Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

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

1 files changed, 2168 insertions, 0 deletions

diff --git a/js/src/vm/Iteration.cpp b/js/src/vm/Iteration.cpp
new file mode 100644
index 0000000000..a7d7287770
--- /dev/null
+++ b/js/src/vm/Iteration.cpp
@@ -0,0 +1,2168 @@
+/* -*- 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/. */
+
+/* JavaScript iterators. */
+
+#include "vm/Iteration.h"
+
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Likely.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/PodOperations.h"
+
+#include <algorithm>
+#include <new>
+
+#include "jsapi.h"
+#include "jstypes.h"
+
+#include "builtin/Array.h"
+#include "builtin/SelfHostingDefines.h"
+#include "ds/Sort.h"
+#include "gc/GCContext.h"
+#include "js/ForOfIterator.h"         // JS::ForOfIterator
+#include "js/friend/ErrorMessages.h"  // js::GetErrorMessage, JSMSG_*
+#include "js/PropertySpec.h"
+#include "util/DifferentialTesting.h"
+#include "util/Poison.h"
+#include "vm/GlobalObject.h"
+#include "vm/Interpreter.h"
+#include "vm/JSContext.h"
+#include "vm/JSObject.h"
+#include "vm/NativeObject.h"  // js::PlainObject
+#include "vm/Shape.h"
+#include "vm/StringType.h"
+#include "vm/TypedArrayObject.h"
+#include "vm/WellKnownAtom.h"  // js_*_str
+
+#ifdef ENABLE_RECORD_TUPLE
+#  include "builtin/RecordObject.h"
+#  include "builtin/TupleObject.h"
+#endif
+
+#include "vm/NativeObject-inl.h"
+#include "vm/PlainObject-inl.h"  // js::PlainObject::createWithTemplate
+
+using namespace js;
+
+using mozilla::ArrayEqual;
+using mozilla::DebugOnly;
+using mozilla::Maybe;
+using mozilla::PodCopy;
+
+using RootedPropertyIteratorObject = Rooted<PropertyIteratorObject*>;
+
+static const gc::AllocKind ITERATOR_FINALIZE_KIND =
+    gc::AllocKind::OBJECT2_BACKGROUND;
+
+// Beware!  This function may have to trace incompletely-initialized
+// |NativeIterator| allocations if the |IdToString| in that constructor recurs
+// into this code.
+void NativeIterator::trace(JSTracer* trc) {
+  TraceNullableEdge(trc, &objectBeingIterated_, "objectBeingIterated_");
+  TraceNullableEdge(trc, &iterObj_, "iterObj");
+
+  // The limits below are correct at every instant of |NativeIterator|
+  // initialization, with the end-pointer incremented as each new shape is
+  // created, so they're safe to use here.
+  std::for_each(shapesBegin(), shapesEnd(), [trc](GCPtr<Shape*>& shape) {
+    TraceEdge(trc, &shape, "iterator_shape");
+  });
+
+  // But as properties must be created *before* shapes, |propertiesBegin()|
+  // that depends on |shapesEnd()| having its final value can't safely be
+  // used.  Until this is fully initialized, use |propertyCursor_| instead,
+  // which points at the start of properties even in partially initialized
+  // |NativeIterator|s.  (|propertiesEnd()| is safe at all times with respect
+  // to the properly-chosen beginning.)
+  //
+  // Note that we must trace all properties (not just those not yet visited,
+  // or just visited, due to |NativeIterator::previousPropertyWas|) for
+  // |NativeIterator|s to be reusable.
+  GCPtr<JSLinearString*>* begin =
+      MOZ_LIKELY(isInitialized()) ? propertiesBegin() : propertyCursor_;
+  std::for_each(begin, propertiesEnd(), [trc](GCPtr<JSLinearString*>& prop) {
+    // Properties begin life non-null and never *become*
+    // null.  (Deletion-suppression will shift trailing
+    // properties over a deleted property in the properties
+    // array, but it doesn't null them out.)
+    TraceEdge(trc, &prop, "prop");
+  });
+}
+
+using PropertyKeySet = GCHashSet<PropertyKey, DefaultHasher<PropertyKey>>;
+
+class PropertyEnumerator {
+  RootedObject obj_;
+  MutableHandleIdVector props_;
+  PropertyIndexVector* indices_;
+
+  uint32_t flags_;
+  Rooted<PropertyKeySet> visited_;
+
+  bool enumeratingProtoChain_ = false;
+
+  enum class IndicesState {
+    // Every property that has been enumerated so far can be represented as a
+    // PropertyIndex, but we are not currently producing a list of indices. If
+    // the state is Valid when we are done enumerating, then the resulting
+    // iterator can be marked as NativeIteratorIndices::AvailableOnRequest.
+    Valid,
+
+    // Every property that has been enumerated so far can be represented as a
+    // PropertyIndex, and |indices_| points to a PropertyIndexVector containing
+    // those indices. This is used when we want to create a NativeIterator with
+    // valid indices.
+    Allocating,
+
+    // It is not possible to represent every property of the object being
+    // enumerated as a PropertyIndex. For example, enumerated properties on the
+    // prototype chain are unsupported. We can transition to this state from
+    // either of the other two.
+    Unsupported
+  };
+  IndicesState indicesState_;
+
+ public:
+  PropertyEnumerator(JSContext* cx, JSObject* obj, uint32_t flags,
+                     MutableHandleIdVector props,
+                     PropertyIndexVector* indices = nullptr)
+      : obj_(cx, obj),
+        props_(props),
+        indices_(indices),
+        flags_(flags),
+        visited_(cx, PropertyKeySet(cx)),
+        indicesState_(indices ? IndicesState::Allocating
+                              : IndicesState::Valid) {}
+
+  bool snapshot(JSContext* cx);
+
+  void markIndicesUnsupported() { indicesState_ = IndicesState::Unsupported; }
+  bool supportsIndices() const {
+    return indicesState_ != IndicesState::Unsupported;
+  }
+  bool allocatingIndices() const {
+    return indicesState_ == IndicesState::Allocating;
+  }
+
+ private:
+  template <bool CheckForDuplicates>
+  bool enumerate(JSContext* cx, jsid id, bool enumerable,
+                 PropertyIndex index = PropertyIndex::Invalid());
+
+  bool enumerateExtraProperties(JSContext* cx);
+
+  template <bool CheckForDuplicates>
+  bool enumerateNativeProperties(JSContext* cx);
+
+  bool enumerateNativeProperties(JSContext* cx, bool checkForDuplicates) {
+    if (checkForDuplicates) {
+      return enumerateNativeProperties<true>(cx);
+    }
+    return enumerateNativeProperties<false>(cx);
+  }
+
+  template <bool CheckForDuplicates>
+  bool enumerateProxyProperties(JSContext* cx);
+
+  void reversePropsAndIndicesAfter(size_t initialLength) {
+    // We iterate through prop maps in descending order of property creation,
+    // but we need our return value to be in ascending order. If we are tracking
+    // property indices, make sure to keep them in sync.
