
+  // Steps 1-2.
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return CallNonGenericMethod<IsLocale, Locale_language>(cx, args);
+}
+
+// get Intl.Locale.prototype.script
+static bool Locale_script(JSContext* cx, const CallArgs& args) {
+  MOZ_ASSERT(IsLocale(args.thisv()));
+
+  // Step 3.
+  auto* locale = &args.thisv().toObject().as<LocaleObject>();
+  JSLinearString* baseName = locale->baseName()->ensureLinear(cx);
+  if (!baseName) {
+    return false;
+  }
+
+  // Step 4 (Unnecessary assertion).
+
+  auto script = BaseNameParts(baseName).script;
+
+  // Step 5.
+  if (!script) {
+    args.rval().setUndefined();
+    return true;
+  }
+
+  size_t index = script->index;
+  size_t length = script->length;
+
+  // Step 6.
+  JSString* str = NewDependentString(cx, baseName, index, length);
+  if (!str) {
+    return false;
+  }
+
+  args.rval().setString(str);
+  return true;
+}
+
+// get Intl.Locale.prototype.script
+static bool Locale_script(JSContext* cx, unsigned argc, Value* vp) {
+  // Steps 1-2.
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return CallNonGenericMethod<IsLocale, Locale_script>(cx, args);
+}
+
+// get Intl.Locale.prototype.region
+static bool Locale_region(JSContext* cx, const CallArgs& args) {
+  MOZ_ASSERT(IsLocale(args.thisv()));
+
+  // Step 3.
+  auto* locale = &args.thisv().toObject().as<LocaleObject>();
+  JSLinearString* baseName = locale->baseName()->ensureLinear(cx);
+  if (!baseName) {
+    return false;
+  }
+
+  // Step 4 (Unnecessary assertion).
+
+  auto region = BaseNameParts(baseName).region;
+
+  // Step 5.
+  if (!region) {
+    args.rval().setUndefined();
+    return true;
+  }
+
+  size_t index = region->index;
+  size_t length = region->length;
+
+  // Step 6.
+  JSString* str = NewDependentString(cx, baseName, index, length);
+  if (!str) {
+    return false;
+  }
+
+  args.rval().setString(str);
+  return true;
+}
+
+// get Intl.Locale.prototype.region
+static bool Locale_region(JSContext* cx, unsigned argc, Value* vp) {
+  // Steps 1-2.
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return CallNonGenericMethod<IsLocale, Locale_region>(cx, args);
+}
+
+static bool Locale_toSource(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  args.rval().setString(cx->names().Locale);
+  return true;
+}
+
+static const JSFunctionSpec locale_methods[] = {
+    JS_FN("maximize", Locale_maximize, 0, 0),
+    JS_FN("minimize", Locale_minimize, 0, 0),
+    JS_FN(js_toString_str, Locale_toString, 0, 0),
+    JS_FN(js_toSource_str, Locale_toSource, 0, 0), JS_FS_END};
+
+static const JSPropertySpec locale_properties[] = {
+    JS_PSG("baseName", Locale_baseName, 0),
+    JS_PSG("calendar", Locale_calendar, 0),
+    JS_PSG("caseFirst", Locale_caseFirst, 0),
+    JS_PSG("collation", Locale_collation, 0),
+    JS_PSG("hourCycle", Locale_hourCycle, 0),
+    JS_PSG("numeric", Locale_numeric, 0),
+    JS_PSG("numberingSystem", Locale_numberingSystem, 0),
+    JS_PSG("language", Locale_language, 0),
+    JS_PSG("script", Locale_script, 0),
+    JS_PSG("region", Locale_region, 0),
+    JS_STRING_SYM_PS(toStringTag, "Intl.Locale", JSPROP_READONLY),
+    JS_PS_END};
+
+const ClassSpec LocaleObject::classSpec_ = {
+    GenericCreateConstructor<Locale, 1, gc::AllocKind::FUNCTION>,
+    GenericCreatePrototype<LocaleObject>,
+    nullptr,
+    nullptr,
+    locale_methods,
+    locale_properties,
+    nullptr,
+    ClassSpec::DontDefineConstructor};
+
+bool js::intl_ValidateAndCanonicalizeLanguageTag(JSContext* cx, unsigned argc,
+                                                 Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 2);
+
+  HandleValue tagValue = args[0];
+  bool applyToString = args[1].toBoolean();
+
+  if (tagValue.isObject()) {
+    JSString* tagStr;
+    JS_TRY_VAR_OR_RETURN_FALSE(
+        cx, tagStr,
+        LanguageTagFromMaybeWrappedLocale(cx, &tagValue.toObject()));
+    if (tagStr) {
+      args.rval().setString(tagStr);
+      return true;
+    }
+  }
+
+  if (!applyToString && !tagValue.isString()) {
+    args.rval().setNull();
+    return true;
+  }
+
+  JSString* tagStr = ToString(cx, tagValue);
+  if (!tagStr) {
+    return false;
+  }
+
+  Rooted<JSLinearString*> tagLinearStr(cx, tagStr->ensureLinear(cx));
+  if (!tagLinearStr) {
+    return false;
+  }
+
+  // Handle the common case (a standalone language) first.
+  // Only the following Unicode BCP 47 locale identifier subset is accepted:
+  //   unicode_locale_id = unicode_language_id
+  //   unicode_language_id = unicode_language_subtag
+  //   unicode_language_subtag = alpha{2,3}
+  JSString* language;
+  JS_TRY_VAR_OR_RETURN_FALSE(
+      cx, language, intl::ParseStandaloneISO639LanguageTag(cx, tagLinearStr));
+  if (language) {
+    args.rval().setString(language);
+    return true;
+  }
+
+  mozilla::intl::Locale tag;
+  if (!intl::ParseLocale(cx, tagLinearStr, tag)) {
+    return false;
+  }
+
+  auto result = tag.Canonicalize();
+  if (result.isErr()) {
+    if (result.unwrapErr() ==
+        mozilla::intl::Locale::CanonicalizationError::DuplicateVariant) {
+      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                JSMSG_DUPLICATE_VARIANT_SUBTAG);
+    } else {
+      intl::ReportInternalError(cx);
+    }
+    return false;
+  }
+
+  intl::FormatBuffer<char, intl::INITIAL_CHAR_BUFFER_SIZE> buffer(cx);
+  if (auto result = tag.ToString(buffer); result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return false;
+  }
+
+  JSString* resultStr = buffer.toAsciiString(cx);
+  if (!resultStr) {
+    return false;
+  }
+
+  args.rval().setString(resultStr);
+  return true;
+}
+
+bool js::intl_TryValidateAndCanonicalizeLanguageTag(JSContext* cx,
+                                                    unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+
+  Rooted<JSLinearString*> linear(cx, args[0].toString()->ensureLinear(cx));
+  if (!linear) {
+    return false;
+  }
+
+  mozilla::intl::Locale tag;
+  {
+    if (!StringIsAscii(linear)) {
+      // The caller handles invalid inputs.
+      args.rval().setNull();
+      return true;
+    }
+
+    intl::StringAsciiChars chars(linear);
+    if (!chars.init(cx)) {
+      return false;
+    }
+
+    if (mozilla::intl::LocaleParser::TryParse(chars, tag).isErr()) {
+      // The caller handles invalid inputs.
+      args.rval().setNull();
+      return true;
+    }
+  }
+
+  auto result = tag.Canonicalize();
+  if (result.isErr()) {
+    if (result.unwrapErr() ==
+        mozilla::intl::Locale::CanonicalizationError::DuplicateVariant) {
+      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                JSMSG_DUPLICATE_VARIANT_SUBTAG);
+    } else {
+      intl::ReportInternalError(cx);
+    }
+    return false;
+  }
+
+  intl::FormatBuffer<char, intl::INITIAL_CHAR_BUFFER_SIZE> buffer(cx);
+  if (auto result = tag.ToString(buffer); result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return false;
+  }
+
+  JSString* resultStr = buffer.toAsciiString(cx);
+  if (!resultStr) {
+    return false;
+  }
+  args.rval().setString(resultStr);
+  return true;
+}
+
+bool js::intl_ValidateAndCanonicalizeUnicodeExtensionType(JSContext* cx,
+                                                          unsigned argc,
+                                                          Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 3);
+
+  HandleValue typeArg = args[0];
+  MOZ_ASSERT(typeArg.isString(), "type must be a string");
+
+  HandleValue optionArg = args[1];
+  MOZ_ASSERT(optionArg.isString(), "option name must be a string");
+
+  HandleValue keyArg = args[2];
+  MOZ_ASSERT(keyArg.isString(), "key must be a string");
+
+  Rooted<JSLinearString*> unicodeType(cx, typeArg.toString()->ensureLinear(cx));
+  if (!unicodeType) {
+    return false;
+  }
+
+  bool isValid;
+  if (!IsValidUnicodeExtensionValue(cx, unicodeType, &isValid)) {
+    return false;
+  }
+  if (!isValid) {
+    UniqueChars optionChars = EncodeAscii(cx, optionArg.toString());
+    if (!optionChars) {
+      return false;
+    }
+
+    UniqueChars unicodeTypeChars = QuoteString(cx, unicodeType, '"');
+    if (!unicodeTypeChars) {
+      return false;
+    }
+
+    JS_ReportErrorNumberASCII(cx, js::GetErrorMessage, nullptr,
+                              JSMSG_INVALID_OPTION_VALUE, optionChars.get(),
+                              unicodeTypeChars.get());
+    return false;
+  }
+
+  char unicodeKey[UnicodeKeyLength];
+  {
+    JSLinearString* str = keyArg.toString()->ensureLinear(cx);
+    if (!str) {
+      return false;
+    }
+    MOZ_ASSERT(str->length() == UnicodeKeyLength);
+
+    for (size_t i = 0; i < UnicodeKeyLength; i++) {
+      char16_t ch = str->latin1OrTwoByteChar(i);
+      MOZ_ASSERT(mozilla::IsAscii(ch));
+      unicodeKey[i] = char(ch);
+    }
+  }
+
+  UniqueChars unicodeTypeChars = EncodeAscii(cx, unicodeType);
+  if (!unicodeTypeChars) {
+    return false;
+  }
+
+  size_t unicodeTypeLength = unicodeType->length();
+  MOZ_ASSERT(strlen(unicodeTypeChars.get()) == unicodeTypeLength);
+
+  // Convert into canonical case before searching for replacements.
+  mozilla::intl::AsciiToLowerCase(unicodeTypeChars.get(), unicodeTypeLength,
+                                  unicodeTypeChars.get());
+
+  auto key = mozilla::Span(unicodeKey, UnicodeKeyLength);
+  auto type = mozilla::Span(unicodeTypeChars.get(), unicodeTypeLength);
+
+  // Search if there's a replacement for the current Unicode keyword.
+  JSString* result;
+  if (const char* replacement =
+          mozilla::intl::Locale::ReplaceUnicodeExtensionType(key, type)) {
+    result = NewStringCopyZ<CanGC>(cx, replacement);
+  } else {
+    result = StringToLowerCase(cx, unicodeType);
+  }
+  if (!result) {
+    return false;
+  }
+
+  args.rval().setString(result);
+  return true;
+}
diff --git a/js/src/builtin/intl/Locale.h b/js/src/builtin/intl/Locale.h
new file mode 100644
index 0000000000..93b618528a
--- /dev/null
+++ b/js/src/builtin/intl/Locale.h
@@ -0,0 +1,61 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_Locale_h
+#define builtin_intl_Locale_h
+
+#include <stdint.h>
+
+#include "js/Class.h"
+#include "vm/NativeObject.h"
+
+namespace js {
+
+class LocaleObject : public NativeObject {
+ public:
+  static const JSClass class_;
+  static const JSClass& protoClass_;
+
+  static constexpr uint32_t LANGUAGE_TAG_SLOT = 0;
+  static constexpr uint32_t BASENAME_SLOT = 1;
+  static constexpr uint32_t UNICODE_EXTENSION_SLOT = 2;
+  static constexpr uint32_t SLOT_COUNT = 3;
+
+  /**
+   * Returns the complete language tag, including any extensions and privateuse
+   * subtags.
+   */
+  JSString* languageTag() const {
+    return getFixedSlot(LANGUAGE_TAG_SLOT).toString();
+  }
+
+  /**
+   * Returns the basename subtags, i.e. excluding any extensions and privateuse
+   * subtags.
+   */
+  JSString* baseName() const { return getFixedSlot(BASENAME_SLOT).toString(); }
+
+  const Value& unicodeExtension() const {
+    return getFixedSlot(UNICODE_EXTENSION_SLOT);
+  }
+
+ private:
+  static const ClassSpec classSpec_;
+};
+
+[[nodiscard]] extern bool intl_ValidateAndCanonicalizeLanguageTag(JSContext* cx,
+                                                                  unsigned argc,
+                                                                  Value* vp);
+
+[[nodiscard]] extern bool intl_TryValidateAndCanonicalizeLanguageTag(
+    JSContext* cx, unsigned argc, Value* vp);
+
+[[nodiscard]] extern bool intl_ValidateAndCanonicalizeUnicodeExtensionType(
+    JSContext* cx, unsigned argc, Value* vp);
+
+}  // namespace js
+
+#endif /* builtin_intl_Locale_h */
diff --git a/js/src/builtin/intl/NumberFormat.cpp b/js/src/builtin/intl/NumberFormat.cpp
new file mode 100644
index 0000000000..21b0db030e
--- /dev/null
+++ b/js/src/builtin/intl/NumberFormat.cpp
@@ -0,0 +1,1374 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Intl.NumberFormat implementation. */
+
+#include "builtin/intl/NumberFormat.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Casting.h"
+#include "mozilla/FloatingPoint.h"
+#include "mozilla/intl/Locale.h"
+#include "mozilla/intl/MeasureUnit.h"
+#include "mozilla/intl/MeasureUnitGenerated.h"
+#include "mozilla/intl/NumberFormat.h"
+#include "mozilla/intl/NumberingSystem.h"
+#include "mozilla/intl/NumberRangeFormat.h"
+#include "mozilla/Span.h"
+#include "mozilla/TextUtils.h"
+#include "mozilla/UniquePtr.h"
+
+#include <algorithm>
+#include <stddef.h>
+#include <stdint.h>
+#include <string>
+#include <string_view>
+#include <type_traits>
+
+#include "builtin/Array.h"
+#include "builtin/intl/CommonFunctions.h"
+#include "builtin/intl/DecimalNumber.h"
+#include "builtin/intl/FormatBuffer.h"
+#include "builtin/intl/LanguageTag.h"
+#include "builtin/intl/RelativeTimeFormat.h"
+#include "gc/GCContext.h"
+#include "js/CharacterEncoding.h"
+#include "js/PropertySpec.h"
+#include "js/RootingAPI.h"
+#include "js/TypeDecls.h"
+#include "util/Text.h"
+#include "vm/BigIntType.h"
+#include "vm/GlobalObject.h"
+#include "vm/JSContext.h"
+#include "vm/PlainObject.h"  // js::PlainObject
+#include "vm/StringType.h"
+#include "vm/WellKnownAtom.h"  // js_*_str
+
+#include "vm/GeckoProfiler-inl.h"
+#include "vm/JSObject-inl.h"
+#include "vm/NativeObject-inl.h"
+
+using namespace js;
+
+using mozilla::AssertedCast;
+
+using js::intl::DateTimeFormatOptions;
+using js::intl::FieldType;
+
+const JSClassOps NumberFormatObject::classOps_ = {
+    nullptr,                       // addProperty
+    nullptr,                       // delProperty
+    nullptr,                       // enumerate
+    nullptr,                       // newEnumerate
+    nullptr,                       // resolve
+    nullptr,                       // mayResolve
+    NumberFormatObject::finalize,  // finalize
+    nullptr,                       // call
+    nullptr,                       // construct
+    nullptr,                       // trace
+};
+
+const JSClass NumberFormatObject::class_ = {
+    "Intl.NumberFormat",
+    JSCLASS_HAS_RESERVED_SLOTS(NumberFormatObject::SLOT_COUNT) |
+        JSCLASS_HAS_CACHED_PROTO(JSProto_NumberFormat) |
+        JSCLASS_FOREGROUND_FINALIZE,
+    &NumberFormatObject::classOps_, &NumberFormatObject::classSpec_};
+
+const JSClass& NumberFormatObject::protoClass_ = PlainObject::class_;
+
+static bool numberFormat_toSource(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  args.rval().setString(cx->names().NumberFormat);
+  return true;
+}
+
+static const JSFunctionSpec numberFormat_static_methods[] = {
+    JS_SELF_HOSTED_FN("supportedLocalesOf",
+                      "Intl_NumberFormat_supportedLocalesOf", 1, 0),
+    JS_FS_END,
+};
+
+static const JSFunctionSpec numberFormat_methods[] = {
+    JS_SELF_HOSTED_FN("resolvedOptions", "Intl_NumberFormat_resolvedOptions", 0,
+                      0),
+    JS_SELF_HOSTED_FN("formatToParts", "Intl_NumberFormat_formatToParts", 1, 0),
+#ifdef NIGHTLY_BUILD
+    JS_SELF_HOSTED_FN("formatRange", "Intl_NumberFormat_formatRange", 2, 0),
+    JS_SELF_HOSTED_FN("formatRangeToParts",
+                      "Intl_NumberFormat_formatRangeToParts", 2, 0),
+#endif
+    JS_FN(js_toSource_str, numberFormat_toSource, 0, 0),
+    JS_FS_END,
+};
+
+static const JSPropertySpec numberFormat_properties[] = {
+    JS_SELF_HOSTED_GET("format", "$Intl_NumberFormat_format_get", 0),
+    JS_STRING_SYM_PS(toStringTag, "Intl.NumberFormat", JSPROP_READONLY),
+    JS_PS_END,
+};
+
+static bool NumberFormat(JSContext* cx, unsigned argc, Value* vp);
+
+const ClassSpec NumberFormatObject::classSpec_ = {
+    GenericCreateConstructor<NumberFormat, 0, gc::AllocKind::FUNCTION>,
+    GenericCreatePrototype<NumberFormatObject>,
+    numberFormat_static_methods,
+    nullptr,
+    numberFormat_methods,
+    numberFormat_properties,
+    nullptr,
+    ClassSpec::DontDefineConstructor};
+
+/**
+ * 11.2.1 Intl.NumberFormat([ locales [, options]])
+ *
+ * ES2017 Intl draft rev 94045d234762ad107a3d09bb6f7381a65f1a2f9b
+ */
+static bool NumberFormat(JSContext* cx, const CallArgs& args, bool construct) {
+  AutoJSConstructorProfilerEntry pseudoFrame(cx, "Intl.NumberFormat");
+
+  // Step 1 (Handled by OrdinaryCreateFromConstructor fallback code).
+
+  // Step 2 (Inlined 9.1.14, OrdinaryCreateFromConstructor).
+  RootedObject proto(cx);
+  if (!GetPrototypeFromBuiltinConstructor(cx, args, JSProto_NumberFormat,
+                                          &proto)) {
+    return false;
+  }
+
+  Rooted<NumberFormatObject*> numberFormat(cx);
+  numberFormat = NewObjectWithClassProto<NumberFormatObject>(cx, proto);
+  if (!numberFormat) {
+    return false;
+  }
+
+  RootedValue thisValue(cx,
+                        construct ? ObjectValue(*numberFormat) : args.thisv());
+  HandleValue locales = args.get(0);
+  HandleValue options = args.get(1);
+
+  // Step 3.
+  return intl::LegacyInitializeObject(
+      cx, numberFormat, cx->names().InitializeNumberFormat, thisValue, locales,
+      options, DateTimeFormatOptions::Standard, args.rval());
+}
+
+static bool NumberFormat(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  return NumberFormat(cx, args, args.isConstructing());
+}
+
+bool js::intl_NumberFormat(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 2);
+  MOZ_ASSERT(!args.isConstructing());
+  // intl_NumberFormat is an intrinsic for self-hosted JavaScript, so it
+  // cannot be used with "new", but it still has to be treated as a
+  // constructor.
+  return NumberFormat(cx, args, true);
+}
+
+void js::NumberFormatObject::finalize(JS::GCContext* gcx, JSObject* obj) {
+  MOZ_ASSERT(gcx->onMainThread());
+
+  auto* numberFormat = &obj->as<NumberFormatObject>();
+  mozilla::intl::NumberFormat* nf = numberFormat->getNumberFormatter();
+  mozilla::intl::NumberRangeFormat* nrf =
+      numberFormat->getNumberRangeFormatter();
+
+  if (nf) {
+    intl::RemoveICUCellMemory(gcx, obj, NumberFormatObject::EstimatedMemoryUse);
+    // This was allocated using `new` in mozilla::intl::NumberFormat, so we
+    // delete here.
+    delete nf;
+  }
+
+  if (nrf) {
+    intl::RemoveICUCellMemory(gcx, obj, EstimatedRangeFormatterMemoryUse);
+    // This was allocated using `new` in mozilla::intl::NumberRangeFormat, so we
+    // delete here.
+    delete nrf;
+  }
+}
+
+bool js::intl_numberingSystem(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+  MOZ_ASSERT(args[0].isString());
+
+  UniqueChars locale = intl::EncodeLocale(cx, args[0].toString());
+  if (!locale) {
+    return false;
+  }
+
+  auto numberingSystem =
+      mozilla::intl::NumberingSystem::TryCreate(locale.get());
+  if (numberingSystem.isErr()) {
+    intl::ReportInternalError(cx, numberingSystem.unwrapErr());
+    return false;
+  }
+
+  auto name = numberingSystem.inspect()->GetName();
+  if (name.isErr()) {
+    intl::ReportInternalError(cx, name.unwrapErr());
+    return false;
+  }
+
+  JSString* jsname = NewStringCopy<CanGC>(cx, name.unwrap());
+  if (!jsname) {
+    return false;
+  }
+
+  args.rval().setString(jsname);
+  return true;
+}
+
+#if DEBUG || MOZ_SYSTEM_ICU
+bool js::intl_availableMeasurementUnits(JSContext* cx, unsigned argc,
+                                        Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 0);
+
+  RootedObject measurementUnits(cx, NewPlainObjectWithProto(cx, nullptr));
+  if (!measurementUnits) {
+    return false;
+  }
+
+  auto units = mozilla::intl::MeasureUnit::GetAvailable();
+  if (units.isErr()) {
+    intl::ReportInternalError(cx, units.unwrapErr());
+    return false;
+  }
+
+  Rooted<JSAtom*> unitAtom(cx);
+  for (auto unit : units.unwrap()) {
+    if (unit.isErr()) {
+      intl::ReportInternalError(cx);
+      return false;
+    }
+    auto unitIdentifier = unit.unwrap();
+
+    unitAtom = Atomize(cx, unitIdentifier.data(), unitIdentifier.size());
+    if (!unitAtom) {
+      return false;
+    }
+
+    if (!DefineDataProperty(cx, measurementUnits, unitAtom->asPropertyName(),
+                            TrueHandleValue)) {
+      return false;
+    }
+  }
+
+  args.rval().setObject(*measurementUnits);
+  return true;
+}
+#endif
+
+static constexpr size_t MaxUnitLength() {
+  size_t length = 0;
+  for (const auto& unit : mozilla::intl::simpleMeasureUnits) {
+    length = std::max(length, std::char_traits<char>::length(unit.name));
+  }
+  return length * 2 + std::char_traits<char>::length("-per-");
+}
+
+static UniqueChars NumberFormatLocale(JSContext* cx, HandleObject internals) {
+  RootedValue value(cx);
+  if (!GetProperty(cx, internals, internals, cx->names().locale, &value)) {
+    return nullptr;
+  }
+
+  // ICU expects numberingSystem as a Unicode locale extensions on locale.
+
+  mozilla::intl::Locale tag;
+  {
+    Rooted<JSLinearString*> locale(cx, value.toString()->ensureLinear(cx));
+    if (!locale) {
+      return nullptr;
+    }
+
+    if (!intl::ParseLocale(cx, locale, tag)) {
+      return nullptr;
+    }
+  }
+
+  JS::RootedVector<intl::UnicodeExtensionKeyword> keywords(cx);
+
+  if (!GetProperty(cx, internals, internals, cx->names().numberingSystem,
+                   &value)) {
+    return nullptr;
+  }
+
+  {
+    JSLinearString* numberingSystem = value.toString()->ensureLinear(cx);
+    if (!numberingSystem) {
+      return nullptr;
+    }
+
+    if (!keywords.emplaceBack("nu", numberingSystem)) {
+      return nullptr;
+    }
+  }
+
+  // |ApplyUnicodeExtensionToTag| applies the new keywords to the front of
+  // the Unicode extension subtag. We're then relying on ICU to follow RFC
+  // 6067, which states that any trailing keywords using the same key
+  // should be ignored.
+  if (!intl::ApplyUnicodeExtensionToTag(cx, tag, keywords)) {
+    return nullptr;
+  }
+
+  intl::FormatBuffer<char> buffer(cx);
+  if (auto result = tag.ToString(buffer); result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return nullptr;
+  }
+  return buffer.extractStringZ();
+}
+
+struct NumberFormatOptions : public mozilla::intl::NumberRangeFormatOptions {
+  static_assert(std::is_base_of_v<mozilla::intl::NumberFormatOptions,
+                                  mozilla::intl::NumberRangeFormatOptions>);
+
+  char currencyChars[3] = {};
+  char unitChars[MaxUnitLength()] = {};
+};
+
+static bool FillNumberFormatOptions(JSContext* cx, HandleObject internals,
+                                    NumberFormatOptions& options) {
+  RootedValue value(cx);
+  if (!GetProperty(cx, internals, internals, cx->names().style, &value)) {
+    return false;
+  }
+
+  bool accountingSign = false;
+  {
+    JSLinearString* style = value.toString()->ensureLinear(cx);
+    if (!style) {
+      return false;
+    }
+
+    if (StringEqualsLiteral(style, "currency")) {
+      if (!GetProperty(cx, internals, internals, cx->names().currency,
+                       &value)) {
+        return false;
+      }
+      JSLinearString* currency = value.toString()->ensureLinear(cx);
+      if (!currency) {
+        return false;
+      }
+
+      MOZ_RELEASE_ASSERT(
+          currency->length() == 3,
+          "IsWellFormedCurrencyCode permits only length-3 strings");
+      MOZ_ASSERT(StringIsAscii(currency),
+                 "IsWellFormedCurrencyCode permits only ASCII strings");
+      CopyChars(reinterpret_cast<Latin1Char*>(options.currencyChars),
+                *currency);
+
+      if (!GetProperty(cx, internals, internals, cx->names().currencyDisplay,
+                       &value)) {
+        return false;
+      }
+      JSLinearString* currencyDisplay = value.toString()->ensureLinear(cx);
+      if (!currencyDisplay) {
+        return false;
+      }
+
+      using CurrencyDisplay =
+          mozilla::intl::NumberFormatOptions::CurrencyDisplay;
+
+      CurrencyDisplay display;
+      if (StringEqualsLiteral(currencyDisplay, "code")) {
+        display = CurrencyDisplay::Code;
+      } else if (StringEqualsLiteral(currencyDisplay, "symbol")) {
+        display = CurrencyDisplay::Symbol;
+      } else if (StringEqualsLiteral(currencyDisplay, "narrowSymbol")) {
+        display = CurrencyDisplay::NarrowSymbol;
+      } else {
+        MOZ_ASSERT(StringEqualsLiteral(currencyDisplay, "name"));
+        display = CurrencyDisplay::Name;
+      }
+
+      if (!GetProperty(cx, internals, internals, cx->names().currencySign,
+                       &value)) {
+        return false;
+      }
+      JSLinearString* currencySign = value.toString()->ensureLinear(cx);
+      if (!currencySign) {
+        return false;
+      }
+
+      if (StringEqualsLiteral(currencySign, "accounting")) {
+        accountingSign = true;
+      } else {
+        MOZ_ASSERT(StringEqualsLiteral(currencySign, "standard"));
+      }
+
+      options.mCurrency = mozilla::Some(
+          std::make_pair(std::string_view(options.currencyChars, 3), display));
+    } else if (StringEqualsLiteral(style, "percent")) {
+      options.mPercent = true;
+    } else if (StringEqualsLiteral(style, "unit")) {
+      if (!GetProperty(cx, internals, internals, cx->names().unit, &value)) {
+        return false;
+      }
+      JSLinearString* unit = value.toString()->ensureLinear(cx);
+      if (!unit) {
+        return false;
+      }
+
+      size_t unit_str_length = unit->length();
+
+      MOZ_ASSERT(StringIsAscii(unit));
+      MOZ_RELEASE_ASSERT(unit_str_length <= MaxUnitLength());
+      CopyChars(reinterpret_cast<Latin1Char*>(options.unitChars), *unit);
+
+      if (!GetProperty(cx, internals, internals, cx->names().unitDisplay,
+                       &value)) {
+        return false;
+      }
+      JSLinearString* unitDisplay = value.toString()->ensureLinear(cx);
+      if (!unitDisplay) {
+        return false;
+      }
+
+      using UnitDisplay = mozilla::intl::NumberFormatOptions::UnitDisplay;
+
+      UnitDisplay display;
+      if (StringEqualsLiteral(unitDisplay, "short")) {
+        display = UnitDisplay::Short;
+      } else if (StringEqualsLiteral(unitDisplay, "narrow")) {
+        display = UnitDisplay::Narrow;
+      } else {
+        MOZ_ASSERT(StringEqualsLiteral(unitDisplay, "long"));
+        display = UnitDisplay::Long;
+      }
+
+      options.mUnit = mozilla::Some(std::make_pair(
+          std::string_view(options.unitChars, unit_str_length), display));
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(style, "decimal"));
+    }
+  }
+
+  bool hasMinimumSignificantDigits;
+  if (!HasProperty(cx, internals, cx->names().minimumSignificantDigits,
+                   &hasMinimumSignificantDigits)) {
+    return false;
+  }
+
+  if (hasMinimumSignificantDigits) {
+    if (!GetProperty(cx, internals, internals,
+                     cx->names().minimumSignificantDigits, &value)) {
+      return false;
+    }
+    uint32_t minimumSignificantDigits = AssertedCast<uint32_t>(value.toInt32());
+
+    if (!GetProperty(cx, internals, internals,
+                     cx->names().maximumSignificantDigits, &value)) {
+      return false;
+    }
+    uint32_t maximumSignificantDigits = AssertedCast<uint32_t>(value.toInt32());
+
+    options.mSignificantDigits = mozilla::Some(
+        std::make_pair(minimumSignificantDigits, maximumSignificantDigits));
+  }
+
+  bool hasMinimumFractionDigits;
+  if (!HasProperty(cx, internals, cx->names().minimumFractionDigits,
+                   &hasMinimumFractionDigits)) {
+    return false;
+  }
+
+  if (hasMinimumFractionDigits) {
+    if (!GetProperty(cx, internals, internals,
+                     cx->names().minimumFractionDigits, &value)) {
+      return false;
+    }
+    uint32_t minimumFractionDigits = AssertedCast<uint32_t>(value.toInt32());
+
+    if (!GetProperty(cx, internals, internals,
+                     cx->names().maximumFractionDigits, &value)) {
+      return false;
+    }
+    uint32_t maximumFractionDigits = AssertedCast<uint32_t>(value.toInt32());
+
+    options.mFractionDigits = mozilla::Some(
+        std::make_pair(minimumFractionDigits, maximumFractionDigits));
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().roundingPriority,
+                   &value)) {
+    return false;
+  }
+
+  {
+    JSLinearString* roundingPriority = value.toString()->ensureLinear(cx);
+    if (!roundingPriority) {
+      return false;
+    }
+
+    using RoundingPriority =
+        mozilla::intl::NumberFormatOptions::RoundingPriority;
+
+    RoundingPriority priority;
+    if (StringEqualsLiteral(roundingPriority, "auto")) {
+      priority = RoundingPriority::Auto;
+    } else if (StringEqualsLiteral(roundingPriority, "morePrecision")) {
+      priority = RoundingPriority::MorePrecision;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(roundingPriority, "lessPrecision"));
+      priority = RoundingPriority::LessPrecision;
+    }
+
+    options.mRoundingPriority = priority;
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().minimumIntegerDigits,
+                   &value)) {
+    return false;
+  }
+  options.mMinIntegerDigits =
+      mozilla::Some(AssertedCast<uint32_t>(value.toInt32()));
+
+  if (!GetProperty(cx, internals, internals, cx->names().useGrouping, &value)) {
+    return false;
+  }
+
+  if (value.isString()) {
+    JSLinearString* useGrouping = value.toString()->ensureLinear(cx);
+    if (!useGrouping) {
+      return false;
+    }
+
+    using Grouping = mozilla::intl::NumberFormatOptions::Grouping;
+
+    Grouping grouping;
+    if (StringEqualsLiteral(useGrouping, "auto")) {
+      grouping = Grouping::Auto;
+    } else if (StringEqualsLiteral(useGrouping, "always")) {
+      grouping = Grouping::Always;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(useGrouping, "min2"));
+      grouping = Grouping::Min2;
+    }
+
+    options.mGrouping = grouping;
+  } else {
+    MOZ_ASSERT(value.isBoolean());
+#ifdef NIGHTLY_BUILD
+    // The caller passes the string "always" instead of |true| when the
+    // NumberFormat V3 spec is being used.
+    MOZ_ASSERT(value.toBoolean() == false);
+#endif
+
+    using Grouping = mozilla::intl::NumberFormatOptions::Grouping;
+
+    Grouping grouping;
+    if (value.toBoolean()) {
+      grouping = Grouping::Auto;
+    } else {
+      grouping = Grouping::Never;
+    }
+
+    options.mGrouping = grouping;
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().notation, &value)) {
+    return false;
+  }
+
+  {
+    JSLinearString* notation = value.toString()->ensureLinear(cx);
+    if (!notation) {
+      return false;
+    }
+
+    using Notation = mozilla::intl::NumberFormatOptions::Notation;
+
+    Notation style;
+    if (StringEqualsLiteral(notation, "standard")) {
+      style = Notation::Standard;
+    } else if (StringEqualsLiteral(notation, "scientific")) {
+      style = Notation::Scientific;
+    } else if (StringEqualsLiteral(notation, "engineering")) {
+      style = Notation::Engineering;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(notation, "compact"));
+
+      if (!GetProperty(cx, internals, internals, cx->names().compactDisplay,
+                       &value)) {
+        return false;
+      }
+
+      JSLinearString* compactDisplay = value.toString()->ensureLinear(cx);
+      if (!compactDisplay) {
+        return false;
+      }
+
+      if (StringEqualsLiteral(compactDisplay, "short")) {
+        style = Notation::CompactShort;
+      } else {
+        MOZ_ASSERT(StringEqualsLiteral(compactDisplay, "long"));
+        style = Notation::CompactLong;
+      }
+    }
+
+    options.mNotation = style;
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().signDisplay, &value)) {
+    return false;
+  }
+
+  {
+    JSLinearString* signDisplay = value.toString()->ensureLinear(cx);
+    if (!signDisplay) {
+      return false;
+    }
+
+    using SignDisplay = mozilla::intl::NumberFormatOptions::SignDisplay;
+
+    SignDisplay display;
+    if (StringEqualsLiteral(signDisplay, "auto")) {
+      if (accountingSign) {
+        display = SignDisplay::Accounting;
+      } else {
+        display = SignDisplay::Auto;
+      }
+    } else if (StringEqualsLiteral(signDisplay, "never")) {
+      display = SignDisplay::Never;
+    } else if (StringEqualsLiteral(signDisplay, "always")) {
+      if (accountingSign) {
+        display = SignDisplay::AccountingAlways;
+      } else {
+        display = SignDisplay::Always;
+      }
+    } else if (StringEqualsLiteral(signDisplay, "exceptZero")) {
+      if (accountingSign) {
+        display = SignDisplay::AccountingExceptZero;
+      } else {
+        display = SignDisplay::ExceptZero;
+      }
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(signDisplay, "negative"));
+      if (accountingSign) {
+        display = SignDisplay::AccountingNegative;
+      } else {
+        display = SignDisplay::Negative;
+      }
+    }
+
+    options.mSignDisplay = display;
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().roundingIncrement,
+                   &value)) {
+    return false;
+  }
+  options.mRoundingIncrement = AssertedCast<uint32_t>(value.toInt32());
+
+  if (!GetProperty(cx, internals, internals, cx->names().roundingMode,
+                   &value)) {
+    return false;
+  }
+
+  {
+    JSLinearString* roundingMode = value.toString()->ensureLinear(cx);
+    if (!roundingMode) {
+      return false;
+    }
+
+    using RoundingMode = mozilla::intl::NumberFormatOptions::RoundingMode;
+
+    RoundingMode rounding;
+    if (StringEqualsLiteral(roundingMode, "halfExpand")) {
+      // "halfExpand" is the default mode, so we handle it first.
+      rounding = RoundingMode::HalfExpand;
+    } else if (StringEqualsLiteral(roundingMode, "ceil")) {
+      rounding = RoundingMode::Ceil;
+    } else if (StringEqualsLiteral(roundingMode, "floor")) {
+      rounding = RoundingMode::Floor;
+    } else if (StringEqualsLiteral(roundingMode, "expand")) {
+      rounding = RoundingMode::Expand;
+    } else if (StringEqualsLiteral(roundingMode, "trunc")) {
+      rounding = RoundingMode::Trunc;
+    } else if (StringEqualsLiteral(roundingMode, "halfCeil")) {
+      rounding = RoundingMode::HalfCeil;
+    } else if (StringEqualsLiteral(roundingMode, "halfFloor")) {
+      rounding = RoundingMode::HalfFloor;
+    } else if (StringEqualsLiteral(roundingMode, "halfTrunc")) {
+      rounding = RoundingMode::HalfTrunc;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(roundingMode, "halfEven"));
+      rounding = RoundingMode::HalfEven;
+    }
+
+    options.mRoundingMode = rounding;
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().trailingZeroDisplay,
+                   &value)) {
+    return false;
+  }
+
+  {
+    JSLinearString* trailingZeroDisplay = value.toString()->ensureLinear(cx);
+    if (!trailingZeroDisplay) {
+      return false;
+    }
+
+    if (StringEqualsLiteral(trailingZeroDisplay, "auto")) {
+      options.mStripTrailingZero = false;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(trailingZeroDisplay, "stripIfInteger"));
+      options.mStripTrailingZero = true;
+    }
+  }
+
+  return true;
+}
+
+/**
+ * Returns a new mozilla::intl::Number[Range]Format with the locale and number
+ * formatting options of the given NumberFormat, or a nullptr if
+ * initialization failed.
+ */
+template <class Formatter>
+static Formatter* NewNumberFormat(JSContext* cx,
+                                  Handle<NumberFormatObject*> numberFormat) {
+  RootedObject internals(cx, intl::GetInternalsObject(cx, numberFormat));
+  if (!internals) {
+    return nullptr;
+  }
+
+  UniqueChars locale = NumberFormatLocale(cx, internals);
+  if (!locale) {
+    return nullptr;
+  }
+
+  NumberFormatOptions options;
+  if (!FillNumberFormatOptions(cx, internals, options)) {
+    return nullptr;
+  }
+
+  options.mRangeCollapse = NumberFormatOptions::RangeCollapse::Auto;
+  options.mRangeIdentityFallback =
+      NumberFormatOptions::RangeIdentityFallback::Approximately;
+
+  mozilla::Result<mozilla::UniquePtr<Formatter>, mozilla::intl::ICUError>
+      result = Formatter::TryCreate(locale.get(), options);
+
+  if (result.isOk()) {
+    return result.unwrap().release();
+  }
+
+  intl::ReportInternalError(cx, result.unwrapErr());
+  return nullptr;
+}
+
+static mozilla::intl::NumberFormat* GetOrCreateNumberFormat(
+    JSContext* cx, Handle<NumberFormatObject*> numberFormat) {
+  // Obtain a cached mozilla::intl::NumberFormat object.
+  mozilla::intl::NumberFormat* nf = numberFormat->getNumberFormatter();
+  if (nf) {
+    return nf;
+  }
+
+  nf = NewNumberFormat<mozilla::intl::NumberFormat>(cx, numberFormat);
+  if (!nf) {
+    return nullptr;
+  }
+  numberFormat->setNumberFormatter(nf);
+
+  intl::AddICUCellMemory(numberFormat, NumberFormatObject::EstimatedMemoryUse);
+  return nf;
+}
+
+static mozilla::intl::NumberRangeFormat* GetOrCreateNumberRangeFormat(
+    JSContext* cx, Handle<NumberFormatObject*> numberFormat) {
+  // Obtain a cached mozilla::intl::NumberRangeFormat object.
+  mozilla::intl::NumberRangeFormat* nrf =
+      numberFormat->getNumberRangeFormatter();
+  if (nrf) {
+    return nrf;
+  }
+
+  nrf = NewNumberFormat<mozilla::intl::NumberRangeFormat>(cx, numberFormat);
+  if (!nrf) {
+    return nullptr;
+  }
+  numberFormat->setNumberRangeFormatter(nrf);
+
+  intl::AddICUCellMemory(numberFormat,
+                         NumberFormatObject::EstimatedRangeFormatterMemoryUse);
+  return nrf;
+}
+
+static FieldType GetFieldTypeForNumberPartType(
+    mozilla::intl::NumberPartType type) {
+  switch (type) {
+    case mozilla::intl::NumberPartType::ApproximatelySign:
+      return &JSAtomState::approximatelySign;
+    case mozilla::intl::NumberPartType::Compact:
+      return &JSAtomState::compact;
+    case mozilla::intl::NumberPartType::Currency:
+      return &JSAtomState::currency;
+    case mozilla::intl::NumberPartType::Decimal:
+      return &JSAtomState::decimal;
+    case mozilla::intl::NumberPartType::ExponentInteger:
+      return &JSAtomState::exponentInteger;
+    case mozilla::intl::NumberPartType::ExponentMinusSign:
+      return &JSAtomState::exponentMinusSign;
+    case mozilla::intl::NumberPartType::ExponentSeparator:
+      return &JSAtomState::exponentSeparator;
+    case mozilla::intl::NumberPartType::Fraction:
+      return &JSAtomState::fraction;
+    case mozilla::intl::NumberPartType::Group:
+      return &JSAtomState::group;
+    case mozilla::intl::NumberPartType::Infinity:
+      return &JSAtomState::infinity;
+    case mozilla::intl::NumberPartType::Integer:
+      return &JSAtomState::integer;
+    case mozilla::intl::NumberPartType::Literal:
+      return &JSAtomState::literal;
+    case mozilla::intl::NumberPartType::MinusSign:
+      return &JSAtomState::minusSign;
+    case mozilla::intl::NumberPartType::Nan:
+      return &JSAtomState::nan;
+    case mozilla::intl::NumberPartType::Percent:
+      return &JSAtomState::percentSign;
+    case mozilla::intl::NumberPartType::PlusSign:
+      return &JSAtomState::plusSign;
+    case mozilla::intl::NumberPartType::Unit:
+      return &JSAtomState::unit;
+  }
+
+  MOZ_ASSERT_UNREACHABLE(
+      "unenumerated, undocumented format field returned by iterator");
+  return nullptr;
+}
+
+static FieldType GetFieldTypeForNumberPartSource(
+    mozilla::intl::NumberPartSource source) {
+  switch (source) {
+    case mozilla::intl::NumberPartSource::Shared:
+      return &JSAtomState::shared;
+    case mozilla::intl::NumberPartSource::Start:
+      return &JSAtomState::startRange;
+    case mozilla::intl::NumberPartSource::End:
+      return &JSAtomState::endRange;
+  }
+
+  MOZ_CRASH("unexpected number part source");
+}
+
+enum class DisplayNumberPartSource : bool { No, Yes };
+
+static bool FormattedNumberToParts(JSContext* cx, HandleString str,
+                                   const mozilla::intl::NumberPartVector& parts,
+                                   DisplayNumberPartSource displaySource,
+                                   FieldType unitType,
+                                   MutableHandleValue result) {
+  size_t lastEndIndex = 0;
+
+  RootedObject singlePart(cx);
+  RootedValue propVal(cx);
+
+  Rooted<ArrayObject*> partsArray(
+      cx, NewDenseFullyAllocatedArray(cx, parts.length()));
+  if (!partsArray) {
+    return false;
+  }
+  partsArray->ensureDenseInitializedLength(0, parts.length());
+
+  size_t index = 0;
+  for (const auto& part : parts) {
+    FieldType type = GetFieldTypeForNumberPartType(part.type);
+    size_t endIndex = part.endIndex;
+
+    MOZ_ASSERT(lastEndIndex < endIndex);
+
+    singlePart = NewPlainObject(cx);
+    if (!singlePart) {
+      return false;
+    }
+
+    propVal.setString(cx->names().*type);
+    if (!DefineDataProperty(cx, singlePart, cx->names().type, propVal)) {
+      return false;
+    }
+
+    JSLinearString* partSubstr =
+        NewDependentString(cx, str, lastEndIndex, endIndex - lastEndIndex);
+    if (!partSubstr) {
+      return false;
+    }
+
+    propVal.setString(partSubstr);
+    if (!DefineDataProperty(cx, singlePart, cx->names().value, propVal)) {
+      return false;
+    }
+
+    if (displaySource == DisplayNumberPartSource::Yes) {
+      FieldType source = GetFieldTypeForNumberPartSource(part.source);
+
+      propVal.setString(cx->names().*source);
+      if (!DefineDataProperty(cx, singlePart, cx->names().source, propVal)) {
+        return false;
+      }
+    }
+
+    if (unitType != nullptr && type != &JSAtomState::literal) {
+      propVal.setString(cx->names().*unitType);
+      if (!DefineDataProperty(cx, singlePart, cx->names().unit, propVal)) {
+        return false;
+      }
+    }
+
+    partsArray->initDenseElement(index++, ObjectValue(*singlePart));
+
+    lastEndIndex = endIndex;
+  }
+
+  MOZ_ASSERT(index == parts.length());
+  MOZ_ASSERT(lastEndIndex == str->length(),
+             "result array must partition the entire string");
+
+  result.setObject(*partsArray);
+  return true;
+}
+
+bool js::intl::FormattedRelativeTimeToParts(
+    JSContext* cx, HandleString str,
+    const mozilla::intl::NumberPartVector& parts, FieldType relativeTimeUnit,
+    MutableHandleValue result) {
+  return FormattedNumberToParts(cx, str, parts, DisplayNumberPartSource::No,
+                                relativeTimeUnit, result);
+}
+
+// Return true if the string starts with "0[bBoOxX]", possibly skipping over
+// leading whitespace.