+    MOZ_ASSERT(props_.begin() + initialLength <= props_.end());
+    MOZ_ASSERT_IF(allocatingIndices(), props_.length() == indices_->length());
+
+    std::reverse(props_.begin() + initialLength, props_.end());
+    if (allocatingIndices()) {
+      std::reverse(indices_->begin() + initialLength, indices_->end());
+    }
+  }
+};
+
+template <bool CheckForDuplicates>
+bool PropertyEnumerator::enumerate(JSContext* cx, jsid id, bool enumerable,
+                                   PropertyIndex index) {
+  if (CheckForDuplicates) {
+    // If we've already seen this, we definitely won't add it.
+    PropertyKeySet::AddPtr p = visited_.lookupForAdd(id);
+    if (MOZ_UNLIKELY(!!p)) {
+      return true;
+    }
+
+    // It's not necessary to add properties to the hash set at the end of
+    // the prototype chain, but custom enumeration behaviors might return
+    // duplicated properties, so always add in such cases.
+    if (obj_->is<ProxyObject>() || obj_->staticPrototype() ||
+        obj_->getClass()->getNewEnumerate()) {
+      if (!visited_.add(p, id)) {
+        return false;
+      }
+    }
+  }
+
+  if (!enumerable && !(flags_ & JSITER_HIDDEN)) {
+    return true;
+  }
+
+  // Symbol-keyed properties and nonenumerable properties are skipped unless
+  // the caller specifically asks for them. A caller can also filter out
+  // non-symbols by asking for JSITER_SYMBOLSONLY. PrivateName symbols are
+  // skipped unless JSITER_PRIVATE is passed.
+  if (id.isSymbol()) {
+    if (!(flags_ & JSITER_SYMBOLS)) {
+      return true;
+    }
+    if (!(flags_ & JSITER_PRIVATE) && id.isPrivateName()) {
+      return true;
+    }
+  } else {
+    if ((flags_ & JSITER_SYMBOLSONLY)) {
+      return true;
+    }
+  }
+
+  MOZ_ASSERT_IF(allocatingIndices(), indices_->length() == props_.length());
+  if (!props_.append(id)) {
+    return false;
+  }
+
+  if (!supportsIndices()) {
+    return true;
+  }
+  if (index.kind() == PropertyIndex::Kind::Invalid || enumeratingProtoChain_) {
+    markIndicesUnsupported();
+    return true;
+  }
+
+  if (allocatingIndices() && !indices_->append(index)) {
+    return false;
+  }
+
+  return true;
+}
+
+bool PropertyEnumerator::enumerateExtraProperties(JSContext* cx) {
+  MOZ_ASSERT(obj_->getClass()->getNewEnumerate());
+
+  RootedIdVector properties(cx);
+  bool enumerableOnly = !(flags_ & JSITER_HIDDEN);
+  if (!obj_->getClass()->getNewEnumerate()(cx, obj_, &properties,
+                                           enumerableOnly)) {
+    return false;
+  }
+
+  RootedId id(cx);
+  for (size_t n = 0; n < properties.length(); n++) {
+    id = properties[n];
+
+    // The enumerate hook does not indicate whether the properties
+    // it returns are enumerable or not. Since we already passed
+    // `enumerableOnly` to the hook to filter out non-enumerable
+    // properties, it doesn't really matter what we pass here.
+    bool enumerable = true;
+    if (!enumerate<true>(cx, id, enumerable)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+static bool SortComparatorIntegerIds(jsid a, jsid b, bool* lessOrEqualp) {
+  uint32_t indexA, indexB;
+  MOZ_ALWAYS_TRUE(IdIsIndex(a, &indexA));
+  MOZ_ALWAYS_TRUE(IdIsIndex(b, &indexB));
+  *lessOrEqualp = (indexA <= indexB);
+  return true;
+}
+
+template <bool CheckForDuplicates>
+bool PropertyEnumerator::enumerateNativeProperties(JSContext* cx) {
+  Handle<NativeObject*> pobj = obj_.as<NativeObject>();
+
+  // We don't need to iterate over the shape's properties if we're only
+  // interested in enumerable properties and the object is known to have no
+  // enumerable properties.
+  //
+  // Don't optimize if CheckForDuplicates is true, because non-enumerable
+  // properties still have to participate in duplicate-property checking.
+  const bool iterShapeProperties = CheckForDuplicates ||
+                                   (flags_ & JSITER_HIDDEN) ||
+                                   pobj->hasEnumerableProperty();
+
+  bool enumerateSymbols;
+  if (flags_ & JSITER_SYMBOLSONLY) {
+    if (!iterShapeProperties) {
+      return true;
+    }
+    enumerateSymbols = true;
+  } else {
+    // Collect any dense elements from this object.
+    size_t firstElemIndex = props_.length();
+    size_t initlen = pobj->getDenseInitializedLength();
+    const Value* elements = pobj->getDenseElements();
+    bool hasHoles = false;
+    for (uint32_t i = 0; i < initlen; ++i) {
+      if (elements[i].isMagic(JS_ELEMENTS_HOLE)) {
+        hasHoles = true;
+      } else {
+        // Dense arrays never get so large that i would not fit into an
+        // integer id.
+        if (!enumerate<CheckForDuplicates>(cx, PropertyKey::Int(i),
+                                           /* enumerable = */ true,
+                                           PropertyIndex::ForElement(i))) {
+          return false;
+        }
+      }
+    }
+
+    // Collect any typed array or shared typed array elements from this
+    // object.
+    if (pobj->is<TypedArrayObject>()) {
+      size_t len = pobj->as<TypedArrayObject>().length();
+
+      // Fail early if the typed array is enormous, because this will be very
+      // slow and will likely report OOM. This also means we don't need to
+      // handle indices greater than PropertyKey::IntMax in the loop below.
+      static_assert(PropertyKey::IntMax == INT32_MAX);
+      if (len > INT32_MAX) {
+        ReportOutOfMemory(cx);
+        return false;
+      }
+
+      for (uint32_t i = 0; i < len; i++) {
+        if (!enumerate<CheckForDuplicates>(cx, PropertyKey::Int(i),
+                                           /* enumerable = */ true)) {
+          return false;
+        }
+      }
+    }
+#ifdef ENABLE_RECORD_TUPLE
+    else {
+      Rooted<RecordType*> rec(cx);
+      if (RecordObject::maybeUnbox(pobj, &rec)) {
+        Rooted<ArrayObject*> keys(cx, rec->keys());
+
+        for (size_t i = 0; i < keys->length(); i++) {
+          JSAtom* key = &keys->getDenseElement(i).toString()->asAtom();
+          PropertyKey id = AtomToId(key);
+          if (!enumerate<CheckForDuplicates>(cx, id,
+                                             /* enumerable = */ true)) {
+            return false;
+          }
+        }
+
+        return true;
+      } else {
+        mozilla::Maybe<TupleType&> tup = TupleObject::maybeUnbox(pobj);
+        if (tup) {
+          uint32_t len = tup->length();
+
+          for (size_t i = 0; i < len; i++) {
+            // We expect tuple indices not to get so large that `i` won't
+            // fit into an `int32_t`.
+            MOZ_ASSERT(PropertyKey::fitsInInt(i));
+            PropertyKey id = PropertyKey::Int(i);
+            if (!enumerate<CheckForDuplicates>(cx, id,
+                                               /* enumerable = */ true)) {
+              return false;
+            }
+          }
+
+          return true;
+        }
+      }
+    }
+#endif
+
+    // The code below enumerates shape properties (including sparse elements) so
+    // if we can ignore those we're done.
+    if (!iterShapeProperties) {
+      return true;
+    }
+
+    // Collect any sparse elements from this object.
+    bool isIndexed = pobj->isIndexed();
+    if (isIndexed) {
+      // If the dense elements didn't have holes, we don't need to include
+      // them in the sort.
+      if (!hasHoles) {
+        firstElemIndex = props_.length();
+      }
+
+      for (ShapePropertyIter<NoGC> iter(pobj->shape()); !iter.done(); iter++) {
+        jsid id = iter->key();
+        uint32_t dummy;
+        if (IdIsIndex(id, &dummy)) {
+          if (!enumerate<CheckForDuplicates>(cx, id, iter->enumerable())) {
+            return false;
+          }
+        }
+      }
+
+      MOZ_ASSERT(firstElemIndex <= props_.length());
+
+      jsid* ids = props_.begin() + firstElemIndex;
+      size_t n = props_.length() - firstElemIndex;
+
+      RootedIdVector tmp(cx);
+      if (!tmp.resize(n)) {
+        return false;
+      }
+      PodCopy(tmp.begin(), ids, n);
+
+      if (!MergeSort(ids, n, tmp.begin(), SortComparatorIntegerIds)) {
+        return false;
+      }
+    }
+
+    size_t initialLength = props_.length();
+
+    /* Collect all unique property names from this object's shape. */
+    bool symbolsFound = false;
+    for (ShapePropertyIter<NoGC> iter(pobj->shape()); !iter.done(); iter++) {
+      jsid id = iter->key();
+
+      if (id.isSymbol()) {
+        symbolsFound = true;
+        continue;
+      }
+
+      uint32_t dummy;
+      if (isIndexed && IdIsIndex(id, &dummy)) {
+        continue;
+      }
+
+      PropertyIndex index = iter->isDataProperty()
+                                ? PropertyIndex::ForSlot(pobj, iter->slot())
+                                : PropertyIndex::Invalid();
+      if (!enumerate<CheckForDuplicates>(cx, id, iter->enumerable(), index)) {
+        return false;
+      }
+    }
+    reversePropsAndIndicesAfter(initialLength);
+
+    enumerateSymbols = symbolsFound && (flags_ & JSITER_SYMBOLS);
+  }
+
+  if (enumerateSymbols) {
+    MOZ_ASSERT(iterShapeProperties);
+    MOZ_ASSERT(!allocatingIndices());
+
+    // Do a second pass to collect symbols. The spec requires that all symbols
+    // appear after all strings in [[OwnPropertyKeys]] for ordinary objects:
+    // https://tc39.es/ecma262/#sec-ordinaryownpropertykeys
+    size_t initialLength = props_.length();
+    for (ShapePropertyIter<NoGC> iter(pobj->shape()); !iter.done(); iter++) {
+      jsid id = iter->key();
+      if (id.isSymbol()) {
+        if (!enumerate<CheckForDuplicates>(cx, id, iter->enumerable())) {
+          return false;
+        }
+      }
+    }
+    reversePropsAndIndicesAfter(initialLength);
+  }
+
+  return true;
+}
+
+template <bool CheckForDuplicates>
+bool PropertyEnumerator::enumerateProxyProperties(JSContext* cx) {
+  MOZ_ASSERT(obj_->is<ProxyObject>());
+
+  RootedIdVector proxyProps(cx);
+
+  if (flags_ & JSITER_HIDDEN || flags_ & JSITER_SYMBOLS) {
+    // This gets all property keys, both strings and symbols. The call to
+    // enumerate in the loop below will filter out unwanted keys, per the
+    // flags.
+    if (!Proxy::ownPropertyKeys(cx, obj_, &proxyProps)) {
+      return false;
+    }
+
+    Rooted<mozilla::Maybe<PropertyDescriptor>> desc(cx);
+    for (size_t n = 0, len = proxyProps.length(); n < len; n++) {
+      bool enumerable = false;
+
+      // We need to filter, if the caller just wants enumerable symbols.
+      if (!(flags_ & JSITER_HIDDEN)) {
+        if (!Proxy::getOwnPropertyDescriptor(cx, obj_, proxyProps[n], &desc)) {
+          return false;
+        }
+        enumerable = desc.isSome() && desc->enumerable();
+      }
+
+      if (!enumerate<CheckForDuplicates>(cx, proxyProps[n], enumerable)) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  // Returns enumerable property names (no symbols).
+  if (!Proxy::getOwnEnumerablePropertyKeys(cx, obj_, &proxyProps)) {
+    return false;
+  }
+
+  for (size_t n = 0, len = proxyProps.length(); n < len; n++) {
+    if (!enumerate<CheckForDuplicates>(cx, proxyProps[n], true)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+#ifdef DEBUG
+
+struct SortComparatorIds {
+  JSContext* const cx;
+
+  explicit SortComparatorIds(JSContext* cx) : cx(cx) {}
+
+  bool operator()(jsid aArg, jsid bArg, bool* lessOrEqualp) {
+    RootedId a(cx, aArg);
+    RootedId b(cx, bArg);
+
+    // Pick an arbitrary order on jsids that is as stable as possible
+    // across executions.