+template <typename CharT>
+static bool IsNonDecimalNumber(mozilla::Range<const CharT> chars) {
+  const CharT* end = chars.begin().get() + chars.length();
+  const CharT* start = SkipSpace(chars.begin().get(), end);
+
+  if (end - start >= 2 && start[0] == '0') {
+    CharT ch = start[1];
+    return ch == 'b' || ch == 'B' || ch == 'o' || ch == 'O' || ch == 'x' ||
+           ch == 'X';
+  }
+  return false;
+}
+
+static bool IsNonDecimalNumber(JSLinearString* str) {
+  JS::AutoCheckCannotGC nogc;
+  return str->hasLatin1Chars() ? IsNonDecimalNumber(str->latin1Range(nogc))
+                               : IsNonDecimalNumber(str->twoByteRange(nogc));
+}
+
+static bool ToIntlMathematicalValue(JSContext* cx, MutableHandleValue value) {
+  if (!ToPrimitive(cx, JSTYPE_NUMBER, value)) {
+    return false;
+  }
+
+  // Maximum exponent supported by ICU. Exponents larger than this value will
+  // cause ICU to report an error.
+  // See also "intl/icu/source/i18n/decContext.h".
+  constexpr int32_t maximumExponent = 999'999'999;
+
+  // We further limit the maximum positive exponent to avoid spending multiple
+  // seconds or even minutes in ICU when formatting large numbers.
+  constexpr int32_t maximumPositiveExponent = 9'999'999;
+
+  // Compute the maximum BigInt digit length from the maximum positive exponent.
+  //
+  // BigInts are stored with base |2 ** BigInt::DigitBits|, so we have:
+  //
+  //   |maximumPositiveExponent| * Log_DigitBase(10)
+  // = |maximumPositiveExponent| * Log2(10) / Log2(2 ** BigInt::DigitBits)
+  // = |maximumPositiveExponent| * Log2(10) / BigInt::DigitBits
+  // = 33219277.626945525... / BigInt::DigitBits
+  constexpr size_t maximumBigIntLength = 33219277.626945525 / BigInt::DigitBits;
+
+  if (!value.isString()) {
+    if (!ToNumeric(cx, value)) {
+      return false;
+    }
+
+    if (value.isBigInt() &&
+        value.toBigInt()->digitLength() > maximumBigIntLength) {
+      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                JSMSG_EXPONENT_TOO_LARGE);
+      return false;
+    }
+
+    return true;
+  }
+
+  JSLinearString* str = value.toString()->ensureLinear(cx);
+  if (!str) {
+    return false;
+  }
+
+  // Parse the string as a number.
+  double number = LinearStringToNumber(str);
+
+  bool exponentTooLarge = false;
+  if (mozilla::IsNaN(number)) {
+    // Set to NaN if the input can't be parsed as a number.
+    value.setNaN();
+  } else if (IsNonDecimalNumber(str)) {
+    // ICU doesn't accept non-decimal numbers, so we have to convert the input
+    // into a base-10 string.
+
+    MOZ_ASSERT(!mozilla::IsNegative(number),
+               "non-decimal numbers can't be negative");
+
+    if (number < DOUBLE_INTEGRAL_PRECISION_LIMIT) {
+      // Fast-path if we can guarantee there was no loss of precision.
+      value.setDouble(number);
+    } else {
+      // For the slow-path convert the string into a BigInt.
+
+      // StringToBigInt can't fail (other than OOM) when StringToNumber already
+      // succeeded.
+      RootedString rooted(cx, str);
+      BigInt* bi;
+      JS_TRY_VAR_OR_RETURN_FALSE(cx, bi, StringToBigInt(cx, rooted));
+      MOZ_ASSERT(bi);
+
+      if (bi->digitLength() > maximumBigIntLength) {
+        exponentTooLarge = true;
+      } else {
+        value.setBigInt(bi);
+      }
+    }
+  } else {
+    JS::AutoCheckCannotGC nogc;
+    if (auto decimal = intl::DecimalNumber::from(str, nogc)) {
+      if (decimal->isZero()) {
+        // Normalize positive/negative zero.
+        MOZ_ASSERT(number == 0);
+
+        value.setDouble(number);
+      } else if (decimal->exponentTooLarge() ||
+                 std::abs(decimal->exponent()) >= maximumExponent ||
+                 decimal->exponent() > maximumPositiveExponent) {
+        exponentTooLarge = true;
+      }
+    } else {
+      // If we can't parse the string as a decimal, it must be ±Infinity.
+      MOZ_ASSERT(mozilla::IsInfinite(number));
+      MOZ_ASSERT(StringFindPattern(str, cx->names().Infinity, 0) >= 0);
+
+      value.setDouble(number);
+    }
+  }
+
+  if (exponentTooLarge) {
+    // Throw an error if the exponent is too large.
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_EXPONENT_TOO_LARGE);
+    return false;
+  }
+
+  return true;
+}
+
+// Return the number part of the input by removing leading and trailing
+// whitespace.
+template <typename CharT>
+static mozilla::Span<const CharT> NumberPart(const CharT* chars,
+                                             size_t length) {
+  const CharT* start = chars;
+  const CharT* end = chars + length;
+
+  start = SkipSpace(start, end);
+
+  // |SkipSpace| only supports forward iteration, so inline the backwards
+  // iteration here.
+  MOZ_ASSERT(start <= end);
+  while (end > start && unicode::IsSpace(end[-1])) {
+    end--;
+  }
+
+  // The number part is a non-empty, ASCII-only substring.
+  MOZ_ASSERT(start < end);
+  MOZ_ASSERT(mozilla::IsAscii(mozilla::Span(start, end)));
+
+  return {start, end};
+}
+
+static bool NumberPart(JSContext* cx, JSLinearString* str,
+                       const JS::AutoCheckCannotGC& nogc,
+                       JS::UniqueChars& latin1, std::string_view& result) {
+  if (str->hasLatin1Chars()) {
+    auto span = NumberPart(
+        reinterpret_cast<const char*>(str->latin1Chars(nogc)), str->length());
+
+    result = {span.data(), span.size()};
+    return true;
+  }
+
+  auto span = NumberPart(str->twoByteChars(nogc), str->length());
+
+  latin1.reset(JS::LossyTwoByteCharsToNewLatin1CharsZ(cx, span).c_str());
+  if (!latin1) {
+    return false;
+  }
+
+  result = {latin1.get(), span.size()};
+  return true;
+}
+
+bool js::intl_FormatNumber(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 3);
+  MOZ_ASSERT(args[0].isObject());
+#ifndef NIGHTLY_BUILD
+  MOZ_ASSERT(args[1].isNumeric());
+#endif
+  MOZ_ASSERT(args[2].isBoolean());
+
+  Rooted<NumberFormatObject*> numberFormat(
+      cx, &args[0].toObject().as<NumberFormatObject>());
+
+  RootedValue value(cx, args[1]);
+#ifdef NIGHTLY_BUILD
+  if (!ToIntlMathematicalValue(cx, &value)) {
+    return false;
+  }
+#endif
+
+  mozilla::intl::NumberFormat* nf = GetOrCreateNumberFormat(cx, numberFormat);
+  if (!nf) {
+    return false;
+  }
+
+  // Actually format the number
+  using ICUError = mozilla::intl::ICUError;
+
+  bool formatToParts = args[2].toBoolean();
+  mozilla::Result<std::u16string_view, ICUError> result =
+      mozilla::Err(ICUError::InternalError);
+  mozilla::intl::NumberPartVector parts;
+  if (value.isNumber()) {
+    double num = value.toNumber();
+    if (formatToParts) {
+      result = nf->formatToParts(num, parts);
+    } else {
+      result = nf->format(num);
+    }
+  } else if (value.isBigInt()) {
+    RootedBigInt bi(cx, value.toBigInt());
+
+    int64_t num;
+    if (BigInt::isInt64(bi, &num)) {
+      if (formatToParts) {
+        result = nf->formatToParts(num, parts);
+      } else {
+        result = nf->format(num);
+      }
+    } else {
+      JSLinearString* str = BigInt::toString<CanGC>(cx, bi, 10);
+      if (!str) {
+        return false;
+      }
+      MOZ_RELEASE_ASSERT(str->hasLatin1Chars());
+
+      JS::AutoCheckCannotGC nogc;
+
+      const char* chars = reinterpret_cast<const char*>(str->latin1Chars(nogc));
+      if (formatToParts) {
+        result =
+            nf->formatToParts(std::string_view(chars, str->length()), parts);
+      } else {
+        result = nf->format(std::string_view(chars, str->length()));
+      }
+    }
+  } else {
+    JSLinearString* str = value.toString()->ensureLinear(cx);
+    if (!str) {
+      return false;
+    }
+
+    JS::AutoCheckCannotGC nogc;
+
+    // Two-byte strings have to be copied into a separate |char| buffer.
+    JS::UniqueChars latin1;
+
+    std::string_view sv;
+    if (!NumberPart(cx, str, nogc, latin1, sv)) {
+      return false;
+    }
+
+    if (formatToParts) {
+      result = nf->formatToParts(sv, parts);
+    } else {
+      result = nf->format(sv);
+    }
+  }
+
+  if (result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return false;
+  }
+
+  RootedString str(cx, NewStringCopy<CanGC>(cx, result.unwrap()));
+  if (!str) {
+    return false;
+  }
+
+  if (formatToParts) {
+    return FormattedNumberToParts(cx, str, parts, DisplayNumberPartSource::No,
+                                  nullptr, args.rval());
+  }
+
+  args.rval().setString(str);
+  return true;
+}
+
+static JSLinearString* ToLinearString(JSContext* cx, HandleValue val) {
+  // Special case to preserve negative zero.
+  if (val.isDouble() && mozilla::IsNegativeZero(val.toDouble())) {
+    constexpr std::string_view negativeZero = "-0";
+    return NewStringCopy<CanGC>(cx, negativeZero);
+  }
+
+  JSString* str = ToString(cx, val);
+  return str ? str->ensureLinear(cx) : nullptr;
+};
+
+bool js::intl_FormatNumberRange(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 4);
+  MOZ_ASSERT(args[0].isObject());
+  MOZ_ASSERT(!args[1].isUndefined());
+  MOZ_ASSERT(!args[2].isUndefined());
+  MOZ_ASSERT(args[3].isBoolean());
+
+  Rooted<NumberFormatObject*> numberFormat(
+      cx, &args[0].toObject().as<NumberFormatObject>());
+  bool formatToParts = args[3].toBoolean();
+
+  RootedValue start(cx, args[1]);
+  if (!ToIntlMathematicalValue(cx, &start)) {
+    return false;
+  }
+
+  RootedValue end(cx, args[2]);
+  if (!ToIntlMathematicalValue(cx, &end)) {
+    return false;
+  }
+
+  // PartitionNumberRangePattern, step 1.
+  if (start.isDouble() && mozilla::IsNaN(start.toDouble())) {
+    JS_ReportErrorNumberASCII(
+        cx, GetErrorMessage, nullptr, JSMSG_NAN_NUMBER_RANGE, "start",
+        "NumberFormat", formatToParts ? "formatRangeToParts" : "formatRange");
+    return false;
+  }
+  if (end.isDouble() && mozilla::IsNaN(end.toDouble())) {
+    JS_ReportErrorNumberASCII(
+        cx, GetErrorMessage, nullptr, JSMSG_NAN_NUMBER_RANGE, "end",
+        "NumberFormat", formatToParts ? "formatRangeToParts" : "formatRange");
+    return false;
+  }
+
+  using NumberRangeFormat = mozilla::intl::NumberRangeFormat;
+  NumberRangeFormat* nf = GetOrCreateNumberRangeFormat(cx, numberFormat);
+  if (!nf) {
+    return false;
+  }
+
+  auto valueRepresentableAsDouble = [](const Value& val, double* num) {
+    if (val.isNumber()) {
+      *num = val.toNumber();
+      return true;
+    }
+    if (val.isBigInt()) {
+      int64_t i64;
+      if (BigInt::isInt64(val.toBigInt(), &i64) &&
+          i64 < int64_t(DOUBLE_INTEGRAL_PRECISION_LIMIT) &&
+          i64 > -int64_t(DOUBLE_INTEGRAL_PRECISION_LIMIT)) {
+        *num = double(i64);
+        return true;
+      }
+    }
+    return false;
+  };
+
+  // Actually format the number range.
+  using ICUError = mozilla::intl::ICUError;
+
+  mozilla::Result<std::u16string_view, ICUError> result =
+      mozilla::Err(ICUError::InternalError);
+  mozilla::intl::NumberPartVector parts;
+
+  double numStart, numEnd;
+  if (valueRepresentableAsDouble(start, &numStart) &&
+      valueRepresentableAsDouble(end, &numEnd)) {
+    if (formatToParts) {
+      result = nf->formatToParts(numStart, numEnd, parts);
+    } else {
+      result = nf->format(numStart, numEnd);
+    }
+  } else {
+    Rooted<JSLinearString*> strStart(cx, ToLinearString(cx, start));
+    if (!strStart) {
+      return false;
+    }
+
+    Rooted<JSLinearString*> strEnd(cx, ToLinearString(cx, end));
+    if (!strEnd) {
+      return false;
+    }
+
+    JS::AutoCheckCannotGC nogc;
+
+    // Two-byte strings have to be copied into a separate |char| buffer.
+    JS::UniqueChars latin1Start;
+    JS::UniqueChars latin1End;
+
+    std::string_view svStart;
+    if (!NumberPart(cx, strStart, nogc, latin1Start, svStart)) {
+      return false;
+    }
+
+    std::string_view svEnd;
+    if (!NumberPart(cx, strEnd, nogc, latin1End, svEnd)) {
+      return false;
+    }
+
+    if (formatToParts) {
+      result = nf->formatToParts(svStart, svEnd, parts);
+    } else {
+      result = nf->format(svStart, svEnd);
+    }
+  }
+
+  if (result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return false;
+  }
+
+  RootedString str(cx, NewStringCopy<CanGC>(cx, result.unwrap()));
+  if (!str) {
+    return false;
+  }
+
+  if (formatToParts) {
+    return FormattedNumberToParts(cx, str, parts, DisplayNumberPartSource::Yes,
+                                  nullptr, args.rval());
+  }
+
+  args.rval().setString(str);
+  return true;
+}
diff --git a/js/src/builtin/intl/NumberFormat.h b/js/src/builtin/intl/NumberFormat.h
new file mode 100644
index 0000000000..a0feb4cb4b
--- /dev/null
+++ b/js/src/builtin/intl/NumberFormat.h
@@ -0,0 +1,129 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_NumberFormat_h
+#define builtin_intl_NumberFormat_h
+
+#include <stdint.h>
+
+#include "builtin/SelfHostingDefines.h"
+#include "js/Class.h"
+#include "vm/NativeObject.h"
+
+namespace mozilla::intl {
+class NumberFormat;
+class NumberRangeFormat;
+}  // namespace mozilla::intl
+
+namespace js {
+
+class NumberFormatObject : public NativeObject {
+ public:
+  static const JSClass class_;
+  static const JSClass& protoClass_;
+
+  static constexpr uint32_t INTERNALS_SLOT = 0;
+  static constexpr uint32_t UNUMBER_FORMATTER_SLOT = 1;
+  static constexpr uint32_t UNUMBER_RANGE_FORMATTER_SLOT = 2;
+  static constexpr uint32_t SLOT_COUNT = 3;
+
+  static_assert(INTERNALS_SLOT == INTL_INTERNALS_OBJECT_SLOT,
+                "INTERNALS_SLOT must match self-hosting define for internals "
+                "object slot");
+
+  // Estimated memory use for UNumberFormatter and UFormattedNumber
+  // (see IcuMemoryUsage).
+  static constexpr size_t EstimatedMemoryUse = 972;
+
+  // Estimated memory use for UNumberRangeFormatter and UFormattedNumberRange
+  // (see IcuMemoryUsage).
+  static constexpr size_t EstimatedRangeFormatterMemoryUse = 14143;
+
+  mozilla::intl::NumberFormat* getNumberFormatter() const {
+    const auto& slot = getFixedSlot(UNUMBER_FORMATTER_SLOT);
+    if (slot.isUndefined()) {
+      return nullptr;
+    }
+    return static_cast<mozilla::intl::NumberFormat*>(slot.toPrivate());
+  }
+
+  void setNumberFormatter(mozilla::intl::NumberFormat* formatter) {
+    setFixedSlot(UNUMBER_FORMATTER_SLOT, PrivateValue(formatter));
+  }
+
+  mozilla::intl::NumberRangeFormat* getNumberRangeFormatter() const {
+    const auto& slot = getFixedSlot(UNUMBER_RANGE_FORMATTER_SLOT);
+    if (slot.isUndefined()) {
+      return nullptr;
+    }
+    return static_cast<mozilla::intl::NumberRangeFormat*>(slot.toPrivate());
+  }
+
+  void setNumberRangeFormatter(mozilla::intl::NumberRangeFormat* formatter) {
+    setFixedSlot(UNUMBER_RANGE_FORMATTER_SLOT, PrivateValue(formatter));
+  }
+
+ private:
+  static const JSClassOps classOps_;
+  static const ClassSpec classSpec_;
+
+  static void finalize(JS::GCContext* gcx, JSObject* obj);
+};
+
+/**
+ * Returns a new instance of the standard built-in NumberFormat constructor.
+ * Self-hosted code cannot cache this constructor (as it does for others in
+ * Utilities.js) because it is initialized after self-hosted code is compiled.
+ *
+ * Usage: numberFormat = intl_NumberFormat(locales, options)
+ */
+[[nodiscard]] extern bool intl_NumberFormat(JSContext* cx, unsigned argc,
+                                            Value* vp);
+
+/**
+ * Returns the numbering system type identifier per Unicode
+ * Technical Standard 35, Unicode Locale Data Markup Language, for the
+ * default numbering system for the given locale.
+ *
+ * Usage: defaultNumberingSystem = intl_numberingSystem(locale)
+ */
+[[nodiscard]] extern bool intl_numberingSystem(JSContext* cx, unsigned argc,
+                                               Value* vp);
+
+/**
+ * Returns a string representing the number x according to the effective
+ * locale and the formatting options of the given NumberFormat.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 11.3.2.
+ *
+ * Usage: formatted = intl_FormatNumber(numberFormat, x, formatToParts)
+ */
+[[nodiscard]] extern bool intl_FormatNumber(JSContext* cx, unsigned argc,
+                                            Value* vp);
+
+/**
+ * Returns a string representing the number range «x - y» according to the
+ * effective locale and the formatting options of the given NumberFormat.
+ *
+ * Usage: formatted = intl_FormatNumberRange(numberFormat, x, y, formatToParts)
+ */
+[[nodiscard]] extern bool intl_FormatNumberRange(JSContext* cx, unsigned argc,
+                                                 Value* vp);
+
+#if DEBUG || MOZ_SYSTEM_ICU
+/**
+ * Returns an object with all available measurement units.
+ *
+ * Usage: units = intl_availableMeasurementUnits()
+ */
+[[nodiscard]] extern bool intl_availableMeasurementUnits(JSContext* cx,
+                                                         unsigned argc,
+                                                         Value* vp);
+#endif
+
+}  // namespace js
+
+#endif /* builtin_intl_NumberFormat_h */
diff --git a/js/src/builtin/intl/NumberFormat.js b/js/src/builtin/intl/NumberFormat.js
new file mode 100644
index 0000000000..d381fc97a8
--- /dev/null
+++ b/js/src/builtin/intl/NumberFormat.js
@@ -0,0 +1,1210 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Portions Copyright Norbert Lindenberg 2011-2012. */
+
+#include "NumberingSystemsGenerated.h"
+
+/**
+ * NumberFormat internal properties.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 9.1 and 11.3.3.
+ */
+var numberFormatInternalProperties = {
+  localeData: numberFormatLocaleData,
+  relevantExtensionKeys: ["nu"],
+};
+
+/**
+ * Compute an internal properties object from |lazyNumberFormatData|.
+ */
+function resolveNumberFormatInternals(lazyNumberFormatData) {
+  assert(IsObject(lazyNumberFormatData), "lazy data not an object?");
+
+  var internalProps = std_Object_create(null);
+
+  var NumberFormat = numberFormatInternalProperties;
+
+  // Compute effective locale.
+
+  // Step 7.
+  var localeData = NumberFormat.localeData;
+
+  // Step 8.
+  var r = ResolveLocale(
+    "NumberFormat",
+    lazyNumberFormatData.requestedLocales,
+    lazyNumberFormatData.opt,
+    NumberFormat.relevantExtensionKeys,
+    localeData
+  );
+
+  // Steps 9-10. (Step 11 is not relevant to our implementation.)
+  internalProps.locale = r.locale;
+  internalProps.numberingSystem = r.nu;
+
+  // Compute formatting options.
+  // Step 13.
+  var style = lazyNumberFormatData.style;
+  internalProps.style = style;
+
+  // Steps 17, 19.
+  if (style === "currency") {
+    internalProps.currency = lazyNumberFormatData.currency;
+    internalProps.currencyDisplay = lazyNumberFormatData.currencyDisplay;
+    internalProps.currencySign = lazyNumberFormatData.currencySign;
+  }
+
+  // Intl.NumberFormat Unified API Proposal
+  if (style === "unit") {
+    internalProps.unit = lazyNumberFormatData.unit;
+    internalProps.unitDisplay = lazyNumberFormatData.unitDisplay;
+  }
+
+  // Intl.NumberFormat Unified API Proposal
+  var notation = lazyNumberFormatData.notation;
+  internalProps.notation = notation;
+
+  // Step 22.
+  internalProps.minimumIntegerDigits =
+    lazyNumberFormatData.minimumIntegerDigits;
+
+  if ("minimumFractionDigits" in lazyNumberFormatData) {
+    // Note: Intl.NumberFormat.prototype.resolvedOptions() exposes the
+    // actual presence (versus undefined-ness) of these properties.
+    assert(
+      "maximumFractionDigits" in lazyNumberFormatData,
+      "min/max frac digits mismatch"
+    );
+    internalProps.minimumFractionDigits =
+      lazyNumberFormatData.minimumFractionDigits;
+    internalProps.maximumFractionDigits =
+      lazyNumberFormatData.maximumFractionDigits;
+  }
+
+  if ("minimumSignificantDigits" in lazyNumberFormatData) {
+    // Note: Intl.NumberFormat.prototype.resolvedOptions() exposes the
+    // actual presence (versus undefined-ness) of these properties.
+    assert(
+      "maximumSignificantDigits" in lazyNumberFormatData,
+      "min/max sig digits mismatch"
+    );
+    internalProps.minimumSignificantDigits =
+      lazyNumberFormatData.minimumSignificantDigits;
+    internalProps.maximumSignificantDigits =
+      lazyNumberFormatData.maximumSignificantDigits;
+  }
+
+  // Intl.NumberFormat v3 Proposal
+  internalProps.trailingZeroDisplay = lazyNumberFormatData.trailingZeroDisplay;
+  internalProps.roundingIncrement = lazyNumberFormatData.roundingIncrement;
+
+  // Intl.NumberFormat Unified API Proposal
+  if (notation === "compact") {
+    internalProps.compactDisplay = lazyNumberFormatData.compactDisplay;
+  }
+
+  // Step 24.
+  internalProps.useGrouping = lazyNumberFormatData.useGrouping;
+
+  // Intl.NumberFormat Unified API Proposal
+  internalProps.signDisplay = lazyNumberFormatData.signDisplay;
+
+  // Intl.NumberFormat v3 Proposal
+  internalProps.roundingMode = lazyNumberFormatData.roundingMode;
+
+  // Intl.NumberFormat v3 Proposal
+  internalProps.roundingPriority = lazyNumberFormatData.roundingPriority;
+
+  // The caller is responsible for associating |internalProps| with the right
+  // object using |setInternalProperties|.
+  return internalProps;
+}
+
+/**
+ * Returns an object containing the NumberFormat internal properties of |obj|.
+ */
+function getNumberFormatInternals(obj) {
+  assert(IsObject(obj), "getNumberFormatInternals called with non-object");
+  assert(
+    intl_GuardToNumberFormat(obj) !== null,
+    "getNumberFormatInternals called with non-NumberFormat"
+  );
+
+  var internals = getIntlObjectInternals(obj);
+  assert(
+    internals.type === "NumberFormat",
+    "bad type escaped getIntlObjectInternals"
+  );
+
+  // If internal properties have already been computed, use them.
+  var internalProps = maybeInternalProperties(internals);
+  if (internalProps) {
+    return internalProps;
+  }
+
+  // Otherwise it's time to fully create them.
+  internalProps = resolveNumberFormatInternals(internals.lazyData);
+  setInternalProperties(internals, internalProps);
+  return internalProps;
+}
+
+/**
+ * 11.1.11 UnwrapNumberFormat( nf )
+ */
+function UnwrapNumberFormat(nf) {
+  // Steps 2 and 4 (error handling moved to caller).
+  if (
+    IsObject(nf) &&
+    intl_GuardToNumberFormat(nf) === null &&
+    !intl_IsWrappedNumberFormat(nf) &&
+    callFunction(
+      std_Object_isPrototypeOf,
+      GetBuiltinPrototype("NumberFormat"),
+      nf
+    )
+  ) {
+    nf = nf[intlFallbackSymbol()];
+  }
+  return nf;
+}
+
+/**
+ * Applies digit options used for number formatting onto the intl object.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 11.1.1.
+ */
+function SetNumberFormatDigitOptions(
+  lazyData,
+  options,
+  mnfdDefault,
+  mxfdDefault,
+  notation
+) {
+  // We skip step 1 because we set the properties on a lazyData object.
+
+  // Steps 2-4.
+  assert(IsObject(options), "SetNumberFormatDigitOptions");
+  assert(typeof mnfdDefault === "number", "SetNumberFormatDigitOptions");
+  assert(typeof mxfdDefault === "number", "SetNumberFormatDigitOptions");
+  assert(mnfdDefault <= mxfdDefault, "SetNumberFormatDigitOptions");
+  assert(typeof notation === "string", "SetNumberFormatDigitOptions");
+
+  // Steps 5-9.
+  const mnid = GetNumberOption(options, "minimumIntegerDigits", 1, 21, 1);
+  let mnfd = options.minimumFractionDigits;
+  let mxfd = options.maximumFractionDigits;
+  let mnsd = options.minimumSignificantDigits;
+  let mxsd = options.maximumSignificantDigits;
+
+  // Step 10.
+  lazyData.minimumIntegerDigits = mnid;
+
+#ifdef NIGHTLY_BUILD
+  // Intl.NumberFormat v3 Proposal
+  var roundingPriority = GetOption(
+    options,
+    "roundingPriority",
+    "string",
+    ["auto", "morePrecision", "lessPrecision"],
+    "auto"
+  );
+#else
+  var roundingPriority = "auto";
+#endif
+
+  const hasSignificantDigits = mnsd !== undefined || mxsd !== undefined;
+  const hasFractionDigits = mnfd !== undefined || mxfd !== undefined;
+
+  const needSignificantDigits =
+    roundingPriority !== "auto" || hasSignificantDigits;
+  const needFractionalDigits =
+    roundingPriority !== "auto" ||
+    !(hasSignificantDigits || (!hasFractionDigits && notation === "compact"));
+
+  if (needSignificantDigits) {
+    // Step 11.
+    if (hasSignificantDigits) {
+      // Step 11.a (Omitted).
+
+      // Step 11.b.
+      mnsd = DefaultNumberOption(mnsd, 1, 21, 1);
+
+      // Step 11.c.
+      mxsd = DefaultNumberOption(mxsd, mnsd, 21, 21);
+
+      // Step 11.d.
+      lazyData.minimumSignificantDigits = mnsd;
+
+      // Step 11.e.
+      lazyData.maximumSignificantDigits = mxsd;
+    } else {
+      lazyData.minimumSignificantDigits = 1;
+      lazyData.maximumSignificantDigits = 21;
+    }
+  }
+
+  if (needFractionalDigits) {
+    // Step 12.
+    if (hasFractionDigits) {
+      // Step 12.a (Omitted).
+
+      // Step 12.b.
+      mnfd = DefaultNumberOption(mnfd, 0, 20, undefined);
+
+      // Step 12.c.
+      mxfd = DefaultNumberOption(mxfd, 0, 20, undefined);
+
+      // Step 12.d.
+      if (mnfd === undefined) {
+        assert(
+          mxfd !== undefined,
+          "mxfd isn't undefined when mnfd is undefined"
+        );
+        mnfd = std_Math_min(mnfdDefault, mxfd);
+      }
+
+      // Step 12.e.
+      else if (mxfd === undefined) {
+        mxfd = std_Math_max(mxfdDefault, mnfd);
+      }
+
+      // Step 12.f.
+      else if (mnfd > mxfd) {
+        ThrowRangeError(JSMSG_INVALID_DIGITS_VALUE, mxfd);
+      }
+
+      // Step 12.g.
+      lazyData.minimumFractionDigits = mnfd;
+
+      // Step 12.h.
+      lazyData.maximumFractionDigits = mxfd;
+    } else {
+      // Step 14.a (Omitted).
+
+      // Step 14.b.
+      lazyData.minimumFractionDigits = mnfdDefault;
+
+      // Step 14.c.
+      lazyData.maximumFractionDigits = mxfdDefault;
+    }
+  }
+
+  if (needSignificantDigits || needFractionalDigits) {
+    lazyData.roundingPriority = roundingPriority;
+  } else {
+    assert(!hasSignificantDigits, "bad significant digits in fallback case");
+    assert(
+      roundingPriority === "auto",
+      `bad rounding in fallback case: ${roundingPriority}`
+    );
+    assert(
+      notation === "compact",
+      `bad notation in fallback case: ${notation}`
+    );
+
+    lazyData.roundingPriority = "morePrecision";
+    lazyData.minimumFractionDigits = 0;
+    lazyData.maximumFractionDigits = 0;
+    lazyData.minimumSignificantDigits = 1;
+    lazyData.maximumSignificantDigits = 2;
+  }
+}
+
+/**
+ * Convert s to upper case, but limited to characters a-z.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 6.1.
+ */
+function toASCIIUpperCase(s) {
+  assert(typeof s === "string", "toASCIIUpperCase");
+
+  // String.prototype.toUpperCase may map non-ASCII characters into ASCII,
+  // so go character by character (actually code unit by code unit, but
+  // since we only care about ASCII characters here, that's OK).
+  var result = "";
+  for (var i = 0; i < s.length; i++) {
+    var c = callFunction(std_String_charCodeAt, s, i);
+    result +=
+      0x61 <= c && c <= 0x7a
+        ? callFunction(std_String_fromCharCode, null, c & ~0x20)
+        : s[i];
+  }
+  return result;
+}
+
+/**
+ * Verifies that the given string is a well-formed ISO 4217 currency code.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 6.3.1.
+ */
+function IsWellFormedCurrencyCode(currency) {
+  assert(typeof currency === "string", "currency is a string value");
+
+  return currency.length === 3 && IsASCIIAlphaString(currency);
+}
+
+/**
+ * Verifies that the given string is a well-formed core unit identifier as
+ * defined in UTS #35, Part 2, Section 6. In addition to obeying the UTS #35
+ * core unit identifier syntax, |unitIdentifier| must be one of the identifiers
+ * sanctioned by UTS #35 or be a compound unit composed of two sanctioned simple
+ * units.
+ *
+ * Intl.NumberFormat Unified API Proposal
+ */
+function IsWellFormedUnitIdentifier(unitIdentifier) {
+  assert(
+    typeof unitIdentifier === "string",
+    "unitIdentifier is a string value"
+  );
+
+  // Step 1.
+  if (IsSanctionedSimpleUnitIdentifier(unitIdentifier)) {
+    return true;
+  }
+
+  // Step 2.
+  var pos = callFunction(std_String_indexOf, unitIdentifier, "-per-");
+  if (pos < 0) {
+    return false;
+  }
+
+  var next = pos + "-per-".length;
+
+  // Steps 3 and 5.
+  var numerator = Substring(unitIdentifier, 0, pos);
+  var denominator = Substring(
+    unitIdentifier,
+    next,
+    unitIdentifier.length - next
+  );
+
+  // Steps 4 and 6.
+  return (
+    IsSanctionedSimpleUnitIdentifier(numerator) &&
+    IsSanctionedSimpleUnitIdentifier(denominator)
+  );
+}
+
+#if DEBUG || MOZ_SYSTEM_ICU
+var availableMeasurementUnits = {
+  value: null,
+};
+#endif
+
+/**
+ * Verifies that the given string is a sanctioned simple core unit identifier.
+ *
+ * Intl.NumberFormat Unified API Proposal
+ *
+ * Also see: https://unicode.org/reports/tr35/tr35-general.html#Unit_Elements
+ */
+function IsSanctionedSimpleUnitIdentifier(unitIdentifier) {
+  assert(
+    typeof unitIdentifier === "string",
+    "unitIdentifier is a string value"
+  );
+
+  var isSanctioned = hasOwn(unitIdentifier, sanctionedSimpleUnitIdentifiers);
+
+#if DEBUG || MOZ_SYSTEM_ICU
+  if (isSanctioned) {
+    if (availableMeasurementUnits.value === null) {
+      availableMeasurementUnits.value = intl_availableMeasurementUnits();
+    }
+
+    var isSupported = hasOwn(unitIdentifier, availableMeasurementUnits.value);
+
+#if MOZ_SYSTEM_ICU
+    // A system ICU may support fewer measurement units, so we need to make
+    // sure the unit is actually supported.
+    isSanctioned = isSupported;
+#else
+    // Otherwise just assert that the sanctioned unit is also supported.
+    assert(
+      isSupported,
+      `"${unitIdentifier}" is sanctioned but not supported. Did you forget to update
+      intl/icu/data_filter.json to include the unit (and any implicit compound units)?
+      For example "speed/kilometer-per-hour" is implied by "length/kilometer" and
+      "duration/hour" and must therefore also be present.`
+    );
+#endif
+  }
+#endif
+
+  return isSanctioned;
+}
+
+/* eslint-disable complexity */
+/**
+ * Initializes an object as a NumberFormat.
+ *
+ * This method is complicated a moderate bit by its implementing initialization
+ * as a *lazy* concept.  Everything that must happen now, does -- but we defer
+ * all the work we can until the object is actually used as a NumberFormat.
+ * This later work occurs in |resolveNumberFormatInternals|; steps not noted
+ * here occur there.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 11.1.2.
+ */
+function InitializeNumberFormat(numberFormat, thisValue, locales, options) {
+  assert(
+    IsObject(numberFormat),
+    "InitializeNumberFormat called with non-object"
+  );
+  assert(
+    intl_GuardToNumberFormat(numberFormat) !== null,
+    "InitializeNumberFormat called with non-NumberFormat"
+  );
+
+  // Lazy NumberFormat data has the following structure:
+  //
+  //   {
+  //     requestedLocales: List of locales,
+  //     style: "decimal" / "percent" / "currency" / "unit",
+  //
+  //     // fields present only if style === "currency":
+  //     currency: a well-formed currency code (IsWellFormedCurrencyCode),
+  //     currencyDisplay: "code" / "symbol" / "narrowSymbol" / "name",
+  //     currencySign: "standard" / "accounting",
+  //
+  //     // fields present only if style === "unit":
+  //     unit: a well-formed unit identifier (IsWellFormedUnitIdentifier),
+  //     unitDisplay: "short" / "narrow" / "long",
+  //
+  //     opt: // opt object computed in InitializeNumberFormat
+  //       {
+  //         localeMatcher: "lookup" / "best fit",
+  //
+  //         nu: string matching a Unicode extension type, // optional
+  //       }
+  //
+  //     minimumIntegerDigits: integer ∈ [1, 21],
+  //
+  //     // optional, mutually exclusive with the significant-digits option
+  //     minimumFractionDigits: integer ∈ [0, 20],
+  //     maximumFractionDigits: integer ∈ [0, 20],
+  //
+  //     // optional, mutually exclusive with the fraction-digits option
+  //     minimumSignificantDigits: integer ∈ [1, 21],
+  //     maximumSignificantDigits: integer ∈ [1, 21],
+  //
+  //     roundingPriority: "auto" / "lessPrecision" / "morePrecision",
+  //
+  //     // accepts different values when Intl.NumberFormat v3 proposal is enabled
+  //     useGrouping: true / false,
+  //     useGrouping: "auto" / "always" / "min2" / false,
+  //
+  //     notation: "standard" / "scientific" / "engineering" / "compact",
+  //
+  //     // optional, if notation is "compact"
+  //     compactDisplay: "short" / "long",
+  //
+  //     signDisplay: "auto" / "never" / "always" / "exceptZero",
+  //
+  //     trailingZeroDisplay: "auto" / "stripIfInteger",
+  //
+  //     roundingIncrement: integer ∈ (1, 2, 5,
+  //                                   10, 20, 25, 50,
+  //                                   100, 200, 250, 500,
+  //                                   1000, 2000, 2500, 5000),
+  //
+  //     roundingMode: "ceil" / "floor" / "expand" / "trunc" /
+  //                   "halfCeil" / "halfFloor" / "halfExpand" / "halfTrunc" / "halfEven",
+  //   }
+  //
+  // Note that lazy data is only installed as a final step of initialization,
+  // so every NumberFormat lazy data object has *all* these properties, never a
+  // subset of them.
+  var lazyNumberFormatData = std_Object_create(null);
+
+  // Step 1.
+  var requestedLocales = CanonicalizeLocaleList(locales);
+  lazyNumberFormatData.requestedLocales = requestedLocales;
+
+  // Steps 2-3.
+  //
+  // If we ever need more speed here at startup, we should try to detect the
+  // case where |options === undefined| and then directly use the default
+  // value for each option.  For now, just keep it simple.
+  if (options === undefined) {
+    options = std_Object_create(null);
+  } else {
+    options = ToObject(options);
+  }
+
+  // Compute options that impact interpretation of locale.
+  // Step 4.
+  var opt = new_Record();
+  lazyNumberFormatData.opt = opt;
+
+  // Steps 5-6.
+  var matcher = GetOption(
+    options,
+    "localeMatcher",
+    "string",
+    ["lookup", "best fit"],
+    "best fit"
+  );
+  opt.localeMatcher = matcher;
+
+  var numberingSystem = GetOption(
+    options,
+    "numberingSystem",
+    "string",
+    undefined,
+    undefined
+  );
+
+  if (numberingSystem !== undefined) {
+    numberingSystem = intl_ValidateAndCanonicalizeUnicodeExtensionType(
+      numberingSystem,
+      "numberingSystem",
+      "nu"
+    );
+  }
+
+  opt.nu = numberingSystem;
+
+  // Compute formatting options.
+  // Step 12.
+  var style = GetOption(
+    options,
+    "style",
+    "string",
+    ["decimal", "percent", "currency", "unit"],
+    "decimal"
+  );
+  lazyNumberFormatData.style = style;
+
+  // Steps 14-17.
+  var currency = GetOption(options, "currency", "string", undefined, undefined);
+
+  // Per the Intl.NumberFormat Unified API Proposal, this check should only
+  // happen for |style === "currency"|, which seems inconsistent, given that
+  // we normally validate all options when present, even the ones which are
+  // unused.
+  // TODO: File issue at <https://github.com/tc39/proposal-unified-intl-numberformat>.
+  if (currency !== undefined && !IsWellFormedCurrencyCode(currency)) {
+    ThrowRangeError(JSMSG_INVALID_CURRENCY_CODE, currency);
+  }
+
+  var cDigits;
+  if (style === "currency") {
+    if (currency === undefined) {
+      ThrowTypeError(JSMSG_UNDEFINED_CURRENCY);
+    }
+
+    // Steps 19.a-c.
+    currency = toASCIIUpperCase(currency);
+    lazyNumberFormatData.currency = currency;
+    cDigits = CurrencyDigits(currency);
+  }
+
+  // Step 18.
+  var currencyDisplay = GetOption(
+    options,
+    "currencyDisplay",
+    "string",
+    ["code", "symbol", "narrowSymbol", "name"],
+    "symbol"
+  );
+  if (style === "currency") {
+    lazyNumberFormatData.currencyDisplay = currencyDisplay;
+  }
+
+  // Intl.NumberFormat Unified API Proposal
+  var currencySign = GetOption(
+    options,
+    "currencySign",
+    "string",
+    ["standard", "accounting"],
+    "standard"
+  );
+  if (style === "currency") {
+    lazyNumberFormatData.currencySign = currencySign;
+  }
+
+  // Intl.NumberFormat Unified API Proposal
+  var unit = GetOption(options, "unit", "string", undefined, undefined);
+
+  // Aligned with |currency| check from above, see note about spec issue there.
+  if (unit !== undefined && !IsWellFormedUnitIdentifier(unit)) {
+    ThrowRangeError(JSMSG_INVALID_UNIT_IDENTIFIER, unit);
+  }
+
+  var unitDisplay = GetOption(
+    options,
+    "unitDisplay",
+    "string",
+    ["short", "narrow", "long"],
+    "short"
+  );
+
+  if (style === "unit") {
+    if (unit === undefined) {
+      ThrowTypeError(JSMSG_UNDEFINED_UNIT);
+    }
+
+    lazyNumberFormatData.unit = unit;
+    lazyNumberFormatData.unitDisplay = unitDisplay;
+  }
+
+  // Steps 20-21.
+  var mnfdDefault, mxfdDefault;
+  if (style === "currency") {
+    mnfdDefault = cDigits;
+    mxfdDefault = cDigits;
+  } else {
+    mnfdDefault = 0;
+    mxfdDefault = style === "percent" ? 0 : 3;
+  }
+
+  // Intl.NumberFormat Unified API Proposal
+  var notation = GetOption(
+    options,
+    "notation",
+    "string",
+    ["standard", "scientific", "engineering", "compact"],
+    "standard"
+  );
+  lazyNumberFormatData.notation = notation;
+
+  // Step 22.
+  SetNumberFormatDigitOptions(
+    lazyNumberFormatData,
+    options,
+    mnfdDefault,
+    mxfdDefault,
+    notation
+  );
+
+#ifdef NIGHTLY_BUILD
+  // Intl.NumberFormat v3 Proposal
+  var roundingIncrement = GetNumberOption(
+    options,
+    "roundingIncrement",
+    1,
+    5000,
+    1
+  );
+  switch (roundingIncrement) {
+    case 1:
+    case 2:
+    case 5:
+    case 10:
+    case 20:
+    case 25:
+    case 50:
+    case 100:
+    case 200:
+    case 250:
+    case 500:
+    case 1000:
+    case 2000:
+    case 2500:
+    case 5000:
+      break;
+    default:
+      ThrowRangeError(
+        JSMSG_INVALID_OPTION_VALUE,
+        "roundingIncrement",
+        roundingIncrement
+      );
+  }
+  lazyNumberFormatData.roundingIncrement = roundingIncrement;
+
+  if (roundingIncrement !== 1) {
+    // [[RoundingType]] must be `fractionDigits`.
+    if (lazyNumberFormatData.roundingPriority !== "auto") {
+      ThrowTypeError(
+        JSMSG_INVALID_NUMBER_OPTION,
+        "roundingIncrement",
+        "roundingPriority"
+      );
+    }
+    if (hasOwn("minimumSignificantDigits", lazyNumberFormatData)) {
+      ThrowTypeError(
+        JSMSG_INVALID_NUMBER_OPTION,
+        "roundingIncrement",
+        "minimumSignificantDigits"
+      );
+    }
+
+    // Minimum and maximum fraction digits must be equal.
+    if (
+      lazyNumberFormatData.minimumFractionDigits !==
+      lazyNumberFormatData.maximumFractionDigits
+    ) {
+      ThrowRangeError(JSMSG_UNEQUAL_FRACTION_DIGITS);
+    }
+  }
+#else
+  lazyNumberFormatData.roundingIncrement = 1;
+#endif
+
+#ifdef NIGHTLY_BUILD
+  // Intl.NumberFormat v3 Proposal
+  var trailingZeroDisplay = GetOption(
+    options,
+    "trailingZeroDisplay",
+    "string",
+    ["auto", "stripIfInteger"],
+    "auto"
+  );
+  lazyNumberFormatData.trailingZeroDisplay = trailingZeroDisplay;
+#else
+  lazyNumberFormatData.trailingZeroDisplay = "auto";
+#endif
+
+  // Intl.NumberFormat Unified API Proposal
+  var compactDisplay = GetOption(
+    options,
+    "compactDisplay",
+    "string",
+    ["short", "long"],
+    "short"
+  );
+  if (notation === "compact") {
+    lazyNumberFormatData.compactDisplay = compactDisplay;
+  }
+
+  // Steps 23.
+#ifdef NIGHTLY_BUILD
+  var defaultUseGrouping = notation !== "compact" ? "auto" : "min2";
+  var useGrouping = GetStringOrBooleanOption(
+    options,
+    "useGrouping",
+    ["min2", "auto", "always"],
+    "always",
+    false,
+    defaultUseGrouping
+  );
+#else
+  var useGrouping = GetOption(
+    options,
+    "useGrouping",
+    "boolean",
+    undefined,
+    true
+  );
+#endif
+  lazyNumberFormatData.useGrouping = useGrouping;
+
+  // Intl.NumberFormat Unified API Proposal
+  var signDisplay = GetOption(
+    options,
+    "signDisplay",
+    "string",
+#ifdef NIGHTLY_BUILD
+    ["auto", "never", "always", "exceptZero", "negative"],
+#else
+    ["auto", "never", "always", "exceptZero"],
+#endif
+    "auto"
+  );
+  lazyNumberFormatData.signDisplay = signDisplay;
+
+#ifdef NIGHTLY_BUILD
+  // Intl.NumberFormat v3 Proposal
+  var roundingMode = GetOption(
+    options,
+    "roundingMode",
+    "string",
+    [
+      "ceil",
+      "floor",
+      "expand",
+      "trunc",
+      "halfCeil",
+      "halfFloor",
+      "halfExpand",
+      "halfTrunc",
+      "halfEven",
+    ],
+    "halfExpand"
+  );
+  lazyNumberFormatData.roundingMode = roundingMode;
+#else
+  lazyNumberFormatData.roundingMode = "halfExpand";
+#endif
+
+  // Step 31.
+  //
+  // We've done everything that must be done now: mark the lazy data as fully
+  // computed and install it.
+  initializeIntlObject(numberFormat, "NumberFormat", lazyNumberFormatData);
+
+  // 11.2.1, steps 4-5.
+  if (
+    numberFormat !== thisValue &&
+    callFunction(
+      std_Object_isPrototypeOf,
+      GetBuiltinPrototype("NumberFormat"),
+      thisValue
+    )
+  ) {
+    DefineDataProperty(
+      thisValue,
+      intlFallbackSymbol(),
+      numberFormat,
+      ATTR_NONENUMERABLE | ATTR_NONCONFIGURABLE | ATTR_NONWRITABLE
+    );
+
+    return thisValue;
+  }
+
+  // 11.2.1, step 6.
+  return numberFormat;
+}
+/* eslint-enable complexity */
+
+/**
+ * Returns the number of decimal digits to be used for the given currency.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 11.1.3.
+ */
+function CurrencyDigits(currency) {
+  assert(typeof currency === "string", "currency is a string value");
+  assert(IsWellFormedCurrencyCode(currency), "currency is well-formed");
+  assert(currency === toASCIIUpperCase(currency), "currency is all upper-case");
+
+  if (hasOwn(currency, currencyDigits)) {
+    return currencyDigits[currency];
+  }
+  return 2;
+}
+
+/**
+ * Returns the subset of the given locale list for which this locale list has a
+ * matching (possibly fallback) locale. Locales appear in the same order in the
+ * returned list as in the input list.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 11.3.2.
+ */
+function Intl_NumberFormat_supportedLocalesOf(locales /*, options*/) {
+  var options = arguments.length > 1 ? arguments[1] : undefined;
+
+  // Step 1.
+  var availableLocales = "NumberFormat";
+
+  // Step 2.
+  var requestedLocales = CanonicalizeLocaleList(locales);
+
+  // Step 3.
+  return SupportedLocales(availableLocales, requestedLocales, options);
+}
+
+function getNumberingSystems(locale) {
+  // ICU doesn't have an API to determine the set of numbering systems
+  // supported for a locale; it generally pretends that any numbering system
+  // can be used with any locale. Supporting a decimal numbering system
+  // (where only the digits are replaced) is easy, so we offer them all here.
+  // Algorithmic numbering systems are typically tied to one locale, so for
+  // lack of information we don't offer them.
+  // The one thing we can find out from ICU is the default numbering system
+  // for a locale.
+  var defaultNumberingSystem = intl_numberingSystem(locale);
+  return [defaultNumberingSystem, NUMBERING_SYSTEMS_WITH_SIMPLE_DIGIT_MAPPINGS];
+}
+
+function numberFormatLocaleData() {
+  return {
+    nu: getNumberingSystems,
+    default: {
+      nu: intl_numberingSystem,
+    },
+  };
+}
+
+/**
+ * Create function to be cached and returned by Intl.NumberFormat.prototype.format.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 11.1.4.
+ */
+function createNumberFormatFormat(nf) {
+  // This function is not inlined in $Intl_NumberFormat_format_get to avoid
+  // creating a call-object on each call to $Intl_NumberFormat_format_get.
+  return function(value) {
+    // Step 1 (implicit).
+
+    // Step 2.
+    assert(IsObject(nf), "InitializeNumberFormat called with non-object");
+    assert(
+      intl_GuardToNumberFormat(nf) !== null,
+      "InitializeNumberFormat called with non-NumberFormat"
+    );
+
+#ifdef NIGHTLY_BUILD
+    var x = value;
+#else
+    // Steps 3-4.
+    var x = ToNumeric(value);
+#endif
+
+    // Step 5.
+    return intl_FormatNumber(nf, x, /* formatToParts = */ false);
+  };
+}
+
+/**
+ * Returns a function bound to this NumberFormat that returns a String value
+ * representing the result of calling ToNumber(value) according to the
+ * effective locale and the formatting options of this NumberFormat.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 11.4.3.
+ */
+// Uncloned functions with `$` prefix are allocated as extended function
+// to store the original name in `SetCanonicalName`.
+function $Intl_NumberFormat_format_get() {
+  // Steps 1-3.
+  var thisArg = UnwrapNumberFormat(this);
+  var nf = thisArg;
+  if (!IsObject(nf) || (nf = intl_GuardToNumberFormat(nf)) === null) {
+    return callFunction(
+      intl_CallNumberFormatMethodIfWrapped,
+      thisArg,
+      "$Intl_NumberFormat_format_get"
+    );
+  }
+
+  var internals = getNumberFormatInternals(nf);
+
+  // Step 4.
+  if (internals.boundFormat === undefined) {
+    // Steps 4.a-c.
+    internals.boundFormat = createNumberFormatFormat(nf);
+  }
+
+  // Step 5.
+  return internals.boundFormat;
+}
+SetCanonicalName($Intl_NumberFormat_format_get, "get format");
+
+/**
+ * 11.4.4 Intl.NumberFormat.prototype.formatToParts ( value )
+ */
+function Intl_NumberFormat_formatToParts(value) {
+  // Step 1.
+  var nf = this;
+
+  // Steps 2-3.
+  if (!IsObject(nf) || (nf = intl_GuardToNumberFormat(nf)) === null) {
+    return callFunction(
+      intl_CallNumberFormatMethodIfWrapped,
+      this,
+      value,
+      "Intl_NumberFormat_formatToParts"
+    );
+  }
+
+#ifdef NIGHTLY_BUILD
+  var x = value;
+#else
+  // Step 4.
+  var x = ToNumeric(value);
+#endif
+
+  // Step 5.
+  return intl_FormatNumber(nf, x, /* formatToParts = */ true);
+}
+
+/**
+ * Intl.NumberFormat.prototype.formatRange ( start, end )
+ */
+function Intl_NumberFormat_formatRange(start, end) {
+  // Step 1.
+  var nf = this;
+
+  // Step 2.
+  if (!IsObject(nf) || (nf = intl_GuardToNumberFormat(nf)) === null) {
+    return callFunction(
+      intl_CallNumberFormatMethodIfWrapped,
+      this,
+      start,
+      end,
+      "Intl_NumberFormat_formatRange"
+    );
+  }
+
+  // Step 3.
+  if (start === undefined || end === undefined) {
+    ThrowTypeError(
+      JSMSG_UNDEFINED_NUMBER,
+      start === undefined ? "start" : "end",
+      "NumberFormat",
+      "formatRange"
+    );
+  }
+
+  // Steps 4-6.
+  return intl_FormatNumberRange(nf, start, end, /* formatToParts = */ false);
+}
+
+/**
+ * Intl.NumberFormat.prototype.formatRangeToParts ( start, end )
+ */
+function Intl_NumberFormat_formatRangeToParts(start, end) {
+  // Step 1.
+  var nf = this;
+
+  // Step 2.
+  if (!IsObject(nf) || (nf = intl_GuardToNumberFormat(nf)) === null) {
+    return callFunction(
+      intl_CallNumberFormatMethodIfWrapped,
+      this,
+      start,
+      end,
+      "Intl_NumberFormat_formatRangeToParts"
+    );
+  }
+
+  // Step 3.
+  if (start === undefined || end === undefined) {
+    ThrowTypeError(
+      JSMSG_UNDEFINED_NUMBER,
+      start === undefined ? "start" : "end",
+      "NumberFormat",
+      "formatRangeToParts"
+    );
+  }
+
+  // Steps 4-6.
+  return intl_FormatNumberRange(nf, start, end, /* formatToParts = */ true);
+}
+
+/**
+ * Returns the resolved options for a NumberFormat object.
+ *
+ * Spec: ECMAScript Internationalization API Specification, 11.4.5.
+ */
+function Intl_NumberFormat_resolvedOptions() {
+  // Steps 1-3.
+  var thisArg = UnwrapNumberFormat(this);
+  var nf = thisArg;
+  if (!IsObject(nf) || (nf = intl_GuardToNumberFormat(nf)) === null) {
+    return callFunction(
+      intl_CallNumberFormatMethodIfWrapped,
+      thisArg,
+      "Intl_NumberFormat_resolvedOptions"
+    );
+  }
+
+  var internals = getNumberFormatInternals(nf);
+
+  // Steps 4-5.
+  var result = {
+    locale: internals.locale,
+    numberingSystem: internals.numberingSystem,
+    style: internals.style,
+  };
+
+  // currency, currencyDisplay, and currencySign are only present for currency
+  // formatters.
+  assert(
+    hasOwn("currency", internals) === (internals.style === "currency"),
+    "currency is present iff style is 'currency'"
+  );
+  assert(
+    hasOwn("currencyDisplay", internals) === (internals.style === "currency"),
+    "currencyDisplay is present iff style is 'currency'"
+  );
+  assert(
+    hasOwn("currencySign", internals) === (internals.style === "currency"),
+    "currencySign is present iff style is 'currency'"
+  );
+
+  if (hasOwn("currency", internals)) {
+    DefineDataProperty(result, "currency", internals.currency);
+    DefineDataProperty(result, "currencyDisplay", internals.currencyDisplay);
+    DefineDataProperty(result, "currencySign", internals.currencySign);
+  }
+
+  // unit and unitDisplay are only present for unit formatters.
+  assert(
+    hasOwn("unit", internals) === (internals.style === "unit"),
+    "unit is present iff style is 'unit'"
+  );
+  assert(
+    hasOwn("unitDisplay", internals) === (internals.style === "unit"),
+    "unitDisplay is present iff style is 'unit'"
+  );
+
+  if (hasOwn("unit", internals)) {
+    DefineDataProperty(result, "unit", internals.unit);
+    DefineDataProperty(result, "unitDisplay", internals.unitDisplay);
+  }
+
+  DefineDataProperty(
+    result,
+    "minimumIntegerDigits",
+    internals.minimumIntegerDigits
+  );
+
+  // Min/Max fraction digits are either both present or not present at all.
+  assert(
+    hasOwn("minimumFractionDigits", internals) ===
+      hasOwn("maximumFractionDigits", internals),
+    "minimumFractionDigits is present iff maximumFractionDigits is present"
+  );
+
+  if (hasOwn("minimumFractionDigits", internals)) {
+    DefineDataProperty(
+      result,
+      "minimumFractionDigits",
+      internals.minimumFractionDigits
+    );
+    DefineDataProperty(
+      result,
+      "maximumFractionDigits",
+      internals.maximumFractionDigits
+    );
+  }
+
+  // Min/Max significant digits are either both present or not present at all.
+  assert(
+    hasOwn("minimumSignificantDigits", internals) ===
+      hasOwn("maximumSignificantDigits", internals),
+    "minimumSignificantDigits is present iff maximumSignificantDigits is present"
+  );
+
+  if (hasOwn("minimumSignificantDigits", internals)) {
+    DefineDataProperty(
+      result,
+      "minimumSignificantDigits",
+      internals.minimumSignificantDigits
+    );
+    DefineDataProperty(
+      result,
+      "maximumSignificantDigits",
+      internals.maximumSignificantDigits
+    );
+  }
+
+  DefineDataProperty(result, "useGrouping", internals.useGrouping);
+
+  var notation = internals.notation;
+  DefineDataProperty(result, "notation", notation);
+
+  if (notation === "compact") {
+    DefineDataProperty(result, "compactDisplay", internals.compactDisplay);
+  }
+
+  DefineDataProperty(result, "signDisplay", internals.signDisplay);
+
+#ifdef NIGHTLY_BUILD
+  DefineDataProperty(result, "roundingMode", internals.roundingMode);
+  DefineDataProperty(result, "roundingIncrement", internals.roundingIncrement);
+  DefineDataProperty(
+    result,
+    "trailingZeroDisplay",
+    internals.trailingZeroDisplay
+  );
+  DefineDataProperty(result, "roundingPriority", internals.roundingPriority);
+#endif
+
+  // Step 6.
+  return result;
+}
diff --git a/js/src/builtin/intl/NumberingSystems.yaml b/js/src/builtin/intl/NumberingSystems.yaml
new file mode 100644
index 0000000000..db287c10ef
--- /dev/null
+++ b/js/src/builtin/intl/NumberingSystems.yaml
@@ -0,0 +1,82 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# 12.1.7 PartitionNotationSubPattern ( numberFormat, x, n, exponent )
+#
+# Numbering systems with simple digit mappings
+#
+# https://tc39.es/ecma402/#table-numbering-system-digits
+
+# Run |make_intl_data numbering| to regenerate all files which reference this list
+# of numbering systems.
+
+- adlm
+- ahom
+- arab
+- arabext
+- bali
+- beng
+- bhks
+- brah
+- cakm
+- cham
+- deva
+- diak
+- fullwide
+- gong
+- gonm
+- gujr
+- guru
+- hanidec
+- hmng
+- hmnp
+- java
+- kali
+- kawi
+- khmr
+- knda
+- lana
+- lanatham
+- laoo
+- latn
+- lepc
+- limb
+- mathbold
+- mathdbl
+- mathmono
+- mathsanb
+- mathsans
+- mlym
+- modi
+- mong
+- mroo
+- mtei
+- mymr
+- mymrshan
+- mymrtlng
+- nagm
+- newa
+- nkoo
+- olck
+- orya
+- osma
+- rohg
+- saur
+- segment
+- shrd
+- sind
+- sinh
+- sora
+- sund
+- takr
+- talu
+- tamldec
+- telu
+- thai
+- tibt
+- tirh
+- tnsa
+- vaii
+- wara
+- wcho
diff --git a/js/src/builtin/intl/NumberingSystemsGenerated.h b/js/src/builtin/intl/NumberingSystemsGenerated.h
new file mode 100644
index 0000000000..f51d0f9c53
--- /dev/null
+++ b/js/src/builtin/intl/NumberingSystemsGenerated.h
@@ -0,0 +1,83 @@
+// Generated by make_intl_data.py. DO NOT EDIT.
+
+/**
+ * The list of numbering systems with simple digit mappings.
+ */
+
+#ifndef builtin_intl_NumberingSystemsGenerated_h
+#define builtin_intl_NumberingSystemsGenerated_h
+
+// clang-format off
+#define NUMBERING_SYSTEMS_WITH_SIMPLE_DIGIT_MAPPINGS \
+  "adlm", \
+  "ahom", \
+  "arab", \
+  "arabext", \
+  "bali", \
+  "beng", \
+  "bhks", \
+  "brah", \
+  "cakm", \
+  "cham", \
+  "deva", \
+  "diak", \
+  "fullwide", \
+  "gong", \
+  "gonm", \
+  "gujr", \
+  "guru", \
+  "hanidec", \
+  "hmng", \
+  "hmnp", \
+  "java", \
+  "kali", \
+  "kawi", \
+  "khmr", \
+  "knda", \
+  "lana", \
+  "lanatham", \
+  "laoo", \
+  "latn", \
+  "lepc", \
+  "limb", \
+  "mathbold", \
+  "mathdbl", \
+  "mathmono", \
+  "mathsanb", \
+  "mathsans", \
+  "mlym", \
+  "modi", \
+  "mong", \
+  "mroo", \
+  "mtei", \
+  "mymr", \
+  "mymrshan", \
+  "mymrtlng", \
+  "nagm", \
+  "newa", \
+  "nkoo", \
+  "olck", \
+  "orya", \
+  "osma", \
+  "rohg", \
+  "saur", \
+  "segment", \
+  "shrd", \
+  "sind", \
+  "sinh", \
+  "sora", \
+  "sund", \
+  "takr", \
+  "talu", \
+  "tamldec", \
+  "telu", \
+  "thai", \
+  "tibt", \
+  "tirh", \
+  "tnsa", \
+  "vaii", \
+  "wara", \
+  "wcho"
+// clang-format on
+
+#endif  // builtin_intl_NumberingSystemsGenerated_h
diff --git a/js/src/builtin/intl/PluralRules.cpp b/js/src/builtin/intl/PluralRules.cpp
new file mode 100644
index 0000000000..b7f9d3c6ca
--- /dev/null
+++ b/js/src/builtin/intl/PluralRules.cpp
@@ -0,0 +1,423 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Implementation of the Intl.PluralRules proposal. */
+
+#include "builtin/intl/PluralRules.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Casting.h"
+#include "mozilla/intl/PluralRules.h"
+
+#include "builtin/Array.h"
+#include "builtin/intl/CommonFunctions.h"
+#include "gc/GCContext.h"
+#include "js/PropertySpec.h"
+#include "vm/GlobalObject.h"
+#include "vm/JSContext.h"
+#include "vm/PlainObject.h"  // js::PlainObject
+#include "vm/StringType.h"
+#include "vm/WellKnownAtom.h"  // js_*_str
+
+#include "vm/JSObject-inl.h"
+#include "vm/NativeObject-inl.h"
+
+using namespace js;
+
+using mozilla::AssertedCast;
+
+const JSClassOps PluralRulesObject::classOps_ = {
+    nullptr,                      // addProperty
+    nullptr,                      // delProperty
+    nullptr,                      // enumerate
+    nullptr,                      // newEnumerate
+    nullptr,                      // resolve
+    nullptr,                      // mayResolve
+    PluralRulesObject::finalize,  // finalize
+    nullptr,                      // call
+    nullptr,                      // construct
+    nullptr,                      // trace
+};
+
+const JSClass PluralRulesObject::class_ = {
+    "Intl.PluralRules",
+    JSCLASS_HAS_RESERVED_SLOTS(PluralRulesObject::SLOT_COUNT) |
+        JSCLASS_HAS_CACHED_PROTO(JSProto_PluralRules) |
+        JSCLASS_FOREGROUND_FINALIZE,
+    &PluralRulesObject::classOps_, &PluralRulesObject::classSpec_};
+
+const JSClass& PluralRulesObject::protoClass_ = PlainObject::class_;
+
+static bool pluralRules_toSource(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  args.rval().setString(cx->names().PluralRules);
+  return true;
+}
+
+static const JSFunctionSpec pluralRules_static_methods[] = {
+    JS_SELF_HOSTED_FN("supportedLocalesOf",
+                      "Intl_PluralRules_supportedLocalesOf", 1, 0),
+    JS_FS_END};
+
+static const JSFunctionSpec pluralRules_methods[] = {
+    JS_SELF_HOSTED_FN("resolvedOptions", "Intl_PluralRules_resolvedOptions", 0,
+                      0),
+    JS_SELF_HOSTED_FN("select", "Intl_PluralRules_select", 1, 0),
+#ifdef NIGHTLY_BUILD
+    JS_SELF_HOSTED_FN("selectRange", "Intl_PluralRules_selectRange", 2, 0),
+#endif
+    JS_FN(js_toSource_str, pluralRules_toSource, 0, 0), JS_FS_END};
+
+static const JSPropertySpec pluralRules_properties[] = {
+    JS_STRING_SYM_PS(toStringTag, "Intl.PluralRules", JSPROP_READONLY),
+    JS_PS_END};
+
+static bool PluralRules(JSContext* cx, unsigned argc, Value* vp);
+
+const ClassSpec PluralRulesObject::classSpec_ = {
+    GenericCreateConstructor<PluralRules, 0, gc::AllocKind::FUNCTION>,
+    GenericCreatePrototype<PluralRulesObject>,
+    pluralRules_static_methods,
+    nullptr,
+    pluralRules_methods,
+    pluralRules_properties,
+    nullptr,
+    ClassSpec::DontDefineConstructor};
+
+/**
+ * PluralRules constructor.
+ * Spec: ECMAScript 402 API, PluralRules, 13.2.1
+ */
+static bool PluralRules(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+
+  // Step 1.
+  if (!ThrowIfNotConstructing(cx, args, "Intl.PluralRules")) {
+    return false;
+  }
+
+  // Step 2 (Inlined 9.1.14, OrdinaryCreateFromConstructor).
+  RootedObject proto(cx);
+  if (!GetPrototypeFromBuiltinConstructor(cx, args, JSProto_PluralRules,
+                                          &proto)) {
+    return false;
+  }
+
+  Rooted<PluralRulesObject*> pluralRules(cx);
+  pluralRules = NewObjectWithClassProto<PluralRulesObject>(cx, proto);
+  if (!pluralRules) {
+    return false;
+  }
+
+  HandleValue locales = args.get(0);
+  HandleValue options = args.get(1);
+
+  // Step 3.
+  if (!intl::InitializeObject(cx, pluralRules,
+                              cx->names().InitializePluralRules, locales,
+                              options)) {
+    return false;
+  }
+
+  args.rval().setObject(*pluralRules);
+  return true;
+}
+
+void js::PluralRulesObject::finalize(JS::GCContext* gcx, JSObject* obj) {
+  MOZ_ASSERT(gcx->onMainThread());
+
+  auto* pluralRules = &obj->as<PluralRulesObject>();
+  if (mozilla::intl::PluralRules* pr = pluralRules->getPluralRules()) {
+    intl::RemoveICUCellMemory(
+        gcx, obj, PluralRulesObject::UPluralRulesEstimatedMemoryUse);
+    delete pr;
+  }
+}
+
+static JSString* KeywordToString(mozilla::intl::PluralRules::Keyword keyword,
+                                 JSContext* cx) {
+  using Keyword = mozilla::intl::PluralRules::Keyword;
+  switch (keyword) {
+    case Keyword::Zero: {
+      return cx->names().zero;
+    }
+    case Keyword::One: {
+      return cx->names().one;
+    }
+    case Keyword::Two: {
+      return cx->names().two;
+    }
+    case Keyword::Few: {
+      return cx->names().few;
+    }
+    case Keyword::Many: {
+      return cx->names().many;
+    }
+    case Keyword::Other: {
+      return cx->names().other;
+    }
+  }
+  MOZ_CRASH("Unexpected PluralRules keyword");
+}
+
+/**
+ * Returns a new intl::PluralRules with the locale and type options of the given
+ * PluralRules.
+ */
+static mozilla::intl::PluralRules* NewPluralRules(
+    JSContext* cx, Handle<PluralRulesObject*> pluralRules) {
+  RootedObject internals(cx, intl::GetInternalsObject(cx, pluralRules));
+  if (!internals) {
+    return nullptr;
+  }
+
+  RootedValue value(cx);
+
+  if (!GetProperty(cx, internals, internals, cx->names().locale, &value)) {
+    return nullptr;
+  }
+  UniqueChars locale = intl::EncodeLocale(cx, value.toString());
+  if (!locale) {
+    return nullptr;
+  }
+
+  using PluralRules = mozilla::intl::PluralRules;
+  mozilla::intl::PluralRulesOptions options;
+
+  if (!GetProperty(cx, internals, internals, cx->names().type, &value)) {
+    return nullptr;
+  }
+
+  {
+    JSLinearString* type = value.toString()->ensureLinear(cx);
+    if (!type) {
+      return nullptr;
+    }
+
+    if (StringEqualsLiteral(type, "ordinal")) {
+      options.mPluralType = PluralRules::Type::Ordinal;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(type, "cardinal"));
+      options.mPluralType = PluralRules::Type::Cardinal;
+    }
+  }
+
+  bool hasMinimumSignificantDigits;
+  if (!HasProperty(cx, internals, cx->names().minimumSignificantDigits,
+                   &hasMinimumSignificantDigits)) {
+    return nullptr;
+  }
+
+  if (hasMinimumSignificantDigits) {
+    if (!GetProperty(cx, internals, internals,
+                     cx->names().minimumSignificantDigits, &value)) {
+      return nullptr;
+    }
+    uint32_t minimumSignificantDigits = AssertedCast<uint32_t>(value.toInt32());
+
+    if (!GetProperty(cx, internals, internals,
+                     cx->names().maximumSignificantDigits, &value)) {
+      return nullptr;
+    }
+    uint32_t maximumSignificantDigits = AssertedCast<uint32_t>(value.toInt32());
+
+    options.mSignificantDigits = mozilla::Some(
+        std::make_pair(minimumSignificantDigits, maximumSignificantDigits));
+  } else {
+    if (!GetProperty(cx, internals, internals,
+                     cx->names().minimumFractionDigits, &value)) {
+      return nullptr;
+    }
+    uint32_t minimumFractionDigits = AssertedCast<uint32_t>(value.toInt32());
+
+    if (!GetProperty(cx, internals, internals,
+                     cx->names().maximumFractionDigits, &value)) {
+      return nullptr;
+    }
+    uint32_t maximumFractionDigits = AssertedCast<uint32_t>(value.toInt32());
+
+    options.mFractionDigits = mozilla::Some(
+        std::make_pair(minimumFractionDigits, maximumFractionDigits));
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().roundingPriority,
+                   &value)) {
+    return nullptr;
+  }
+
+  {
+    JSLinearString* roundingPriority = value.toString()->ensureLinear(cx);
+    if (!roundingPriority) {
+      return nullptr;
+    }
+
+    using RoundingPriority =
+        mozilla::intl::PluralRulesOptions::RoundingPriority;
+
+    RoundingPriority priority;
+    if (StringEqualsLiteral(roundingPriority, "auto")) {
+      priority = RoundingPriority::Auto;
+    } else if (StringEqualsLiteral(roundingPriority, "morePrecision")) {
+      priority = RoundingPriority::MorePrecision;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(roundingPriority, "lessPrecision"));
+      priority = RoundingPriority::LessPrecision;
+    }
+
+    options.mRoundingPriority = priority;
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().minimumIntegerDigits,
+                   &value)) {
+    return nullptr;
+  }
+  options.mMinIntegerDigits =
+      mozilla::Some(AssertedCast<uint32_t>(value.toInt32()));
+
+  auto result = PluralRules::TryCreate(locale.get(), options);
+  if (result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return nullptr;
+  }
+
+  return result.unwrap().release();
+}
+
+static mozilla::intl::PluralRules* GetOrCreatePluralRules(
+    JSContext* cx, Handle<PluralRulesObject*> pluralRules) {
+  // Obtain a cached PluralRules object.
+  mozilla::intl::PluralRules* pr = pluralRules->getPluralRules();
+  if (pr) {
+    return pr;
+  }
+
+  pr = NewPluralRules(cx, pluralRules);
+  if (!pr) {
+    return nullptr;
+  }
+  pluralRules->setPluralRules(pr);
+
+  intl::AddICUCellMemory(pluralRules,
+                         PluralRulesObject::UPluralRulesEstimatedMemoryUse);
+  return pr;
+}
+
+bool js::intl_SelectPluralRule(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 2);
+
+  Rooted<PluralRulesObject*> pluralRules(
+      cx, &args[0].toObject().as<PluralRulesObject>());
+
+  double x = args[1].toNumber();
+
+  using PluralRules = mozilla::intl::PluralRules;
+  PluralRules* pr = GetOrCreatePluralRules(cx, pluralRules);
+  if (!pr) {
+    return false;
+  }
+
+  auto keywordResult = pr->Select(x);
+  if (keywordResult.isErr()) {
+    intl::ReportInternalError(cx, keywordResult.unwrapErr());
+    return false;
+  }
+
+  JSString* str = KeywordToString(keywordResult.unwrap(), cx);
+  MOZ_ASSERT(str);
+
+  args.rval().setString(str);
+  return true;
+}
+
+/**
+ * ResolvePluralRange ( pluralRules, x, y )
+ * PluralRuleSelectRange ( locale, type, xp, yp )
+ */
+bool js::intl_SelectPluralRuleRange(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 3);
+
+  // Steps 1-2.
+  Rooted<PluralRulesObject*> pluralRules(
+      cx, &args[0].toObject().as<PluralRulesObject>());
+
+  // Steps 3-4.
+  double x = args[1].toNumber();
+  double y = args[2].toNumber();
+
+  // Step 5.
+  if (mozilla::IsNaN(x)) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_NAN_NUMBER_RANGE, "start", "PluralRules",
+                              "selectRange");
+    return false;
+  }
+  if (mozilla::IsNaN(y)) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_NAN_NUMBER_RANGE, "end", "PluralRules",
+                              "selectRange");
+    return false;
+  }
+
+  using PluralRules = mozilla::intl::PluralRules;
+  PluralRules* pr = GetOrCreatePluralRules(cx, pluralRules);
+  if (!pr) {
+    return false;
+  }
+
+  // Steps 6-10.
+  auto keywordResult = pr->SelectRange(x, y);
+  if (keywordResult.isErr()) {
+    intl::ReportInternalError(cx, keywordResult.unwrapErr());
+    return false;
+  }
+
+  JSString* str = KeywordToString(keywordResult.unwrap(), cx);
+  MOZ_ASSERT(str);
+
+  args.rval().setString(str);
+  return true;
+}
+
+bool js::intl_GetPluralCategories(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+
+  Rooted<PluralRulesObject*> pluralRules(
+      cx, &args[0].toObject().as<PluralRulesObject>());
+
+  using PluralRules = mozilla::intl::PluralRules;
+  PluralRules* pr = GetOrCreatePluralRules(cx, pluralRules);
+  if (!pr) {
+    return false;
+  }
+
+  auto categoriesResult = pr->Categories();
+  if (categoriesResult.isErr()) {
+    intl::ReportInternalError(cx, categoriesResult.unwrapErr());
+    return false;
+  }
+  auto categories = categoriesResult.unwrap();
+
+  ArrayObject* res = NewDenseFullyAllocatedArray(cx, categories.size());
+  if (!res) {
+    return false;
+  }
+  res->setDenseInitializedLength(categories.size());
+
+  size_t index = 0;
+  for (PluralRules::Keyword keyword : categories) {
+    JSString* str = KeywordToString(keyword, cx);
+    MOZ_ASSERT(str);
+
+    res->initDenseElement(index++, StringValue(str));
+  }
+  MOZ_ASSERT(index == categories.size());
+
+  args.rval().setObject(*res);
+  return true;
+}
diff --git a/js/src/builtin/intl/PluralRules.h b/js/src/builtin/intl/PluralRules.h
new file mode 100644
index 0000000000..86d8ec105d
--- /dev/null
+++ b/js/src/builtin/intl/PluralRules.h
@@ -0,0 +1,98 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_PluralRules_h
+#define builtin_intl_PluralRules_h
+
+#include "builtin/SelfHostingDefines.h"
+#include "js/Class.h"
+#include "vm/NativeObject.h"
+
+namespace mozilla::intl {
+class PluralRules;
+}
+
+namespace js {
+
+class PluralRulesObject : public NativeObject {
+ public:
+  static const JSClass class_;
+  static const JSClass& protoClass_;
+
+  static constexpr uint32_t INTERNALS_SLOT = 0;
+  static constexpr uint32_t PLURAL_RULES_SLOT = 1;
+  static constexpr uint32_t SLOT_COUNT = 2;
+
+  static_assert(INTERNALS_SLOT == INTL_INTERNALS_OBJECT_SLOT,
+                "INTERNALS_SLOT must match self-hosting define for internals "
+                "object slot");
+
+  // Estimated memory use for UPluralRules (see IcuMemoryUsage).
+  // Includes usage for UNumberFormat and UNumberRangeFormatter since our
+  // PluralRules implementations contains a NumberFormat and a NumberRangeFormat
+  // object.
+  static constexpr size_t UPluralRulesEstimatedMemoryUse = 5736;
+
+  mozilla::intl::PluralRules* getPluralRules() const {
+    const auto& slot = getFixedSlot(PLURAL_RULES_SLOT);
+    if (slot.isUndefined()) {
+      return nullptr;
+    }
+    return static_cast<mozilla::intl::PluralRules*>(slot.toPrivate());
+  }
+
+  void setPluralRules(mozilla::intl::PluralRules* pluralRules) {
+    setFixedSlot(PLURAL_RULES_SLOT, PrivateValue(pluralRules));
+  }
+
+ private:
+  static const JSClassOps classOps_;
+  static const ClassSpec classSpec_;
+
+  static void finalize(JS::GCContext* gcx, JSObject* obj);
+};
+
+/**
+ * Returns a plural rule for the number x according to the effective
+ * locale and the formatting options of the given PluralRules.
+ *
+ * A plural rule is a grammatical category that expresses count distinctions
+ * (such as "one", "two", "few" etc.).
+ *
+ * Usage: rule = intl_SelectPluralRule(pluralRules, x)
+ */
+[[nodiscard]] extern bool intl_SelectPluralRule(JSContext* cx, unsigned argc,
+                                                JS::Value* vp);
+
+/**
+ * Returns a plural rule for the number range «x - y» according to the effective
+ * locale and the formatting options of the given PluralRules.
+ *
+ * A plural rule is a grammatical category that expresses count distinctions
+ * (such as "one", "two", "few" etc.).
+ *
+ * Usage: rule = intl_SelectPluralRuleRange(pluralRules, x, y)
+ */
+[[nodiscard]] extern bool intl_SelectPluralRuleRange(JSContext* cx,
+                                                     unsigned argc,
+                                                     JS::Value* vp);
+
+/**
+ * Returns an array of plural rules categories for a given pluralRules object.
+ *
+ * Usage: categories = intl_GetPluralCategories(pluralRules)
+ *
+ * Example:
+ *
+ * pluralRules = new Intl.PluralRules('pl', {type: 'cardinal'});
+ * intl_getPluralCategories(pluralRules); // ['one', 'few', 'many', 'other']
+ */
+[[nodiscard]] extern bool intl_GetPluralCategories(JSContext* cx, unsigned argc,
+                                                   JS::Value* vp);
+
+}  // namespace js
+
+#endif /* builtin_intl_PluralRules_h */
diff --git a/js/src/builtin/intl/PluralRules.js b/js/src/builtin/intl/PluralRules.js
new file mode 100644
index 0000000000..f9d8e310a2
--- /dev/null
+++ b/js/src/builtin/intl/PluralRules.js
@@ -0,0 +1,390 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * PluralRules internal properties.
+ *
+ * Spec: ECMAScript 402 API, PluralRules, 13.3.3.
+ */
+var pluralRulesInternalProperties = {
+  localeData: pluralRulesLocaleData,
+  relevantExtensionKeys: [],
+};
+
+function pluralRulesLocaleData() {
+  // PluralRules don't support any extension keys.
+  return {};
+}
+
+/**
+ * Compute an internal properties object from |lazyPluralRulesData|.
+ */
+function resolvePluralRulesInternals(lazyPluralRulesData) {
+  assert(IsObject(lazyPluralRulesData), "lazy data not an object?");
+
+  var internalProps = std_Object_create(null);
+
+  var PluralRules = pluralRulesInternalProperties;
+
+  // Compute effective locale.
+
+  // Step 10.
+  var localeData = PluralRules.localeData;
+
+  // Step 11.
+  const r = ResolveLocale(
+    "PluralRules",
+    lazyPluralRulesData.requestedLocales,
+    lazyPluralRulesData.opt,
+    PluralRules.relevantExtensionKeys,
+    localeData
+  );
+
+  // Step 12.
+  internalProps.locale = r.locale;
+
+  // Step 8.
+  internalProps.type = lazyPluralRulesData.type;
+
+  // Step 9.
+  internalProps.minimumIntegerDigits = lazyPluralRulesData.minimumIntegerDigits;
+
+  if ("minimumFractionDigits" in lazyPluralRulesData) {
+    assert(
+      "maximumFractionDigits" in lazyPluralRulesData,
+      "min/max frac digits mismatch"
+    );
+    internalProps.minimumFractionDigits =
+      lazyPluralRulesData.minimumFractionDigits;
+    internalProps.maximumFractionDigits =
+      lazyPluralRulesData.maximumFractionDigits;
+  }
+
+  if ("minimumSignificantDigits" in lazyPluralRulesData) {
+    assert(
+      "maximumSignificantDigits" in lazyPluralRulesData,
+      "min/max sig digits mismatch"
+    );
+    internalProps.minimumSignificantDigits =
+      lazyPluralRulesData.minimumSignificantDigits;
+    internalProps.maximumSignificantDigits =
+      lazyPluralRulesData.maximumSignificantDigits;
+  }
+
+  // Intl.NumberFormat v3 Proposal
+  internalProps.roundingPriority = lazyPluralRulesData.roundingPriority;
+
+  // Step 13 (lazily computed on first access).
+  internalProps.pluralCategories = null;
+
+  return internalProps;
+}
+
+/**
+ * Returns an object containing the PluralRules internal properties of |obj|.
+ */
+function getPluralRulesInternals(obj) {
+  assert(IsObject(obj), "getPluralRulesInternals called with non-object");
+  assert(
+    intl_GuardToPluralRules(obj) !== null,
+    "getPluralRulesInternals called with non-PluralRules"
+  );
+
+  var internals = getIntlObjectInternals(obj);
+  assert(
+    internals.type === "PluralRules",
+    "bad type escaped getIntlObjectInternals"
+  );
+
+  var internalProps = maybeInternalProperties(internals);
+  if (internalProps) {
+    return internalProps;
+  }
+
+  internalProps = resolvePluralRulesInternals(internals.lazyData);
+  setInternalProperties(internals, internalProps);
+  return internalProps;
+}
+
+/**
+ * Initializes an object as a PluralRules.
+ *
+ * This method is complicated a moderate bit by its implementing initialization
+ * as a *lazy* concept.  Everything that must happen now, does -- but we defer
+ * all the work we can until the object is actually used as a PluralRules.
+ * This later work occurs in |resolvePluralRulesInternals|; steps not noted
+ * here occur there.
+ *
+ * Spec: ECMAScript 402 API, PluralRules, 13.1.1.
+ */
+function InitializePluralRules(pluralRules, locales, options) {
+  assert(IsObject(pluralRules), "InitializePluralRules called with non-object");
+  assert(
+    intl_GuardToPluralRules(pluralRules) !== null,
+    "InitializePluralRules called with non-PluralRules"
+  );
+
+  // Lazy PluralRules data has the following structure:
+  //
+  //   {
+  //     requestedLocales: List of locales,
+  //     type: "cardinal" / "ordinal",
+  //
+  //     opt: // opt object computer in InitializePluralRules
+  //       {
+  //         localeMatcher: "lookup" / "best fit",
+  //       }
+  //
+  //     minimumIntegerDigits: integer ∈ [1, 21],
+  //
+  //     // optional, mutually exclusive with the significant-digits option
+  //     minimumFractionDigits: integer ∈ [0, 20],
+  //     maximumFractionDigits: integer ∈ [0, 20],
+  //
+  //     // optional, mutually exclusive with the fraction-digits option
+  //     minimumSignificantDigits: integer ∈ [1, 21],
+  //     maximumSignificantDigits: integer ∈ [1, 21],
+  //
+  //     roundingPriority: "auto" / "lessPrecision" / "morePrecision",
+  //   }
+  //
+  // Note that lazy data is only installed as a final step of initialization,
+  // so every PluralRules lazy data object has *all* these properties, never a
+  // subset of them.
+  const lazyPluralRulesData = std_Object_create(null);
+
+  // Step 1.
+  let requestedLocales = CanonicalizeLocaleList(locales);
+  lazyPluralRulesData.requestedLocales = requestedLocales;
+
+  // Steps 2-3.
+  if (options === undefined) {
+    options = std_Object_create(null);
+  } else {
+    options = ToObject(options);
+  }
+
+  // Step 4.
+  let opt = new_Record();
+  lazyPluralRulesData.opt = opt;
+
+  // Steps 5-6.
+  let matcher = GetOption(
+    options,
+    "localeMatcher",
+    "string",
+    ["lookup", "best fit"],
+    "best fit"
+  );
+  opt.localeMatcher = matcher;
+
+  // Step 7.
+  const type = GetOption(
+    options,
+    "type",
+    "string",
+    ["cardinal", "ordinal"],
+    "cardinal"
+  );
+  lazyPluralRulesData.type = type;
+
+  // Step 9.
+  SetNumberFormatDigitOptions(lazyPluralRulesData, options, 0, 3, "standard");
+
+  // Step 15.
+  //
+  // We've done everything that must be done now: mark the lazy data as fully
+  // computed and install it.
+  initializeIntlObject(pluralRules, "PluralRules", lazyPluralRulesData);
+}
+
+/**
+ * Returns the subset of the given locale list for which this locale list has a
+ * matching (possibly fallback) locale. Locales appear in the same order in the
+ * returned list as in the input list.
+ *
+ * Spec: ECMAScript 402 API, PluralRules, 13.3.2.
+ */
+function Intl_PluralRules_supportedLocalesOf(locales /*, options*/) {
+  var options = arguments.length > 1 ? arguments[1] : undefined;
+
+  // Step 1.
+  var availableLocales = "PluralRules";
+
+  // Step 2.
+  let requestedLocales = CanonicalizeLocaleList(locales);
+
+  // Step 3.
+  return SupportedLocales(availableLocales, requestedLocales, options);
+}
+
+/**
+ * Returns a String value representing the plural category matching
+ * the number passed as value according to the
+ * effective locale and the formatting options of this PluralRules.
+ *
+ * Spec: ECMAScript 402 API, PluralRules, 13.4.3.
+ */
+function Intl_PluralRules_select(value) {
+  // Step 1.
+  let pluralRules = this;
+
+  // Steps 2-3.
+  if (
+    !IsObject(pluralRules) ||
+    (pluralRules = intl_GuardToPluralRules(pluralRules)) === null
+  ) {
+    return callFunction(
+      intl_CallPluralRulesMethodIfWrapped,
+      this,
+      value,
+      "Intl_PluralRules_select"
+    );
+  }
+
+  // Step 4.
+  let n = ToNumber(value);
+
+  // Ensure the PluralRules internals are resolved.
+  getPluralRulesInternals(pluralRules);
+
+  // Step 5.