+    if (a == b) {
+      *lessOrEqualp = true;
+      return true;
+    }
+
+    enum class KeyType { Void, Int, String, Symbol };
+
+    auto keyType = [](PropertyKey key) {
+      if (key.isString()) {
+        return KeyType::String;
+      }
+      if (key.isInt()) {
+        return KeyType::Int;
+      }
+      if (key.isSymbol()) {
+        return KeyType::Symbol;
+      }
+      MOZ_ASSERT(key.isVoid());
+      return KeyType::Void;
+    };
+
+    if (keyType(a) != keyType(b)) {
+      *lessOrEqualp = (keyType(a) <= keyType(b));
+      return true;
+    }
+
+    if (a.isInt()) {
+      *lessOrEqualp = (a.toInt() <= b.toInt());
+      return true;
+    }
+
+    RootedString astr(cx), bstr(cx);
+    if (a.isSymbol()) {
+      MOZ_ASSERT(b.isSymbol());
+      JS::SymbolCode ca = a.toSymbol()->code();
+      JS::SymbolCode cb = b.toSymbol()->code();
+      if (ca != cb) {
+        *lessOrEqualp = uint32_t(ca) <= uint32_t(cb);
+        return true;
+      }
+      MOZ_ASSERT(ca == JS::SymbolCode::PrivateNameSymbol ||
+                 ca == JS::SymbolCode::InSymbolRegistry ||
+                 ca == JS::SymbolCode::UniqueSymbol);
+      astr = a.toSymbol()->description();
+      bstr = b.toSymbol()->description();
+      if (!astr || !bstr) {
+        *lessOrEqualp = !astr;
+        return true;
+      }
+
+      // Fall through to string comparison on the descriptions. The sort
+      // order is nondeterministic if two different unique symbols have
+      // the same description.
+    } else {
+      astr = IdToString(cx, a);
+      if (!astr) {
+        return false;
+      }
+      bstr = IdToString(cx, b);
+      if (!bstr) {
+        return false;
+      }
+    }
+
+    int32_t result;
+    if (!CompareStrings(cx, astr, bstr, &result)) {
+      return false;
+    }
+
+    *lessOrEqualp = (result <= 0);
+    return true;
+  }
+};
+
+#endif /* DEBUG */
+
+static void AssertNoEnumerableProperties(NativeObject* obj) {
+#ifdef DEBUG
+  // Verify the object has no enumerable properties if the HasEnumerable
+  // ObjectFlag is not set.
+
+  MOZ_ASSERT(!obj->hasEnumerableProperty());
+
+  static constexpr size_t MaxPropsToCheck = 5;
+
+  size_t count = 0;
+  for (ShapePropertyIter<NoGC> iter(obj->shape()); !iter.done(); iter++) {
+    MOZ_ASSERT(!iter->enumerable());
+    if (++count > MaxPropsToCheck) {
+      break;
+    }
+  }
+#endif  // DEBUG
+}
+
+// Typed arrays and classes with an enumerate hook can have extra properties not
+// included in the shape's property map or the object's dense elements.
+static bool ClassCanHaveExtraEnumeratedProperties(const JSClass* clasp) {
+  return IsTypedArrayClass(clasp) || clasp->getNewEnumerate() ||
+         clasp->getEnumerate();
+}
+
+static bool ProtoMayHaveEnumerableProperties(JSObject* obj) {
+  if (!obj->is<NativeObject>()) {
+    return true;
+  }
+
+  JSObject* proto = obj->as<NativeObject>().staticPrototype();
+  while (proto) {
+    if (!proto->is<NativeObject>()) {
+      return true;
+    }
+    NativeObject* nproto = &proto->as<NativeObject>();
+    if (nproto->hasEnumerableProperty() ||
+        nproto->getDenseInitializedLength() > 0 ||
+        ClassCanHaveExtraEnumeratedProperties(nproto->getClass())) {
+      return true;
+    }
+    AssertNoEnumerableProperties(nproto);
+    proto = nproto->staticPrototype();
+  }
+
+  return false;
+}
+
+bool PropertyEnumerator::snapshot(JSContext* cx) {
+  // If we're only interested in enumerable properties and the proto chain has
+  // no enumerable properties (the common case), we can optimize this to ignore
+  // the proto chain. This also lets us take advantage of the no-duplicate-check
+  // optimization below.
+  if (!(flags_ & JSITER_HIDDEN) && !(flags_ & JSITER_OWNONLY) &&
+      !ProtoMayHaveEnumerableProperties(obj_)) {
+    flags_ |= JSITER_OWNONLY;
+  }
+
+  // Don't check for duplicates if we're only interested in own properties.
+  // This does the right thing for most objects: native objects don't have
+  // duplicate property ids and we allow the [[OwnPropertyKeys]] proxy trap to
+  // return duplicates.
+  //
+  // The only special case is when the object has a newEnumerate hook: it
+  // can return duplicate properties and we have to filter them. This is
+  // handled below.
+  bool checkForDuplicates = !(flags_ & JSITER_OWNONLY);
+
+  do {
+    if (obj_->getClass()->getNewEnumerate()) {
+      markIndicesUnsupported();
+
+      if (!enumerateExtraProperties(cx)) {
+        return false;
+      }
+
+      if (obj_->is<NativeObject>()) {
+        if (!enumerateNativeProperties(cx, /*checkForDuplicates*/ true)) {
+          return false;
+        }
+      }
+
+    } else if (obj_->is<NativeObject>()) {
+      // Give the object a chance to resolve all lazy properties
+      if (JSEnumerateOp enumerateOp = obj_->getClass()->getEnumerate()) {
+        markIndicesUnsupported();
+        if (!enumerateOp(cx, obj_.as<NativeObject>())) {
+          return false;
+        }
+      }
+      if (!enumerateNativeProperties(cx, checkForDuplicates)) {
+        return false;
+      }
+    } else if (obj_->is<ProxyObject>()) {
+      markIndicesUnsupported();
+      if (checkForDuplicates) {
+        if (!enumerateProxyProperties<true>(cx)) {
+          return false;
+        }
+      } else {
+        if (!enumerateProxyProperties<false>(cx)) {
+          return false;
+        }
+      }
+    } else {
+      MOZ_CRASH("non-native objects must have an enumerate op");
+    }
+
+    if (flags_ & JSITER_OWNONLY) {
+      break;
+    }
+
+    if (!GetPrototype(cx, obj_, &obj_)) {
+      return false;
+    }
+    enumeratingProtoChain_ = true;
+
+    // The [[Prototype]] chain might be cyclic.