+  return intl_SelectPluralRule(pluralRules, n);
+}
+
+/**
+ * Returns a String value representing the plural category matching the input
+ * number range according to the effective locale and the formatting options
+ * of this PluralRules.
+ */
+function Intl_PluralRules_selectRange(start, end) {
+  // Step 1.
+  var pluralRules = this;
+
+  // Step 2.
+  if (
+    !IsObject(pluralRules) ||
+    (pluralRules = intl_GuardToPluralRules(pluralRules)) === null
+  ) {
+    return callFunction(
+      intl_CallPluralRulesMethodIfWrapped,
+      this,
+      start,
+      end,
+      "Intl_PluralRules_selectRange"
+    );
+  }
+
+  // Step 3.
+  if (start === undefined || end === undefined) {
+    ThrowTypeError(
+      JSMSG_UNDEFINED_NUMBER,
+      start === undefined ? "start" : "end",
+      "PluralRules",
+      "selectRange"
+    );
+  }
+
+  // Step 4.
+  var x = ToNumber(start);
+
+  // Step 5.
+  var y = ToNumber(end);
+
+  // Step 6.
+  return intl_SelectPluralRuleRange(pluralRules, x, y);
+}
+
+/**
+ * Returns the resolved options for a PluralRules object.
+ *
+ * Spec: ECMAScript 402 API, PluralRules, 13.4.4.
+ */
+function Intl_PluralRules_resolvedOptions() {
+  // Step 1.
+  var pluralRules = this;
+
+  // Steps 2-3.
+  if (
+    !IsObject(pluralRules) ||
+    (pluralRules = intl_GuardToPluralRules(pluralRules)) === null
+  ) {
+    return callFunction(
+      intl_CallPluralRulesMethodIfWrapped,
+      this,
+      "Intl_PluralRules_resolvedOptions"
+    );
+  }
+
+  var internals = getPluralRulesInternals(pluralRules);
+
+  // Steps 4-5.
+  var result = {
+    locale: internals.locale,
+    type: internals.type,
+    minimumIntegerDigits: internals.minimumIntegerDigits,
+  };
+
+  // Min/Max fraction digits are either both present or not present at all.
+  assert(
+    hasOwn("minimumFractionDigits", internals) ===
+      hasOwn("maximumFractionDigits", internals),
+    "minimumFractionDigits is present iff maximumFractionDigits is present"
+  );
+
+  if (hasOwn("minimumFractionDigits", internals)) {
+    DefineDataProperty(
+      result,
+      "minimumFractionDigits",
+      internals.minimumFractionDigits
+    );
+    DefineDataProperty(
+      result,
+      "maximumFractionDigits",
+      internals.maximumFractionDigits
+    );
+  }
+
+  // Min/Max significant digits are either both present or not present at all.
+  assert(
+    hasOwn("minimumSignificantDigits", internals) ===
+      hasOwn("maximumSignificantDigits", internals),
+    "minimumSignificantDigits is present iff maximumSignificantDigits is present"
+  );
+
+  if (hasOwn("minimumSignificantDigits", internals)) {
+    DefineDataProperty(
+      result,
+      "minimumSignificantDigits",
+      internals.minimumSignificantDigits
+    );
+    DefineDataProperty(
+      result,
+      "maximumSignificantDigits",
+      internals.maximumSignificantDigits
+    );
+  }
+
+  // Step 6.
+  var internalsPluralCategories = internals.pluralCategories;
+  if (internalsPluralCategories === null) {
+    internalsPluralCategories = intl_GetPluralCategories(pluralRules);
+    internals.pluralCategories = internalsPluralCategories;
+  }
+
+  var pluralCategories = [];
+  for (var i = 0; i < internalsPluralCategories.length; i++) {
+    DefineDataProperty(pluralCategories, i, internalsPluralCategories[i]);
+  }
+
+  // Step 7.
+  DefineDataProperty(result, "pluralCategories", pluralCategories);
+
+#ifdef NIGHTLY_BUILD
+  DefineDataProperty(result, "roundingPriority", internals.roundingPriority);
+#endif
+
+  // Step 8.
+  return result;
+}
diff --git a/js/src/builtin/intl/RelativeTimeFormat.cpp b/js/src/builtin/intl/RelativeTimeFormat.cpp
new file mode 100644
index 0000000000..c847fe0e6c
--- /dev/null
+++ b/js/src/builtin/intl/RelativeTimeFormat.cpp
@@ -0,0 +1,403 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Implementation of the Intl.RelativeTimeFormat proposal. */
+
+#include "builtin/intl/RelativeTimeFormat.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/FloatingPoint.h"
+#include "mozilla/intl/RelativeTimeFormat.h"
+
+#include "builtin/intl/CommonFunctions.h"
+#include "builtin/intl/FormatBuffer.h"
+#include "builtin/intl/LanguageTag.h"
+#include "gc/GCContext.h"
+#include "js/friend/ErrorMessages.h"  // js::GetErrorMessage, JSMSG_*
+#include "js/PropertySpec.h"
+#include "vm/GlobalObject.h"
+#include "vm/JSContext.h"
+#include "vm/PlainObject.h"  // js::PlainObject
+#include "vm/Printer.h"
+#include "vm/StringType.h"
+#include "vm/WellKnownAtom.h"  // js_*_str
+
+#include "vm/NativeObject-inl.h"
+
+using namespace js;
+
+/**************** RelativeTimeFormat *****************/
+
+const JSClassOps RelativeTimeFormatObject::classOps_ = {
+    nullptr,                             // addProperty
+    nullptr,                             // delProperty
+    nullptr,                             // enumerate
+    nullptr,                             // newEnumerate
+    nullptr,                             // resolve
+    nullptr,                             // mayResolve
+    RelativeTimeFormatObject::finalize,  // finalize
+    nullptr,                             // call
+    nullptr,                             // construct
+    nullptr,                             // trace
+};
+
+const JSClass RelativeTimeFormatObject::class_ = {
+    "Intl.RelativeTimeFormat",
+    JSCLASS_HAS_RESERVED_SLOTS(RelativeTimeFormatObject::SLOT_COUNT) |
+        JSCLASS_HAS_CACHED_PROTO(JSProto_RelativeTimeFormat) |
+        JSCLASS_FOREGROUND_FINALIZE,
+    &RelativeTimeFormatObject::classOps_,
+    &RelativeTimeFormatObject::classSpec_};
+
+const JSClass& RelativeTimeFormatObject::protoClass_ = PlainObject::class_;
+
+static bool relativeTimeFormat_toSource(JSContext* cx, unsigned argc,
+                                        Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  args.rval().setString(cx->names().RelativeTimeFormat);
+  return true;
+}
+
+static const JSFunctionSpec relativeTimeFormat_static_methods[] = {
+    JS_SELF_HOSTED_FN("supportedLocalesOf",
+                      "Intl_RelativeTimeFormat_supportedLocalesOf", 1, 0),
+    JS_FS_END};
+
+static const JSFunctionSpec relativeTimeFormat_methods[] = {
+    JS_SELF_HOSTED_FN("resolvedOptions",
+                      "Intl_RelativeTimeFormat_resolvedOptions", 0, 0),
+    JS_SELF_HOSTED_FN("format", "Intl_RelativeTimeFormat_format", 2, 0),
+    JS_SELF_HOSTED_FN("formatToParts", "Intl_RelativeTimeFormat_formatToParts",
+                      2, 0),
+    JS_FN(js_toSource_str, relativeTimeFormat_toSource, 0, 0), JS_FS_END};
+
+static const JSPropertySpec relativeTimeFormat_properties[] = {
+    JS_STRING_SYM_PS(toStringTag, "Intl.RelativeTimeFormat", JSPROP_READONLY),
+    JS_PS_END};
+
+static bool RelativeTimeFormat(JSContext* cx, unsigned argc, Value* vp);
+
+const ClassSpec RelativeTimeFormatObject::classSpec_ = {
+    GenericCreateConstructor<RelativeTimeFormat, 0, gc::AllocKind::FUNCTION>,
+    GenericCreatePrototype<RelativeTimeFormatObject>,
+    relativeTimeFormat_static_methods,
+    nullptr,
+    relativeTimeFormat_methods,
+    relativeTimeFormat_properties,
+    nullptr,
+    ClassSpec::DontDefineConstructor};
+
+/**
+ * RelativeTimeFormat constructor.
+ * Spec: ECMAScript 402 API, RelativeTimeFormat, 1.1
+ */
+static bool RelativeTimeFormat(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+
+  // Step 1.
+  if (!ThrowIfNotConstructing(cx, args, "Intl.RelativeTimeFormat")) {
+    return false;
+  }
+
+  // Step 2 (Inlined 9.1.14, OrdinaryCreateFromConstructor).
+  RootedObject proto(cx);
+  if (!GetPrototypeFromBuiltinConstructor(cx, args, JSProto_RelativeTimeFormat,
+                                          &proto)) {
+    return false;
+  }
+
+  Rooted<RelativeTimeFormatObject*> relativeTimeFormat(cx);
+  relativeTimeFormat =
+      NewObjectWithClassProto<RelativeTimeFormatObject>(cx, proto);
+  if (!relativeTimeFormat) {
+    return false;
+  }
+
+  HandleValue locales = args.get(0);
+  HandleValue options = args.get(1);
+
+  // Step 3.
+  if (!intl::InitializeObject(cx, relativeTimeFormat,
+                              cx->names().InitializeRelativeTimeFormat, locales,
+                              options)) {
+    return false;
+  }
+
+  args.rval().setObject(*relativeTimeFormat);
+  return true;
+}
+
+void js::RelativeTimeFormatObject::finalize(JS::GCContext* gcx, JSObject* obj) {
+  MOZ_ASSERT(gcx->onMainThread());
+
+  if (mozilla::intl::RelativeTimeFormat* rtf =
+          obj->as<RelativeTimeFormatObject>().getRelativeTimeFormatter()) {
+    intl::RemoveICUCellMemory(gcx, obj,
+                              RelativeTimeFormatObject::EstimatedMemoryUse);
+
+    // This was allocated using `new` in mozilla::intl::RelativeTimeFormat,
+    // so we delete here.
+    delete rtf;
+  }
+}
+
+/**
+ * Returns a new URelativeDateTimeFormatter with the locale and options of the
+ * given RelativeTimeFormatObject.
+ */
+static mozilla::intl::RelativeTimeFormat* NewRelativeTimeFormatter(
+    JSContext* cx, Handle<RelativeTimeFormatObject*> relativeTimeFormat) {
+  RootedObject internals(cx, intl::GetInternalsObject(cx, relativeTimeFormat));
+  if (!internals) {
+    return nullptr;
+  }
+
+  RootedValue value(cx);
+
+  if (!GetProperty(cx, internals, internals, cx->names().locale, &value)) {
+    return nullptr;
+  }
+
+  // ICU expects numberingSystem as a Unicode locale extensions on locale.
+
+  mozilla::intl::Locale tag;
+  {
+    Rooted<JSLinearString*> locale(cx, value.toString()->ensureLinear(cx));
+    if (!locale) {
+      return nullptr;
+    }
+
+    if (!intl::ParseLocale(cx, locale, tag)) {
+      return nullptr;
+    }
+  }
+
+  JS::RootedVector<intl::UnicodeExtensionKeyword> keywords(cx);
+
+  if (!GetProperty(cx, internals, internals, cx->names().numberingSystem,
+                   &value)) {
+    return nullptr;
+  }
+
+  {
+    JSLinearString* numberingSystem = value.toString()->ensureLinear(cx);
+    if (!numberingSystem) {
+      return nullptr;
+    }
+
+    if (!keywords.emplaceBack("nu", numberingSystem)) {
+      return nullptr;
+    }
+  }
+
+  // |ApplyUnicodeExtensionToTag| applies the new keywords to the front of the
+  // Unicode extension subtag. We're then relying on ICU to follow RFC 6067,
+  // which states that any trailing keywords using the same key should be
+  // ignored.
+  if (!intl::ApplyUnicodeExtensionToTag(cx, tag, keywords)) {
+    return nullptr;
+  }
+
+  intl::FormatBuffer<char> buffer(cx);
+  if (auto result = tag.ToString(buffer); result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return nullptr;
+  }
+
+  UniqueChars locale = buffer.extractStringZ();
+  if (!locale) {
+    return nullptr;
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().style, &value)) {
+    return nullptr;
+  }
+
+  using RelativeTimeFormatOptions = mozilla::intl::RelativeTimeFormatOptions;
+  RelativeTimeFormatOptions options;
+  {
+    JSLinearString* style = value.toString()->ensureLinear(cx);
+    if (!style) {
+      return nullptr;
+    }
+
+    if (StringEqualsLiteral(style, "short")) {
+      options.style = RelativeTimeFormatOptions::Style::Short;
+    } else if (StringEqualsLiteral(style, "narrow")) {
+      options.style = RelativeTimeFormatOptions::Style::Narrow;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(style, "long"));
+      options.style = RelativeTimeFormatOptions::Style::Long;
+    }
+  }
+
+  if (!GetProperty(cx, internals, internals, cx->names().numeric, &value)) {
+    return nullptr;
+  }
+
+  {
+    JSLinearString* numeric = value.toString()->ensureLinear(cx);
+    if (!numeric) {
+      return nullptr;
+    }
+
+    if (StringEqualsLiteral(numeric, "auto")) {
+      options.numeric = RelativeTimeFormatOptions::Numeric::Auto;
+    } else {
+      MOZ_ASSERT(StringEqualsLiteral(numeric, "always"));
+      options.numeric = RelativeTimeFormatOptions::Numeric::Always;
+    }
+  }
+
+  using RelativeTimeFormat = mozilla::intl::RelativeTimeFormat;
+  mozilla::Result<mozilla::UniquePtr<RelativeTimeFormat>,
+                  mozilla::intl::ICUError>
+      result = RelativeTimeFormat::TryCreate(locale.get(), options);
+
+  if (result.isOk()) {
+    return result.unwrap().release();
+  }
+
+  intl::ReportInternalError(cx, result.unwrapErr());
+  return nullptr;
+}
+
+static mozilla::intl::RelativeTimeFormat* GetOrCreateRelativeTimeFormat(
+    JSContext* cx, Handle<RelativeTimeFormatObject*> relativeTimeFormat) {
+  // Obtain a cached RelativeDateTimeFormatter object.
+  mozilla::intl::RelativeTimeFormat* rtf =
+      relativeTimeFormat->getRelativeTimeFormatter();
+  if (rtf) {
+    return rtf;
+  }
+
+  rtf = NewRelativeTimeFormatter(cx, relativeTimeFormat);
+  if (!rtf) {
+    return nullptr;
+  }
+  relativeTimeFormat->setRelativeTimeFormatter(rtf);
+
+  intl::AddICUCellMemory(relativeTimeFormat,
+                         RelativeTimeFormatObject::EstimatedMemoryUse);
+  return rtf;
+}
+
+bool js::intl_FormatRelativeTime(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 4);
+  MOZ_ASSERT(args[0].isObject());
+  MOZ_ASSERT(args[1].isNumber());
+  MOZ_ASSERT(args[2].isString());
+  MOZ_ASSERT(args[3].isBoolean());
+
+  Rooted<RelativeTimeFormatObject*> relativeTimeFormat(cx);
+  relativeTimeFormat = &args[0].toObject().as<RelativeTimeFormatObject>();
+
+  bool formatToParts = args[3].toBoolean();
+
+  // PartitionRelativeTimePattern, step 4.
+  double t = args[1].toNumber();
+  if (!mozilla::IsFinite(t)) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_DATE_NOT_FINITE, "RelativeTimeFormat",
+                              formatToParts ? "formatToParts" : "format");
+    return false;
+  }
+
+  mozilla::intl::RelativeTimeFormat* rtf =
+      GetOrCreateRelativeTimeFormat(cx, relativeTimeFormat);
+  if (!rtf) {
+    return false;
+  }
+
+  intl::FieldType jsUnitType;
+  using FormatUnit = mozilla::intl::RelativeTimeFormat::FormatUnit;
+  FormatUnit relTimeUnit;
+  {
+    JSLinearString* unit = args[2].toString()->ensureLinear(cx);
+    if (!unit) {
+      return false;
+    }
+
+    // PartitionRelativeTimePattern, step 5.
+    if (StringEqualsLiteral(unit, "second") ||
+        StringEqualsLiteral(unit, "seconds")) {
+      jsUnitType = &JSAtomState::second;
+      relTimeUnit = FormatUnit::Second;
+    } else if (StringEqualsLiteral(unit, "minute") ||
+               StringEqualsLiteral(unit, "minutes")) {
+      jsUnitType = &JSAtomState::minute;
+      relTimeUnit = FormatUnit::Minute;
+    } else if (StringEqualsLiteral(unit, "hour") ||
+               StringEqualsLiteral(unit, "hours")) {
+      jsUnitType = &JSAtomState::hour;
+      relTimeUnit = FormatUnit::Hour;
+    } else if (StringEqualsLiteral(unit, "day") ||
+               StringEqualsLiteral(unit, "days")) {
+      jsUnitType = &JSAtomState::day;
+      relTimeUnit = FormatUnit::Day;
+    } else if (StringEqualsLiteral(unit, "week") ||
+               StringEqualsLiteral(unit, "weeks")) {
+      jsUnitType = &JSAtomState::week;
+      relTimeUnit = FormatUnit::Week;
+    } else if (StringEqualsLiteral(unit, "month") ||
+               StringEqualsLiteral(unit, "months")) {
+      jsUnitType = &JSAtomState::month;
+      relTimeUnit = FormatUnit::Month;
+    } else if (StringEqualsLiteral(unit, "quarter") ||
+               StringEqualsLiteral(unit, "quarters")) {
+      jsUnitType = &JSAtomState::quarter;
+      relTimeUnit = FormatUnit::Quarter;
+    } else if (StringEqualsLiteral(unit, "year") ||
+               StringEqualsLiteral(unit, "years")) {
+      jsUnitType = &JSAtomState::year;
+      relTimeUnit = FormatUnit::Year;
+    } else {
+      if (auto unitChars = QuoteString(cx, unit, '"')) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_INVALID_OPTION_VALUE, "unit",
+                                  unitChars.get());
+      }
+      return false;
+    }
+  }
+
+  using ICUError = mozilla::intl::ICUError;
+  if (formatToParts) {
+    mozilla::intl::NumberPartVector parts;
+    mozilla::Result<mozilla::Span<const char16_t>, ICUError> result =
+        rtf->formatToParts(t, relTimeUnit, parts);
+
+    if (result.isErr()) {
+      intl::ReportInternalError(cx, result.unwrapErr());
+      return false;
+    }
+
+    RootedString str(cx, NewStringCopy<CanGC>(cx, result.unwrap()));
+    if (!str) {
+      return false;
+    }
+
+    return js::intl::FormattedRelativeTimeToParts(cx, str, parts, jsUnitType,
+                                                  args.rval());
+  }
+
+  js::intl::FormatBuffer<char16_t, intl::INITIAL_CHAR_BUFFER_SIZE> buffer(cx);
+  mozilla::Result<Ok, ICUError> result = rtf->format(t, relTimeUnit, buffer);
+
+  if (result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return false;
+  }
+
+  JSString* str = buffer.toString(cx);
+  if (!str) {
+    return false;
+  }
+
+  args.rval().setString(str);
+  return true;
+}
diff --git a/js/src/builtin/intl/RelativeTimeFormat.h b/js/src/builtin/intl/RelativeTimeFormat.h
new file mode 100644
index 0000000000..f46aaa217b
--- /dev/null
+++ b/js/src/builtin/intl/RelativeTimeFormat.h
@@ -0,0 +1,87 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_RelativeTimeFormat_h
+#define builtin_intl_RelativeTimeFormat_h
+
+#include "mozilla/intl/NumberPart.h"
+
+#include <stdint.h>
+
+#include "builtin/SelfHostingDefines.h"
+#include "gc/Barrier.h"
+#include "js/Class.h"
+#include "vm/NativeObject.h"
+
+namespace mozilla::intl {
+class RelativeTimeFormat;
+}
+
+namespace js {
+
+class RelativeTimeFormatObject : public NativeObject {
+ public:
+  static const JSClass class_;
+  static const JSClass& protoClass_;
+
+  static constexpr uint32_t INTERNALS_SLOT = 0;
+  static constexpr uint32_t URELATIVE_TIME_FORMAT_SLOT = 1;
+  static constexpr uint32_t SLOT_COUNT = 2;
+
+  static_assert(INTERNALS_SLOT == INTL_INTERNALS_OBJECT_SLOT,
+                "INTERNALS_SLOT must match self-hosting define for internals "
+                "object slot");
+
+  // Estimated memory use for URelativeDateTimeFormatter (see IcuMemoryUsage).
+  static constexpr size_t EstimatedMemoryUse = 10896;
+
+  mozilla::intl::RelativeTimeFormat* getRelativeTimeFormatter() const {
+    const auto& slot = getFixedSlot(URELATIVE_TIME_FORMAT_SLOT);
+    if (slot.isUndefined()) {
+      return nullptr;
+    }
+    return static_cast<mozilla::intl::RelativeTimeFormat*>(slot.toPrivate());
+  }
+
+  void setRelativeTimeFormatter(mozilla::intl::RelativeTimeFormat* rtf) {
+    setFixedSlot(URELATIVE_TIME_FORMAT_SLOT, PrivateValue(rtf));
+  }
+
+ private:
+  static const JSClassOps classOps_;
+  static const ClassSpec classSpec_;
+
+  static void finalize(JS::GCContext* gcx, JSObject* obj);
+};
+
+/**
+ * Returns a relative time as a string formatted according to the effective
+ * locale and the formatting options of the given RelativeTimeFormat.
+ *
+ * |t| should be a number representing a number to be formatted.
+ * |unit| should be "second", "minute", "hour", "day", "week", "month",
+ *                  "quarter", or "year".
+ * |numeric| should be "always" or "auto".
+ *
+ * Usage: formatted = intl_FormatRelativeTime(relativeTimeFormat, t,
+ *                                            unit, numeric, formatToParts)
+ */
+[[nodiscard]] extern bool intl_FormatRelativeTime(JSContext* cx, unsigned argc,
+                                                  JS::Value* vp);
+
+namespace intl {
+
+using FieldType = js::ImmutableTenuredPtr<PropertyName*> JSAtomState::*;
+
+[[nodiscard]] bool FormattedRelativeTimeToParts(
+    JSContext* cx, HandleString str,
+    const mozilla::intl::NumberPartVector& parts, FieldType relativeTimeUnit,
+    MutableHandleValue result);
+
+}  // namespace intl
+}  // namespace js
+
+#endif /* builtin_intl_RelativeTimeFormat_h */
diff --git a/js/src/builtin/intl/RelativeTimeFormat.js b/js/src/builtin/intl/RelativeTimeFormat.js
new file mode 100644
index 0000000000..5b23650e73
--- /dev/null
+++ b/js/src/builtin/intl/RelativeTimeFormat.js
@@ -0,0 +1,329 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * RelativeTimeFormat internal properties.
+ *
+ * Spec: ECMAScript 402 API, RelativeTimeFormat, 1.3.3.
+ */
+var relativeTimeFormatInternalProperties = {
+  localeData: relativeTimeFormatLocaleData,
+  relevantExtensionKeys: ["nu"],
+};
+
+function relativeTimeFormatLocaleData() {
+  return {
+    nu: getNumberingSystems,
+    default: {
+      nu: intl_numberingSystem,
+    },
+  };
+}
+
+/**
+ * Compute an internal properties object from |lazyRelativeTimeFormatData|.
+ */
+function resolveRelativeTimeFormatInternals(lazyRelativeTimeFormatData) {
+  assert(IsObject(lazyRelativeTimeFormatData), "lazy data not an object?");
+
+  var internalProps = std_Object_create(null);
+
+  var RelativeTimeFormat = relativeTimeFormatInternalProperties;
+
+  // Steps 10-11.
+  const r = ResolveLocale(
+    "RelativeTimeFormat",
+    lazyRelativeTimeFormatData.requestedLocales,
+    lazyRelativeTimeFormatData.opt,
+    RelativeTimeFormat.relevantExtensionKeys,
+    RelativeTimeFormat.localeData
+  );
+
+  // Steps 12-13.
+  internalProps.locale = r.locale;
+
+  // Step 14.
+  internalProps.numberingSystem = r.nu;
+
+  // Step 15 (Not relevant in our implementation).
+
+  // Step 17.
+  internalProps.style = lazyRelativeTimeFormatData.style;
+
+  // Step 19.
+  internalProps.numeric = lazyRelativeTimeFormatData.numeric;
+
+  // Steps 20-24 (Not relevant in our implementation).
+
+  return internalProps;
+}
+
+/**
+ * Returns an object containing the RelativeTimeFormat internal properties of |obj|.
+ */
+function getRelativeTimeFormatInternals(obj) {
+  assert(
+    IsObject(obj),
+    "getRelativeTimeFormatInternals called with non-object"
+  );
+  assert(
+    intl_GuardToRelativeTimeFormat(obj) !== null,
+    "getRelativeTimeFormatInternals called with non-RelativeTimeFormat"
+  );
+
+  var internals = getIntlObjectInternals(obj);
+  assert(
+    internals.type === "RelativeTimeFormat",
+    "bad type escaped getIntlObjectInternals"
+  );
+
+  var internalProps = maybeInternalProperties(internals);
+  if (internalProps) {
+    return internalProps;
+  }
+
+  internalProps = resolveRelativeTimeFormatInternals(internals.lazyData);
+  setInternalProperties(internals, internalProps);
+  return internalProps;
+}
+
+/**
+ * Initializes an object as a RelativeTimeFormat.
+ *
+ * This method is complicated a moderate bit by its implementing initialization
+ * as a *lazy* concept.  Everything that must happen now, does -- but we defer
+ * all the work we can until the object is actually used as a RelativeTimeFormat.
+ * This later work occurs in |resolveRelativeTimeFormatInternals|; steps not noted
+ * here occur there.
+ *
+ * Spec: ECMAScript 402 API, RelativeTimeFormat, 1.1.1.
+ */
+function InitializeRelativeTimeFormat(relativeTimeFormat, locales, options) {
+  assert(
+    IsObject(relativeTimeFormat),
+    "InitializeRelativeimeFormat called with non-object"
+  );
+  assert(
+    intl_GuardToRelativeTimeFormat(relativeTimeFormat) !== null,
+    "InitializeRelativeTimeFormat called with non-RelativeTimeFormat"
+  );
+
+  // Lazy RelativeTimeFormat data has the following structure:
+  //
+  //   {
+  //     requestedLocales: List of locales,
+  //     style: "long" / "short" / "narrow",
+  //     numeric: "always" / "auto",
+  //
+  //     opt: // opt object computed in InitializeRelativeTimeFormat
+  //       {
+  //         localeMatcher: "lookup" / "best fit",
+  //       }
+  //   }
+  //
+  // Note that lazy data is only installed as a final step of initialization,
+  // so every RelativeTimeFormat lazy data object has *all* these properties, never a
+  // subset of them.
+  const lazyRelativeTimeFormatData = std_Object_create(null);
+
+  // Step 1.
+  let requestedLocales = CanonicalizeLocaleList(locales);
+  lazyRelativeTimeFormatData.requestedLocales = requestedLocales;
+
+  // Steps 2-3.
+  if (options === undefined) {
+    options = std_Object_create(null);
+  } else {
+    options = ToObject(options);
+  }
+
+  // Step 4.
+  let opt = new_Record();
+
+  // Steps 5-6.
+  let matcher = GetOption(
+    options,
+    "localeMatcher",
+    "string",
+    ["lookup", "best fit"],
+    "best fit"
+  );
+  opt.localeMatcher = matcher;
+
+  // Steps 7-9.
+  let numberingSystem = GetOption(
+    options,
+    "numberingSystem",
+    "string",
+    undefined,
+    undefined
+  );
+  if (numberingSystem !== undefined) {
+    numberingSystem = intl_ValidateAndCanonicalizeUnicodeExtensionType(
+      numberingSystem,
+      "numberingSystem",
+      "nu"
+    );
+  }
+  opt.nu = numberingSystem;
+
+  lazyRelativeTimeFormatData.opt = opt;
+
+  // Steps 16-17.
+  const style = GetOption(
+    options,
+    "style",
+    "string",
+    ["long", "short", "narrow"],
+    "long"
+  );
+  lazyRelativeTimeFormatData.style = style;
+
+  // Steps 18-19.
+  const numeric = GetOption(
+    options,
+    "numeric",
+    "string",
+    ["always", "auto"],
+    "always"
+  );
+  lazyRelativeTimeFormatData.numeric = numeric;
+
+  initializeIntlObject(
+    relativeTimeFormat,
+    "RelativeTimeFormat",
+    lazyRelativeTimeFormatData
+  );
+}
+
+/**
+ * Returns the subset of the given locale list for which this locale list has a
+ * matching (possibly fallback) locale. Locales appear in the same order in the
+ * returned list as in the input list.
+ *
+ * Spec: ECMAScript 402 API, RelativeTimeFormat, 1.3.2.
+ */
+function Intl_RelativeTimeFormat_supportedLocalesOf(locales /*, options*/) {
+  var options = arguments.length > 1 ? arguments[1] : undefined;
+
+  // Step 1.
+  var availableLocales = "RelativeTimeFormat";
+
+  // Step 2.
+  let requestedLocales = CanonicalizeLocaleList(locales);
+
+  // Step 3.
+  return SupportedLocales(availableLocales, requestedLocales, options);
+}
+
+/**
+ * Returns a String value representing the written form of a relative date
+ * formatted according to the effective locale and the formatting options
+ * of this RelativeTimeFormat object.
+ *
+ * Spec: ECMAScript 402 API, RelativeTImeFormat, 1.4.3.
+ */
+function Intl_RelativeTimeFormat_format(value, unit) {
+  // Step 1.
+  let relativeTimeFormat = this;
+
+  // Step 2.
+  if (
+    !IsObject(relativeTimeFormat) ||
+    (relativeTimeFormat = intl_GuardToRelativeTimeFormat(
+      relativeTimeFormat
+    )) === null
+  ) {
+    return callFunction(
+      intl_CallRelativeTimeFormatMethodIfWrapped,
+      this,
+      value,
+      unit,
+      "Intl_RelativeTimeFormat_format"
+    );
+  }
+
+  // Step 3.
+  let t = ToNumber(value);
+
+  // Step 4.
+  let u = ToString(unit);
+
+  // Step 5.
+  return intl_FormatRelativeTime(relativeTimeFormat, t, u, false);
+}
+
+/**
+ * Returns an Array composed of the components of a relative date formatted
+ * according to the effective locale and the formatting options of this
+ * RelativeTimeFormat object.
+ *
+ * Spec: ECMAScript 402 API, RelativeTImeFormat, 1.4.4.
+ */
+function Intl_RelativeTimeFormat_formatToParts(value, unit) {
+  // Step 1.
+  let relativeTimeFormat = this;
+
+  // Step 2.
+  if (
+    !IsObject(relativeTimeFormat) ||
+    (relativeTimeFormat = intl_GuardToRelativeTimeFormat(
+      relativeTimeFormat
+    )) === null
+  ) {
+    return callFunction(
+      intl_CallRelativeTimeFormatMethodIfWrapped,
+      this,
+      value,
+      unit,
+      "Intl_RelativeTimeFormat_formatToParts"
+    );
+  }
+
+  // Step 3.
+  let t = ToNumber(value);
+
+  // Step 4.
+  let u = ToString(unit);
+
+  // Step 5.
+  return intl_FormatRelativeTime(relativeTimeFormat, t, u, true);
+}
+
+/**
+ * Returns the resolved options for a RelativeTimeFormat object.
+ *
+ * Spec: ECMAScript 402 API, RelativeTimeFormat, 1.4.5.
+ */
+function Intl_RelativeTimeFormat_resolvedOptions() {
+  // Step 1.
+  var relativeTimeFormat = this;
+
+  // Steps 2-3.
+  if (
+    !IsObject(relativeTimeFormat) ||
+    (relativeTimeFormat = intl_GuardToRelativeTimeFormat(
+      relativeTimeFormat
+    )) === null
+  ) {
+    return callFunction(
+      intl_CallRelativeTimeFormatMethodIfWrapped,
+      this,
+      "Intl_RelativeTimeFormat_resolvedOptions"
+    );
+  }
+
+  var internals = getRelativeTimeFormatInternals(relativeTimeFormat);
+
+  // Steps 4-5.
+  var result = {
+    locale: internals.locale,
+    style: internals.style,
+    numeric: internals.numeric,
+    numberingSystem: internals.numberingSystem,
+  };
+
+  // Step 6.
+  return result;
+}
diff --git a/js/src/builtin/intl/SanctionedSimpleUnitIdentifiers.yaml b/js/src/builtin/intl/SanctionedSimpleUnitIdentifiers.yaml
new file mode 100644
index 0000000000..97cb44c12c
--- /dev/null
+++ b/js/src/builtin/intl/SanctionedSimpleUnitIdentifiers.yaml
@@ -0,0 +1,58 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# 6.5.2 IsSanctionedSimpleUnitIdentifier ( unitIdentifier )
+#
+# Simple units sanctioned for use in ECMAScript
+#
+# https://tc39.es/ecma402/#table-sanctioned-simple-unit-identifiers
+
+# Run |make_intl_data units| to regenerate all files which reference this list
+# of sanctioned unit identifiers.
+
+- acre
+- bit
+- byte
+- celsius
+- centimeter
+- day
+- degree
+- fahrenheit
+- fluid-ounce
+- foot
+- gallon
+- gigabit
+- gigabyte
+- gram
+- hectare
+- hour
+- inch
+- kilobit
+- kilobyte
+- kilogram
+- kilometer
+- liter
+- megabit
+- megabyte
+- meter
+- microsecond
+- mile
+- mile-scandinavian
+- milliliter
+- millimeter
+- millisecond
+- minute
+- month
+- nanosecond
+- ounce
+- percent
+- petabyte
+- pound
+- second
+- stone
+- terabit
+- terabyte
+- week
+- yard
+- year
diff --git a/js/src/builtin/intl/SanctionedSimpleUnitIdentifiersGenerated.js b/js/src/builtin/intl/SanctionedSimpleUnitIdentifiersGenerated.js
new file mode 100644
index 0000000000..7f762910ee
--- /dev/null
+++ b/js/src/builtin/intl/SanctionedSimpleUnitIdentifiersGenerated.js
@@ -0,0 +1,56 @@
+// Generated by make_intl_data.py. DO NOT EDIT.
+
+/**
+ * The list of currently supported simple unit identifiers.
+ *
+ * Intl.NumberFormat Unified API Proposal
+ */
+/* eslint-disable prettier/prettier */
+var sanctionedSimpleUnitIdentifiers = {
+  "acre": true,
+  "bit": true,
+  "byte": true,
+  "celsius": true,
+  "centimeter": true,
+  "day": true,
+  "degree": true,
+  "fahrenheit": true,
+  "fluid-ounce": true,
+  "foot": true,
+  "gallon": true,
+  "gigabit": true,
+  "gigabyte": true,
+  "gram": true,
+  "hectare": true,
+  "hour": true,
+  "inch": true,
+  "kilobit": true,
+  "kilobyte": true,
+  "kilogram": true,
+  "kilometer": true,
+  "liter": true,
+  "megabit": true,
+  "megabyte": true,
+  "meter": true,
+  "microsecond": true,
+  "mile": true,
+  "mile-scandinavian": true,
+  "milliliter": true,
+  "millimeter": true,
+  "millisecond": true,
+  "minute": true,
+  "month": true,
+  "nanosecond": true,
+  "ounce": true,
+  "percent": true,
+  "petabyte": true,
+  "pound": true,
+  "second": true,
+  "stone": true,
+  "terabit": true,
+  "terabyte": true,
+  "week": true,
+  "yard": true,
+  "year": true
+};
+/* eslint-enable prettier/prettier */
diff --git a/js/src/builtin/intl/SharedIntlData.cpp b/js/src/builtin/intl/SharedIntlData.cpp
new file mode 100644
index 0000000000..18e87b6399
--- /dev/null
+++ b/js/src/builtin/intl/SharedIntlData.cpp
@@ -0,0 +1,754 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Runtime-wide Intl data shared across compartments. */
+
+#include "builtin/intl/SharedIntlData.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/HashFunctions.h"
+#include "mozilla/intl/Collator.h"
+#include "mozilla/intl/DateTimeFormat.h"
+#include "mozilla/intl/DateTimePatternGenerator.h"
+#include "mozilla/intl/Locale.h"
+#include "mozilla/intl/NumberFormat.h"
+#include "mozilla/intl/TimeZone.h"
+#include "mozilla/Span.h"
+#include "mozilla/TextUtils.h"
+
+#include <algorithm>
+#include <stdint.h>
+#include <string>
+#include <string.h>
+#include <string_view>
+#include <utility>
+
+#include "builtin/Array.h"
+#include "builtin/intl/CommonFunctions.h"
+#include "builtin/intl/TimeZoneDataGenerated.h"
+#include "js/Utility.h"
+#include "js/Vector.h"
+#include "vm/ArrayObject.h"
+#include "vm/JSAtom.h"
+#include "vm/JSContext.h"
+#include "vm/StringType.h"
+
+using js::HashNumber;
+
+template <typename Char>
+static constexpr Char ToUpperASCII(Char c) {
+  return mozilla::IsAsciiLowercaseAlpha(c) ? (c - 0x20) : c;
+}
+
+static_assert(ToUpperASCII('a') == 'A', "verifying 'a' uppercases correctly");
+static_assert(ToUpperASCII('m') == 'M', "verifying 'm' uppercases correctly");
+static_assert(ToUpperASCII('z') == 'Z', "verifying 'z' uppercases correctly");
+static_assert(ToUpperASCII(u'a') == u'A',
+              "verifying u'a' uppercases correctly");
+static_assert(ToUpperASCII(u'k') == u'K',
+              "verifying u'k' uppercases correctly");
+static_assert(ToUpperASCII(u'z') == u'Z',
+              "verifying u'z' uppercases correctly");
+
+template <typename Char>
+static HashNumber HashStringIgnoreCaseASCII(const Char* s, size_t length) {
+  uint32_t hash = 0;
+  for (size_t i = 0; i < length; i++) {
+    hash = mozilla::AddToHash(hash, ToUpperASCII(s[i]));
+  }
+  return hash;
+}
+
+js::intl::SharedIntlData::TimeZoneHasher::Lookup::Lookup(
+    JSLinearString* timeZone)
+    : js::intl::SharedIntlData::LinearStringLookup(timeZone) {
+  if (isLatin1) {
+    hash = HashStringIgnoreCaseASCII(latin1Chars, length);
+  } else {
+    hash = HashStringIgnoreCaseASCII(twoByteChars, length);
+  }
+}
+
+template <typename Char1, typename Char2>
+static bool EqualCharsIgnoreCaseASCII(const Char1* s1, const Char2* s2,
+                                      size_t len) {
+  for (const Char1* s1end = s1 + len; s1 < s1end; s1++, s2++) {
+    if (ToUpperASCII(*s1) != ToUpperASCII(*s2)) {
+      return false;
+    }
+  }
+  return true;
+}
+
+bool js::intl::SharedIntlData::TimeZoneHasher::match(TimeZoneName key,
+                                                     const Lookup& lookup) {
+  if (key->length() != lookup.length) {
+    return false;
+  }
+
+  // Compare time zone names ignoring ASCII case differences.
+  if (key->hasLatin1Chars()) {
+    const Latin1Char* keyChars = key->latin1Chars(lookup.nogc);
+    if (lookup.isLatin1) {
+      return EqualCharsIgnoreCaseASCII(keyChars, lookup.latin1Chars,
+                                       lookup.length);
+    }
+    return EqualCharsIgnoreCaseASCII(keyChars, lookup.twoByteChars,
+                                     lookup.length);
+  }
+
+  const char16_t* keyChars = key->twoByteChars(lookup.nogc);
+  if (lookup.isLatin1) {
+    return EqualCharsIgnoreCaseASCII(lookup.latin1Chars, keyChars,
+                                     lookup.length);
+  }
+  return EqualCharsIgnoreCaseASCII(keyChars, lookup.twoByteChars,
+                                   lookup.length);
+}
+
+static bool IsLegacyICUTimeZone(mozilla::Span<const char> timeZone) {
+  std::string_view timeZoneView(timeZone.data(), timeZone.size());
+  for (const auto& legacyTimeZone : js::timezone::legacyICUTimeZones) {
+    if (timeZoneView == legacyTimeZone) {
+      return true;
+    }
+  }
+  return false;
+}
+
+bool js::intl::SharedIntlData::ensureTimeZones(JSContext* cx) {
+  if (timeZoneDataInitialized) {
+    return true;
+  }
+
+  // If ensureTimeZones() was called previously, but didn't complete due to
+  // OOM, clear all sets/maps and start from scratch.
+  availableTimeZones.clearAndCompact();
+
+  auto timeZones = mozilla::intl::TimeZone::GetAvailableTimeZones();
+  if (timeZones.isErr()) {
+    ReportInternalError(cx, timeZones.unwrapErr());
+    return false;
+  }
+
+  Rooted<JSAtom*> timeZone(cx);
+  for (auto timeZoneName : timeZones.unwrap()) {
+    if (timeZoneName.isErr()) {
+      ReportInternalError(cx);
+      return false;
+    }
+    auto timeZoneSpan = timeZoneName.unwrap();
+
+    // Skip legacy ICU time zone names.
+    if (IsLegacyICUTimeZone(timeZoneSpan)) {
+      continue;
+    }
+
+    timeZone = Atomize(cx, timeZoneSpan.data(), timeZoneSpan.size());
+    if (!timeZone) {
+      return false;
+    }
+
+    TimeZoneHasher::Lookup lookup(timeZone);
+    TimeZoneSet::AddPtr p = availableTimeZones.lookupForAdd(lookup);
+
+    // ICU shouldn't report any duplicate time zone names, but if it does,
+    // just ignore the duplicate name.
+    if (!p && !availableTimeZones.add(p, timeZone)) {
+      ReportOutOfMemory(cx);
+      return false;
+    }
+  }
+
+  ianaZonesTreatedAsLinksByICU.clearAndCompact();
+
+  for (const char* rawTimeZone : timezone::ianaZonesTreatedAsLinksByICU) {
+    MOZ_ASSERT(rawTimeZone != nullptr);
+    timeZone = Atomize(cx, rawTimeZone, strlen(rawTimeZone));
+    if (!timeZone) {
+      return false;
+    }
+
+    TimeZoneHasher::Lookup lookup(timeZone);
+    TimeZoneSet::AddPtr p = ianaZonesTreatedAsLinksByICU.lookupForAdd(lookup);
+    MOZ_ASSERT(!p, "Duplicate entry in timezone::ianaZonesTreatedAsLinksByICU");
+
+    if (!ianaZonesTreatedAsLinksByICU.add(p, timeZone)) {
+      ReportOutOfMemory(cx);
+      return false;
+    }
+  }
+
+  ianaLinksCanonicalizedDifferentlyByICU.clearAndCompact();
+
+  Rooted<JSAtom*> linkName(cx);
+  Rooted<JSAtom*>& target = timeZone;
+  for (const auto& linkAndTarget :
+       timezone::ianaLinksCanonicalizedDifferentlyByICU) {
+    const char* rawLinkName = linkAndTarget.link;
+    const char* rawTarget = linkAndTarget.target;
+
+    MOZ_ASSERT(rawLinkName != nullptr);
+    linkName = Atomize(cx, rawLinkName, strlen(rawLinkName));
+    if (!linkName) {
+      return false;
+    }
+
+    MOZ_ASSERT(rawTarget != nullptr);
+    target = Atomize(cx, rawTarget, strlen(rawTarget));
+    if (!target) {
+      return false;
+    }
+
+    TimeZoneHasher::Lookup lookup(linkName);
+    TimeZoneMap::AddPtr p =
+        ianaLinksCanonicalizedDifferentlyByICU.lookupForAdd(lookup);
+    MOZ_ASSERT(
+        !p,
+        "Duplicate entry in timezone::ianaLinksCanonicalizedDifferentlyByICU");
+
+    if (!ianaLinksCanonicalizedDifferentlyByICU.add(p, linkName, target)) {
+      ReportOutOfMemory(cx);
+      return false;
+    }
+  }
+
+  MOZ_ASSERT(!timeZoneDataInitialized,
+             "ensureTimeZones is neither reentrant nor thread-safe");
+  timeZoneDataInitialized = true;
+
+  return true;
+}
+
+bool js::intl::SharedIntlData::validateTimeZoneName(
+    JSContext* cx, HandleString timeZone, MutableHandle<JSAtom*> result) {
+  if (!ensureTimeZones(cx)) {
+    return false;
+  }
+
+  Rooted<JSLinearString*> timeZoneLinear(cx, timeZone->ensureLinear(cx));
+  if (!timeZoneLinear) {
+    return false;
+  }
+
+  TimeZoneHasher::Lookup lookup(timeZoneLinear);
+  if (TimeZoneSet::Ptr p = availableTimeZones.lookup(lookup)) {
+    result.set(*p);
+  }
+
+  return true;
+}
+
+bool js::intl::SharedIntlData::tryCanonicalizeTimeZoneConsistentWithIANA(
+    JSContext* cx, HandleString timeZone, MutableHandle<JSAtom*> result) {
+  if (!ensureTimeZones(cx)) {
+    return false;
+  }
+
+  Rooted<JSLinearString*> timeZoneLinear(cx, timeZone->ensureLinear(cx));
+  if (!timeZoneLinear) {
+    return false;
+  }
+
+  TimeZoneHasher::Lookup lookup(timeZoneLinear);
+  MOZ_ASSERT(availableTimeZones.has(lookup), "Invalid time zone name");
+
+  if (TimeZoneMap::Ptr p =
+          ianaLinksCanonicalizedDifferentlyByICU.lookup(lookup)) {
+    // The effectively supported time zones aren't known at compile time,
+    // when
+    // 1. SpiderMonkey was compiled with "--with-system-icu".
+    // 2. ICU's dynamic time zone data loading feature was used.
+    //    (ICU supports loading time zone files at runtime through the
+    //    ICU_TIMEZONE_FILES_DIR environment variable.)
+    // Ensure ICU supports the new target zone before applying the update.
+    TimeZoneName targetTimeZone = p->value();
+    TimeZoneHasher::Lookup targetLookup(targetTimeZone);
+    if (availableTimeZones.has(targetLookup)) {
+      result.set(targetTimeZone);
+    }
+  } else if (TimeZoneSet::Ptr p = ianaZonesTreatedAsLinksByICU.lookup(lookup)) {
+    result.set(*p);
+  }
+
+  return true;
+}
+
+JS::Result<js::intl::SharedIntlData::TimeZoneSet::Iterator>
+js::intl::SharedIntlData::availableTimeZonesIteration(JSContext* cx) {
+  if (!ensureTimeZones(cx)) {
+    return cx->alreadyReportedError();
+  }
+  return availableTimeZones.iter();
+}
+
+js::intl::SharedIntlData::LocaleHasher::Lookup::Lookup(JSLinearString* locale)
+    : js::intl::SharedIntlData::LinearStringLookup(locale) {
+  if (isLatin1) {
+    hash = mozilla::HashString(latin1Chars, length);
+  } else {
+    hash = mozilla::HashString(twoByteChars, length);
+  }
+}
+
+js::intl::SharedIntlData::LocaleHasher::Lookup::Lookup(const char* chars,
+                                                       size_t length)
+    : js::intl::SharedIntlData::LinearStringLookup(chars, length) {
+  hash = mozilla::HashString(latin1Chars, length);
+}
+
+bool js::intl::SharedIntlData::LocaleHasher::match(Locale key,
+                                                   const Lookup& lookup) {
+  if (key->length() != lookup.length) {
+    return false;
+  }
+
+  if (key->hasLatin1Chars()) {
+    const Latin1Char* keyChars = key->latin1Chars(lookup.nogc);
+    if (lookup.isLatin1) {
+      return EqualChars(keyChars, lookup.latin1Chars, lookup.length);
+    }
+    return EqualChars(keyChars, lookup.twoByteChars, lookup.length);
+  }
+
+  const char16_t* keyChars = key->twoByteChars(lookup.nogc);
+  if (lookup.isLatin1) {
+    return EqualChars(lookup.latin1Chars, keyChars, lookup.length);
+  }
+  return EqualChars(keyChars, lookup.twoByteChars, lookup.length);
+}
+
+template <class AvailableLocales>
+bool js::intl::SharedIntlData::getAvailableLocales(
+    JSContext* cx, LocaleSet& locales,
+    const AvailableLocales& availableLocales) {
+  auto addLocale = [cx, &locales](const char* locale, size_t length) {
+    JSAtom* atom = Atomize(cx, locale, length);
+    if (!atom) {
+      return false;
+    }
+
+    LocaleHasher::Lookup lookup(atom);
+    LocaleSet::AddPtr p = locales.lookupForAdd(lookup);
+
+    // ICU shouldn't report any duplicate locales, but if it does, just
+    // ignore the duplicated locale.
+    if (!p && !locales.add(p, atom)) {
+      ReportOutOfMemory(cx);
+      return false;
+    }
+
+    return true;
+  };
+
+  js::Vector<char, 16> lang(cx);
+
+  for (const char* locale : availableLocales) {
+    size_t length = strlen(locale);
+
+    lang.clear();
+    if (!lang.append(locale, length)) {
+      return false;
+    }
+    MOZ_ASSERT(lang.length() == length);
+
+    std::replace(lang.begin(), lang.end(), '_', '-');
+
+    if (!addLocale(lang.begin(), length)) {
+      return false;
+    }
+
+    // From <https://tc39.es/ecma402/#sec-internal-slots>:
+    //
+    // For locales that include a script subtag in addition to language and
+    // region, the corresponding locale without a script subtag must also be
+    // supported; that is, if an implementation recognizes "zh-Hant-TW", it is
+    // also expected to recognize "zh-TW".
+
+    //   2 * Alpha language subtag
+    // + 1 separator
+    // + 4 * Alphanum script subtag
+    // + 1 separator
+    // + 2 * Alpha region subtag
+    using namespace mozilla::intl::LanguageTagLimits;
+    static constexpr size_t MinLanguageLength = 2;
+    static constexpr size_t MinLengthForScriptAndRegion =
+        MinLanguageLength + 1 + ScriptLength + 1 + AlphaRegionLength;
+
+    // Fast case: Skip locales without script subtags.
+    if (length < MinLengthForScriptAndRegion) {
+      continue;
+    }
+
+    // We don't need the full-fledged language tag parser when we just want to
+    // remove the script subtag.
+
+    // Find the separator between the language and script subtags.
+    const char* sep = std::char_traits<char>::find(lang.begin(), length, '-');
+    if (!sep) {
+      continue;
+    }
+
+    // Possible |script| subtag start position.
+    const char* script = sep + 1;
+
+    // Find the separator between the script and region subtags.
+    sep = std::char_traits<char>::find(script, lang.end() - script, '-');
+    if (!sep) {
+      continue;
+    }
+
+    // Continue with the next locale if we didn't find a script subtag.
+    size_t scriptLength = sep - script;
+    if (!mozilla::intl::IsStructurallyValidScriptTag<char>(
+            {script, scriptLength})) {
+      continue;
+    }
+
+    // Possible |region| subtag start position.
+    const char* region = sep + 1;
+
+    // Search if there's yet another subtag after the region subtag.
+    sep = std::char_traits<char>::find(region, lang.end() - region, '-');
+
+    // Continue with the next locale if we didn't find a region subtag.
+    size_t regionLength = (sep ? sep : lang.end()) - region;
+    if (!mozilla::intl::IsStructurallyValidRegionTag<char>(
+            {region, regionLength})) {
+      continue;
+    }
+
+    // We've found a script and a region subtag.
+
+    static constexpr size_t ScriptWithSeparatorLength = ScriptLength + 1;
+
+    // Remove the script subtag. Note: erase() needs non-const pointers, which
+    // means we can't directly pass |script|.
+    char* p = const_cast<char*>(script);
+    lang.erase(p, p + ScriptWithSeparatorLength);
+
+    MOZ_ASSERT(lang.length() == length - ScriptWithSeparatorLength);
+
+    // Add the locale with the script subtag removed.
+    if (!addLocale(lang.begin(), lang.length())) {
+      return false;
+    }
+  }
+
+  // Forcibly add an entry for the last-ditch locale, in case ICU doesn't
+  // directly support it (but does support it through fallback, e.g. supporting
+  // "en-GB" indirectly using "en" support).
+  {
+    const char* lastDitch = intl::LastDitchLocale();
+    MOZ_ASSERT(strcmp(lastDitch, "en-GB") == 0);
+
+#ifdef DEBUG
+    static constexpr char lastDitchParent[] = "en";
+
+    LocaleHasher::Lookup lookup(lastDitchParent, strlen(lastDitchParent));
+    MOZ_ASSERT(locales.has(lookup),
+               "shouldn't be a need to add every locale implied by the "
+               "last-ditch locale, merely just the last-ditch locale");
+#endif
+
+    if (!addLocale(lastDitch, strlen(lastDitch))) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+#ifdef DEBUG
+template <class AvailableLocales1, class AvailableLocales2>
+static bool IsSameAvailableLocales(const AvailableLocales1& availableLocales1,
+                                   const AvailableLocales2& availableLocales2) {
+  return std::equal(std::begin(availableLocales1), std::end(availableLocales1),
+                    std::begin(availableLocales2), std::end(availableLocales2),
+                    [](const char* a, const char* b) {
+                      // Intentionally comparing pointer equivalence.
+                      return a == b;
+                    });
+}
+#endif
+
+bool js::intl::SharedIntlData::ensureSupportedLocales(JSContext* cx) {
+  if (supportedLocalesInitialized) {
+    return true;
+  }
+
+  // If ensureSupportedLocales() was called previously, but didn't complete due
+  // to OOM, clear all data and start from scratch.
+  supportedLocales.clearAndCompact();
+  collatorSupportedLocales.clearAndCompact();
+
+  if (!getAvailableLocales(cx, supportedLocales,
+                           mozilla::intl::Locale::GetAvailableLocales())) {
+    return false;
+  }
+  if (!getAvailableLocales(cx, collatorSupportedLocales,
+                           mozilla::intl::Collator::GetAvailableLocales())) {
+    return false;
+  }
+
+  MOZ_ASSERT(IsSameAvailableLocales(
+      mozilla::intl::Locale::GetAvailableLocales(),
+      mozilla::intl::DateTimeFormat::GetAvailableLocales()));
+
+  MOZ_ASSERT(IsSameAvailableLocales(
+      mozilla::intl::Locale::GetAvailableLocales(),
+      mozilla::intl::NumberFormat::GetAvailableLocales()));
+
+  MOZ_ASSERT(!supportedLocalesInitialized,
+             "ensureSupportedLocales is neither reentrant nor thread-safe");
+  supportedLocalesInitialized = true;
+
+  return true;
+}
+
+bool js::intl::SharedIntlData::isSupportedLocale(JSContext* cx,
+                                                 SupportedLocaleKind kind,
+                                                 HandleString locale,
+                                                 bool* supported) {
+  if (!ensureSupportedLocales(cx)) {
+    return false;
+  }
+
+  Rooted<JSLinearString*> localeLinear(cx, locale->ensureLinear(cx));
+  if (!localeLinear) {
+    return false;
+  }
+
+  LocaleHasher::Lookup lookup(localeLinear);
+
+  switch (kind) {
+    case SupportedLocaleKind::Collator:
+      *supported = collatorSupportedLocales.has(lookup);
+      return true;
+    case SupportedLocaleKind::DateTimeFormat:
+    case SupportedLocaleKind::DisplayNames:
+    case SupportedLocaleKind::ListFormat:
+    case SupportedLocaleKind::NumberFormat:
+    case SupportedLocaleKind::PluralRules:
+    case SupportedLocaleKind::RelativeTimeFormat:
+      *supported = supportedLocales.has(lookup);
+      return true;
+  }
+  MOZ_CRASH("Invalid Intl constructor");
+}
+
+js::ArrayObject* js::intl::SharedIntlData::availableLocalesOf(
+    JSContext* cx, SupportedLocaleKind kind) {
+  if (!ensureSupportedLocales(cx)) {
+    return nullptr;
+  }
+
+  LocaleSet* localeSet = nullptr;
+  switch (kind) {
+    case SupportedLocaleKind::Collator:
+      localeSet = &collatorSupportedLocales;
+      break;
+    case SupportedLocaleKind::DateTimeFormat:
+    case SupportedLocaleKind::DisplayNames:
+    case SupportedLocaleKind::ListFormat:
+    case SupportedLocaleKind::NumberFormat:
+    case SupportedLocaleKind::PluralRules:
+    case SupportedLocaleKind::RelativeTimeFormat:
+      localeSet = &supportedLocales;
+      break;
+    default:
+      MOZ_CRASH("Invalid Intl constructor");
+  }
+
+  const uint32_t count = localeSet->count();
+  ArrayObject* result = NewDenseFullyAllocatedArray(cx, count);
+  if (!result) {
+    return nullptr;
+  }
+  result->setDenseInitializedLength(count);
+
+  uint32_t index = 0;
+  for (auto range = localeSet->iter(); !range.done(); range.next()) {
+    JSAtom* locale = range.get();
+    cx->markAtom(locale);
+
+    result->initDenseElement(index++, StringValue(locale));
+  }
+  MOZ_ASSERT(index == count);
+
+  return result;
+}
+
+#if DEBUG || MOZ_SYSTEM_ICU
+bool js::intl::SharedIntlData::ensureUpperCaseFirstLocales(JSContext* cx) {
+  if (upperCaseFirstInitialized) {
+    return true;
+  }
+
+  // If ensureUpperCaseFirstLocales() was called previously, but didn't
+  // complete due to OOM, clear all data and start from scratch.
+  upperCaseFirstLocales.clearAndCompact();
+
+  Rooted<JSAtom*> locale(cx);
+  for (const char* rawLocale : mozilla::intl::Collator::GetAvailableLocales()) {
+    auto collator = mozilla::intl::Collator::TryCreate(rawLocale);
+    if (collator.isErr()) {
+      ReportInternalError(cx, collator.unwrapErr());
+      return false;
+    }
+
+    auto caseFirst = collator.unwrap()->GetCaseFirst();
+    if (caseFirst.isErr()) {
+      ReportInternalError(cx, caseFirst.unwrapErr());
+      return false;
+    }
+
+    if (caseFirst.unwrap() != mozilla::intl::Collator::CaseFirst::Upper) {
+      continue;
+    }
+
+    locale = Atomize(cx, rawLocale, strlen(rawLocale));
+    if (!locale) {
+      return false;
+    }
+
+    LocaleHasher::Lookup lookup(locale);
+    LocaleSet::AddPtr p = upperCaseFirstLocales.lookupForAdd(lookup);
+
+    // ICU shouldn't report any duplicate locales, but if it does, just
+    // ignore the duplicated locale.
+    if (!p && !upperCaseFirstLocales.add(p, locale)) {
+      ReportOutOfMemory(cx);
+      return false;
+    }
+  }
+
+  MOZ_ASSERT(
+      !upperCaseFirstInitialized,
+      "ensureUpperCaseFirstLocales is neither reentrant nor thread-safe");
+  upperCaseFirstInitialized = true;
+
+  return true;
+}
+#endif  // DEBUG || MOZ_SYSTEM_ICU
+
+bool js::intl::SharedIntlData::isUpperCaseFirst(JSContext* cx,
+                                                HandleString locale,
+                                                bool* isUpperFirst) {
+#if DEBUG || MOZ_SYSTEM_ICU
+  if (!ensureUpperCaseFirstLocales(cx)) {
+    return false;
+  }
+#endif
+
+  Rooted<JSLinearString*> localeLinear(cx, locale->ensureLinear(cx));
+  if (!localeLinear) {
+    return false;
+  }
+
+#if !MOZ_SYSTEM_ICU
+  // "da" (Danish) and "mt" (Maltese) are the only two supported locales using
+  // upper-case first. CLDR also lists "cu" (Church Slavic) as an upper-case
+  // first locale, but since it's not supported in ICU, we don't care about it
+  // here.
+  bool isDefaultUpperCaseFirstLocale =
+      js::StringEqualsLiteral(localeLinear, "da") ||
+      js::StringEqualsLiteral(localeLinear, "mt");
+#endif
+
+#if DEBUG || MOZ_SYSTEM_ICU
+  LocaleHasher::Lookup lookup(localeLinear);
+  *isUpperFirst = upperCaseFirstLocales.has(lookup);
+#else
+  *isUpperFirst = isDefaultUpperCaseFirstLocale;
+#endif
+
+#if !MOZ_SYSTEM_ICU
+  MOZ_ASSERT(*isUpperFirst == isDefaultUpperCaseFirstLocale,
+             "upper-case first locales don't match hard-coded list");
+#endif
+
+  return true;
+}
+
+void js::intl::DateTimePatternGeneratorDeleter::operator()(
+    mozilla::intl::DateTimePatternGenerator* ptr) {
+  delete ptr;
+}
+
+static bool StringsAreEqual(const char* s1, const char* s2) {
+  return !strcmp(s1, s2);
+}
+
+mozilla::intl::DateTimePatternGenerator*
+js::intl::SharedIntlData::getDateTimePatternGenerator(JSContext* cx,
+                                                      const char* locale) {
+  // Return the cached instance if the requested locale matches the locale
+  // of the cached generator.
+  if (dateTimePatternGeneratorLocale &&
+      StringsAreEqual(dateTimePatternGeneratorLocale.get(), locale)) {
+    return dateTimePatternGenerator.get();
+  }
+
+  auto result = mozilla::intl::DateTimePatternGenerator::TryCreate(locale);
+  if (result.isErr()) {
+    intl::ReportInternalError(cx, result.unwrapErr());
+    return nullptr;
+  }
+  // The UniquePtr needs to be recreated as it's using a different Deleter in
+  // order to be able to forward declare DateTimePatternGenerator in
+  // SharedIntlData.h.
+  UniqueDateTimePatternGenerator gen(result.unwrap().release());
+
+  JS::UniqueChars localeCopy = js::DuplicateString(cx, locale);
+  if (!localeCopy) {
+    return nullptr;
+  }
+
+  dateTimePatternGenerator = std::move(gen);
+  dateTimePatternGeneratorLocale = std::move(localeCopy);
+
+  return dateTimePatternGenerator.get();
+}
+
+void js::intl::SharedIntlData::destroyInstance() {
+  availableTimeZones.clearAndCompact();
+  ianaZonesTreatedAsLinksByICU.clearAndCompact();
+  ianaLinksCanonicalizedDifferentlyByICU.clearAndCompact();
+  supportedLocales.clearAndCompact();
+  collatorSupportedLocales.clearAndCompact();
+#if DEBUG || MOZ_SYSTEM_ICU
+  upperCaseFirstLocales.clearAndCompact();
+#endif
+}
+
+void js::intl::SharedIntlData::trace(JSTracer* trc) {
+  // Atoms are always tenured.
+  if (!JS::RuntimeHeapIsMinorCollecting()) {
+    availableTimeZones.trace(trc);
+    ianaZonesTreatedAsLinksByICU.trace(trc);
+    ianaLinksCanonicalizedDifferentlyByICU.trace(trc);
+    supportedLocales.trace(trc);
+    collatorSupportedLocales.trace(trc);
+#if DEBUG || MOZ_SYSTEM_ICU
+    upperCaseFirstLocales.trace(trc);
+#endif
+  }
+}
+
+size_t js::intl::SharedIntlData::sizeOfExcludingThis(
+    mozilla::MallocSizeOf mallocSizeOf) const {
+  return availableTimeZones.shallowSizeOfExcludingThis(mallocSizeOf) +
+         ianaZonesTreatedAsLinksByICU.shallowSizeOfExcludingThis(mallocSizeOf) +
+         ianaLinksCanonicalizedDifferentlyByICU.shallowSizeOfExcludingThis(
+             mallocSizeOf) +
+         supportedLocales.shallowSizeOfExcludingThis(mallocSizeOf) +
+         collatorSupportedLocales.shallowSizeOfExcludingThis(mallocSizeOf) +
+#if DEBUG || MOZ_SYSTEM_ICU
+         upperCaseFirstLocales.shallowSizeOfExcludingThis(mallocSizeOf) +
+#endif
+         mallocSizeOf(dateTimePatternGeneratorLocale.get());
+}
diff --git a/js/src/builtin/intl/SharedIntlData.h b/js/src/builtin/intl/SharedIntlData.h
new file mode 100644
index 0000000000..8cada7c61b
--- /dev/null
+++ b/js/src/builtin/intl/SharedIntlData.h
@@ -0,0 +1,335 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_SharedIntlData_h
+#define builtin_intl_SharedIntlData_h
+
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/UniquePtr.h"
+
+#include <stddef.h>
+
+#include "js/AllocPolicy.h"
+#include "js/GCAPI.h"
+#include "js/GCHashTable.h"
+#include "js/Result.h"
+#include "js/RootingAPI.h"
+#include "js/Utility.h"
+#include "vm/StringType.h"
+
+namespace mozilla::intl {
+class DateTimePatternGenerator;
+}  // namespace mozilla::intl
+
+namespace js {
+
+class ArrayObject;
+
+namespace intl {
+
+/**
+ * This deleter class exists so that mozilla::intl::DateTimePatternGenerator
+ * can be a forward declaration, but still be used inside of a UniquePtr.
+ */
+class DateTimePatternGeneratorDeleter {
+ public:
+  void operator()(mozilla::intl::DateTimePatternGenerator* ptr);
+};
+
+/**
+ * Stores Intl data which can be shared across compartments (but not contexts).
+ *
+ * Used for data which is expensive when computed repeatedly or is not
+ * available through ICU.
+ */
+class SharedIntlData {
+  struct LinearStringLookup {
+    union {
+      const JS::Latin1Char* latin1Chars;
+      const char16_t* twoByteChars;
+    };
+    bool isLatin1;
+    size_t length;
+    JS::AutoCheckCannotGC nogc;
+    HashNumber hash = 0;
+
+    explicit LinearStringLookup(JSLinearString* string)
+        : isLatin1(string->hasLatin1Chars()), length(string->length()) {
+      if (isLatin1) {
+        latin1Chars = string->latin1Chars(nogc);
+      } else {
+        twoByteChars = string->twoByteChars(nogc);
+      }
+    }
+
+    LinearStringLookup(const char* chars, size_t length)
+        : isLatin1(true), length(length) {
+      latin1Chars = reinterpret_cast<const JS::Latin1Char*>(chars);
+    }
+  };
+
+ public:
+  /**
+   * Information tracking the set of the supported time zone names, derived
+   * from the IANA time zone database <https://www.iana.org/time-zones>.
+   *
+   * There are two kinds of IANA time zone names: Zone and Link (denoted as
+   * such in database source files). Zone names are the canonical, preferred
+   * name for a time zone, e.g. Asia/Kolkata. Link names simply refer to
+   * target Zone names for their meaning, e.g. Asia/Calcutta targets
+   * Asia/Kolkata. That a name is a Link doesn't *necessarily* reflect a
+   * sense of deprecation: some Link names also exist partly for convenience,
+   * e.g. UTC and GMT as Link names targeting the Zone name Etc/UTC.
+   *
+   * Two data sources determine the time zone names we support: those ICU
+   * supports and IANA's zone information.
+   *
+   * Unfortunately the names ICU and IANA support, and their Link
+   * relationships from name to target, aren't identical, so we can't simply
+   * implicitly trust ICU's name handling. We must perform various
+   * preprocessing of user-provided zone names and post-processing of
+   * ICU-provided zone names to implement ECMA-402's IANA-consistent behavior.
+   *
+   * Also see <https://ssl.icu-project.org/trac/ticket/12044> and
+   * <http://unicode.org/cldr/trac/ticket/9892>.
+   */
+
+  using TimeZoneName = JSAtom*;
+
+  struct TimeZoneHasher {
+    struct Lookup : LinearStringLookup {
+      explicit Lookup(JSLinearString* timeZone);
+    };
+
+    static js::HashNumber hash(const Lookup& lookup) { return lookup.hash; }
+    static bool match(TimeZoneName key, const Lookup& lookup);
+  };
+
+  using TimeZoneSet =
+      GCHashSet<TimeZoneName, TimeZoneHasher, SystemAllocPolicy>;
+  using TimeZoneMap =
+      GCHashMap<TimeZoneName, TimeZoneName, TimeZoneHasher, SystemAllocPolicy>;
+
+ private:
+  /**
+   * As a threshold matter, available time zones are those time zones ICU
+   * supports, via ucal_openTimeZones. But ICU supports additional non-IANA
+   * time zones described in intl/icu/source/tools/tzcode/icuzones (listed in
+   * IntlTimeZoneData.cpp's |legacyICUTimeZones|) for its own backwards
+   * compatibility purposes. This set consists of ICU's supported time zones,
+   * minus all backwards-compatibility time zones.
+   */
+  TimeZoneSet availableTimeZones;
+
+  /**
+   * IANA treats some time zone names as Zones, that ICU instead treats as
+   * Links. For example, IANA considers "America/Indiana/Indianapolis" to be
+   * a Zone and "America/Fort_Wayne" a Link that targets it, but ICU
+   * considers the former a Link that targets "America/Indianapolis" (which
+   * IANA treats as a Link).
+   *
+   * ECMA-402 requires that we respect IANA data, so if we're asked to
+   * canonicalize a time zone name in this set, we must *not* return ICU's
+   * canonicalization.
+   */
+  TimeZoneSet ianaZonesTreatedAsLinksByICU;
+
+  /**
+   * IANA treats some time zone names as Links to one target, that ICU
+   * instead treats as either Zones, or Links to different targets. An
+   * example of the former is "Asia/Calcutta, which IANA assigns the target
+   * "Asia/Kolkata" but ICU considers its own Zone. An example of the latter
+   * is "America/Virgin", which IANA assigns the target
+   * "America/Port_of_Spain" but ICU assigns the target "America/St_Thomas".
+   *
+   * ECMA-402 requires that we respect IANA data, so if we're asked to
+   * canonicalize a time zone name that's a key in this map, we *must* return
+   * the corresponding value and *must not* return ICU's canonicalization.
+   */
+  TimeZoneMap ianaLinksCanonicalizedDifferentlyByICU;
+
+  bool timeZoneDataInitialized = false;
+
+  /**
+   * Precomputes the available time zone names, because it's too expensive to
+   * call ucal_openTimeZones() repeatedly.
+   */
+  bool ensureTimeZones(JSContext* cx);
+
+ public:
+  /**
+   * Returns the validated time zone name in |result|. If the input time zone
+   * isn't a valid IANA time zone name, |result| remains unchanged.
+   */
+  bool validateTimeZoneName(JSContext* cx, JS::Handle<JSString*> timeZone,
+                            JS::MutableHandle<JSAtom*> result);
+
+  /**
+   * Returns the canonical time zone name in |result|. If no canonical name
+   * was found, |result| remains unchanged.
+   *
+   * This method only handles time zones which are canonicalized differently
+   * by ICU when compared to IANA.
+   */
+  bool tryCanonicalizeTimeZoneConsistentWithIANA(
+      JSContext* cx, JS::Handle<JSString*> timeZone,
+      JS::MutableHandle<JSAtom*> result);
+
+  /**
+   * Returns an iterator over all available time zones supported by ICU. The
+   * returned time zone names aren't canonicalized.
+   */
+  JS::Result<TimeZoneSet::Iterator> availableTimeZonesIteration(JSContext* cx);
+
+ private:
+  using Locale = JSAtom*;
+
+  struct LocaleHasher {
+    struct Lookup : LinearStringLookup {
+      explicit Lookup(JSLinearString* locale);
+      Lookup(const char* chars, size_t length);
+    };
+
+    static js::HashNumber hash(const Lookup& lookup) { return lookup.hash; }
+    static bool match(Locale key, const Lookup& lookup);
+  };
+
+  using LocaleSet = GCHashSet<Locale, LocaleHasher, SystemAllocPolicy>;
+
+  // Set of supported locales for all Intl service constructors except Collator,
+  // which uses its own set.
+  //
+  // UDateFormat:
+  // udat_[count,get]Available() return the same results as their
+  // uloc_[count,get]Available() counterparts.
+  //
+  // UNumberFormatter:
+  // unum_[count,get]Available() return the same results as their
+  // uloc_[count,get]Available() counterparts.
+  //
+  // UListFormatter, UPluralRules, and URelativeDateTimeFormatter:
+  // We're going to use ULocale availableLocales as per ICU recommendation:
+  // https://unicode-org.atlassian.net/browse/ICU-12756
+  LocaleSet supportedLocales;
+
+  // ucol_[count,get]Available() return different results compared to
+  // uloc_[count,get]Available(), we can't use |supportedLocales| here.
+  LocaleSet collatorSupportedLocales;
+
+  bool supportedLocalesInitialized = false;
+
+  // CountAvailable and GetAvailable describe the signatures used for ICU API
+  // to determine available locales for various functionality.
+  using CountAvailable = int32_t (*)();
+  using GetAvailable = const char* (*)(int32_t localeIndex);
+
+  template <class AvailableLocales>
+  static bool getAvailableLocales(JSContext* cx, LocaleSet& locales,
+                                  const AvailableLocales& availableLocales);
+
+  /**
+   * Precomputes the available locales sets.
+   */
+  bool ensureSupportedLocales(JSContext* cx);
+
+ public:
+  enum class SupportedLocaleKind {
+    Collator,
+    DateTimeFormat,
+    DisplayNames,
+    ListFormat,
+    NumberFormat,
+    PluralRules,
+    RelativeTimeFormat
+  };
+
+  /**
+   * Sets |supported| to true if |locale| is supported by the requested Intl
+   * service constructor. Otherwise sets |supported| to false.
+   */
+  [[nodiscard]] bool isSupportedLocale(JSContext* cx, SupportedLocaleKind kind,
+                                       JS::Handle<JSString*> locale,
+                                       bool* supported);
+
+  /**
+   * Returns all available locales for |kind|.
+   */
+  ArrayObject* availableLocalesOf(JSContext* cx, SupportedLocaleKind kind);
+
+ private:
+  /**
+   * The case first parameter (BCP47 key "kf") allows to switch the order of
+   * upper- and lower-case characters. ICU doesn't directly provide an API
+   * to query the default case first value of a given locale, but instead
+   * requires to instantiate a collator object and then query the case first
+   * attribute (UCOL_CASE_FIRST).
+   * To avoid instantiating an additional collator object whenever we need
+   * to retrieve the default case first value of a specific locale, we
+   * compute the default case first value for every supported locale only
+   * once and then keep a list of all locales which don't use the default
+   * case first setting.
+   * There is almost no difference between lower-case first and when case
+   * first is disabled (UCOL_LOWER_FIRST resp. UCOL_OFF), so we only need to
+   * track locales which use upper-case first as their default setting.
+   *
+   * Instantiating collator objects for each available locale is slow
+   * (bug 1527879), therefore we're hardcoding the two locales using upper-case
+   * first ("da" (Danish) and "mt" (Maltese)) and only assert in debug-mode
+   * these two locales match the upper-case first locales returned by ICU. A
+   * system-ICU may support a different set of locales, therefore we're always
+   * calling into ICU to find the upper-case first locales in that case.
+   */
+
+#if DEBUG || MOZ_SYSTEM_ICU
+  LocaleSet upperCaseFirstLocales;
+
+  bool upperCaseFirstInitialized = false;
+
+  /**
+   * Precomputes the available locales which use upper-case first sorting.
+   */
+  bool ensureUpperCaseFirstLocales(JSContext* cx);
+#endif
+
+ public:
+  /**
+   * Sets |isUpperFirst| to true if |locale| sorts upper-case characters
+   * before lower-case characters.
+   */
+  bool isUpperCaseFirst(JSContext* cx, JS::Handle<JSString*> locale,
+                        bool* isUpperFirst);
+
+ private:
+  using UniqueDateTimePatternGenerator =
+      mozilla::UniquePtr<mozilla::intl::DateTimePatternGenerator,
+                         DateTimePatternGeneratorDeleter>;
+
+  UniqueDateTimePatternGenerator dateTimePatternGenerator;
+  JS::UniqueChars dateTimePatternGeneratorLocale;
+
+ public:
+  /**
+   * Get a non-owned cached instance of the DateTimePatternGenerator, which is
+   * expensive to instantiate.
+   *
+   * See: https://bugzilla.mozilla.org/show_bug.cgi?id=1549578
+   */
+  mozilla::intl::DateTimePatternGenerator* getDateTimePatternGenerator(
+      JSContext* cx, const char* locale);
+
+ public:
+  void destroyInstance();
+
+  void trace(JSTracer* trc);
+
+  size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const;
+};
+
+}  // namespace intl
+
+}  // namespace js
+
+#endif /* builtin_intl_SharedIntlData_h */
diff --git a/js/src/builtin/intl/StringAsciiChars.h b/js/src/builtin/intl/StringAsciiChars.h
new file mode 100644
index 0000000000..3323544d8c
--- /dev/null
+++ b/js/src/builtin/intl/StringAsciiChars.h
@@ -0,0 +1,77 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_intl_StringAsciiChars_h
+#define builtin_intl_StringAsciiChars_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Span.h"
+
+#include <stddef.h>
+
+#include "js/GCAPI.h"
+#include "js/TypeDecls.h"
+#include "js/Vector.h"
+
+#include "vm/StringType.h"
+
+namespace js::intl {
+
+/**
+ * String view of an ASCII-only string.
+ *
+ * This holds a reference to a JSLinearString and can produce a string view
+ * into that string. If the string is represented by Latin1 characters, the
+ * span is returned directly. If the string is represented by UTF-16
+ * characters, it copies the char16_t characters into a char array, and then
+ * returns a span based on the copy.
+ *
+ * This allows us to avoid copying for the common use case that the ASCII
+ * characters are represented in Latin1.
+ */
+class MOZ_STACK_CLASS StringAsciiChars final {
+  // When copying string characters, use this many bytes of inline storage.
+  static const size_t InlineCapacity = 24;
+
+  JS::AutoCheckCannotGC nogc_;
+
+  JSLinearString* str_;
+
+  mozilla::Maybe<Vector<Latin1Char, InlineCapacity>> ownChars_;
+
+ public:
+  explicit StringAsciiChars(JSLinearString* str) : str_(str) {
+    MOZ_ASSERT(StringIsAscii(str));
+  }
+
+  operator mozilla::Span<const char>() const {
+    if (str_->hasLatin1Chars()) {
+      return mozilla::AsChars(str_->latin1Range(nogc_));
+    }
+    return mozilla::AsChars(mozilla::Span<const Latin1Char>(*ownChars_));
+  }
+
+  [[nodiscard]] bool init(JSContext* cx) {
+    if (str_->hasLatin1Chars()) {
+      return true;
+    }
+
+    ownChars_.emplace(cx);
+    if (!ownChars_->resize(str_->length())) {
+      return false;
+    }
+
+    js::CopyChars(ownChars_->begin(), *str_);
+
+    return true;
+  }
+};
+
+}  // namespace js::intl
+
+#endif  // builtin_intl_StringAsciiChars_h
diff --git a/js/src/builtin/intl/TimeZoneDataGenerated.h b/js/src/builtin/intl/TimeZoneDataGenerated.h
new file mode 100644
index 0000000000..04954ffbc7
--- /dev/null
+++ b/js/src/builtin/intl/TimeZoneDataGenerated.h
@@ -0,0 +1,142 @@
+// Generated by make_intl_data.py. DO NOT EDIT.
+// tzdata version = 2022g
+
+#ifndef builtin_intl_TimeZoneDataGenerated_h
+#define builtin_intl_TimeZoneDataGenerated_h
+
+namespace js {
+namespace timezone {
+
+// Format:
+// "ZoneName" // ICU-Name [time zone file]
+const char* const ianaZonesTreatedAsLinksByICU[] = {
+    "Africa/Asmara", // Africa/Asmera [backzone]
+    "Africa/Timbuktu", // Africa/Bamako [backzone]
+    "America/Argentina/Buenos_Aires", // America/Buenos_Aires [southamerica]
+    "America/Argentina/Catamarca", // America/Catamarca [southamerica]
+    "America/Argentina/ComodRivadavia", // America/Catamarca [backzone]
+    "America/Argentina/Cordoba", // America/Cordoba [southamerica]
+    "America/Argentina/Jujuy", // America/Jujuy [southamerica]
+    "America/Argentina/Mendoza", // America/Mendoza [southamerica]
+    "America/Atikokan", // America/Coral_Harbour [backzone]
+    "America/Ensenada", // America/Tijuana [backzone]
+    "America/Indiana/Indianapolis", // America/Indianapolis [northamerica]
+    "America/Kentucky/Louisville", // America/Louisville [northamerica]
+    "America/Nuuk", // America/Godthab [europe]
+    "America/Rosario", // America/Cordoba [backzone]
+    "Asia/Chongqing", // Asia/Shanghai [backzone]
+    "Asia/Harbin", // Asia/Shanghai [backzone]
+    "Asia/Ho_Chi_Minh", // Asia/Saigon [asia]
+    "Asia/Kashgar", // Asia/Urumqi [backzone]
+    "Asia/Kathmandu", // Asia/Katmandu [asia]
+    "Asia/Kolkata", // Asia/Calcutta [asia]
+    "Asia/Tel_Aviv", // Asia/Jerusalem [backzone]
+    "Asia/Yangon", // Asia/Rangoon [asia]
+    "Atlantic/Faroe", // Atlantic/Faeroe [europe]
+    "Atlantic/Jan_Mayen", // Arctic/Longyearbyen [backzone]
+    "EST", // Etc/GMT+5 [northamerica]
+    "Europe/Belfast", // Europe/London [backzone]
+    "Europe/Kyiv", // Europe/Kiev [europe]
+    "Europe/Tiraspol", // Europe/Chisinau [backzone]
+    "HST", // Etc/GMT+10 [northamerica]
+    "MST", // Etc/GMT+7 [northamerica]
+    "Pacific/Chuuk", // Pacific/Truk [backzone]
+    "Pacific/Kanton", // Pacific/Enderbury [australasia]
+    "Pacific/Pohnpei", // Pacific/Ponape [backzone]
+};
+
+// Format:
+// "LinkName", "Target" // ICU-Target [time zone file]
+struct LinkAndTarget
+{
+    const char* const link;
+    const char* const target;
+};
+
+const LinkAndTarget ianaLinksCanonicalizedDifferentlyByICU[] = {
+    { "Africa/Asmera", "Africa/Asmara" }, // Africa/Asmera [backward]
+    { "America/Buenos_Aires", "America/Argentina/Buenos_Aires" }, // America/Buenos_Aires [backward]
+    { "America/Catamarca", "America/Argentina/Catamarca" }, // America/Catamarca [backward]
+    { "America/Cordoba", "America/Argentina/Cordoba" }, // America/Cordoba [backward]
+    { "America/Fort_Wayne", "America/Indiana/Indianapolis" }, // America/Indianapolis [backward]
+    { "America/Godthab", "America/Nuuk" }, // America/Godthab [backward]
+    { "America/Indianapolis", "America/Indiana/Indianapolis" }, // America/Indianapolis [backward]
+    { "America/Jujuy", "America/Argentina/Jujuy" }, // America/Jujuy [backward]
+    { "America/Kralendijk", "America/Curacao" }, // America/Kralendijk [backward]
+    { "America/Louisville", "America/Kentucky/Louisville" }, // America/Louisville [backward]
+    { "America/Lower_Princes", "America/Curacao" }, // America/Lower_Princes [backward]
+    { "America/Marigot", "America/Port_of_Spain" }, // America/Marigot [backward]
+    { "America/Mendoza", "America/Argentina/Mendoza" }, // America/Mendoza [backward]
+    { "America/Santa_Isabel", "America/Tijuana" }, // America/Santa_Isabel [backward]
+    { "America/St_Barthelemy", "America/Port_of_Spain" }, // America/St_Barthelemy [backward]
+    { "Antarctica/South_Pole", "Antarctica/McMurdo" }, // Pacific/Auckland [backward]
+    { "Arctic/Longyearbyen", "Europe/Oslo" }, // Arctic/Longyearbyen [backward]
+    { "Asia/Calcutta", "Asia/Kolkata" }, // Asia/Calcutta [backward]
+    { "Asia/Chungking", "Asia/Chongqing" }, // Asia/Shanghai [backward]
+    { "Asia/Katmandu", "Asia/Kathmandu" }, // Asia/Katmandu [backward]
+    { "Asia/Rangoon", "Asia/Yangon" }, // Asia/Rangoon [backward]
+    { "Asia/Saigon", "Asia/Ho_Chi_Minh" }, // Asia/Saigon [backward]
+    { "Atlantic/Faeroe", "Atlantic/Faroe" }, // Atlantic/Faeroe [backward]
+    { "Europe/Bratislava", "Europe/Prague" }, // Europe/Bratislava [backward]
+    { "Europe/Busingen", "Europe/Zurich" }, // Europe/Busingen [backward]
+    { "Europe/Kiev", "Europe/Kyiv" }, // Europe/Kiev [backward]
+    { "Europe/Mariehamn", "Europe/Helsinki" }, // Europe/Mariehamn [backward]
+    { "Europe/Podgorica", "Europe/Belgrade" }, // Europe/Podgorica [backward]
+    { "Europe/San_Marino", "Europe/Rome" }, // Europe/San_Marino [backward]
+    { "Europe/Vatican", "Europe/Rome" }, // Europe/Vatican [backward]
+    { "Pacific/Ponape", "Pacific/Pohnpei" }, // Pacific/Ponape [backward]
+    { "Pacific/Truk", "Pacific/Chuuk" }, // Pacific/Truk [backward]
+    { "Pacific/Yap", "Pacific/Chuuk" }, // Pacific/Truk [backward]
+    { "US/East-Indiana", "America/Indiana/Indianapolis" }, // America/Indianapolis [backward]
+};
+
+// Legacy ICU time zones, these are not valid IANA time zone names. We also
+// disallow the old and deprecated System V time zones.
+// https://ssl.icu-project.org/repos/icu/trunk/icu4c/source/tools/tzcode/icuzones
+const char* const legacyICUTimeZones[] = {
+    "ACT",
+    "AET",
+    "AGT",
+    "ART",
+    "AST",
+    "BET",
+    "BST",
+    "CAT",
+    "CNT",
+    "CST",
+    "CTT",
+    "Canada/East-Saskatchewan",
+    "EAT",
+    "ECT",
+    "IET",
+    "IST",
+    "JST",
+    "MIT",
+    "NET",
+    "NST",
+    "PLT",
+    "PNT",
+    "PRT",
+    "PST",
+    "SST",
+    "US/Pacific-New",
+    "VST",
+    "SystemV/AST4",
+    "SystemV/AST4ADT",
+    "SystemV/CST6",
+    "SystemV/CST6CDT",
+    "SystemV/EST5",
+    "SystemV/EST5EDT",
+    "SystemV/HST10",
+    "SystemV/MST7",
+    "SystemV/MST7MDT",
+    "SystemV/PST8",
+    "SystemV/PST8PDT",
+    "SystemV/YST9",
+    "SystemV/YST9YDT",
+};
+
+} // namespace timezone
+} // namespace js
+
+#endif /* builtin_intl_TimeZoneDataGenerated_h */
diff --git a/js/src/builtin/intl/make_intl_data.py b/js/src/builtin/intl/make_intl_data.py
new file mode 100755
index 0000000000..1f9d7ff30e
--- /dev/null
+++ b/js/src/builtin/intl/make_intl_data.py
@@ -0,0 +1,4123 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+""" Usage:
+    make_intl_data.py langtags [cldr_core.zip]
+    make_intl_data.py tzdata
+    make_intl_data.py currency
+    make_intl_data.py units
+    make_intl_data.py numbering
+
+
+    Target "langtags":
+    This script extracts information about 1) mappings between deprecated and
+    current Unicode BCP 47 locale identifiers, and 2) deprecated and current
+    BCP 47 Unicode extension value from CLDR, and converts it to C++ mapping
+    code in intl/components/LocaleGenerated.cpp. The code is used in
+    intl/components/Locale.cpp.
+
+
+    Target "tzdata":
+    This script computes which time zone informations are not up-to-date in ICU
+    and provides the necessary mappings to workaround this problem.
+    https://ssl.icu-project.org/trac/ticket/12044
+
+
+    Target "currency":
+    Generates the mapping from currency codes to decimal digits used for them.
+
+
+    Target "units":
+    Generate source and test files using the list of so-called "sanctioned unit
+    identifiers" and verifies that the ICU data filter includes these units.
+
+
+    Target "numbering":
+    Generate source and test files using the list of numbering systems with
+    simple digit mappings and verifies that it's in sync with ICU/CLDR.