+    if (!CheckForInterrupt(cx)) {
+      return false;
+    }
+  } while (obj_ != nullptr);
+
+#ifdef DEBUG
+  if (js::SupportDifferentialTesting() && !supportsIndices()) {
+    /*
+     * In some cases the enumeration order for an object depends on the
+     * execution mode (interpreter vs. JIT), especially for native objects
+     * with a class enumerate hook (where resolving a property changes the
+     * resulting enumeration order). These aren't really bugs, but the
+     * differences can change the generated output and confuse correctness
+     * fuzzers, so we sort the ids if such a fuzzer is running.
+     *
+     * We don't do this in the general case because (a) doing so is slow,
+     * and (b) it also breaks the web, which expects enumeration order to
+     * follow the order in which properties are added, in certain cases.
+     * Since ECMA does not specify an enumeration order for objects, both
+     * behaviors are technically correct to do.
+     */
+
+    jsid* ids = props_.begin();
+    size_t n = props_.length();
+
+    RootedIdVector tmp(cx);
+    if (!tmp.resize(n)) {
+      return false;
+    }
+    PodCopy(tmp.begin(), ids, n);
+
+    if (!MergeSort(ids, n, tmp.begin(), SortComparatorIds(cx))) {
+      return false;
+    }
+  }
+#endif
+
+  return true;
+}
+
+JS_PUBLIC_API bool js::GetPropertyKeys(JSContext* cx, HandleObject obj,
+                                       unsigned flags,
+                                       MutableHandleIdVector props) {
+  uint32_t validFlags =
+      flags & (JSITER_OWNONLY | JSITER_HIDDEN | JSITER_SYMBOLS |
+               JSITER_SYMBOLSONLY | JSITER_PRIVATE);
+
+  PropertyEnumerator enumerator(cx, obj, validFlags, props);
+  return enumerator.snapshot(cx);
+}
+
+static inline void RegisterEnumerator(JSContext* cx, NativeIterator* ni) {
+  MOZ_ASSERT(ni->objectBeingIterated());
+
+  // Register non-escaping native enumerators (for-in) with the current
+  // context.
+  ni->link(cx->compartment()->enumeratorsAddr());
+
+  MOZ_ASSERT(!ni->isActive());
+  ni->markActive();
+}
+
+static PropertyIteratorObject* NewPropertyIteratorObject(JSContext* cx) {
+  const JSClass* clasp = &PropertyIteratorObject::class_;
+  Rooted<SharedShape*> shape(
+      cx,
+      SharedShape::getInitialShape(cx, clasp, cx->realm(), TaggedProto(nullptr),
+                                   ITERATOR_FINALIZE_KIND));
+  if (!shape) {
+    return nullptr;
+  }
+
+  JSObject* obj = NativeObject::create(
+      cx, ITERATOR_FINALIZE_KIND, GetInitialHeap(GenericObject, clasp), shape);
+  if (!obj) {
+    return nullptr;
+  }
+
+  PropertyIteratorObject* res = &obj->as<PropertyIteratorObject>();
+
+  // CodeGenerator::visitIteratorStartO assumes the iterator object is not
+  // inside the nursery when deciding whether a barrier is necessary.
+  MOZ_ASSERT(!js::gc::IsInsideNursery(res));
+  return res;
+}
+
+static inline size_t NumTrailingBytes(size_t propertyCount, size_t shapeCount,
+                                      bool hasIndices) {
+  static_assert(alignof(GCPtr<JSLinearString*>) <= alignof(NativeIterator));
+  static_assert(alignof(GCPtr<Shape*>) <= alignof(GCPtr<JSLinearString*>));
+  static_assert(alignof(PropertyIndex) <= alignof(GCPtr<Shape*>));
+  size_t result = propertyCount * sizeof(GCPtr<JSLinearString*>) +
+                  shapeCount * sizeof(GCPtr<Shape*>);
+  if (hasIndices) {
+    result += propertyCount * sizeof(PropertyIndex);
+  }
+  return result;
+}
+
+static inline size_t AllocationSize(size_t propertyCount, size_t shapeCount,
+                                    bool hasIndices) {
+  return sizeof(NativeIterator) +
+         NumTrailingBytes(propertyCount, shapeCount, hasIndices);
+}
+
+static PropertyIteratorObject* CreatePropertyIterator(
+    JSContext* cx, Handle<JSObject*> objBeingIterated, HandleIdVector props,
+    bool supportsIndices, PropertyIndexVector* indices,
+    uint32_t cacheableProtoChainLength) {
+  MOZ_ASSERT_IF(indices, supportsIndices);
+  if (props.length() > NativeIterator::PropCountLimit) {
+    ReportAllocationOverflow(cx);
+    return nullptr;
+  }
+
+  bool hasIndices = !!indices;
+
+  // If the iterator is cacheable, we store the shape of each object
+  // along the proto chain in the iterator. If the iterator is not
+  // cacheable, but has indices, then we store one shape (the shape of
+  // the object being iterated.)
+  uint32_t numShapes = cacheableProtoChainLength;
+  if (numShapes == 0 && hasIndices) {
+    numShapes = 1;
+  }
+
+  Rooted<PropertyIteratorObject*> propIter(cx, NewPropertyIteratorObject(cx));
+  if (!propIter) {
+    return nullptr;
+  }
+
+  void* mem = cx->pod_malloc_with_extra<NativeIterator, uint8_t>(
+      NumTrailingBytes(props.length(), numShapes, hasIndices));
+  if (!mem) {
+    return nullptr;
+  }
+
+  // This also registers |ni| with |propIter|.
+  bool hadError = false;
+  new (mem) NativeIterator(cx, propIter, objBeingIterated, props,
+                           supportsIndices, indices, numShapes, &hadError);
+  if (hadError) {
+    return nullptr;
+  }
+
+  return propIter;
+}
+
+static HashNumber HashIteratorShape(Shape* shape) {
+  return DefaultHasher<Shape*>::hash(shape);
+}
+
+/**
+ * Initialize a fresh NativeIterator.
+ *
+ * This definition is a bit tricky: some parts of initializing are fallible, so
+ * as we initialize, we must carefully keep this in GC-safe state (see
+ * NativeIterator::trace).
+ */
+NativeIterator::NativeIterator(JSContext* cx,
+                               Handle<PropertyIteratorObject*> propIter,
+                               Handle<JSObject*> objBeingIterated,
+                               HandleIdVector props, bool supportsIndices,
+                               PropertyIndexVector* indices, uint32_t numShapes,
+                               bool* hadError)
+    : objectBeingIterated_(objBeingIterated),
+      iterObj_(propIter),
+      // NativeIterator initially acts (before full initialization) as if it
+      // contains no shapes...
+      shapesEnd_(shapesBegin()),
+      // ...and no properties.