+"""
+
+import io
+import json
+import os
+import re
+import sys
+import tarfile
+import tempfile
+from contextlib import closing
+from functools import partial, total_ordering
+from itertools import chain, groupby, tee
+from operator import attrgetter, itemgetter
+from zipfile import ZipFile
+
+import yaml
+
+if sys.version_info.major == 2:
+    from itertools import ifilter as filter
+    from itertools import ifilterfalse as filterfalse
+    from itertools import imap as map
+    from itertools import izip_longest as zip_longest
+
+    from urllib2 import Request as UrlRequest
+    from urllib2 import urlopen
+    from urlparse import urlsplit
+else:
+    from itertools import filterfalse, zip_longest
+    from urllib.parse import urlsplit
+    from urllib.request import Request as UrlRequest
+    from urllib.request import urlopen
+
+
+# From https://docs.python.org/3/library/itertools.html
+def grouper(iterable, n, fillvalue=None):
+    "Collect data into fixed-length chunks or blocks"
+    # grouper('ABCDEFG', 3, 'x') --> ABC DEF Gxx"
+    args = [iter(iterable)] * n
+    return zip_longest(*args, fillvalue=fillvalue)
+
+
+def writeMappingHeader(println, description, source, url):
+    if type(description) is not list:
+        description = [description]
+    for desc in description:
+        println("// {0}".format(desc))
+    println("// Derived from {0}.".format(source))
+    println("// {0}".format(url))
+
+
+def writeMappingsVar(println, mapping, name, description, source, url):
+    """Writes a variable definition with a mapping table.
+
+    Writes the contents of dictionary |mapping| through the |println|
+    function with the given variable name and a comment with description,
+    fileDate, and URL.
+    """
+    println("")
+    writeMappingHeader(println, description, source, url)
+    println("var {0} = {{".format(name))
+    for (key, value) in sorted(mapping.items(), key=itemgetter(0)):
+        println('    "{0}": "{1}",'.format(key, value))
+    println("};")
+
+
+def writeMappingsBinarySearch(
+    println,
+    fn_name,
+    type_name,
+    name,
+    validate_fn,
+    validate_case_fn,
+    mappings,
+    tag_maxlength,
+    description,
+    source,
+    url,
+):
+    """Emit code to perform a binary search on language tag subtags.
+
+    Uses the contents of |mapping|, which can either be a dictionary or set,
+    to emit a mapping function to find subtag replacements.
+    """
+    println("")
+    writeMappingHeader(println, description, source, url)
+    println(
+        """
+bool mozilla::intl::Locale::{0}({1} {2}) {{
+  MOZ_ASSERT({3}({2}.Span()));
+  MOZ_ASSERT({4}({2}.Span()));
+""".format(
+            fn_name, type_name, name, validate_fn, validate_case_fn
+        ).strip()
+    )
+    writeMappingsBinarySearchBody(println, name, name, mappings, tag_maxlength)
+
+    println(
+        """
+}""".lstrip(
+            "\n"
+        )
+    )
+
+
+def writeMappingsBinarySearchBody(
+    println, source_name, target_name, mappings, tag_maxlength
+):
+    def write_array(subtags, name, length, fixed):
+        if fixed:
+            println(
+                "    static const char {}[{}][{}] = {{".format(
+                    name, len(subtags), length + 1
+                )
+            )
+        else:
+            println("    static const char* {}[{}] = {{".format(name, len(subtags)))
+
+        # Group in pairs of ten to not exceed the 80 line column limit.
+        for entries in grouper(subtags, 10):
+            entries = (
+                '"{}"'.format(tag).rjust(length + 2)
+                for tag in entries
+                if tag is not None
+            )
+            println("      {},".format(", ".join(entries)))
+
+        println("    };")
+
+    trailing_return = True
+
+    # Sort the subtags by length. That enables using an optimized comparator
+    # for the binary search, which only performs a single |memcmp| for multiple
+    # of two subtag lengths.
+    mappings_keys = mappings.keys() if type(mappings) == dict else mappings
+    for (length, subtags) in groupby(sorted(mappings_keys, key=len), len):
+        # Omit the length check if the current length is the maximum length.
+        if length != tag_maxlength:
+            println(
+                """
+  if ({}.Length() == {}) {{
+""".format(
+                    source_name, length
+                ).rstrip(
+                    "\n"
+                )
+            )
+        else:
+            trailing_return = False
+            println(
+                """
+  {
+""".rstrip(
+                    "\n"
+                )
+            )
+
+        # The subtags need to be sorted for binary search to work.
+        subtags = sorted(subtags)
+
+        def equals(subtag):
+            return """{}.EqualTo("{}")""".format(source_name, subtag)
+
+        # Don't emit a binary search for short lists.
+        if len(subtags) == 1:
+            if type(mappings) == dict:
+                println(
+                    """
+    if ({}) {{
+      {}.Set(mozilla::MakeStringSpan("{}"));
+      return true;
+    }}
+    return false;
+""".format(
+                        equals(subtags[0]), target_name, mappings[subtags[0]]
+                    ).strip(
+                        "\n"
+                    )
+                )
+            else:
+                println(
+                    """
+    return {};
+""".format(
+                        equals(subtags[0])
+                    ).strip(
+                        "\n"
+                    )
+                )
+        elif len(subtags) <= 4:
+            if type(mappings) == dict:
+                for subtag in subtags:
+                    println(
+                        """
+    if ({}) {{
+      {}.Set("{}");
+      return true;
+    }}
+""".format(
+                            equals(subtag), target_name, mappings[subtag]
+                        ).strip(
+                            "\n"
+                        )
+                    )
+
+                println(
+                    """
+    return false;
+""".strip(
+                        "\n"
+                    )
+                )
+            else:
+                cond = (equals(subtag) for subtag in subtags)
+                cond = (" ||\n" + " " * (4 + len("return "))).join(cond)
+                println(
+                    """
+    return {};
+""".format(
+                        cond
+                    ).strip(
+                        "\n"
+                    )
+                )
+        else:
+            write_array(subtags, source_name + "s", length, True)
+
+            if type(mappings) == dict:
+                write_array([mappings[k] for k in subtags], "aliases", length, False)
+
+                println(
+                    """
+    if (const char* replacement = SearchReplacement({0}s, aliases, {0})) {{
+      {1}.Set(mozilla::MakeStringSpan(replacement));
+      return true;
+    }}
+    return false;
+""".format(
+                        source_name, target_name
+                    ).rstrip()
+                )
+            else:
+                println(
+                    """
+    return HasReplacement({0}s, {0});
+""".format(
+                        source_name
+                    ).rstrip()
+                )
+
+        println(
+            """
+  }
+""".strip(
+                "\n"
+            )
+        )
+
+    if trailing_return:
+        println(
+            """
+  return false;"""
+        )
+
+
+def writeComplexLanguageTagMappings(
+    println, complex_language_mappings, description, source, url
+):
+    println("")
+    writeMappingHeader(println, description, source, url)
+    println(
+        """
+void mozilla::intl::Locale::PerformComplexLanguageMappings() {
+  MOZ_ASSERT(IsStructurallyValidLanguageTag(Language().Span()));
+  MOZ_ASSERT(IsCanonicallyCasedLanguageTag(Language().Span()));
+""".lstrip()
+    )
+
+    # Merge duplicate language entries.
+    language_aliases = {}
+    for (deprecated_language, (language, script, region)) in sorted(
+        complex_language_mappings.items(), key=itemgetter(0)
+    ):
+        key = (language, script, region)
+        if key not in language_aliases:
+            language_aliases[key] = []
+        else:
+            language_aliases[key].append(deprecated_language)
+
+    first_language = True
+    for (deprecated_language, (language, script, region)) in sorted(
+        complex_language_mappings.items(), key=itemgetter(0)
+    ):
+        key = (language, script, region)
+        if deprecated_language in language_aliases[key]:
+            continue
+
+        if_kind = "if" if first_language else "else if"
+        first_language = False
+
+        cond = (
+            'Language().EqualTo("{}")'.format(lang)
+            for lang in [deprecated_language] + language_aliases[key]
+        )
+        cond = (" ||\n" + " " * (2 + len(if_kind) + 2)).join(cond)
+
+        println(
+            """
+  {} ({}) {{""".format(
+                if_kind, cond
+            ).strip(
+                "\n"
+            )
+        )
+
+        println(
+            """
+    SetLanguage("{}");""".format(
+                language
+            ).strip(
+                "\n"
+            )
+        )
+
+        if script is not None:
+            println(
+                """
+    if (Script().Missing()) {{
+      SetScript("{}");
+    }}""".format(
+                    script
+                ).strip(
+                    "\n"
+                )
+            )
+        if region is not None:
+            println(
+                """
+    if (Region().Missing()) {{
+      SetRegion("{}");
+    }}""".format(
+                    region
+                ).strip(
+                    "\n"
+                )
+            )
+        println(
+            """
+  }""".strip(
+                "\n"
+            )
+        )
+
+    println(
+        """
+}
+""".strip(
+            "\n"
+        )
+    )
+
+
+def writeComplexRegionTagMappings(
+    println, complex_region_mappings, description, source, url
+):
+    println("")
+    writeMappingHeader(println, description, source, url)
+    println(
+        """
+void mozilla::intl::Locale::PerformComplexRegionMappings() {
+  MOZ_ASSERT(IsStructurallyValidLanguageTag(Language().Span()));
+  MOZ_ASSERT(IsCanonicallyCasedLanguageTag(Language().Span()));
+  MOZ_ASSERT(IsStructurallyValidRegionTag(Region().Span()));
+  MOZ_ASSERT(IsCanonicallyCasedRegionTag(Region().Span()));
+""".lstrip()
+    )
+
+    # |non_default_replacements| is a list and hence not hashable. Convert it
+    # to a string to get a proper hashable value.
+    def hash_key(default, non_default_replacements):
+        return (default, str(sorted(str(v) for v in non_default_replacements)))
+
+    # Merge duplicate region entries.
+    region_aliases = {}
+    for (deprecated_region, (default, non_default_replacements)) in sorted(
+        complex_region_mappings.items(), key=itemgetter(0)
+    ):
+        key = hash_key(default, non_default_replacements)
+        if key not in region_aliases:
+            region_aliases[key] = []
+        else:
+            region_aliases[key].append(deprecated_region)
+
+    first_region = True
+    for (deprecated_region, (default, non_default_replacements)) in sorted(
+        complex_region_mappings.items(), key=itemgetter(0)
+    ):
+        key = hash_key(default, non_default_replacements)
+        if deprecated_region in region_aliases[key]:
+            continue
+
+        if_kind = "if" if first_region else "else if"
+        first_region = False
+
+        cond = (
+            'Region().EqualTo("{}")'.format(region)
+            for region in [deprecated_region] + region_aliases[key]
+        )
+        cond = (" ||\n" + " " * (2 + len(if_kind) + 2)).join(cond)
+
+        println(
+            """
+  {} ({}) {{""".format(
+                if_kind, cond
+            ).strip(
+                "\n"
+            )
+        )
+
+        replacement_regions = sorted(
+            {region for (_, _, region) in non_default_replacements}
+        )
+
+        first_case = True
+        for replacement_region in replacement_regions:
+            replacement_language_script = sorted(
+                (language, script)
+                for (language, script, region) in (non_default_replacements)
+                if region == replacement_region
+            )
+
+            if_kind = "if" if first_case else "else if"
+            first_case = False
+
+            def compare_tags(language, script):
+                if script is None:
+                    return 'Language().EqualTo("{}")'.format(language)
+                return '(Language().EqualTo("{}") && Script().EqualTo("{}"))'.format(
+                    language, script
+                )
+
+            cond = (
+                compare_tags(language, script)
+                for (language, script) in replacement_language_script
+            )
+            cond = (" ||\n" + " " * (4 + len(if_kind) + 2)).join(cond)
+
+            println(
+                """
+    {} ({}) {{
+      SetRegion("{}");
+    }}""".format(
+                    if_kind, cond, replacement_region
+                )
+                .rstrip()
+                .strip("\n")
+            )
+
+        println(
+            """
+    else {{
+      SetRegion("{}");
+    }}
+  }}""".format(
+                default
+            )
+            .rstrip()
+            .strip("\n")
+        )
+
+    println(
+        """
+}
+""".strip(
+            "\n"
+        )
+    )
+
+
+def writeVariantTagMappings(println, variant_mappings, description, source, url):
+    """Writes a function definition that maps variant subtags."""
+    println(
+        """
+static const char* ToCharPointer(const char* str) {
+  return str;
+}
+
+static const char* ToCharPointer(const mozilla::intl::UniqueChars& str) {
+  return str.get();
+}
+
+template <typename T, typename U = T>
+static bool IsLessThan(const T& a, const U& b) {
+  return strcmp(ToCharPointer(a), ToCharPointer(b)) < 0;
+}
+"""
+    )
+    writeMappingHeader(println, description, source, url)
+    println(
+        """
+bool mozilla::intl::Locale::PerformVariantMappings() {
+  // The variant subtags need to be sorted for binary search.
+  MOZ_ASSERT(std::is_sorted(mVariants.begin(), mVariants.end(),
+                            IsLessThan<decltype(mVariants)::ElementType>));
+
+  auto removeVariantAt = [&](size_t index) {
+    mVariants.erase(mVariants.begin() + index);
+  };
+
+  auto insertVariantSortedIfNotPresent = [&](const char* variant) {
+    auto* p = std::lower_bound(
+        mVariants.begin(), mVariants.end(), variant,
+        IsLessThan<decltype(mVariants)::ElementType, decltype(variant)>);
+
+    // Don't insert the replacement when already present.
+    if (p != mVariants.end() && strcmp(p->get(), variant) == 0) {
+      return true;
+    }
+
+    // Insert the preferred variant in sort order.
+    auto preferred = DuplicateStringToUniqueChars(variant);
+    return !!mVariants.insert(p, std::move(preferred));
+  };
+
+  for (size_t i = 0; i < mVariants.length();) {
+    const char* variant = mVariants[i].get();
+    MOZ_ASSERT(IsCanonicallyCasedVariantTag(mozilla::MakeStringSpan(variant)));
+""".lstrip()
+    )
+
+    (no_alias, with_alias) = partition(
+        variant_mappings.items(), lambda item: item[1] is None
+    )
+
+    no_replacements = " ||\n        ".join(
+        f"""strcmp(variant, "{deprecated_variant}") == 0"""
+        for (deprecated_variant, _) in sorted(no_alias, key=itemgetter(0))
+    )
+
+    println(
+        f"""
+    if ({no_replacements}) {{
+      removeVariantAt(i);
+    }}
+""".strip(
+            "\n"
+        )
+    )
+
+    for (deprecated_variant, (type, replacement)) in sorted(
+        with_alias, key=itemgetter(0)
+    ):
+        println(
+            f"""
+    else if (strcmp(variant, "{deprecated_variant}") == 0) {{
+      removeVariantAt(i);
+""".strip(
+                "\n"
+            )
+        )
+
+        if type == "language":
+            println(
+                f"""
+      SetLanguage("{replacement}");
+""".strip(
+                    "\n"
+                )
+            )
+        elif type == "region":
+            println(
+                f"""
+      SetRegion("{replacement}");
+""".strip(
+                    "\n"
+                )
+            )
+        else:
+            assert type == "variant"
+            println(
+                f"""
+      if (!insertVariantSortedIfNotPresent("{replacement}")) {{
+        return false;
+      }}
+""".strip(
+                    "\n"
+                )
+            )
+
+        println(
+            """
+    }
+""".strip(
+                "\n"
+            )
+        )
+
+    println(
+        """
+    else {
+      i++;
+    }
+  }
+  return true;
+}
+""".strip(
+            "\n"
+        )
+    )
+
+
+def writeLegacyMappingsFunction(println, legacy_mappings, description, source, url):
+    """Writes a function definition that maps legacy language tags."""
+    println("")
+    writeMappingHeader(println, description, source, url)
+    println(
+        """\
+bool mozilla::intl::Locale::UpdateLegacyMappings() {
+  // We're mapping legacy tags to non-legacy form here.
+  // Other tags remain unchanged.
+  //
+  // Legacy tags are either sign language tags ("sgn") or have one or multiple
+  // variant subtags. Therefore we can quickly exclude most tags by checking
+  // these two subtags.
+
+  MOZ_ASSERT(IsCanonicallyCasedLanguageTag(Language().Span()));
+
+  if (!Language().EqualTo("sgn") && mVariants.length() == 0) {
+    return true;
+  }
+
+#ifdef DEBUG
+  for (const auto& variant : Variants()) {
+    MOZ_ASSERT(IsStructurallyValidVariantTag(variant));
+    MOZ_ASSERT(IsCanonicallyCasedVariantTag(variant));
+  }
+#endif
+
+  // The variant subtags need to be sorted for binary search.
+  MOZ_ASSERT(std::is_sorted(mVariants.begin(), mVariants.end(),
+                            IsLessThan<decltype(mVariants)::ElementType>));
+
+  auto findVariant = [this](const char* variant) {
+    auto* p = std::lower_bound(mVariants.begin(), mVariants.end(), variant,
+                               IsLessThan<decltype(mVariants)::ElementType,
+                                          decltype(variant)>);
+
+    if (p != mVariants.end() && strcmp(p->get(), variant) == 0) {
+      return p;
+    }
+    return static_cast<decltype(p)>(nullptr);
+  };
+
+  auto insertVariantSortedIfNotPresent = [&](const char* variant) {
+    auto* p = std::lower_bound(mVariants.begin(), mVariants.end(), variant,
+                               IsLessThan<decltype(mVariants)::ElementType,
+                                          decltype(variant)>);
+
+    // Don't insert the replacement when already present.
+    if (p != mVariants.end() && strcmp(p->get(), variant) == 0) {
+      return true;
+    }
+
+    // Insert the preferred variant in sort order.
+    auto preferred = DuplicateStringToUniqueChars(variant);
+    return !!mVariants.insert(p, std::move(preferred));
+  };
+
+  auto removeVariant = [&](auto* p) {
+    size_t index = std::distance(mVariants.begin(), p);
+    mVariants.erase(mVariants.begin() + index);
+  };
+
+  auto removeVariants = [&](auto* p, auto* q) {
+    size_t pIndex = std::distance(mVariants.begin(), p);
+    size_t qIndex = std::distance(mVariants.begin(), q);
+    MOZ_ASSERT(pIndex < qIndex, "variant subtags are sorted");
+
+    mVariants.erase(mVariants.begin() + qIndex);
+    mVariants.erase(mVariants.begin() + pIndex);
+  };"""
+    )
+
+    # Helper class for pattern matching.
+    class AnyClass:
+        def __eq__(self, obj):
+            return obj is not None
+
+    Any = AnyClass()
+
+    # Group the mappings by language.
+    legacy_mappings_by_language = {}
+    for (type, replacement) in legacy_mappings.items():
+        (language, _, _, _) = type
+        legacy_mappings_by_language.setdefault(language, {})[type] = replacement
+
+    # Handle the empty language case first.
+    if None in legacy_mappings_by_language:
+        # Get the mappings and remove them from the dict.
+        mappings = legacy_mappings_by_language.pop(None)
+
+        # This case only applies for the "hepburn-heploc" -> "alalc97"
+        # mapping, so just inline it here.
+        from_tag = (None, None, None, "hepburn-heploc")
+        to_tag = (None, None, None, "alalc97")
+
+        assert len(mappings) == 1
+        assert mappings[from_tag] == to_tag
+
+        println(
+            """
+  if (mVariants.length() >= 2) {
+    if (auto* hepburn = findVariant("hepburn")) {
+      if (auto* heploc = findVariant("heploc")) {
+        removeVariants(hepburn, heploc);
+
+        if (!insertVariantSortedIfNotPresent("alalc97")) {
+          return false;
+        }
+      }
+    }
+  }
+"""
+        )
+
+    # Handle sign languages next.
+    if "sgn" in legacy_mappings_by_language:
+        mappings = legacy_mappings_by_language.pop("sgn")
+
+        # Legacy sign language mappings have the form "sgn-XX" where "XX" is
+        # some region code.
+        assert all(type == ("sgn", None, Any, None) for type in mappings.keys())
+
+        # Legacy sign languages are mapped to a single language subtag.
+        assert all(
+            replacement == (Any, None, None, None) for replacement in mappings.values()
+        )
+
+        println(
+            """
+  if (Language().EqualTo("sgn")) {
+    if (Region().Present() && SignLanguageMapping(mLanguage, Region())) {
+      mRegion.Set(mozilla::MakeStringSpan(""));
+    }
+  }
+""".rstrip().lstrip(
+                "\n"
+            )
+        )
+
+    # Finally handle all remaining cases.
+
+    # The remaining mappings have neither script nor region subtags in the source locale.
+    assert all(
+        type == (Any, None, None, Any)
+        for mappings in legacy_mappings_by_language.values()
+        for type in mappings.keys()
+    )
+
+    # And they have neither script nor region nor variant subtags in the target locale.
+    assert all(
+        replacement == (Any, None, None, None)
+        for mappings in legacy_mappings_by_language.values()
+        for replacement in mappings.values()
+    )
+
+    # Compact the mappings table by removing empty fields.
+    legacy_mappings_by_language = {
+        lang: {
+            variants: r_language
+            for ((_, _, _, variants), (r_language, _, _, _)) in mappings.items()
+        }
+        for (lang, mappings) in legacy_mappings_by_language.items()
+    }
+
+    # Try to combine the remaining cases.
+    legacy_mappings_compact = {}
+
+    # Python can't hash dicts or lists, so use the string representation as the hash key.
+    def hash_key(mappings):
+        return str(sorted(mappings.items(), key=itemgetter(0)))
+
+    for (lang, mappings) in sorted(
+        legacy_mappings_by_language.items(), key=itemgetter(0)
+    ):
+        key = hash_key(mappings)
+        legacy_mappings_compact.setdefault(key, []).append(lang)
+
+    for langs in legacy_mappings_compact.values():
+        language_equal_to = (
+            f"""Language().EqualTo("{lang}")""" for lang in sorted(langs)
+        )
+        cond = f""" ||\n{" " * len("  else if (")}""".join(language_equal_to)
+
+        println(
+            f"""
+  else if ({cond}) {{
+""".rstrip().lstrip(
+                "\n"
+            )
+        )
+
+        mappings = legacy_mappings_by_language[langs[0]]
+
+        # Count the variant subtags to determine the sort order.
+        def variant_size(m):
+            (k, _) = m
+            return len(k.split("-"))
+
+        # Alias rules are applied by largest union size first.
+        for (size, mappings_by_size) in groupby(
+            sorted(mappings.items(), key=variant_size, reverse=True), key=variant_size
+        ):
+
+            # Convert grouper object to dict.
+            mappings_by_size = dict(mappings_by_size)
+
+            is_first = True
+            chain_if = size == 1
+
+            # Alias rules are applied in alphabetical order
+            for (variants, r_language) in sorted(
+                mappings_by_size.items(), key=itemgetter(0)
+            ):
+                sorted_variants = sorted(variants.split("-"))
+                len_variants = len(sorted_variants)
+
+                maybe_else = "else " if chain_if and not is_first else ""
+                is_first = False
+
+                for (i, variant) in enumerate(sorted_variants):
+                    println(
+                        f"""
+    {"  " * i}{maybe_else}if (auto* {variant} = findVariant("{variant}")) {{
+""".rstrip().lstrip(
+                            "\n"
+                        )
+                    )
+
+                indent = "  " * len_variants
+
+                println(
+                    f"""
+    {indent}removeVariant{"s" if len_variants > 1 else ""}({", ".join(sorted_variants)});
+    {indent}SetLanguage("{r_language}");
+    {indent}{"return true;" if not chain_if else ""}
+""".rstrip().lstrip(
+                        "\n"
+                    )
+                )
+
+                for i in range(len_variants, 0, -1):
+                    println(
+                        f"""
+    {"  " * (i - 1)}}}
+""".rstrip().lstrip(
+                            "\n"
+                        )
+                    )
+
+        println(
+            """
+  }
+""".rstrip().lstrip(
+                "\n"
+            )
+        )
+
+    println(
+        """
+  return true;
+}"""
+    )
+
+
+def writeSignLanguageMappingsFunction(
+    println, legacy_mappings, description, source, url
+):
+    """Writes a function definition that maps legacy sign language tags."""
+    println("")
+    writeMappingHeader(println, description, source, url)
+    println(
+        """\
+bool mozilla::intl::Locale::SignLanguageMapping(LanguageSubtag& language,
+                                                const RegionSubtag& region) {
+  MOZ_ASSERT(language.EqualTo("sgn"));
+  MOZ_ASSERT(IsStructurallyValidRegionTag(region.Span()));
+  MOZ_ASSERT(IsCanonicallyCasedRegionTag(region.Span()));
+""".rstrip()
+    )
+
+    region_mappings = {
+        rg: lg
+        for ((lang, _, rg, _), (lg, _, _, _)) in legacy_mappings.items()
+        if lang == "sgn"
+    }
+
+    source_name = "region"
+    target_name = "language"
+    tag_maxlength = 3
+    writeMappingsBinarySearchBody(
+        println, source_name, target_name, region_mappings, tag_maxlength
+    )
+
+    println(
+        """
+}""".lstrip()
+    )
+
+
+def readSupplementalData(core_file):
+    """Reads CLDR Supplemental Data and extracts information for Intl.js.
+
+    Information extracted:
+    - legacyMappings: mappings from legacy tags to preferred complete language tags
+    - languageMappings: mappings from language subtags to preferred subtags
+    - complexLanguageMappings: mappings from language subtags with complex rules
+    - regionMappings: mappings from region subtags to preferred subtags
+    - complexRegionMappings: mappings from region subtags with complex rules
+    - variantMappings: mappings from variant subtags to preferred subtags
+    - likelySubtags: likely subtags used for generating test data only
+    Returns these mappings as dictionaries.
+    """
+    import xml.etree.ElementTree as ET
+
+    # From Unicode BCP 47 locale identifier <https://unicode.org/reports/tr35/>.
+    re_unicode_language_id = re.compile(
+        r"""
+        ^
+        # unicode_language_id = unicode_language_subtag
+        #     unicode_language_subtag = alpha{2,3} | alpha{5,8}
+        (?P<language>[a-z]{2,3}|[a-z]{5,8})
+
+        # (sep unicode_script_subtag)?
+        #     unicode_script_subtag = alpha{4}
+        (?:-(?P<script>[a-z]{4}))?
+
+        # (sep unicode_region_subtag)?
+        #     unicode_region_subtag = (alpha{2} | digit{3})
+        (?:-(?P<region>([a-z]{2}|[0-9]{3})))?
+
+        # (sep unicode_variant_subtag)*
+        #     unicode_variant_subtag = (alphanum{5,8} | digit alphanum{3})
+        (?P<variants>(-([a-z0-9]{5,8}|[0-9][a-z0-9]{3}))+)?
+        $
+        """,
+        re.IGNORECASE | re.VERBOSE,
+    )
+
+    # CLDR uses "_" as the separator for some elements. Replace it with "-".
+    def bcp47_id(cldr_id):
+        return cldr_id.replace("_", "-")
+
+    # Return the tuple (language, script, region, variants) and assert all
+    # subtags are in canonical case.
+    def bcp47_canonical(language, script, region, variants):
+        # Canonical case for language subtags is lower case.
+        assert language is None or language.lower() == language
+
+        # Canonical case for script subtags is title case.
+        assert script is None or script.title() == script
+
+        # Canonical case for region subtags is upper case.
+        assert region is None or region.upper() == region
+
+        # Canonical case for variant subtags is lower case.
+        assert variants is None or variants.lower() == variants
+
+        return (language, script, region, variants[1:] if variants else None)
+
+    # Language ids are interpreted as multi-maps in
+    # <https://www.unicode.org/reports/tr35/#LocaleId_Canonicalization>.
+    #
+    # See UTS35, §Annex C, Definitions - 1. Multimap interpretation.
+    def language_id_to_multimap(language_id):
+        match = re_unicode_language_id.match(language_id)
+        assert (
+            match is not None
+        ), f"{language_id} invalid Unicode BCP 47 locale identifier"
+
+        canonical_language_id = bcp47_canonical(
+            *match.group("language", "script", "region", "variants")
+        )
+        (language, _, _, _) = canonical_language_id
+
+        # Normalize "und" language to None, but keep the rest as is.
+        return (language if language != "und" else None,) + canonical_language_id[1:]
+
+    rules = {}
+    territory_exception_rules = {}
+
+    tree = ET.parse(core_file.open("common/supplemental/supplementalMetadata.xml"))
+
+    # Load the rules from supplementalMetadata.xml.
+    #
+    # See UTS35, §Annex C, Definitions - 2. Alias elements.
+    # See UTS35, §Annex C, Preprocessing.
+    for alias_name in [
+        "languageAlias",
+        "scriptAlias",
+        "territoryAlias",
+        "variantAlias",
+    ]:
+        for alias in tree.iterfind(".//" + alias_name):
+            # Replace '_' by '-'.
+            type = bcp47_id(alias.get("type"))
+            replacement = bcp47_id(alias.get("replacement"))
+
+            # Prefix with "und-".
+            if alias_name != "languageAlias":
+                type = "und-" + type
+
+            # Discard all rules where the type is an invalid languageId.
+            if re_unicode_language_id.match(type) is None:
+                continue
+
+            type = language_id_to_multimap(type)
+
+            # Multiple, whitespace-separated territory replacements may be present.
+            if alias_name == "territoryAlias" and " " in replacement:
+                replacements = replacement.split(" ")
+                replacement_list = [
+                    language_id_to_multimap("und-" + r) for r in replacements
+                ]
+
+                assert (
+                    type not in territory_exception_rules
+                ), f"Duplicate alias rule: {type}"
+
+                territory_exception_rules[type] = replacement_list
+
+                # The first element is the default territory replacement.
+                replacement = replacements[0]
+
+            # Prefix with "und-".
+            if alias_name != "languageAlias":
+                replacement = "und-" + replacement
+
+            replacement = language_id_to_multimap(replacement)
+
+            assert type not in rules, f"Duplicate alias rule: {type}"
+
+            rules[type] = replacement
+
+    # Helper class for pattern matching.
+    class AnyClass:
+        def __eq__(self, obj):
+            return obj is not None
+
+    Any = AnyClass()
+
+    modified_rules = True
+    loop_count = 0
+
+    while modified_rules:
+        modified_rules = False
+        loop_count += 1
+
+        # UTS 35 defines that canonicalization is applied until a fixed point has
+        # been reached. This iterative application of the canonicalization algorithm
+        # is only needed for a relatively small set of rules, so we can precompute
+        # the transitive closure of all rules here and then perform a single pass
+        # when canonicalizing language tags at runtime.
+        transitive_rules = {}
+
+        # Compute the transitive closure.
+        # Any case which currently doesn't occur in the CLDR sources isn't supported
+        # and will lead to throwing an error.
+        for (type, replacement) in rules.items():
+            (language, script, region, variants) = type
+            (r_language, r_script, r_region, r_variants) = replacement
+
+            for (i_type, i_replacement) in rules.items():
+                (i_language, i_script, i_region, i_variants) = i_type
+                (i_r_language, i_r_script, i_r_region, i_r_variants) = i_replacement
+
+                if i_language is not None and i_language == r_language:
+                    # This case currently only occurs when neither script nor region
+                    # subtags are present. A single variant subtags may be present
+                    # in |type|. And |i_type| definitely has a single variant subtag.
+                    # Should this ever change, update this code accordingly.
+                    assert type == (Any, None, None, None) or type == (
+                        Any,
+                        None,
+                        None,
+                        Any,
+                    )
+                    assert replacement == (Any, None, None, None)
+                    assert i_type == (Any, None, None, Any)
+                    assert i_replacement == (Any, None, None, None)
+
+                    # This case happens for the rules
+                    #   "zh-guoyu -> zh",
+                    #   "zh-hakka -> hak", and
+                    #   "und-hakka -> und".
+                    # Given the possible input "zh-guoyu-hakka", the first rule will
+                    # change it to "zh-hakka", and then the second rule can be
+                    # applied. (The third rule isn't applied ever.)
+                    #
+                    # Let's assume there's a hypothetical rule
+                    #   "zh-aaaaa" -> "en"
+                    # And we have the input "zh-aaaaa-hakka", then "zh-aaaaa -> en"
+                    # is applied before "zh-hakka -> hak", because rules are sorted
+                    # alphabetically. That means the overall result is "en":
+                    # "zh-aaaaa-hakka" is first canonicalized to "en-hakka" and then
+                    # "hakka" is removed through the third rule.
+                    #
+                    # No current rule requires to handle this special case, so we
+                    # don't yet support it.
+                    assert variants is None or variants <= i_variants
+
+                    # Combine all variants and remove duplicates.
+                    vars = set(
+                        i_variants.split("-")
+                        + (variants.split("-") if variants else [])
+                    )
+
+                    # Add the variants alphabetically sorted.
+                    n_type = (language, None, None, "-".join(sorted(vars)))
+
+                    assert (
+                        n_type not in transitive_rules
+                        or transitive_rules[n_type] == i_replacement
+                    )
+                    transitive_rules[n_type] = i_replacement
+
+                    continue
+
+                if i_script is not None and i_script == r_script:
+                    # This case currently doesn't occur, so we don't yet support it.
+                    raise ValueError(
+                        f"{type} -> {replacement} :: {i_type} -> {i_replacement}"
+                    )
+                if i_region is not None and i_region == r_region:
+                    # This case currently only applies for sign language
+                    # replacements. Similar to the language subtag case any other
+                    # combination isn't currently supported.
+                    assert type == (None, None, Any, None)
+                    assert replacement == (None, None, Any, None)
+                    assert i_type == ("sgn", None, Any, None)
+                    assert i_replacement == (Any, None, None, None)
+
+                    n_type = ("sgn", None, region, None)
+
+                    assert n_type not in transitive_rules
+                    transitive_rules[n_type] = i_replacement
+
+                    continue
+
+                if i_variants is not None and i_variants == r_variants:
+                    # This case currently doesn't occur, so we don't yet support it.
+                    raise ValueError(
+                        f"{type} -> {replacement} :: {i_type} -> {i_replacement}"
+                    )
+
+        # Ensure there are no contradicting rules.
+        assert all(
+            rules[type] == replacement
+            for (type, replacement) in transitive_rules.items()
+            if type in rules
+        )
+
+        # If |transitive_rules| is not a subset of |rules|, new rules will be added.
+        modified_rules = not (transitive_rules.keys() <= rules.keys())
+
+        # Ensure we only have to iterate more than once for the "guoyo-{hakka,xiang}"
+        # case. Failing this assertion means either there's a bug when computing the
+        # stop condition of this loop or a new kind of legacy language tags was added.
+        if modified_rules and loop_count > 1:
+            new_rules = {k for k in transitive_rules.keys() if k not in rules}
+            for k in new_rules:
+                assert k == (Any, None, None, "guoyu-hakka") or k == (
+                    Any,
+                    None,
+                    None,
+                    "guoyu-xiang",
+                )
+
+        # Merge the transitive rules.
+        rules.update(transitive_rules)
+
+    # Computes the size of the union of all field value sets.
+    def multi_map_size(locale_id):
+        (language, script, region, variants) = locale_id
+
+        return (
+            (1 if language is not None else 0)
+            + (1 if script is not None else 0)
+            + (1 if region is not None else 0)
+            + (len(variants.split("-")) if variants is not None else 0)
+        )
+
+    # Dictionary of legacy mappings, contains raw rules, e.g.
+    # (None, None, None, "hepburn-heploc") -> (None, None, None, "alalc97").
+    legacy_mappings = {}
+
+    # Dictionary of simple language subtag mappings, e.g. "in" -> "id".
+    language_mappings = {}
+
+    # Dictionary of complex language subtag mappings, modifying more than one
+    # subtag, e.g. "sh" -> ("sr", "Latn", None) and "cnr" -> ("sr", None, "ME").
+    complex_language_mappings = {}
+
+    # Dictionary of simple script subtag mappings, e.g. "Qaai" -> "Zinh".
+    script_mappings = {}
+
+    # Dictionary of simple region subtag mappings, e.g. "DD" -> "DE".
+    region_mappings = {}
+
+    # Dictionary of complex region subtag mappings, containing more than one
+    # replacement, e.g. "SU" -> ("RU", ["AM", "AZ", "BY", ...]).
+    complex_region_mappings = {}
+
+    # Dictionary of aliased variant subtags to a tuple of preferred replacement
+    # type and replacement, e.g. "arevela" -> ("language", "hy") or
+    # "aaland" -> ("region", "AX") or "heploc" -> ("variant", "alalc97").
+    variant_mappings = {}
+
+    # Preprocess all rules so we can perform a single lookup per subtag at runtime.
+    for (type, replacement) in rules.items():
+        (language, script, region, variants) = type
+        (r_language, r_script, r_region, r_variants) = replacement
+
+        type_map_size = multi_map_size(type)
+
+        # Most mappings are one-to-one and can be encoded through lookup tables.
+        if type_map_size == 1:
+            if language is not None:
+                assert r_language is not None, "Can't remove a language subtag"
+
+                # We don't yet support this case.
+                assert (
+                    r_variants is None
+                ), f"Unhandled variant replacement in language alias: {replacement}"
+
+                if replacement == (Any, None, None, None):
+                    language_mappings[language] = r_language
+                else:
+                    complex_language_mappings[language] = replacement[:-1]
+            elif script is not None:
+                # We don't support removing script subtags.
+                assert (
+                    r_script is not None
+                ), f"Can't remove a script subtag: {replacement}"
+
+                # We only support one-to-one script mappings for now.
+                assert replacement == (
+                    None,
+                    Any,
+                    None,
+                    None,
+                ), f"Unhandled replacement in script alias: {replacement}"
+
+                script_mappings[script] = r_script
+            elif region is not None:
+                # We don't support removing region subtags.
+                assert (
+                    r_region is not None
+                ), f"Can't remove a region subtag: {replacement}"
+
+                # We only support one-to-one region mappings for now.
+                assert replacement == (
+                    None,
+                    None,
+                    Any,
+                    None,
+                ), f"Unhandled replacement in region alias: {replacement}"
+
+                if type not in territory_exception_rules:
+                    region_mappings[region] = r_region
+                else:
+                    complex_region_mappings[region] = [
+                        r_region
+                        for (_, _, r_region, _) in territory_exception_rules[type]
+                    ]
+            else:
+                assert variants is not None
+                assert len(variants.split("-")) == 1
+
+                # We only support one-to-one variant mappings for now.
+                assert (
+                    multi_map_size(replacement) <= 1
+                ), f"Unhandled replacement in variant alias: {replacement}"
+
+                if r_language is not None:
+                    variant_mappings[variants] = ("language", r_language)
+                elif r_script is not None:
+                    variant_mappings[variants] = ("script", r_script)
+                elif r_region is not None:
+                    variant_mappings[variants] = ("region", r_region)
+                elif r_variants is not None:
+                    assert len(r_variants.split("-")) == 1
+                    variant_mappings[variants] = ("variant", r_variants)
+                else:
+                    variant_mappings[variants] = None
+        else:
+            # Alias rules which have multiple input fields must be processed
+            # first. This applies only to a handful of rules, so our generated
+            # code adds fast paths to skip these rules in the common case.
+
+            # Case 1: Language and at least one variant subtag.
+            if language is not None and variants is not None:
+                pass
+
+            # Case 2: Sign language and a region subtag.
+            elif language == "sgn" and region is not None:
+                pass
+
+            # Case 3: "hepburn-heploc" to "alalc97" canonicalization.
+            elif (
+                language is None
+                and variants is not None
+                and len(variants.split("-")) == 2
+            ):
+                pass
+
+            # Any other combination is currently unsupported.
+            else:
+                raise ValueError(f"{type} -> {replacement}")
+
+            legacy_mappings[type] = replacement
+
+    tree = ET.parse(core_file.open("common/supplemental/likelySubtags.xml"))
+
+    likely_subtags = {}
+
+    for likely_subtag in tree.iterfind(".//likelySubtag"):
+        from_tag = bcp47_id(likely_subtag.get("from"))
+        from_match = re_unicode_language_id.match(from_tag)
+        assert (
+            from_match is not None
+        ), f"{from_tag} invalid Unicode BCP 47 locale identifier"
+        assert (
+            from_match.group("variants") is None
+        ), f"unexpected variant subtags in {from_tag}"
+
+        to_tag = bcp47_id(likely_subtag.get("to"))
+        to_match = re_unicode_language_id.match(to_tag)
+        assert (
+            to_match is not None
+        ), f"{to_tag} invalid Unicode BCP 47 locale identifier"
+        assert (
+            to_match.group("variants") is None
+        ), f"unexpected variant subtags in {to_tag}"
+
+        from_canonical = bcp47_canonical(
+            *from_match.group("language", "script", "region", "variants")
+        )
+
+        to_canonical = bcp47_canonical(
+            *to_match.group("language", "script", "region", "variants")
+        )
+
+        # Remove the empty variant subtags.
+        from_canonical = from_canonical[:-1]
+        to_canonical = to_canonical[:-1]
+
+        likely_subtags[from_canonical] = to_canonical
+
+    complex_region_mappings_final = {}
+
+    for (deprecated_region, replacements) in complex_region_mappings.items():
+        # Find all likely subtag entries which don't already contain a region
+        # subtag and whose target region is in the list of replacement regions.
+        region_likely_subtags = [
+            (from_language, from_script, to_region)
+            for (
+                (from_language, from_script, from_region),
+                (_, _, to_region),
+            ) in likely_subtags.items()
+            if from_region is None and to_region in replacements
+        ]
+
+        # The first replacement entry is the default region.
+        default = replacements[0]
+
+        # Find all likely subtag entries whose region matches the default region.
+        default_replacements = {
+            (language, script)
+            for (language, script, region) in region_likely_subtags
+            if region == default
+        }
+
+        # And finally find those entries which don't use the default region.
+        # These are the entries we're actually interested in, because those need
+        # to be handled specially when selecting the correct preferred region.
+        non_default_replacements = [
+            (language, script, region)
+            for (language, script, region) in region_likely_subtags
+            if (language, script) not in default_replacements
+        ]
+
+        # If there are no non-default replacements, we can handle the region as
+        # part of the simple region mapping.
+        if non_default_replacements:
+            complex_region_mappings_final[deprecated_region] = (
+                default,
+                non_default_replacements,
+            )
+        else:
+            region_mappings[deprecated_region] = default
+
+    return {
+        "legacyMappings": legacy_mappings,
+        "languageMappings": language_mappings,
+        "complexLanguageMappings": complex_language_mappings,
+        "scriptMappings": script_mappings,
+        "regionMappings": region_mappings,
+        "complexRegionMappings": complex_region_mappings_final,
+        "variantMappings": variant_mappings,
+        "likelySubtags": likely_subtags,
+    }
+
+
+def readUnicodeExtensions(core_file):
+    import xml.etree.ElementTree as ET
+
+    # Match all xml-files in the BCP 47 directory.
+    bcpFileRE = re.compile(r"^common/bcp47/.+\.xml$")
+
+    # https://www.unicode.org/reports/tr35/#Unicode_locale_identifier
+    #
+    # type = alphanum{3,8} (sep alphanum{3,8})* ;
+    typeRE = re.compile(r"^[a-z0-9]{3,8}(-[a-z0-9]{3,8})*$")
+
+    # https://www.unicode.org/reports/tr35/#Unicode_language_identifier
+    #
+    # unicode_region_subtag = alpha{2} ;
+    alphaRegionRE = re.compile(r"^[A-Z]{2}$", re.IGNORECASE)
+
+    # Mapping from Unicode extension types to dict of deprecated to
+    # preferred values.
+    mapping = {
+        # Unicode BCP 47 U Extension
+        "u": {},
+        # Unicode BCP 47 T Extension
+        "t": {},
+    }
+
+    def readBCP47File(file):
+        tree = ET.parse(file)
+        for keyword in tree.iterfind(".//keyword/key"):
+            extension = keyword.get("extension", "u")
+            assert (
+                extension == "u" or extension == "t"
+            ), "unknown extension type: {}".format(extension)
+
+            extension_name = keyword.get("name")
+
+            for type in keyword.iterfind("type"):
+                # <https://unicode.org/reports/tr35/#Unicode_Locale_Extension_Data_Files>:
+                #
+                # The key or type name used by Unicode locale extension with 'u' extension
+                # syntax or the 't' extensions syntax. When alias below is absent, this name
+                # can be also used with the old style "@key=type" syntax.
+                name = type.get("name")
+
+                # Ignore the special name:
+                # - <https://unicode.org/reports/tr35/#CODEPOINTS>
+                # - <https://unicode.org/reports/tr35/#REORDER_CODE>
+                # - <https://unicode.org/reports/tr35/#RG_KEY_VALUE>
+                # - <https://unicode.org/reports/tr35/#SCRIPT_CODE>
+                # - <https://unicode.org/reports/tr35/#SUBDIVISION_CODE>
+                # - <https://unicode.org/reports/tr35/#PRIVATE_USE>
+                if name in (
+                    "CODEPOINTS",
+                    "REORDER_CODE",
+                    "RG_KEY_VALUE",
+                    "SCRIPT_CODE",
+                    "SUBDIVISION_CODE",
+                    "PRIVATE_USE",
+                ):
+                    continue
+
+                # All other names should match the 'type' production.
+                assert (
+                    typeRE.match(name) is not None
+                ), "{} matches the 'type' production".format(name)
+
+                # <https://unicode.org/reports/tr35/#Unicode_Locale_Extension_Data_Files>:
+                #
+                # The preferred value of the deprecated key, type or attribute element.
+                # When a key, type or attribute element is deprecated, this attribute is
+                # used for specifying a new canonical form if available.
+                preferred = type.get("preferred")
+
+                # <https://unicode.org/reports/tr35/#Unicode_Locale_Extension_Data_Files>:
+                #
+                # The BCP 47 form is the canonical form, and recommended. Other aliases are
+                # included only for backwards compatibility.
+                alias = type.get("alias")
+
+                # <https://unicode.org/reports/tr35/#Canonical_Unicode_Locale_Identifiers>
+                #
+                # Use the bcp47 data to replace keys, types, tfields, and tvalues by their
+                # canonical forms. See Section 3.6.4 U Extension Data Files) and Section
+                # 3.7.1 T Extension Data Files. The aliases are in the alias attribute
+                # value, while the canonical is in the name attribute value.
+
+                # 'preferred' contains the new preferred name, 'alias' the compatibility
+                # name, but then there's this entry where 'preferred' and 'alias' are the
+                # same. So which one to choose? Assume 'preferred' is the actual canonical
+                # name.
+                #
+                # <type name="islamicc"
+                #       description="Civil (algorithmic) Arabic calendar"
+                #       deprecated="true"
+                #       preferred="islamic-civil"
+                #       alias="islamic-civil"/>
+
+                if preferred is not None:
+                    assert typeRE.match(preferred), preferred
+                    mapping[extension].setdefault(extension_name, {})[name] = preferred
+
+                if alias is not None:
+                    for alias_name in alias.lower().split(" "):
+                        # Ignore alias entries which don't match the 'type' production.
+                        if typeRE.match(alias_name) is None:
+                            continue
+
+                        # See comment above when 'alias' and 'preferred' are both present.
+                        if (
+                            preferred is not None
+                            and name in mapping[extension][extension_name]
+                        ):
+                            continue
+
+                        # Skip over entries where 'name' and 'alias' are equal.
+                        #
+                        # <type name="pst8pdt"
+                        #       description="POSIX style time zone for US Pacific Time"
+                        #       alias="PST8PDT"
+                        #       since="1.8"/>
+                        if name == alias_name:
+                            continue
+
+                        mapping[extension].setdefault(extension_name, {})[
+                            alias_name
+                        ] = name
+
+    def readSupplementalMetadata(file):
+        # Find subdivision and region replacements.
+        #
+        # <https://www.unicode.org/reports/tr35/#Canonical_Unicode_Locale_Identifiers>
+        #
+        # Replace aliases in special key values:
+        #   - If there is an 'sd' or 'rg' key, replace any subdivision alias
+        #     in its value in the same way, using subdivisionAlias data.
+        tree = ET.parse(file)
+        for alias in tree.iterfind(".//subdivisionAlias"):
+            type = alias.get("type")
+            assert (
+                typeRE.match(type) is not None
+            ), "{} matches the 'type' production".format(type)
+
+            # Take the first replacement when multiple ones are present.
+            replacement = alias.get("replacement").split(" ")[0].lower()
+
+            # Append "zzzz" if the replacement is a two-letter region code.
+            if alphaRegionRE.match(replacement) is not None:
+                replacement += "zzzz"
+
+            # Assert the replacement is syntactically correct.
+            assert (
+                typeRE.match(replacement) is not None
+            ), "replacement {} matches the 'type' production".format(replacement)
+
+            # 'subdivisionAlias' applies to 'rg' and 'sd' keys.
+            mapping["u"].setdefault("rg", {})[type] = replacement
+            mapping["u"].setdefault("sd", {})[type] = replacement
+
+    for name in core_file.namelist():
+        if bcpFileRE.match(name):
+            readBCP47File(core_file.open(name))
+
+    readSupplementalMetadata(
+        core_file.open("common/supplemental/supplementalMetadata.xml")
+    )
+
+    return {
+        "unicodeMappings": mapping["u"],
+        "transformMappings": mapping["t"],
+    }
+
+
+def writeCLDRLanguageTagData(println, data, url):
+    """Writes the language tag data to the Intl data file."""
+
+    println(generatedFileWarning)
+    println("// Version: CLDR-{}".format(data["version"]))
+    println("// URL: {}".format(url))
+
+    println(
+        """
+#include "mozilla/Assertions.h"
+#include "mozilla/Span.h"
+#include "mozilla/TextUtils.h"
+
+#include <algorithm>
+#include <cstdint>
+#include <cstring>
+#include <iterator>
+#include <string>
+#include <type_traits>
+
+#include "mozilla/intl/Locale.h"
+
+using namespace mozilla::intl::LanguageTagLimits;
+
+template <size_t Length, size_t TagLength, size_t SubtagLength>
+static inline bool HasReplacement(
+    const char (&subtags)[Length][TagLength],
+    const mozilla::intl::LanguageTagSubtag<SubtagLength>& subtag) {
+  MOZ_ASSERT(subtag.Length() == TagLength - 1,
+             "subtag must have the same length as the list of subtags");
+
+  const char* ptr = subtag.Span().data();
+  return std::binary_search(std::begin(subtags), std::end(subtags), ptr,
+                            [](const char* a, const char* b) {
+                              return memcmp(a, b, TagLength - 1) < 0;
+                            });
+}
+
+template <size_t Length, size_t TagLength, size_t SubtagLength>
+static inline const char* SearchReplacement(
+    const char (&subtags)[Length][TagLength], const char* (&aliases)[Length],
+    const mozilla::intl::LanguageTagSubtag<SubtagLength>& subtag) {
+  MOZ_ASSERT(subtag.Length() == TagLength - 1,
+             "subtag must have the same length as the list of subtags");
+
+  const char* ptr = subtag.Span().data();
+  auto p = std::lower_bound(std::begin(subtags), std::end(subtags), ptr,
+                            [](const char* a, const char* b) {
+                              return memcmp(a, b, TagLength - 1) < 0;
+                            });
+  if (p != std::end(subtags) && memcmp(*p, ptr, TagLength - 1) == 0) {
+    return aliases[std::distance(std::begin(subtags), p)];
+  }
+  return nullptr;
+}
+
+#ifdef DEBUG
+static bool IsAsciiLowercaseAlphanumeric(char c) {
+  return mozilla::IsAsciiLowercaseAlpha(c) || mozilla::IsAsciiDigit(c);
+}
+
+static bool IsAsciiLowercaseAlphanumericOrDash(char c) {
+  return IsAsciiLowercaseAlphanumeric(c) || c == '-';
+}
+
+static bool IsCanonicallyCasedLanguageTag(mozilla::Span<const char> span) {
+  return std::all_of(span.begin(), span.end(),
+                     mozilla::IsAsciiLowercaseAlpha<char>);
+}
+
+static bool IsCanonicallyCasedScriptTag(mozilla::Span<const char> span) {
+  return mozilla::IsAsciiUppercaseAlpha(span[0]) &&
+         std::all_of(span.begin() + 1, span.end(),
+                     mozilla::IsAsciiLowercaseAlpha<char>);
+}
+
+static bool IsCanonicallyCasedRegionTag(mozilla::Span<const char> span) {
+  return std::all_of(span.begin(), span.end(),
+                     mozilla::IsAsciiUppercaseAlpha<char>) ||
+         std::all_of(span.begin(), span.end(), mozilla::IsAsciiDigit<char>);
+}
+
+static bool IsCanonicallyCasedVariantTag(mozilla::Span<const char> span) {
+  return std::all_of(span.begin(), span.end(), IsAsciiLowercaseAlphanumeric);
+}
+
+static bool IsCanonicallyCasedUnicodeKey(mozilla::Span<const char> key) {
+  return std::all_of(key.begin(), key.end(), IsAsciiLowercaseAlphanumeric);
+}
+
+static bool IsCanonicallyCasedUnicodeType(mozilla::Span<const char> type) {
+  return std::all_of(type.begin(), type.end(),
+                     IsAsciiLowercaseAlphanumericOrDash);
+}
+
+static bool IsCanonicallyCasedTransformKey(mozilla::Span<const char> key) {
+  return std::all_of(key.begin(), key.end(), IsAsciiLowercaseAlphanumeric);
+}
+
+static bool IsCanonicallyCasedTransformType(mozilla::Span<const char> type) {
+  return std::all_of(type.begin(), type.end(),
+                     IsAsciiLowercaseAlphanumericOrDash);
+}
+#endif
+""".rstrip()
+    )
+
+    source = "CLDR Supplemental Data, version {}".format(data["version"])
+    legacy_mappings = data["legacyMappings"]
+    language_mappings = data["languageMappings"]
+    complex_language_mappings = data["complexLanguageMappings"]
+    script_mappings = data["scriptMappings"]
+    region_mappings = data["regionMappings"]
+    complex_region_mappings = data["complexRegionMappings"]
+    variant_mappings = data["variantMappings"]
+    unicode_mappings = data["unicodeMappings"]
+    transform_mappings = data["transformMappings"]
+
+    # unicode_language_subtag = alpha{2,3} | alpha{5,8} ;
+    language_maxlength = 8
+
+    # unicode_script_subtag = alpha{4} ;
+    script_maxlength = 4
+
+    # unicode_region_subtag = (alpha{2} | digit{3}) ;
+    region_maxlength = 3
+
+    writeMappingsBinarySearch(
+        println,
+        "LanguageMapping",
+        "LanguageSubtag&",
+        "language",
+        "IsStructurallyValidLanguageTag",
+        "IsCanonicallyCasedLanguageTag",
+        language_mappings,
+        language_maxlength,
+        "Mappings from language subtags to preferred values.",
+        source,
+        url,
+    )
+    writeMappingsBinarySearch(
+        println,
+        "ComplexLanguageMapping",
+        "const LanguageSubtag&",
+        "language",
+        "IsStructurallyValidLanguageTag",
+        "IsCanonicallyCasedLanguageTag",
+        complex_language_mappings.keys(),
+        language_maxlength,
+        "Language subtags with complex mappings.",
+        source,
+        url,
+    )
+    writeMappingsBinarySearch(
+        println,
+        "ScriptMapping",
+        "ScriptSubtag&",
+        "script",
+        "IsStructurallyValidScriptTag",
+        "IsCanonicallyCasedScriptTag",
+        script_mappings,
+        script_maxlength,
+        "Mappings from script subtags to preferred values.",
+        source,
+        url,
+    )
+    writeMappingsBinarySearch(
+        println,
+        "RegionMapping",
+        "RegionSubtag&",
+        "region",
+        "IsStructurallyValidRegionTag",
+        "IsCanonicallyCasedRegionTag",
+        region_mappings,
+        region_maxlength,
+        "Mappings from region subtags to preferred values.",
+        source,
+        url,
+    )
+    writeMappingsBinarySearch(
+        println,
+        "ComplexRegionMapping",
+        "const RegionSubtag&",
+        "region",
+        "IsStructurallyValidRegionTag",
+        "IsCanonicallyCasedRegionTag",
+        complex_region_mappings.keys(),
+        region_maxlength,
+        "Region subtags with complex mappings.",
+        source,
+        url,
+    )
+
+    writeComplexLanguageTagMappings(
+        println,
+        complex_language_mappings,
+        "Language subtags with complex mappings.",
+        source,
+        url,
+    )
+    writeComplexRegionTagMappings(
+        println,
+        complex_region_mappings,
+        "Region subtags with complex mappings.",
+        source,
+        url,
+    )
+
+    writeVariantTagMappings(
+        println,
+        variant_mappings,
+        "Mappings from variant subtags to preferred values.",
+        source,
+        url,
+    )
+
+    writeLegacyMappingsFunction(
+        println, legacy_mappings, "Canonicalize legacy locale identifiers.", source, url
+    )
+
+    writeSignLanguageMappingsFunction(
+        println, legacy_mappings, "Mappings from legacy sign languages.", source, url
+    )
+
+    writeUnicodeExtensionsMappings(println, unicode_mappings, "Unicode")
+    writeUnicodeExtensionsMappings(println, transform_mappings, "Transform")
+
+
+def writeCLDRLanguageTagLikelySubtagsTest(println, data, url):
+    """Writes the likely-subtags test file."""
+
+    println(generatedFileWarning)
+
+    source = "CLDR Supplemental Data, version {}".format(data["version"])
+    language_mappings = data["languageMappings"]
+    complex_language_mappings = data["complexLanguageMappings"]
+    script_mappings = data["scriptMappings"]
+    region_mappings = data["regionMappings"]
+    complex_region_mappings = data["complexRegionMappings"]
+    likely_subtags = data["likelySubtags"]
+
+    def bcp47(tag):
+        (language, script, region) = tag
+        return "{}{}{}".format(
+            language, "-" + script if script else "", "-" + region if region else ""
+        )
+
+    def canonical(tag):
+        (language, script, region) = tag
+
+        # Map deprecated language subtags.
+        if language in language_mappings:
+            language = language_mappings[language]
+        elif language in complex_language_mappings:
+            (language2, script2, region2) = complex_language_mappings[language]
+            (language, script, region) = (
+                language2,
+                script if script else script2,
+                region if region else region2,
+            )
+
+        # Map deprecated script subtags.
+        if script in script_mappings:
+            script = script_mappings[script]
+
+        # Map deprecated region subtags.
+        if region in region_mappings:
+            region = region_mappings[region]
+        else:
+            # Assume no complex region mappings are needed for now.
+            assert (
+                region not in complex_region_mappings
+            ), "unexpected region with complex mappings: {}".format(region)
+
+        return (language, script, region)
+
+    # https://unicode.org/reports/tr35/#Likely_Subtags
+
+    def addLikelySubtags(tag):
+        # Step 1: Canonicalize.
+        (language, script, region) = canonical(tag)
+        if script == "Zzzz":
+            script = None
+        if region == "ZZ":
+            region = None
+
+        # Step 2: Lookup.
+        searches = (
+            (language, script, region),
+            (language, None, region),
+            (language, script, None),
+            (language, None, None),
+            ("und", script, None),
+        )
+        search = next(search for search in searches if search in likely_subtags)
+
+        (language_s, script_s, region_s) = search
+        (language_m, script_m, region_m) = likely_subtags[search]
+
+        # Step 3: Return.
+        return (
+            language if language != language_s else language_m,
+            script if script != script_s else script_m,
+            region if region != region_s else region_m,
+        )
+
+    # https://unicode.org/reports/tr35/#Likely_Subtags
+    def removeLikelySubtags(tag):
+        # Step 1: Add likely subtags.
+        max = addLikelySubtags(tag)
+
+        # Step 2: Remove variants (doesn't apply here).
+
+        # Step 3: Find a match.
+        (language, script, region) = max
+        for trial in (
+            (language, None, None),
+            (language, None, region),
+            (language, script, None),
+        ):
+            if addLikelySubtags(trial) == max:
+                return trial
+
+        # Step 4: Return maximized if no match found.
+        return max
+
+    def likely_canonical(from_tag, to_tag):
+        # Canonicalize the input tag.
+        from_tag = canonical(from_tag)
+
+        # Update the expected result if necessary.
+        if from_tag in likely_subtags:
+            to_tag = likely_subtags[from_tag]
+
+        # Canonicalize the expected output.
+        to_canonical = canonical(to_tag)
+
+        # Sanity check: This should match the result of |addLikelySubtags|.
+        assert to_canonical == addLikelySubtags(from_tag)
+
+        return to_canonical
+
+    # |likely_subtags| contains non-canonicalized tags, so canonicalize it first.
+    likely_subtags_canonical = {
+        k: likely_canonical(k, v) for (k, v) in likely_subtags.items()
+    }
+
+    # Add test data for |Intl.Locale.prototype.maximize()|.
+    writeMappingsVar(
+        println,
+        {bcp47(k): bcp47(v) for (k, v) in likely_subtags_canonical.items()},
+        "maxLikelySubtags",
+        "Extracted from likelySubtags.xml.",
+        source,
+        url,
+    )
+
+    # Use the maximalized tags as the input for the remove likely-subtags test.
+    minimized = {
+        tag: removeLikelySubtags(tag) for tag in likely_subtags_canonical.values()
+    }
+
+    # Add test data for |Intl.Locale.prototype.minimize()|.
+    writeMappingsVar(
+        println,
+        {bcp47(k): bcp47(v) for (k, v) in minimized.items()},
+        "minLikelySubtags",
+        "Extracted from likelySubtags.xml.",
+        source,
+        url,
+    )
+
+    println(
+        """
+for (let [tag, maximal] of Object.entries(maxLikelySubtags)) {
+    assertEq(new Intl.Locale(tag).maximize().toString(), maximal);
+}"""
+    )
+
+    println(
+        """
+for (let [tag, minimal] of Object.entries(minLikelySubtags)) {
+    assertEq(new Intl.Locale(tag).minimize().toString(), minimal);
+}"""
+    )
+
+    println(
+        """
+if (typeof reportCompare === "function")
+    reportCompare(0, 0);"""
+    )
+
+
+def readCLDRVersionFromICU():
+    icuDir = os.path.join(topsrcdir, "intl/icu/source")
+    if not os.path.isdir(icuDir):
+        raise RuntimeError("not a directory: {}".format(icuDir))
+
+    reVersion = re.compile(r'\s*cldrVersion\{"(\d+(?:\.\d+)?)"\}')
+
+    for line in flines(os.path.join(icuDir, "data/misc/supplementalData.txt")):
+        m = reVersion.match(line)
+        if m:
+            version = m.group(1)
+            break
+
+    if version is None:
+        raise RuntimeError("can't resolve CLDR version")
+
+    return version
+
+
+def updateCLDRLangTags(args):
+    """Update the LanguageTagGenerated.cpp file."""
+    version = args.version
+    url = args.url
+    out = args.out
+    filename = args.file
+
+    # Determine current CLDR version from ICU.
+    if version is None:
+        version = readCLDRVersionFromICU()
+
+    url = url.replace("<VERSION>", version)
+
+    print("Arguments:")
+    print("\tCLDR version: %s" % version)
+    print("\tDownload url: %s" % url)
+    if filename is not None:
+        print("\tLocal CLDR core.zip file: %s" % filename)
+    print("\tOutput file: %s" % out)
+    print("")
+
+    data = {
+        "version": version,
+    }
+
+    def readFiles(cldr_file):
+        with ZipFile(cldr_file) as zip_file:
+            data.update(readSupplementalData(zip_file))
+            data.update(readUnicodeExtensions(zip_file))
+
+    print("Processing CLDR data...")
+    if filename is not None:
+        print("Always make sure you have the newest CLDR core.zip!")
+        with open(filename, "rb") as cldr_file:
+            readFiles(cldr_file)
+    else:
+        print("Downloading CLDR core.zip...")
+        with closing(urlopen(url)) as cldr_file:
+            cldr_data = io.BytesIO(cldr_file.read())
+            readFiles(cldr_data)
+
+    print("Writing Intl data...")
+    with io.open(out, mode="w", encoding="utf-8", newline="") as f:
+        println = partial(print, file=f)
+
+        writeCLDRLanguageTagData(println, data, url)
+
+    print("Writing Intl test data...")
+    js_src_builtin_intl_dir = os.path.dirname(os.path.abspath(__file__))
+    test_file = os.path.join(
+        js_src_builtin_intl_dir,
+        "../../tests/non262/Intl/Locale/likely-subtags-generated.js",
+    )
+    with io.open(test_file, mode="w", encoding="utf-8", newline="") as f:
+        println = partial(print, file=f)
+
+        println("// |reftest| skip-if(!this.hasOwnProperty('Intl'))")
+        writeCLDRLanguageTagLikelySubtagsTest(println, data, url)
+
+
+def flines(filepath, encoding="utf-8"):
+    """Open filepath and iterate over its content."""
+    with io.open(filepath, mode="r", encoding=encoding) as f:
+        for line in f:
+            yield line
+
+
+@total_ordering
+class Zone(object):
+    """Time zone with optional file name."""
+
+    def __init__(self, name, filename=""):
+        self.name = name
+        self.filename = filename
+
+    def __eq__(self, other):
+        return hasattr(other, "name") and self.name == other.name
+
+    def __lt__(self, other):
+        return self.name < other.name
+
+    def __hash__(self):
+        return hash(self.name)
+
+    def __str__(self):
+        return self.name
+
+    def __repr__(self):
+        return self.name
+
+
+class TzDataDir(object):
+    """tzdata source from a directory."""
+
+    def __init__(self, obj):
+        self.name = partial(os.path.basename, obj)
+        self.resolve = partial(os.path.join, obj)
+        self.basename = os.path.basename
+        self.isfile = os.path.isfile
+        self.listdir = partial(os.listdir, obj)
+        self.readlines = flines
+
+
+class TzDataFile(object):
+    """tzdata source from a file (tar or gzipped)."""
+
+    def __init__(self, obj):
+        self.name = lambda: os.path.splitext(
+            os.path.splitext(os.path.basename(obj))[0]
+        )[0]
+        self.resolve = obj.getmember
+        self.basename = attrgetter("name")
+        self.isfile = tarfile.TarInfo.isfile
+        self.listdir = obj.getnames
+        self.readlines = partial(self._tarlines, obj)
+
+    def _tarlines(self, tar, m):
+        with closing(tar.extractfile(m)) as f:
+            for line in f:
+                yield line.decode("utf-8")
+
+
+def validateTimeZones(zones, links):
+    """Validate the zone and link entries."""
+    linkZones = set(links.keys())
+    intersect = linkZones.intersection(zones)
+    if intersect:
+        raise RuntimeError("Links also present in zones: %s" % intersect)
+
+    zoneNames = {z.name for z in zones}
+    linkTargets = set(links.values())
+    if not linkTargets.issubset(zoneNames):
+        raise RuntimeError(
+            "Link targets not found: %s" % linkTargets.difference(zoneNames)
+        )
+
+
+def partition(iterable, *predicates):
+    def innerPartition(pred, it):
+        it1, it2 = tee(it)
+        return (filter(pred, it1), filterfalse(pred, it2))
+
+    if len(predicates) == 0:
+        return iterable
+    (left, right) = innerPartition(predicates[0], iterable)
+    if len(predicates) == 1:
+        return (left, right)
+    return tuple([left] + list(partition(right, *predicates[1:])))
+
+
+def listIANAFiles(tzdataDir):
+    def isTzFile(d, m, f):
+        return m(f) and d.isfile(d.resolve(f))
+
+    return filter(
+        partial(isTzFile, tzdataDir, re.compile("^[a-z0-9]+$").match),
+        tzdataDir.listdir(),
+    )
+
+
+def readIANAFiles(tzdataDir, files):
+    """Read all IANA time zone files from the given iterable."""
+    nameSyntax = "[\w/+\-]+"
+    pZone = re.compile(r"Zone\s+(?P<name>%s)\s+.*" % nameSyntax)
+    pLink = re.compile(
+        r"Link\s+(?P<target>%s)\s+(?P<name>%s)(?:\s+#.*)?" % (nameSyntax, nameSyntax)
+    )
+
+    def createZone(line, fname):
+        match = pZone.match(line)
+        name = match.group("name")
+        return Zone(name, fname)
+
+    def createLink(line, fname):
+        match = pLink.match(line)
+        (name, target) = match.group("name", "target")
+        return (Zone(name, fname), target)
+
+    zones = set()
+    links = dict()
+    for filename in files:
+        filepath = tzdataDir.resolve(filename)
+        for line in tzdataDir.readlines(filepath):
+            if line.startswith("Zone"):
+                zones.add(createZone(line, filename))
+            if line.startswith("Link"):
+                (link, target) = createLink(line, filename)
+                links[link] = target
+
+    return (zones, links)
+
+
+def readIANATimeZones(tzdataDir, ignoreBackzone, ignoreFactory):
+    """Read the IANA time zone information from `tzdataDir`."""
+
+    backzoneFiles = {"backzone"}
+    (bkfiles, tzfiles) = partition(listIANAFiles(tzdataDir), backzoneFiles.__contains__)
+
+    # Read zone and link infos.
+    (zones, links) = readIANAFiles(tzdataDir, tzfiles)
+    (backzones, backlinks) = readIANAFiles(tzdataDir, bkfiles)
+
+    # Remove the placeholder time zone "Factory".
+    if ignoreFactory:
+        zones.remove(Zone("Factory"))
+
+    # Merge with backzone data.
+    if not ignoreBackzone:
+        zones |= backzones
+        links = {
+            name: target for name, target in links.items() if name not in backzones
+        }
+        links.update(backlinks)
+
+    validateTimeZones(zones, links)
+
+    return (zones, links)
+
+
+def readICUResourceFile(filename):
+    """Read an ICU resource file.
+
+    Yields (<table-name>, <startOrEnd>, <value>) for each table.
+    """
+
+    numberValue = r"-?\d+"
+    stringValue = r'".+?"'
+
+    def asVector(val):
+        return r"%s(?:\s*,\s*%s)*" % (val, val)
+
+    numberVector = asVector(numberValue)
+    stringVector = asVector(stringValue)
+
+    reNumberVector = re.compile(numberVector)
+    reStringVector = re.compile(stringVector)
+    reNumberValue = re.compile(numberValue)
+    reStringValue = re.compile(stringValue)
+
+    def parseValue(value):
+        m = reNumberVector.match(value)
+        if m:
+            return [int(v) for v in reNumberValue.findall(value)]
+        m = reStringVector.match(value)
+        if m:
+            return [v[1:-1] for v in reStringValue.findall(value)]
+        raise RuntimeError("unknown value type: %s" % value)
+
+    def extractValue(values):
+        if len(values) == 0:
+            return None
+        if len(values) == 1:
+            return values[0]
+        return values
+
+    def line(*args):
+        maybeMultiComments = r"(?:/\*[^*]*\*/)*"
+        maybeSingleComment = r"(?://.*)?"
+        lineStart = "^%s" % maybeMultiComments
+        lineEnd = "%s\s*%s$" % (maybeMultiComments, maybeSingleComment)
+        return re.compile(r"\s*".join(chain([lineStart], args, [lineEnd])))
+
+    tableName = r'(?P<quote>"?)(?P<name>.+?)(?P=quote)'
+    tableValue = r"(?P<value>%s|%s)" % (numberVector, stringVector)
+
+    reStartTable = line(tableName, r"\{")
+    reEndTable = line(r"\}")
+    reSingleValue = line(r",?", tableValue, r",?")
+    reCompactTable = line(tableName, r"\{", tableValue, r"\}")
+    reEmptyLine = line()
+
+    tables = []
+
+    def currentTable():
+        return "|".join(tables)
+
+    values = []
+    for line in flines(filename, "utf-8-sig"):
+        line = line.strip()
+        if line == "":
+            continue
+
+        m = reEmptyLine.match(line)
+        if m:
+            continue
+
+        m = reStartTable.match(line)
+        if m:
+            assert len(values) == 0
+            tables.append(m.group("name"))
+            continue
+
+        m = reEndTable.match(line)
+        if m:
+            yield (currentTable(), extractValue(values))
+            tables.pop()
+            values = []
+            continue
+
+        m = reCompactTable.match(line)
+        if m:
+            assert len(values) == 0
+            tables.append(m.group("name"))
+            yield (currentTable(), extractValue(parseValue(m.group("value"))))
+            tables.pop()
+            continue
+
+        m = reSingleValue.match(line)
+        if m and tables:
+            values.extend(parseValue(m.group("value")))
+            continue
+
+        raise RuntimeError("unknown entry: %s" % line)
+
+
+def readICUTimeZonesFromTimezoneTypes(icuTzDir):
+    """Read the ICU time zone information from `icuTzDir`/timezoneTypes.txt
+    and returns the tuple (zones, links).
+    """
+    typeMapTimeZoneKey = "timezoneTypes:table(nofallback)|typeMap|timezone|"
+    typeAliasTimeZoneKey = "timezoneTypes:table(nofallback)|typeAlias|timezone|"
+
+    def toTimeZone(name):
+        return Zone(name.replace(":", "/"))
+
+    zones = set()
+    links = dict()
+
+    for name, value in readICUResourceFile(os.path.join(icuTzDir, "timezoneTypes.txt")):
+        if name.startswith(typeMapTimeZoneKey):
+            zones.add(toTimeZone(name[len(typeMapTimeZoneKey) :]))
+        if name.startswith(typeAliasTimeZoneKey):
+            links[toTimeZone(name[len(typeAliasTimeZoneKey) :])] = value
+
+    validateTimeZones(zones, links)
+
+    return (zones, links)
+
+
+def readICUTimeZonesFromZoneInfo(icuTzDir):
+    """Read the ICU time zone information from `icuTzDir`/zoneinfo64.txt
+    and returns the tuple (zones, links).
+    """
+    zoneKey = "zoneinfo64:table(nofallback)|Zones:array|:table"
+    linkKey = "zoneinfo64:table(nofallback)|Zones:array|:int"
+    namesKey = "zoneinfo64:table(nofallback)|Names"
+
+    tzId = 0
+    tzLinks = dict()
+    tzNames = []
+
+    for name, value in readICUResourceFile(os.path.join(icuTzDir, "zoneinfo64.txt")):
+        if name == zoneKey:
+            tzId += 1
+        elif name == linkKey:
+            tzLinks[tzId] = int(value)
+            tzId += 1
+        elif name == namesKey:
+            tzNames.extend(value)
+
+    links = {Zone(tzNames[zone]): tzNames[target] for (zone, target) in tzLinks.items()}
+    zones = {Zone(v) for v in tzNames if Zone(v) not in links}
+
+    validateTimeZones(zones, links)
+
+    return (zones, links)
+
+
+def readICUTimeZones(icuDir, icuTzDir, ignoreFactory):
+    # zoneinfo64.txt contains the supported time zones by ICU. This data is
+    # generated from tzdata files, it doesn't include "backzone" in stock ICU.
+    (zoneinfoZones, zoneinfoLinks) = readICUTimeZonesFromZoneInfo(icuTzDir)
+
+    # timezoneTypes.txt contains the canonicalization information for ICU. This
+    # data is generated from CLDR files. It includes data about time zones from
+    # tzdata's "backzone" file.
+    (typesZones, typesLinks) = readICUTimeZonesFromTimezoneTypes(icuTzDir)
+
+    # Remove the placeholder time zone "Factory".
+    # See also <https://github.com/eggert/tz/blob/master/factory>.
+    if ignoreFactory:
+        zoneinfoZones.remove(Zone("Factory"))
+
+    # Remove the ICU placeholder time zone "Etc/Unknown".
+    # See also <https://unicode.org/reports/tr35/#Time_Zone_Identifiers>.
+    for zones in (zoneinfoZones, typesZones):
+        zones.remove(Zone("Etc/Unknown"))
+
+    # Remove any outdated ICU links.
+    for links in (zoneinfoLinks, typesLinks):
+        for zone in otherICULegacyLinks().keys():
+            if zone not in links:
+                raise KeyError(f"Can't remove non-existent link from '{zone}'")
+            del links[zone]
+
+    # Information in zoneinfo64 should be a superset of timezoneTypes.
+    def inZoneInfo64(zone):
+        return zone in zoneinfoZones or zone in zoneinfoLinks
+
+    notFoundInZoneInfo64 = [zone for zone in typesZones if not inZoneInfo64(zone)]
+    if notFoundInZoneInfo64:
+        raise RuntimeError(
+            "Missing time zones in zoneinfo64.txt: %s" % notFoundInZoneInfo64
+        )
+
+    notFoundInZoneInfo64 = [
+        zone for zone in typesLinks.keys() if not inZoneInfo64(zone)
+    ]
+    if notFoundInZoneInfo64:
+        raise RuntimeError(
+            "Missing time zones in zoneinfo64.txt: %s" % notFoundInZoneInfo64
+        )
+
+    # zoneinfo64.txt only defines the supported time zones by ICU, the canonicalization
+    # rules are defined through timezoneTypes.txt. Merge both to get the actual zones
+    # and links used by ICU.
+    icuZones = set(
+        chain(
+            (zone for zone in zoneinfoZones if zone not in typesLinks),
+            (zone for zone in typesZones),
+        )
+    )
+    icuLinks = dict(
+        chain(
+            (
+                (zone, target)
+                for (zone, target) in zoneinfoLinks.items()
+                if zone not in typesZones
+            ),
+            ((zone, target) for (zone, target) in typesLinks.items()),
+        )
+    )
+
+    return (icuZones, icuLinks)
+
+
+def readICULegacyZones(icuDir):
+    """Read the ICU legacy time zones from `icuTzDir`/tools/tzcode/icuzones
+    and returns the tuple (zones, links).
+    """
+    tzdir = TzDataDir(os.path.join(icuDir, "tools/tzcode"))
+
+    # Per spec we must recognize only IANA time zones and links, but ICU
+    # recognizes various legacy, non-IANA time zones and links. Compute these
+    # non-IANA time zones and links.
+
+    # Most legacy, non-IANA time zones and links are in the icuzones file.
+    (zones, links) = readIANAFiles(tzdir, ["icuzones"])
+
+    # Remove the ICU placeholder time zone "Etc/Unknown".
+    # See also <https://unicode.org/reports/tr35/#Time_Zone_Identifiers>.
+    zones.remove(Zone("Etc/Unknown"))
+
+    # A handful of non-IANA zones/links are not in icuzones and must be added
+    # manually so that we won't invoke ICU with them.
+    for (zone, target) in otherICULegacyLinks().items():
+        if zone in links:
+            if links[zone] != target:
+                raise KeyError(
+                    f"Can't overwrite link '{zone} -> {links[zone]}' with '{target}'"
+                )
+            else:
+                print(
+                    f"Info: Link '{zone} -> {target}' can be removed from otherICULegacyLinks()"
+                )
+        links[zone] = target
+
+    return (zones, links)
+
+
+def otherICULegacyLinks():
+    """The file `icuTzDir`/tools/tzcode/icuzones contains all ICU legacy time
+    zones with the exception of time zones which are removed by IANA after an
+    ICU release.
+
+    For example ICU 67 uses tzdata2018i, but tzdata2020b removed the link from
+    "US/Pacific-New" to "America/Los_Angeles". ICU standalone tzdata updates
+    don't include modified icuzones files, so we must manually record any IANA
+    modifications here.
+
+    After an ICU update, we can remove any no longer needed entries from this
+    function by checking if the relevant entries are now included in icuzones.
+    """
+
+    return {
+        # Current ICU is up-to-date with IANA, so this dict is empty.
+    }
+
+
+def icuTzDataVersion(icuTzDir):
+    """Read the ICU time zone version from `icuTzDir`/zoneinfo64.txt."""
+
+    def searchInFile(pattern, f):
+        p = re.compile(pattern)
+        for line in flines(f, "utf-8-sig"):
+            m = p.search(line)
+            if m:
+                return m.group(1)
+        return None
+
+    zoneinfo = os.path.join(icuTzDir, "zoneinfo64.txt")
+    if not os.path.isfile(zoneinfo):
+        raise RuntimeError("file not found: %s" % zoneinfo)
+    version = searchInFile("^//\s+tz version:\s+([0-9]{4}[a-z])$", zoneinfo)
+    if version is None:
+        raise RuntimeError(
+            "%s does not contain a valid tzdata version string" % zoneinfo
+        )
+    return version
+
+
+def findIncorrectICUZones(ianaZones, ianaLinks, icuZones, icuLinks, ignoreBackzone):
+    """Find incorrect ICU zone entries."""
+
+    def isIANATimeZone(zone):
+        return zone in ianaZones or zone in ianaLinks
+
+    def isICUTimeZone(zone):
+        return zone in icuZones or zone in icuLinks
+
+    def isICULink(zone):
+        return zone in icuLinks
+
+    # All IANA zones should be present in ICU.
+    missingTimeZones = [zone for zone in ianaZones if not isICUTimeZone(zone)]
+    # Normally zones in backzone are also present as links in one of the other
+    # time zone files. The only exception to this rule is the Asia/Hanoi time
+    # zone, this zone is only present in the backzone file.
+    expectedMissing = [] if ignoreBackzone else [Zone("Asia/Hanoi")]
+    if missingTimeZones != expectedMissing:
+        raise RuntimeError(
+            "Not all zones are present in ICU, did you forget "
+            "to run intl/update-tzdata.sh? %s" % missingTimeZones
+        )
+
+    # Zones which are only present in ICU?
+    additionalTimeZones = [zone for zone in icuZones if not isIANATimeZone(zone)]
+    if additionalTimeZones:
+        raise RuntimeError(
+            "Additional zones present in ICU, did you forget "
+            "to run intl/update-tzdata.sh? %s" % additionalTimeZones
+        )
+
+    # Zones which are marked as links in ICU.
+    result = ((zone, icuLinks[zone]) for zone in ianaZones if isICULink(zone))
+
+    # Remove unnecessary UTC mappings.
+    utcnames = ["Etc/UTC", "Etc/UCT", "Etc/GMT"]
+    result = ((zone, target) for (zone, target) in result if zone.name not in utcnames)
+
+    return sorted(result, key=itemgetter(0))
+
+
+def findIncorrectICULinks(ianaZones, ianaLinks, icuZones, icuLinks):
+    """Find incorrect ICU link entries."""
+
+    def isIANATimeZone(zone):
+        return zone in ianaZones or zone in ianaLinks
+
+    def isICUTimeZone(zone):
+        return zone in icuZones or zone in icuLinks
+
+    def isICULink(zone):
+        return zone in icuLinks
+
+    def isICUZone(zone):
+        return zone in icuZones
+
+    # All links should be present in ICU.
+    missingTimeZones = [zone for zone in ianaLinks.keys() if not isICUTimeZone(zone)]
+    if missingTimeZones:
+        raise RuntimeError(
+            "Not all zones are present in ICU, did you forget "
+            "to run intl/update-tzdata.sh? %s" % missingTimeZones
+        )
+
+    # Links which are only present in ICU?
+    additionalTimeZones = [zone for zone in icuLinks.keys() if not isIANATimeZone(zone)]
+    if additionalTimeZones:
+        raise RuntimeError(
+            "Additional links present in ICU, did you forget "
+            "to run intl/update-tzdata.sh? %s" % additionalTimeZones
+        )
+
+    result = chain(
+        # IANA links which have a different target in ICU.
+        (
+            (zone, target, icuLinks[zone])
+            for (zone, target) in ianaLinks.items()
+            if isICULink(zone) and target != icuLinks[zone]
+        ),
+        # IANA links which are zones in ICU.
+        (
+            (zone, target, zone.name)
+            for (zone, target) in ianaLinks.items()
+            if isICUZone(zone)
+        ),
+    )
+
+    # Remove unnecessary UTC mappings.
+    utcnames = ["Etc/UTC", "Etc/UCT", "Etc/GMT"]
+    result = (
+        (zone, target, icuTarget)
+        for (zone, target, icuTarget) in result
+        if target not in utcnames or icuTarget not in utcnames
+    )
+
+    return sorted(result, key=itemgetter(0))
+
+
+generatedFileWarning = "// Generated by make_intl_data.py. DO NOT EDIT."
+tzdataVersionComment = "// tzdata version = {0}"
+
+
+def processTimeZones(
+    tzdataDir, icuDir, icuTzDir, version, ignoreBackzone, ignoreFactory, out
+):
+    """Read the time zone info and create a new time zone cpp file."""
+    print("Processing tzdata mapping...")
+    (ianaZones, ianaLinks) = readIANATimeZones(tzdataDir, ignoreBackzone, ignoreFactory)
+    (icuZones, icuLinks) = readICUTimeZones(icuDir, icuTzDir, ignoreFactory)
+    (legacyZones, legacyLinks) = readICULegacyZones(icuDir)
+
+    # Remove all legacy ICU time zones.
+    icuZones = {zone for zone in icuZones if zone not in legacyZones}
+    icuLinks = {
+        zone: target for (zone, target) in icuLinks.items() if zone not in legacyLinks
+    }
+
+    incorrectZones = findIncorrectICUZones(
+        ianaZones, ianaLinks, icuZones, icuLinks, ignoreBackzone
+    )
+    if not incorrectZones:
+        print("<<< No incorrect ICU time zones found, please update Intl.js! >>>")
+        print("<<< Maybe https://ssl.icu-project.org/trac/ticket/12044 was fixed? >>>")
+
+    incorrectLinks = findIncorrectICULinks(ianaZones, ianaLinks, icuZones, icuLinks)
+    if not incorrectLinks:
+        print("<<< No incorrect ICU time zone links found, please update Intl.js! >>>")
+        print("<<< Maybe https://ssl.icu-project.org/trac/ticket/12044 was fixed? >>>")
+
+    print("Writing Intl tzdata file...")
+    with io.open(out, mode="w", encoding="utf-8", newline="") as f:
+        println = partial(print, file=f)
+
+        println(generatedFileWarning)
+        println(tzdataVersionComment.format(version))
+        println("")
+
+        println("#ifndef builtin_intl_TimeZoneDataGenerated_h")
+        println("#define builtin_intl_TimeZoneDataGenerated_h")
+        println("")
+
+        println("namespace js {")
+        println("namespace timezone {")
+        println("")
+
+        println("// Format:")
+        println('// "ZoneName" // ICU-Name [time zone file]')
+        println("const char* const ianaZonesTreatedAsLinksByICU[] = {")
+        for (zone, icuZone) in incorrectZones:
+            println('    "%s", // %s [%s]' % (zone, icuZone, zone.filename))
+        println("};")
+        println("")
+
+        println("// Format:")
+        println('// "LinkName", "Target" // ICU-Target [time zone file]')
+        println("struct LinkAndTarget")
+        println("{")
+        println("    const char* const link;")
+        println("    const char* const target;")
+        println("};")
+        println("")
+        println("const LinkAndTarget ianaLinksCanonicalizedDifferentlyByICU[] = {")
+        for (zone, target, icuTarget) in incorrectLinks:
+            println(
+                '    { "%s", "%s" }, // %s [%s]'
+                % (zone, target, icuTarget, zone.filename)
+            )
+        println("};")
+        println("")
+
+        println(
+            "// Legacy ICU time zones, these are not valid IANA time zone names. We also"
+        )
+        println("// disallow the old and deprecated System V time zones.")
+        println(
+            "// https://ssl.icu-project.org/repos/icu/trunk/icu4c/source/tools/tzcode/icuzones"
+        )  # NOQA: E501
+        println("const char* const legacyICUTimeZones[] = {")
+        for zone in chain(sorted(legacyLinks.keys()), sorted(legacyZones)):
+            println('    "%s",' % zone)
+        println("};")
+        println("")
+
+        println("} // namespace timezone")
+        println("} // namespace js")
+        println("")
+        println("#endif /* builtin_intl_TimeZoneDataGenerated_h */")
+
+
+def updateBackzoneLinks(tzdataDir, links):
+    def withZone(fn):
+        return lambda zone_target: fn(zone_target[0])
+
+    (backzoneZones, backzoneLinks) = readIANAFiles(tzdataDir, ["backzone"])
+    (stableZones, updatedLinks, updatedZones) = partition(
+        links.items(),
+        # Link not changed in backzone.
+        withZone(lambda zone: zone not in backzoneLinks and zone not in backzoneZones),
+        # Link has a new target.
+        withZone(lambda zone: zone in backzoneLinks),
+    )
+    # Keep stable zones and links with updated target.