+      propertyCursor_(
+          reinterpret_cast<GCPtr<JSLinearString*>*>(shapesBegin() + numShapes)),
+      propertiesEnd_(propertyCursor_),
+      shapesHash_(0),
+      flagsAndCount_(
+          initialFlagsAndCount(props.length()))  // note: no Flags::Initialized
+{
+  // If there are shapes, the object and all objects on its prototype chain must
+  // be native objects. See CanCompareIterableObjectToCache.
+  MOZ_ASSERT_IF(numShapes > 0,
+                objBeingIterated && objBeingIterated->is<NativeObject>());
+
+  MOZ_ASSERT(!*hadError);
+
+  bool hasActualIndices = !!indices;
+  MOZ_ASSERT_IF(hasActualIndices, indices->length() == props.length());
+
+  // NOTE: This must be done first thing: The caller can't free `this` on error
+  //       because it has GCPtr fields whose barriers have already fired; the
+  //       store buffer has pointers to them. Only the GC can free `this` (via
+  //       PropertyIteratorObject::finalize).
+  propIter->initNativeIterator(this);
+
+  // The GC asserts on finalization that `this->allocationSize()` matches the
+  // `nbytes` passed to `AddCellMemory`. So once these lines run, we must make
+  // `this->allocationSize()` correct. That means infallibly initializing the
+  // shapes, and ensuring that indicesState_.allocated() is true if we've
+  // allocated space for indices. It's OK for the constructor to fail after
+  // that.
+  size_t nbytes = AllocationSize(props.length(), numShapes, hasActualIndices);
+  AddCellMemory(propIter, nbytes, MemoryUse::NativeIterator);
+  if (supportsIndices) {
+    if (hasActualIndices) {
+      // If the string allocation fails, indicesAllocated() must be true
+      // so that this->allocationSize() is correct. Set it to Disabled. It will
+      // be updated below.
+      setIndicesState(NativeIteratorIndices::Disabled);
+    } else {
+      // This object supports indices (ie it only has own enumerable
+      // properties), but we didn't allocate them because we haven't seen a
+      // consumer yet. We mark the iterator so that potential consumers know to
+      // request a fresh iterator with indices.
+      setIndicesState(NativeIteratorIndices::AvailableOnRequest);
+    }
+  }
+
+  if (numShapes > 0) {
+    // Construct shapes into the shapes array.  Also compute the shapesHash,
+    // which incorporates Shape* addresses that could have changed during a GC
+    // triggered in (among other places) |IdToString| above.
+    JSObject* pobj = objBeingIterated;
+    HashNumber shapesHash = 0;
+    for (uint32_t i = 0; i < numShapes; i++) {
+      MOZ_ASSERT(pobj->is<NativeObject>());
+      Shape* shape = pobj->shape();
+      new (shapesEnd_) GCPtr<Shape*>(shape);
+      shapesEnd_++;
+      shapesHash = mozilla::AddToHash(shapesHash, HashIteratorShape(shape));
+      pobj = pobj->staticPrototype();
+    }
+    shapesHash_ = shapesHash;
+
+    // There are two cases in which we need to store shapes. If this
+    // iterator is cacheable, we store the shapes for the entire proto
+    // chain so we can check that the cached iterator is still valid
+    // (see MacroAssembler::maybeLoadIteratorFromShape). If this iterator
+    // has indices, then even if it isn't cacheable we need to store the
+    // shape of the iterated object itself (see IteratorHasIndicesAndBranch).
+    // In the former case, assert that we're storing the entire proto chain.
+    MOZ_ASSERT_IF(numShapes > 1, pobj == nullptr);
+  }
+  MOZ_ASSERT(static_cast<void*>(shapesEnd_) == propertyCursor_);
+
+  size_t numProps = props.length();
+  for (size_t i = 0; i < numProps; i++) {
+    JSLinearString* str = IdToString(cx, props[i]);
+    if (!str) {
+      *hadError = true;
+      return;
+    }
+    new (propertiesEnd_) GCPtr<JSLinearString*>(str);
+    propertiesEnd_++;
+  }
+
+  if (hasActualIndices) {
+    PropertyIndex* cursor = indicesBegin();
+    for (size_t i = 0; i < numProps; i++) {
+      *cursor++ = (*indices)[i];
+    }
+    MOZ_ASSERT(uintptr_t(cursor) == uintptr_t(this) + nbytes);
+    setIndicesState(NativeIteratorIndices::Valid);
+  }
+
+  markInitialized();
+
+  MOZ_ASSERT(!*hadError);
+}
+
+inline size_t NativeIterator::allocationSize() const {
+  size_t numShapes = shapesEnd() - shapesBegin();
+
+  return AllocationSize(initialPropertyCount(), numShapes, indicesAllocated());
+}
+
+/* static */
+bool IteratorHashPolicy::match(PropertyIteratorObject* obj,
+                               const Lookup& lookup) {
+  NativeIterator* ni = obj->getNativeIterator();
+  if (ni->shapesHash() != lookup.shapesHash ||
+      ni->shapeCount() != lookup.numShapes) {
+    return false;
+  }
+
+  return ArrayEqual(reinterpret_cast<Shape**>(ni->shapesBegin()), lookup.shapes,
+                    ni->shapeCount());
+}
+
+static inline bool CanCompareIterableObjectToCache(JSObject* obj) {
+  if (obj->is<NativeObject>()) {
+    return obj->as<NativeObject>().getDenseInitializedLength() == 0;
+  }
+  return false;
+}
+
+static bool CanStoreInIteratorCache(JSObject* obj) {
+  do {
+    MOZ_ASSERT(obj->as<NativeObject>().getDenseInitializedLength() == 0);
+
+    // Typed arrays have indexed properties not captured by the Shape guard.
+    // Enumerate hooks may add extra properties.
+    if (MOZ_UNLIKELY(ClassCanHaveExtraEnumeratedProperties(obj->getClass()))) {
+      return false;
+    }
+
+    obj = obj->staticPrototype();
+  } while (obj);
+
+  return true;
+}
+
+static MOZ_ALWAYS_INLINE PropertyIteratorObject* LookupInIteratorCache(
+    JSContext* cx, JSObject* obj, uint32_t* cacheableProtoChainLength) {
+  MOZ_ASSERT(*cacheableProtoChainLength == 0);
+
+  if (obj->shape()->cache().isIterator() &&
+      CanCompareIterableObjectToCache(obj)) {
+    PropertyIteratorObject* iterobj = obj->shape()->cache().toIterator();
+    NativeIterator* ni = iterobj->getNativeIterator();
+    MOZ_ASSERT(*ni->shapesBegin() == obj->shape());
+    if (!ni->isReusable()) {
+      return nullptr;
+    }
+
+    // Verify shapes of proto chain.