+    return dict(
+        chain(
+            stableZones,
+            map(withZone(lambda zone: (zone, backzoneLinks[zone])), updatedLinks),
+        )
+    )
+
+
+def generateTzDataLinkTestContent(testDir, version, fileName, description, links):
+    with io.open(
+        os.path.join(testDir, fileName), mode="w", encoding="utf-8", newline=""
+    ) as f:
+        println = partial(print, file=f)
+
+        println('// |reftest| skip-if(!this.hasOwnProperty("Intl"))')
+        println("")
+        println(generatedFileWarning)
+        println(tzdataVersionComment.format(version))
+        println(
+            """
+const tzMapper = [
+    x => x,
+    x => x.toUpperCase(),
+    x => x.toLowerCase(),
+];
+"""
+        )
+
+        println(description)
+        println("const links = {")
+        for (zone, target) in sorted(links, key=itemgetter(0)):
+            println('    "%s": "%s",' % (zone, target))
+        println("};")
+
+        println(
+            """
+for (let [linkName, target] of Object.entries(links)) {
+    if (target === "Etc/UTC" || target === "Etc/GMT")
+        target = "UTC";
+
+    for (let map of tzMapper) {
+        let dtf = new Intl.DateTimeFormat(undefined, {timeZone: map(linkName)});
+        let resolvedTimeZone = dtf.resolvedOptions().timeZone;
+        assertEq(resolvedTimeZone, target, `${linkName} -> ${target}`);
+    }
+}
+"""
+        )
+        println(
+            """
+if (typeof reportCompare === "function")
+    reportCompare(0, 0, "ok");
+"""
+        )
+
+
+def generateTzDataTestBackwardLinks(tzdataDir, version, ignoreBackzone, testDir):
+    (zones, links) = readIANAFiles(tzdataDir, ["backward"])
+    assert len(zones) == 0
+
+    if not ignoreBackzone:
+        links = updateBackzoneLinks(tzdataDir, links)
+
+    generateTzDataLinkTestContent(
+        testDir,
+        version,
+        "timeZone_backward_links.js",
+        "// Link names derived from IANA Time Zone Database, backward file.",
+        links.items(),
+    )
+
+
+def generateTzDataTestNotBackwardLinks(tzdataDir, version, ignoreBackzone, testDir):
+    tzfiles = filterfalse(
+        {"backward", "backzone"}.__contains__, listIANAFiles(tzdataDir)
+    )
+    (zones, links) = readIANAFiles(tzdataDir, tzfiles)
+
+    if not ignoreBackzone:
+        links = updateBackzoneLinks(tzdataDir, links)
+
+    generateTzDataLinkTestContent(
+        testDir,
+        version,
+        "timeZone_notbackward_links.js",
+        "// Link names derived from IANA Time Zone Database, excluding backward file.",
+        links.items(),
+    )
+
+
+def generateTzDataTestBackzone(tzdataDir, version, ignoreBackzone, testDir):
+    backzoneFiles = {"backzone"}
+    (bkfiles, tzfiles) = partition(listIANAFiles(tzdataDir), backzoneFiles.__contains__)
+
+    # Read zone and link infos.
+    (zones, links) = readIANAFiles(tzdataDir, tzfiles)
+    (backzones, backlinks) = readIANAFiles(tzdataDir, bkfiles)
+
+    if not ignoreBackzone:
+        comment = """\
+// This file was generated with historical, pre-1970 backzone information
+// respected. Therefore, every zone key listed below is its own Zone, not
+// a Link to a modern-day target as IANA ignoring backzones would say.
+
+"""
+    else:
+        comment = """\
+// This file was generated while ignoring historical, pre-1970 backzone
+// information. Therefore, every zone key listed below is part of a Link
+// whose target is the corresponding value.
+
+"""
+
+    generateTzDataLinkTestContent(
+        testDir,
+        version,
+        "timeZone_backzone.js",
+        comment + "// Backzone zones derived from IANA Time Zone Database.",
+        (
+            (zone, zone if not ignoreBackzone else links[zone])
+            for zone in backzones
+            if zone in links
+        ),
+    )
+
+
+def generateTzDataTestBackzoneLinks(tzdataDir, version, ignoreBackzone, testDir):
+    backzoneFiles = {"backzone"}
+    (bkfiles, tzfiles) = partition(listIANAFiles(tzdataDir), backzoneFiles.__contains__)
+
+    # Read zone and link infos.
+    (zones, links) = readIANAFiles(tzdataDir, tzfiles)
+    (backzones, backlinks) = readIANAFiles(tzdataDir, bkfiles)
+
+    if not ignoreBackzone:
+        comment = """\
+// This file was generated with historical, pre-1970 backzone information
+// respected. Therefore, every zone key listed below points to a target
+// in the backzone file and not to its modern-day target as IANA ignoring
+// backzones would say.
+
+"""
+    else:
+        comment = """\
+// This file was generated while ignoring historical, pre-1970 backzone
+// information. Therefore, every zone key listed below is part of a Link
+// whose target is the corresponding value ignoring any backzone entries.
+
+"""
+
+    generateTzDataLinkTestContent(
+        testDir,
+        version,
+        "timeZone_backzone_links.js",
+        comment + "// Backzone links derived from IANA Time Zone Database.",
+        (
+            (zone, target if not ignoreBackzone else links[zone])
+            for (zone, target) in backlinks.items()
+        ),
+    )
+
+
+def generateTzDataTestVersion(tzdataDir, version, testDir):
+    fileName = "timeZone_version.js"
+
+    with io.open(
+        os.path.join(testDir, fileName), mode="w", encoding="utf-8", newline=""
+    ) as f:
+        println = partial(print, file=f)
+
+        println('// |reftest| skip-if(!this.hasOwnProperty("Intl"))')
+        println("")
+        println(generatedFileWarning)
+        println(tzdataVersionComment.format(version))
+        println("""const tzdata = "{0}";""".format(version))
+
+        println(
+            """
+if (typeof getICUOptions === "undefined") {
+    var getICUOptions = SpecialPowers.Cu.getJSTestingFunctions().getICUOptions;
+}
+
+var options = getICUOptions();
+
+assertEq(options.tzdata, tzdata);
+
+if (typeof reportCompare === "function")
+    reportCompare(0, 0, "ok");
+"""
+        )
+
+
+def generateTzDataTestCanonicalZones(
+    tzdataDir, version, ignoreBackzone, ignoreFactory, testDir
+):
+    fileName = "supportedValuesOf-timeZones-canonical.js"
+
+    # Read zone and link infos.
+    (ianaZones, _) = readIANATimeZones(tzdataDir, ignoreBackzone, ignoreFactory)
+
+    # Replace Etc/GMT and Etc/UTC with UTC.
+    ianaZones.remove(Zone("Etc/GMT"))
+    ianaZones.remove(Zone("Etc/UTC"))
+    ianaZones.add(Zone("UTC"))
+
+    # See findIncorrectICUZones() for why Asia/Hanoi has to be special-cased.
+    ianaZones.remove(Zone("Asia/Hanoi"))
+
+    if not ignoreBackzone:
+        comment = """\
+// This file was generated with historical, pre-1970 backzone information
+// respected.
+"""
+    else:
+        comment = """\
+// This file was generated while ignoring historical, pre-1970 backzone
+// information.
+"""
+
+    with io.open(
+        os.path.join(testDir, fileName), mode="w", encoding="utf-8", newline=""
+    ) as f:
+        println = partial(print, file=f)
+
+        println('// |reftest| skip-if(!this.hasOwnProperty("Intl"))')
+        println("")
+        println(generatedFileWarning)
+        println(tzdataVersionComment.format(version))
+        println("")
+        println(comment)
+
+        println("const zones = [")
+        for zone in sorted(ianaZones):
+            println(f'  "{zone}",')
+        println("];")
+
+        println(
+            """
+let supported = Intl.supportedValuesOf("timeZone");
+
+assertEqArray(supported, zones);
+
+if (typeof reportCompare === "function")
+    reportCompare(0, 0, "ok");
+"""
+        )
+
+
+def generateTzDataTests(tzdataDir, version, ignoreBackzone, ignoreFactory, testDir):
+    dtfTestDir = os.path.join(testDir, "DateTimeFormat")
+    if not os.path.isdir(dtfTestDir):
+        raise RuntimeError("not a directory: %s" % dtfTestDir)
+
+    generateTzDataTestBackwardLinks(tzdataDir, version, ignoreBackzone, dtfTestDir)
+    generateTzDataTestNotBackwardLinks(tzdataDir, version, ignoreBackzone, dtfTestDir)
+    generateTzDataTestBackzone(tzdataDir, version, ignoreBackzone, dtfTestDir)
+    generateTzDataTestBackzoneLinks(tzdataDir, version, ignoreBackzone, dtfTestDir)
+    generateTzDataTestVersion(tzdataDir, version, dtfTestDir)
+    generateTzDataTestCanonicalZones(
+        tzdataDir, version, ignoreBackzone, ignoreFactory, testDir
+    )
+
+
+def updateTzdata(topsrcdir, args):
+    """Update the time zone cpp file."""
+
+    icuDir = os.path.join(topsrcdir, "intl/icu/source")
+    if not os.path.isdir(icuDir):
+        raise RuntimeError("not a directory: %s" % icuDir)
+
+    icuTzDir = os.path.join(topsrcdir, "intl/tzdata/source")
+    if not os.path.isdir(icuTzDir):
+        raise RuntimeError("not a directory: %s" % icuTzDir)
+
+    intlTestDir = os.path.join(topsrcdir, "js/src/tests/non262/Intl")
+    if not os.path.isdir(intlTestDir):
+        raise RuntimeError("not a directory: %s" % intlTestDir)
+
+    tzDir = args.tz
+    if tzDir is not None and not (os.path.isdir(tzDir) or os.path.isfile(tzDir)):
+        raise RuntimeError("not a directory or file: %s" % tzDir)
+    ignoreBackzone = args.ignore_backzone
+    # TODO: Accept or ignore the placeholder time zone "Factory"?
+    ignoreFactory = False
+    out = args.out
+
+    version = icuTzDataVersion(icuTzDir)
+    url = (
+        "https://www.iana.org/time-zones/repository/releases/tzdata%s.tar.gz" % version
+    )
+
+    print("Arguments:")
+    print("\ttzdata version: %s" % version)
+    print("\ttzdata URL: %s" % url)
+    print("\ttzdata directory|file: %s" % tzDir)
+    print("\tICU directory: %s" % icuDir)
+    print("\tICU timezone directory: %s" % icuTzDir)
+    print("\tIgnore backzone file: %s" % ignoreBackzone)
+    print("\tOutput file: %s" % out)
+    print("")
+
+    def updateFrom(f):
+        if os.path.isfile(f) and tarfile.is_tarfile(f):
+            with tarfile.open(f, "r:*") as tar:
+                processTimeZones(
+                    TzDataFile(tar),
+                    icuDir,
+                    icuTzDir,
+                    version,
+                    ignoreBackzone,
+                    ignoreFactory,
+                    out,
+                )
+                generateTzDataTests(
+                    TzDataFile(tar), version, ignoreBackzone, ignoreFactory, intlTestDir
+                )
+        elif os.path.isdir(f):
+            processTimeZones(
+                TzDataDir(f),
+                icuDir,
+                icuTzDir,
+                version,
+                ignoreBackzone,
+                ignoreFactory,
+                out,
+            )
+            generateTzDataTests(
+                TzDataDir(f), version, ignoreBackzone, ignoreFactory, intlTestDir
+            )
+        else:
+            raise RuntimeError("unknown format")
+
+    if tzDir is None:
+        print("Downloading tzdata file...")
+        with closing(urlopen(url)) as tzfile:
+            fname = urlsplit(tzfile.geturl()).path.split("/")[-1]
+            with tempfile.NamedTemporaryFile(suffix=fname) as tztmpfile:
+                print("File stored in %s" % tztmpfile.name)
+                tztmpfile.write(tzfile.read())
+                tztmpfile.flush()
+                updateFrom(tztmpfile.name)
+    else:
+        updateFrom(tzDir)
+
+
+def readCurrencyFile(tree):
+    reCurrency = re.compile(r"^[A-Z]{3}$")
+    reIntMinorUnits = re.compile(r"^\d+$")
+
+    for country in tree.iterfind(".//CcyNtry"):
+        # Skip entry if no currency information is available.
+        currency = country.findtext("Ccy")
+        if currency is None:
+            continue
+        assert reCurrency.match(currency)
+
+        minorUnits = country.findtext("CcyMnrUnts")
+        assert minorUnits is not None
+
+        # Skip all entries without minorUnits or which use the default minorUnits.
+        if reIntMinorUnits.match(minorUnits) and int(minorUnits) != 2:
+            currencyName = country.findtext("CcyNm")
+            countryName = country.findtext("CtryNm")
+            yield (currency, int(minorUnits), currencyName, countryName)
+
+
+def writeCurrencyFile(published, currencies, out):
+    with io.open(out, mode="w", encoding="utf-8", newline="") as f:
+        println = partial(print, file=f)
+
+        println(generatedFileWarning)
+        println("// Version: {}".format(published))
+
+        println(
+            """
+/**
+ * Mapping from currency codes to the number of decimal digits used for them.
+ * Default is 2 digits.
+ *
+ * Spec: ISO 4217 Currency and Funds Code List.
+ * http://www.currency-iso.org/en/home/tables/table-a1.html
+ */"""
+        )
+        println("var currencyDigits = {")
+        for (currency, entries) in groupby(
+            sorted(currencies, key=itemgetter(0)), itemgetter(0)
+        ):
+            for (_, minorUnits, currencyName, countryName) in entries:
+                println("  // {} ({})".format(currencyName, countryName))
+            println("  {}: {},".format(currency, minorUnits))
+        println("};")
+
+
+def updateCurrency(topsrcdir, args):
+    """Update the CurrencyDataGenerated.js file."""
+    import xml.etree.ElementTree as ET
+    from random import randint
+
+    url = args.url
+    out = args.out
+    filename = args.file
+
+    print("Arguments:")
+    print("\tDownload url: %s" % url)
+    print("\tLocal currency file: %s" % filename)
+    print("\tOutput file: %s" % out)
+    print("")
+
+    def updateFrom(currencyFile):
+        print("Processing currency code list file...")
+        tree = ET.parse(currencyFile)
+        published = tree.getroot().attrib["Pblshd"]
+        currencies = readCurrencyFile(tree)
+
+        print("Writing CurrencyData file...")
+        writeCurrencyFile(published, currencies, out)
+
+    if filename is not None:
+        print("Always make sure you have the newest currency code list file!")
+        updateFrom(filename)
+    else:
+        print("Downloading currency & funds code list...")
+        request = UrlRequest(url)
+        request.add_header(
+            "User-agent",
+            "Mozilla/5.0 (Mobile; rv:{0}.0) Gecko/{0}.0 Firefox/{0}.0".format(
+                randint(1, 999)
+            ),
+        )
+        with closing(urlopen(request)) as currencyFile:
+            fname = urlsplit(currencyFile.geturl()).path.split("/")[-1]
+            with tempfile.NamedTemporaryFile(suffix=fname) as currencyTmpFile:
+                print("File stored in %s" % currencyTmpFile.name)
+                currencyTmpFile.write(currencyFile.read())
+                currencyTmpFile.flush()
+                updateFrom(currencyTmpFile.name)
+
+
+def writeUnicodeExtensionsMappings(println, mapping, extension):
+    println(
+        """
+template <size_t Length>
+static inline bool Is{0}Key(mozilla::Span<const char> key, const char (&str)[Length]) {{
+  static_assert(Length == {0}KeyLength + 1,
+                "{0} extension key is two characters long");
+  return memcmp(key.data(), str, Length - 1) == 0;
+}}
+
+template <size_t Length>
+static inline bool Is{0}Type(mozilla::Span<const char> type, const char (&str)[Length]) {{
+  static_assert(Length > {0}KeyLength + 1,
+                "{0} extension type contains more than two characters");
+  return type.size() == (Length - 1) &&
+         memcmp(type.data(), str, Length - 1) == 0;
+}}
+""".format(
+            extension
+        ).rstrip(
+            "\n"
+        )
+    )
+
+    linear_search_max_length = 4
+
+    needs_binary_search = any(
+        len(replacements.items()) > linear_search_max_length
+        for replacements in mapping.values()
+    )
+
+    if needs_binary_search:
+        println(
+            """
+static int32_t Compare{0}Type(const char* a, mozilla::Span<const char> b) {{
+  MOZ_ASSERT(!std::char_traits<char>::find(b.data(), b.size(), '\\0'),
+             "unexpected null-character in string");
+
+  using UnsignedChar = unsigned char;
+  for (size_t i = 0; i < b.size(); i++) {{
+    // |a| is zero-terminated and |b| doesn't contain a null-terminator. So if
+    // we've reached the end of |a|, the below if-statement will always be true.
+    // That ensures we don't read past the end of |a|.
+    if (int32_t r = UnsignedChar(a[i]) - UnsignedChar(b[i])) {{
+      return r;
+    }}
+  }}
+
+  // Return zero if both strings are equal or a positive number if |b| is a
+  // prefix of |a|.
+  return int32_t(UnsignedChar(a[b.size()]));
+}}
+
+template <size_t Length>
+static inline const char* Search{0}Replacement(
+  const char* (&types)[Length], const char* (&aliases)[Length],
+  mozilla::Span<const char> type) {{
+
+  auto p = std::lower_bound(std::begin(types), std::end(types), type,
+                            [](const auto& a, const auto& b) {{
+                              return Compare{0}Type(a, b) < 0;
+                            }});
+  if (p != std::end(types) && Compare{0}Type(*p, type) == 0) {{
+    return aliases[std::distance(std::begin(types), p)];
+  }}
+  return nullptr;
+}}
+""".format(
+                extension
+            ).rstrip(
+                "\n"
+            )
+        )
+
+    println(
+        """
+/**
+ * Mapping from deprecated BCP 47 {0} extension types to their preferred
+ * values.
+ *
+ * Spec: https://www.unicode.org/reports/tr35/#Unicode_Locale_Extension_Data_Files
+ * Spec: https://www.unicode.org/reports/tr35/#t_Extension
+ */
+const char* mozilla::intl::Locale::Replace{0}ExtensionType(
+    mozilla::Span<const char> key, mozilla::Span<const char> type) {{
+  MOZ_ASSERT(key.size() == {0}KeyLength);
+  MOZ_ASSERT(IsCanonicallyCased{0}Key(key));
+
+  MOZ_ASSERT(type.size() > {0}KeyLength);
+  MOZ_ASSERT(IsCanonicallyCased{0}Type(type));
+""".format(
+            extension
+        )
+    )
+
+    def to_hash_key(replacements):
+        return str(sorted(replacements.items()))
+
+    def write_array(subtags, name, length):
+        max_entries = (80 - len("    ")) // (length + len('"", '))
+
+        println("    static const char* {}[{}] = {{".format(name, len(subtags)))
+
+        for entries in grouper(subtags, max_entries):
+            entries = (
+                '"{}"'.format(tag).center(length + 2)
+                for tag in entries
+                if tag is not None
+            )
+            println("        {},".format(", ".join(entries)))
+
+        println("    };")
+
+    # Merge duplicate keys.
+    key_aliases = {}
+    for (key, replacements) in sorted(mapping.items(), key=itemgetter(0)):
+        hash_key = to_hash_key(replacements)
+        if hash_key not in key_aliases:
+            key_aliases[hash_key] = []
+        else:
+            key_aliases[hash_key].append(key)
+
+    first_key = True
+    for (key, replacements) in sorted(mapping.items(), key=itemgetter(0)):
+        hash_key = to_hash_key(replacements)
+        if key in key_aliases[hash_key]:
+            continue
+
+        cond = (
+            'Is{}Key(key, "{}")'.format(extension, k)
+            for k in [key] + key_aliases[hash_key]
+        )
+
+        if_kind = "if" if first_key else "else if"
+        cond = (" ||\n" + " " * (2 + len(if_kind) + 2)).join(cond)
+        println(
+            """
+  {} ({}) {{""".format(
+                if_kind, cond
+            ).strip(
+                "\n"
+            )
+        )
+        first_key = False
+
+        replacements = sorted(replacements.items(), key=itemgetter(0))
+
+        if len(replacements) > linear_search_max_length:
+            types = [t for (t, _) in replacements]
+            preferred = [r for (_, r) in replacements]
+            max_len = max(len(k) for k in types + preferred)
+
+            write_array(types, "types", max_len)
+            write_array(preferred, "aliases", max_len)
+            println(
+                """
+    return Search{}Replacement(types, aliases, type);
+""".format(
+                    extension
+                ).strip(
+                    "\n"
+                )
+            )
+        else:
+            for (type, replacement) in replacements:
+                println(
+                    """
+    if (Is{}Type(type, "{}")) {{
+      return "{}";
+    }}""".format(
+                        extension, type, replacement
+                    ).strip(
+                        "\n"
+                    )
+                )
+
+        println(
+            """
+  }""".lstrip(
+                "\n"
+            )
+        )
+
+    println(
+        """
+  return nullptr;
+}
+""".strip(
+            "\n"
+        )
+    )
+
+
+def readICUUnitResourceFile(filepath):
+    """Return a set of unit descriptor pairs where the first entry denotes the unit type and the
+    second entry the unit name.
+
+    Example:
+
+    root{
+        units{
+            compound{
+            }
+            coordinate{
+            }
+            length{
+                meter{
+                }
+            }
+        }
+        unitsNarrow:alias{"/LOCALE/unitsShort"}
+        unitsShort{
+            duration{
+                day{
+                }
+                day-person:alias{"/LOCALE/unitsShort/duration/day"}
+            }
+            length{
+                meter{
+                }
+            }
+        }
+    }
+
+    Returns {("length", "meter"), ("duration", "day"), ("duration", "day-person")}
+    """
+
+    start_table_re = re.compile(r"^([\w\-%:\"]+)\{$")
+    end_table_re = re.compile(r"^\}$")
+    table_entry_re = re.compile(r"^([\w\-%:\"]+)\{\"(.*?)\"\}$")
+
+    # The current resource table.
+    table = {}
+
+    # List of parent tables when parsing.
+    parents = []
+
+    # Track multi-line comments state.
+    in_multiline_comment = False
+
+    for line in flines(filepath, "utf-8-sig"):
+        # Remove leading and trailing whitespace.
+        line = line.strip()
+
+        # Skip over comments.
+        if in_multiline_comment:
+            if line.endswith("*/"):
+                in_multiline_comment = False
+            continue
+
+        if line.startswith("//"):
+            continue
+
+        if line.startswith("/*"):
+            in_multiline_comment = True
+            continue
+
+        # Try to match the start of a table, e.g. `length{` or `meter{`.
+        match = start_table_re.match(line)
+        if match:
+            parents.append(table)
+            table_name = match.group(1)
+            new_table = {}
+            table[table_name] = new_table
+            table = new_table
+            continue
+
+        # Try to match the end of a table.
+        match = end_table_re.match(line)
+        if match:
+            table = parents.pop()
+            continue
+
+        # Try to match a table entry, e.g. `dnam{"meter"}`.
+        match = table_entry_re.match(line)
+        if match:
+            entry_key = match.group(1)
+            entry_value = match.group(2)
+            table[entry_key] = entry_value
+            continue
+
+        raise Exception("unexpected line: '{}' in {}".format(line, filepath))
+
+    assert len(parents) == 0, "Not all tables closed"
+    assert len(table) == 1, "More than one root table"
+
+    # Remove the top-level language identifier table.
+    (_, unit_table) = table.popitem()
+
+    # Add all units for the three display formats "units", "unitsNarrow", and "unitsShort".
+    # But exclude the pseudo-units "compound" and "ccoordinate".
+    return {
+        (unit_type, unit_name if not unit_name.endswith(":alias") else unit_name[:-6])
+        for unit_display in ("units", "unitsNarrow", "unitsShort")
+        if unit_display in unit_table
+        for (unit_type, unit_names) in unit_table[unit_display].items()
+        if unit_type != "compound" and unit_type != "coordinate"
+        for unit_name in unit_names.keys()
+    }
+
+
+def computeSupportedUnits(all_units, sanctioned_units):
+    """Given the set of all possible ICU unit identifiers and the set of sanctioned unit
+    identifiers, compute the set of effectively supported ICU unit identifiers.
+    """
+
+    def find_match(unit):
+        unit_match = [
+            (unit_type, unit_name)
+            for (unit_type, unit_name) in all_units
+            if unit_name == unit
+        ]
+        if unit_match:
+            assert len(unit_match) == 1
+            return unit_match[0]
+        return None
+
+    def compound_unit_identifiers():
+        for numerator in sanctioned_units:
+            for denominator in sanctioned_units:
+                yield "{}-per-{}".format(numerator, denominator)
+
+    supported_simple_units = {find_match(unit) for unit in sanctioned_units}
+    assert None not in supported_simple_units
+
+    supported_compound_units = {
+        unit_match
+        for unit_match in (find_match(unit) for unit in compound_unit_identifiers())
+        if unit_match
+    }
+
+    return supported_simple_units | supported_compound_units
+
+
+def readICUDataFilterForUnits(data_filter_file):
+    with io.open(data_filter_file, mode="r", encoding="utf-8") as f:
+        data_filter = json.load(f)
+
+    # Find the rule set for the "unit_tree".
+    unit_tree_rules = [
+        entry["rules"]
+        for entry in data_filter["resourceFilters"]
+        if entry["categories"] == ["unit_tree"]
+    ]
+    assert len(unit_tree_rules) == 1
+
+    # Compute the list of included units from that rule set. The regular expression must match
+    # "+/*/length/meter" and mustn't match either "-/*" or "+/*/compound".
+    included_unit_re = re.compile(r"^\+/\*/(.+?)/(.+)$")
+    filtered_units = (included_unit_re.match(unit) for unit in unit_tree_rules[0])
+
+    return {(unit.group(1), unit.group(2)) for unit in filtered_units if unit}
+
+
+def writeSanctionedSimpleUnitIdentifiersFiles(all_units, sanctioned_units):
+    js_src_builtin_intl_dir = os.path.dirname(os.path.abspath(__file__))
+    intl_components_src_dir = os.path.join(
+        js_src_builtin_intl_dir, "../../../../intl/components/src"
+    )
+
+    def find_unit_type(unit):
+        result = [
+            unit_type for (unit_type, unit_name) in all_units if unit_name == unit
+        ]
+        assert result and len(result) == 1
+        return result[0]
+
+    sanctioned_js_file = os.path.join(
+        js_src_builtin_intl_dir, "SanctionedSimpleUnitIdentifiersGenerated.js"
+    )
+    with io.open(sanctioned_js_file, mode="w", encoding="utf-8", newline="") as f:
+        println = partial(print, file=f)
+
+        sanctioned_units_object = json.dumps(
+            {unit: True for unit in sorted(sanctioned_units)},
+            sort_keys=True,
+            indent=2,
+            separators=(",", ": "),
+        )
+
+        println(generatedFileWarning)
+
+        println(
+            """
+/**
+ * The list of currently supported simple unit identifiers.
+ *
+ * Intl.NumberFormat Unified API Proposal
+ */"""
+        )
+
+        println("/* eslint-disable prettier/prettier */")
+        println(
+            "var sanctionedSimpleUnitIdentifiers = {};".format(sanctioned_units_object)
+        )
+        println("/* eslint-enable prettier/prettier */")
+
+    sanctioned_h_file = os.path.join(intl_components_src_dir, "MeasureUnitGenerated.h")
+    with io.open(sanctioned_h_file, mode="w", encoding="utf-8", newline="") as f:
+        println = partial(print, file=f)
+
+        println(generatedFileWarning)
+
+        println(
+            """
+#ifndef intl_components_MeasureUnitGenerated_h
+#define intl_components_MeasureUnitGenerated_h
+
+namespace mozilla::intl {
+
+struct SimpleMeasureUnit {
+  const char* const type;
+  const char* const name;
+};
+
+/**
+ * The list of currently supported simple unit identifiers.
+ *
+ * The list must be kept in alphabetical order of |name|.
+ */
+inline constexpr SimpleMeasureUnit simpleMeasureUnits[] = {
+    // clang-format off"""
+        )
+
+        for unit_name in sorted(sanctioned_units):
+            println('  {{"{}", "{}"}},'.format(find_unit_type(unit_name), unit_name))
+
+        println(
+            """
+    // clang-format on
+};
+
+}  // namespace mozilla::intl
+
+#endif
+""".strip(
+                "\n"
+            )
+        )
+
+    writeUnitTestFiles(all_units, sanctioned_units)
+
+
+def writeUnitTestFiles(all_units, sanctioned_units):
+    """Generate test files for unit number formatters."""
+
+    js_src_builtin_intl_dir = os.path.dirname(os.path.abspath(__file__))
+    test_dir = os.path.join(
+        js_src_builtin_intl_dir, "../../tests/non262/Intl/NumberFormat"
+    )
+
+    def write_test(file_name, test_content, indent=4):
+        file_path = os.path.join(test_dir, file_name)
+        with io.open(file_path, mode="w", encoding="utf-8", newline="") as f:
+            println = partial(print, file=f)
+
+            println('// |reftest| skip-if(!this.hasOwnProperty("Intl"))')
+            println("")
+            println(generatedFileWarning)
+            println("")
+
+            sanctioned_units_array = json.dumps(
+                [unit for unit in sorted(sanctioned_units)],
+                indent=indent,
+                separators=(",", ": "),
+            )
+
+            println(
+                "const sanctionedSimpleUnitIdentifiers = {};".format(
+                    sanctioned_units_array
+                )
+            )
+
+            println(test_content)
+
+            println(
+                """
+if (typeof reportCompare === "function")
+{}reportCompare(true, true);""".format(
+                    " " * indent
+                )
+            )
+
+    write_test(
+        "unit-compound-combinations.js",
+        """
+// Test all simple unit identifier combinations are allowed.
+
+for (const numerator of sanctionedSimpleUnitIdentifiers) {
+    for (const denominator of sanctionedSimpleUnitIdentifiers) {
+        const unit = `${numerator}-per-${denominator}`;
+        const nf = new Intl.NumberFormat("en", {style: "unit", unit});
+
+        assertEq(nf.format(1), nf.formatToParts(1).map(p => p.value).join(""));
+    }
+}""",
+    )
+
+    all_units_array = json.dumps(
+        ["-".join(unit) for unit in sorted(all_units)], indent=4, separators=(",", ": ")
+    )
+
+    write_test(
+        "unit-well-formed.js",
+        """
+const allUnits = {};
+""".format(
+            all_units_array
+        )
+        + """
+// Test only sanctioned unit identifiers are allowed.
+
+for (const typeAndUnit of allUnits) {
+    const [_, type, unit] = typeAndUnit.match(/(\w+)-(.+)/);
+
+    let allowed;
+    if (unit.includes("-per-")) {
+        const [numerator, denominator] = unit.split("-per-");
+        allowed = sanctionedSimpleUnitIdentifiers.includes(numerator) &&
+                  sanctionedSimpleUnitIdentifiers.includes(denominator);
+    } else {
+        allowed = sanctionedSimpleUnitIdentifiers.includes(unit);
+    }
+
+    if (allowed) {
+        const nf = new Intl.NumberFormat("en", {style: "unit", unit});
+        assertEq(nf.format(1), nf.formatToParts(1).map(p => p.value).join(""));
+    } else {
+        assertThrowsInstanceOf(() => new Intl.NumberFormat("en", {style: "unit", unit}),
+                               RangeError, `Missing error for "${typeAndUnit}"`);
+    }
+}""",
+    )
+
+    write_test(
+        "unit-formatToParts-has-unit-field.js",
+        """
+// Test only English and Chinese to keep the overall runtime reasonable.
+//
+// Chinese is included because it contains more than one "unit" element for
+// certain unit combinations.
+const locales = ["en", "zh"];
+
+// Plural rules for English only differentiate between "one" and "other". Plural
+// rules for Chinese only use "other". That means we only need to test two values
+// per unit.
+const values = [0, 1];
+
+// Ensure unit formatters contain at least one "unit" element.
+
+for (const locale of locales) {
+  for (const unit of sanctionedSimpleUnitIdentifiers) {
+    const nf = new Intl.NumberFormat(locale, {style: "unit", unit});
+
+    for (const value of values) {
+      assertEq(nf.formatToParts(value).some(e => e.type === "unit"), true,
+               `locale=${locale}, unit=${unit}`);
+    }
+  }
+
+  for (const numerator of sanctionedSimpleUnitIdentifiers) {
+    for (const denominator of sanctionedSimpleUnitIdentifiers) {
+      const unit = `${numerator}-per-${denominator}`;
+      const nf = new Intl.NumberFormat(locale, {style: "unit", unit});
+
+      for (const value of values) {
+        assertEq(nf.formatToParts(value).some(e => e.type === "unit"), true,
+                 `locale=${locale}, unit=${unit}`);
+      }
+    }
+  }
+}""",
+        indent=2,
+    )
+
+
+def updateUnits(topsrcdir, args):
+    js_src_builtin_intl_dir = os.path.dirname(os.path.abspath(__file__))
+    icu_path = os.path.join(topsrcdir, "intl", "icu")
+    icu_unit_path = os.path.join(icu_path, "source", "data", "unit")
+
+    with io.open(
+        os.path.join(js_src_builtin_intl_dir, "SanctionedSimpleUnitIdentifiers.yaml"),
+        mode="r",
+        encoding="utf-8",
+    ) as f:
+        sanctioned_units = yaml.safe_load(f)
+
+    # Read all possible ICU unit identifiers from the "unit/root.txt" resource.
+    unit_root_file = os.path.join(icu_unit_path, "root.txt")
+    all_units = readICUUnitResourceFile(unit_root_file)
+
+    # Compute the set of effectively supported ICU unit identifiers.
+    supported_units = computeSupportedUnits(all_units, sanctioned_units)
+
+    # Read the list of units we're including into the ICU data file.
+    data_filter_file = os.path.join(icu_path, "data_filter.json")
+    filtered_units = readICUDataFilterForUnits(data_filter_file)
+
+    # Both sets must match to avoid resource loading errors at runtime.
+    if supported_units != filtered_units:
+
+        def units_to_string(units):
+            return ", ".join("/".join(u) for u in units)
+
+        missing = supported_units - filtered_units
+        if missing:
+            raise RuntimeError("Missing units: {}".format(units_to_string(missing)))
+
+        # Not exactly an error, but we currently don't have a use case where we need to support
+        # more units than required by ECMA-402.
+        extra = filtered_units - supported_units
+        if extra:
+            raise RuntimeError("Unnecessary units: {}".format(units_to_string(extra)))
+
+    writeSanctionedSimpleUnitIdentifiersFiles(all_units, sanctioned_units)
+
+
+def readICUNumberingSystemsResourceFile(filepath):
+    """Returns a dictionary of numbering systems where the key denotes the numbering system name
+    and the value a dictionary with additional numbering system data.
+
+    Example:
+
+    numberingSystems:table(nofallback){
+        numberingSystems{
+            latn{
+                algorithmic:int{0}
+                desc{"0123456789"}
+                radix:int{10}
+            }
+            roman{
+                algorithmic:int{1}
+                desc{"%roman-upper"}
+                radix:int{10}
+            }
+        }
+    }
+
+    Returns {"latn": {"digits": "0123456789", "algorithmic": False},
+             "roman": {"algorithmic": True}}
+    """
+
+    start_table_re = re.compile(r"^(\w+)(?:\:[\w\(\)]+)?\{$")
+    end_table_re = re.compile(r"^\}$")
+    table_entry_re = re.compile(r"^(\w+)(?:\:[\w\(\)]+)?\{(?:(?:\"(.*?)\")|(\d+))\}$")
+
+    # The current resource table.
+    table = {}
+
+    # List of parent tables when parsing.
+    parents = []
+
+    # Track multi-line comments state.
+    in_multiline_comment = False
+
+    for line in flines(filepath, "utf-8-sig"):
+        # Remove leading and trailing whitespace.
+        line = line.strip()
+
+        # Skip over comments.
+        if in_multiline_comment:
+            if line.endswith("*/"):
+                in_multiline_comment = False
+            continue
+
+        if line.startswith("//"):
+            continue
+
+        if line.startswith("/*"):
+            in_multiline_comment = True
+            continue
+
+        # Try to match the start of a table, e.g. `latn{`.
+        match = start_table_re.match(line)
+        if match:
+            parents.append(table)
+            table_name = match.group(1)
+            new_table = {}
+            table[table_name] = new_table
+            table = new_table
+            continue
+
+        # Try to match the end of a table.
+        match = end_table_re.match(line)
+        if match:
+            table = parents.pop()
+            continue
+
+        # Try to match a table entry, e.g. `desc{"0123456789"}`.
+        match = table_entry_re.match(line)
+        if match:
+            entry_key = match.group(1)
+            entry_value = (
+                match.group(2) if match.group(2) is not None else int(match.group(3))
+            )
+            table[entry_key] = entry_value
+            continue
+
+        raise Exception("unexpected line: '{}' in {}".format(line, filepath))
+
+    assert len(parents) == 0, "Not all tables closed"
+    assert len(table) == 1, "More than one root table"
+
+    # Remove the two top-level "numberingSystems" tables.
+    (_, numbering_systems) = table.popitem()
+    (_, numbering_systems) = numbering_systems.popitem()
+
+    # Assert all numbering systems use base 10.
+    assert all(ns["radix"] == 10 for ns in numbering_systems.values())
+
+    # Return the numbering systems.
+    return {
+        key: {"digits": value["desc"], "algorithmic": False}
+        if not bool(value["algorithmic"])
+        else {"algorithmic": True}
+        for (key, value) in numbering_systems.items()
+    }
+
+
+def writeNumberingSystemFiles(numbering_systems):
+    js_src_builtin_intl_dir = os.path.dirname(os.path.abspath(__file__))
+
+    numbering_systems_js_file = os.path.join(
+        js_src_builtin_intl_dir, "NumberingSystemsGenerated.h"
+    )
+    with io.open(
+        numbering_systems_js_file, mode="w", encoding="utf-8", newline=""
+    ) as f:
+        println = partial(print, file=f)
+
+        println(generatedFileWarning)
+
+        println(
+            """
+/**
+ * The list of numbering systems with simple digit mappings.
+ */
+
+#ifndef builtin_intl_NumberingSystemsGenerated_h
+#define builtin_intl_NumberingSystemsGenerated_h
+"""
+        )
+
+        simple_numbering_systems = sorted(
+            name
+            for (name, value) in numbering_systems.items()
+            if not value["algorithmic"]
+        )
+
+        println("// clang-format off")
+        println("#define NUMBERING_SYSTEMS_WITH_SIMPLE_DIGIT_MAPPINGS \\")
+        println(
+            "{}".format(
+                ", \\\n".join(
+                    '  "{}"'.format(name) for name in simple_numbering_systems
+                )
+            )
+        )
+        println("// clang-format on")
+        println("")
+
+        println("#endif  // builtin_intl_NumberingSystemsGenerated_h")
+
+    js_src_builtin_intl_dir = os.path.dirname(os.path.abspath(__file__))
+    test_dir = os.path.join(js_src_builtin_intl_dir, "../../tests/non262/Intl")
+
+    intl_shell_js_file = os.path.join(test_dir, "shell.js")
+
+    with io.open(intl_shell_js_file, mode="w", encoding="utf-8", newline="") as f:
+        println = partial(print, file=f)
+
+        println(generatedFileWarning)
+
+        println(
+            """
+// source: CLDR file common/bcp47/number.xml; version CLDR {}.
+// https://github.com/unicode-org/cldr/blob/master/common/bcp47/number.xml
+// https://github.com/unicode-org/cldr/blob/master/common/supplemental/numberingSystems.xml
+""".format(
+                readCLDRVersionFromICU()
+            ).rstrip()
+        )
+
+        numbering_systems_object = json.dumps(
+            numbering_systems,
+            indent=2,
+            separators=(",", ": "),
+            sort_keys=True,
+            ensure_ascii=False,
+        )
+        println("const numberingSystems = {};".format(numbering_systems_object))
+
+
+def updateNumberingSystems(topsrcdir, args):
+    js_src_builtin_intl_dir = os.path.dirname(os.path.abspath(__file__))
+    icu_path = os.path.join(topsrcdir, "intl", "icu")
+    icu_misc_path = os.path.join(icu_path, "source", "data", "misc")
+
+    with io.open(
+        os.path.join(js_src_builtin_intl_dir, "NumberingSystems.yaml"),
+        mode="r",
+        encoding="utf-8",
+    ) as f:
+        numbering_systems = yaml.safe_load(f)
+
+    # Read all possible ICU unit identifiers from the "misc/numberingSystems.txt" resource.
+    misc_ns_file = os.path.join(icu_misc_path, "numberingSystems.txt")
+    all_numbering_systems = readICUNumberingSystemsResourceFile(misc_ns_file)
+
+    all_numbering_systems_simple_digits = {
+        name
+        for (name, value) in all_numbering_systems.items()
+        if not value["algorithmic"]
+    }
+
+    # Assert ICU includes support for all required numbering systems. If this assertion fails,
+    # something is broken in ICU.
+    assert all_numbering_systems_simple_digits.issuperset(
+        numbering_systems
+    ), "{}".format(numbering_systems.difference(all_numbering_systems_simple_digits))
+
+    # Assert the spec requires support for all numbering systems with simple digit mappings. If
+    # this assertion fails, file a PR at <https://github.com/tc39/ecma402> to include any new
+    # numbering systems.
+    assert all_numbering_systems_simple_digits.issubset(numbering_systems), "{}".format(
+        all_numbering_systems_simple_digits.difference(numbering_systems)
+    )
+
+    writeNumberingSystemFiles(all_numbering_systems)
+
+
+if __name__ == "__main__":
+    import argparse
+
+    # This script must reside in js/src/builtin/intl to work correctly.
+    (thisDir, thisFile) = os.path.split(os.path.abspath(__file__))
+    dirPaths = os.path.normpath(thisDir).split(os.sep)
+    if "/".join(dirPaths[-4:]) != "js/src/builtin/intl":
+        raise RuntimeError("%s must reside in js/src/builtin/intl" % __file__)
+    topsrcdir = "/".join(dirPaths[:-4])
+
+    def EnsureHttps(v):
+        if not v.startswith("https:"):
+            raise argparse.ArgumentTypeError("URL protocol must be https: " % v)
+        return v
+
+    parser = argparse.ArgumentParser(description="Update intl data.")
+    subparsers = parser.add_subparsers(help="Select update mode")
+
+    parser_cldr_tags = subparsers.add_parser(
+        "langtags", help="Update CLDR language tags data"
+    )
+    parser_cldr_tags.add_argument(
+        "--version", metavar="VERSION", help="CLDR version number"
+    )
+    parser_cldr_tags.add_argument(
+        "--url",
+        metavar="URL",
+        default="https://unicode.org/Public/cldr/<VERSION>/core.zip",
+        type=EnsureHttps,
+        help="Download url CLDR data (default: %(default)s)",
+    )
+    parser_cldr_tags.add_argument(
+        "--out",
+        default=os.path.join(
+            topsrcdir, "intl", "components", "src", "LocaleGenerated.cpp"
+        ),
+        help="Output file (default: %(default)s)",
+    )
+    parser_cldr_tags.add_argument(
+        "file", nargs="?", help="Local cldr-core.zip file, if omitted uses <URL>"
+    )
+    parser_cldr_tags.set_defaults(func=updateCLDRLangTags)
+
+    parser_tz = subparsers.add_parser("tzdata", help="Update tzdata")
+    parser_tz.add_argument(
+        "--tz",
+        help="Local tzdata directory or file, if omitted downloads tzdata "
+        "distribution from https://www.iana.org/time-zones/",
+    )
+    # ICU doesn't include the backzone file by default, but we still like to
+    # use the backzone time zone names to avoid user confusion. This does lead
+    # to formatting "historic" dates (pre-1970 era) with the wrong time zone,
+    # but that's probably acceptable for now.
+    parser_tz.add_argument(
+        "--ignore-backzone",
+        action="store_true",
+        help="Ignore tzdata's 'backzone' file. Can be enabled to generate more "
+        "accurate time zone canonicalization reflecting the actual time "
+        "zones as used by ICU.",
+    )
+    parser_tz.add_argument(
+        "--out",
+        default=os.path.join(thisDir, "TimeZoneDataGenerated.h"),
+        help="Output file (default: %(default)s)",
+    )
+    parser_tz.set_defaults(func=partial(updateTzdata, topsrcdir))
+
+    parser_currency = subparsers.add_parser(
+        "currency", help="Update currency digits mapping"
+    )
+    parser_currency.add_argument(
+        "--url",
+        metavar="URL",
+        default="https://www.currency-iso.org/dam/downloads/lists/list_one.xml",  # NOQA: E501
+        type=EnsureHttps,
+        help="Download url for the currency & funds code list (default: "
+        "%(default)s)",
+    )
+    parser_currency.add_argument(
+        "--out",
+        default=os.path.join(thisDir, "CurrencyDataGenerated.js"),
+        help="Output file (default: %(default)s)",
+    )
+    parser_currency.add_argument(
+        "file", nargs="?", help="Local currency code list file, if omitted uses <URL>"
+    )
+    parser_currency.set_defaults(func=partial(updateCurrency, topsrcdir))
+
+    parser_units = subparsers.add_parser(
+        "units", help="Update sanctioned unit identifiers mapping"
+    )
+    parser_units.set_defaults(func=partial(updateUnits, topsrcdir))
+
+    parser_numbering_systems = subparsers.add_parser(
+        "numbering", help="Update numbering systems with simple digit mappings"
+    )
+    parser_numbering_systems.set_defaults(
+        func=partial(updateNumberingSystems, topsrcdir)
+    )
+
+    args = parser.parse_args()
+    args.func(args)