+    JSObject* pobj = obj;
+    for (GCPtr<Shape*>* s = ni->shapesBegin() + 1; s != ni->shapesEnd(); s++) {
+      Shape* shape = *s;
+      pobj = pobj->staticPrototype();
+      if (pobj->shape() != shape) {
+        return nullptr;
+      }
+      if (!CanCompareIterableObjectToCache(pobj)) {
+        return nullptr;
+      }
+    }
+    MOZ_ASSERT(CanStoreInIteratorCache(obj));
+    *cacheableProtoChainLength = ni->shapeCount();
+    return iterobj;
+  }
+
+  Vector<Shape*, 8> shapes(cx);
+  HashNumber shapesHash = 0;
+  JSObject* pobj = obj;
+  do {
+    if (!CanCompareIterableObjectToCache(pobj)) {
+      return nullptr;
+    }
+
+    MOZ_ASSERT(pobj->is<NativeObject>());
+    Shape* shape = pobj->shape();
+    shapesHash = mozilla::AddToHash(shapesHash, HashIteratorShape(shape));
+
+    if (MOZ_UNLIKELY(!shapes.append(shape))) {
+      cx->recoverFromOutOfMemory();
+      return nullptr;
+    }
+
+    pobj = pobj->staticPrototype();
+  } while (pobj);
+
+  MOZ_ASSERT(!shapes.empty());
+  *cacheableProtoChainLength = shapes.length();
+
+  IteratorHashPolicy::Lookup lookup(shapes.begin(), shapes.length(),
+                                    shapesHash);
+  auto p = ObjectRealm::get(obj).iteratorCache.lookup(lookup);
+  if (!p) {
+    return nullptr;
+  }
+
+  PropertyIteratorObject* iterobj = *p;
+  MOZ_ASSERT(iterobj->compartment() == cx->compartment());
+
+  NativeIterator* ni = iterobj->getNativeIterator();
+  if (!ni->isReusable()) {
+    return nullptr;
+  }
+
+  return iterobj;
+}
+
+[[nodiscard]] static bool StoreInIteratorCache(
+    JSContext* cx, JSObject* obj, PropertyIteratorObject* iterobj) {
+  MOZ_ASSERT(CanStoreInIteratorCache(obj));
+
+  NativeIterator* ni = iterobj->getNativeIterator();
+  MOZ_ASSERT(ni->shapeCount() > 0);
+
+  obj->shape()->maybeCacheIterator(cx, iterobj);
+
+  IteratorHashPolicy::Lookup lookup(
+      reinterpret_cast<Shape**>(ni->shapesBegin()), ni->shapeCount(),
+      ni->shapesHash());
+
+  ObjectRealm::IteratorCache& cache = ObjectRealm::get(obj).iteratorCache;
+  bool ok;
+  auto p = cache.lookupForAdd(lookup);
+  if (MOZ_LIKELY(!p)) {
+    ok = cache.add(p, iterobj);
+  } else {
+    // If we weren't able to use an existing cached iterator, just
+    // replace it.
+    cache.remove(p);
+    ok = cache.relookupOrAdd(p, lookup, iterobj);
+  }
+  if (!ok) {
+    ReportOutOfMemory(cx);
+    return false;
+  }
+
+  return true;
+}
+
+bool js::EnumerateProperties(JSContext* cx, HandleObject obj,
+                             MutableHandleIdVector props) {
+  MOZ_ASSERT(props.empty());
+
+  if (MOZ_UNLIKELY(obj->is<ProxyObject>())) {
+    return Proxy::enumerate(cx, obj, props);
+  }
+
+  uint32_t flags = 0;
+  PropertyEnumerator enumerator(cx, obj, flags, props);
+  return enumerator.snapshot(cx);
+}
+
+#ifdef DEBUG
+static bool IndicesAreValid(NativeObject* obj, NativeIterator* ni) {
+  MOZ_ASSERT(ni->hasValidIndices());
+  size_t numDenseElements = obj->getDenseInitializedLength();
+  size_t numFixedSlots = obj->numFixedSlots();
+  const Value* elements = obj->getDenseElements();
+
+  GCPtr<JSLinearString*>* keys = ni->propertiesBegin();
+  PropertyIndex* indices = ni->indicesBegin();
+
+  for (uint32_t i = 0; i < ni->numKeys(); i++) {
+    PropertyIndex index = indices[i];
+    switch (index.kind()) {
+      case PropertyIndex::Kind::Element:
+        // Verify that the dense element exists and is not a hole.
+        if (index.index() >= numDenseElements ||
+            elements[index.index()].isMagic(JS_ELEMENTS_HOLE)) {
+          return false;
+        }
+        break;
+      case PropertyIndex::Kind::FixedSlot: {
+        // Verify that the slot exists and is an enumerable data property with
+        // the expected key.
+        Maybe<PropertyInfo> prop =
+            obj->lookupPure(AtomToId(&keys[i]->asAtom()));
+        if (!prop.isSome() || !prop->hasSlot() || !prop->enumerable() ||
+            !prop->isDataProperty() || prop->slot() != index.index()) {
+          return false;
+        }
+        break;
+      }
+      case PropertyIndex::Kind::DynamicSlot: {
+        // Verify that the slot exists and is an enumerable data property with
+        // the expected key.
+        Maybe<PropertyInfo> prop =
+            obj->lookupPure(AtomToId(&keys[i]->asAtom()));
+        if (!prop.isSome() || !prop->hasSlot() || !prop->enumerable() ||
+            !prop->isDataProperty() ||
+            prop->slot() - numFixedSlots != index.index()) {
+          return false;
+        }
+        break;
+      }
+      case PropertyIndex::Kind::Invalid:
+        return false;
+    }
+  }
+  return true;
+}
+#endif
+
+template <bool WantIndices>
+static PropertyIteratorObject* GetIteratorImpl(JSContext* cx,
+                                               HandleObject obj) {
+  MOZ_ASSERT(!obj->is<PropertyIteratorObject>());
+  MOZ_ASSERT(cx->compartment() == obj->compartment(),
+             "We may end up allocating shapes in the wrong zone!");
+
+  uint32_t cacheableProtoChainLength = 0;
+  if (PropertyIteratorObject* iterobj =
+          LookupInIteratorCache(cx, obj, &cacheableProtoChainLength)) {
+    NativeIterator* ni = iterobj->getNativeIterator();
+    bool recreateWithIndices = WantIndices && ni->indicesAvailableOnRequest();
+    if (!recreateWithIndices) {
+      MOZ_ASSERT_IF(WantIndices && ni->hasValidIndices(),
+                    IndicesAreValid(&obj->as<NativeObject>(), ni));
+      ni->initObjectBeingIterated(*obj);
+      RegisterEnumerator(cx, ni);
+      return iterobj;
+    }
+  }
+
+  if (cacheableProtoChainLength > 0 && !CanStoreInIteratorCache(obj)) {
+    cacheableProtoChainLength = 0;
+  }
+
+  RootedIdVector keys(cx);
+  PropertyIndexVector indices(cx);
+  bool supportsIndices = false;
+
+  if (MOZ_UNLIKELY(obj->is<ProxyObject>())) {
+    if (!Proxy::enumerate(cx, obj, &keys)) {
+      return nullptr;
+    }
+  } else {
+    uint32_t flags = 0;
+    PropertyEnumerator enumerator(cx, obj, flags, &keys, &indices);
+    if (!enumerator.snapshot(cx)) {
+      return nullptr;
+    }
+    supportsIndices = enumerator.supportsIndices();
+    MOZ_ASSERT_IF(WantIndices && supportsIndices,
+                  keys.length() == indices.length());
+  }
+
+  // If the object has dense elements, mark the dense elements as
+  // maybe-in-iteration.
+  //
+  // The iterator is a snapshot so if indexed properties are added after this
+  // point we don't need to do anything. However, the object might have sparse
+  // elements now that can be densified later. To account for this, we set the
+  // maybe-in-iteration flag also in NativeObject::maybeDensifySparseElements.
+  //
+  // In debug builds, AssertDenseElementsNotIterated is used to check the flag
+  // is set correctly.
+  if (obj->is<NativeObject>() &&
+      obj->as<NativeObject>().getDenseInitializedLength() > 0) {
+    obj->as<NativeObject>().markDenseElementsMaybeInIteration();
+  }
+
+  PropertyIndexVector* indicesPtr =
+      WantIndices && supportsIndices ? &indices : nullptr;
+  PropertyIteratorObject* iterobj = CreatePropertyIterator(
+      cx, obj, keys, supportsIndices, indicesPtr, cacheableProtoChainLength);
+  if (!iterobj) {
+    return nullptr;
+  }
+  RegisterEnumerator(cx, iterobj->getNativeIterator());
+
+  cx->check(iterobj);
+  MOZ_ASSERT_IF(
+      WantIndices && supportsIndices,
+      IndicesAreValid(&obj->as<NativeObject>(), iterobj->getNativeIterator()));
+
+#ifdef DEBUG
+  if (obj->is<NativeObject>()) {
+    if (PrototypeMayHaveIndexedProperties(&obj->as<NativeObject>())) {
+      iterobj->getNativeIterator()->setMaybeHasIndexedPropertiesFromProto();
+    }
+  }
+#endif
+
+  // Cache the iterator object.
+  if (cacheableProtoChainLength > 0) {
+    if (!StoreInIteratorCache(cx, obj, iterobj)) {
+      return nullptr;
+    }
+  }
+
+  return iterobj;
+}
+
+PropertyIteratorObject* js::GetIterator(JSContext* cx, HandleObject obj) {
+  return GetIteratorImpl<false>(cx, obj);
+}
+
+PropertyIteratorObject* js::GetIteratorWithIndices(JSContext* cx,
+                                                   HandleObject obj) {
+  return GetIteratorImpl<true>(cx, obj);
+}
+
+PropertyIteratorObject* js::LookupInIteratorCache(JSContext* cx,
+                                                  HandleObject obj) {
+  uint32_t dummy = 0;
+  return LookupInIteratorCache(cx, obj, &dummy);
+}
+
+// ES 2017 draft 7.4.7.
+PlainObject* js::CreateIterResultObject(JSContext* cx, HandleValue value,
+                                        bool done) {
+  // Step 1 (implicit).
+
+  // Step 2.
+  Rooted<PlainObject*> templateObject(
+      cx, GlobalObject::getOrCreateIterResultTemplateObject(cx));
+  if (!templateObject) {
+    return nullptr;
+  }
+
+  PlainObject* resultObj = PlainObject::createWithTemplate(cx, templateObject);
+  if (!resultObj) {
+    return nullptr;
+  }
+
+  // Step 3.
+  resultObj->setSlot(GlobalObject::IterResultObjectValueSlot, value);
+
+  // Step 4.
+  resultObj->setSlot(GlobalObject::IterResultObjectDoneSlot,
+                     done ? TrueHandleValue : FalseHandleValue);
+
+  // Step 5.
+  return resultObj;
+}
+
+PlainObject* GlobalObject::getOrCreateIterResultTemplateObject(JSContext* cx) {
+  HeapPtr<PlainObject*>& obj = cx->global()->data().iterResultTemplate;
+  if (obj) {
+    return obj;
+  }
+
+  PlainObject* templateObj =
+      createIterResultTemplateObject(cx, WithObjectPrototype::Yes);
+  obj.init(templateObj);
+  return obj;
+}
+
+/* static */
+PlainObject* GlobalObject::getOrCreateIterResultWithoutPrototypeTemplateObject(
+    JSContext* cx) {
+  HeapPtr<PlainObject*>& obj =
+      cx->global()->data().iterResultWithoutPrototypeTemplate;
+  if (obj) {
+    return obj;
+  }
+
+  PlainObject* templateObj =
+      createIterResultTemplateObject(cx, WithObjectPrototype::No);
+  obj.init(templateObj);
+  return obj;
+}
+
+/* static */
+PlainObject* GlobalObject::createIterResultTemplateObject(
+    JSContext* cx, WithObjectPrototype withProto) {
+  // Create template plain object
+  Rooted<PlainObject*> templateObject(
+      cx, withProto == WithObjectPrototype::Yes
+              ? NewPlainObject(cx, TenuredObject)
+              : NewPlainObjectWithProto(cx, nullptr));
+  if (!templateObject) {
+    return nullptr;
+  }
+
+  // Set dummy `value` property
+  if (!NativeDefineDataProperty(cx, templateObject, cx->names().value,
+                                UndefinedHandleValue, JSPROP_ENUMERATE)) {
+    return nullptr;
+  }
+
+  // Set dummy `done` property
+  if (!NativeDefineDataProperty(cx, templateObject, cx->names().done,
+                                TrueHandleValue, JSPROP_ENUMERATE)) {
+    return nullptr;
+  }
+
+#ifdef DEBUG
+  // Make sure that the properties are in the right slots.
+  ShapePropertyIter<NoGC> iter(templateObject->shape());
+  MOZ_ASSERT(iter->slot() == GlobalObject::IterResultObjectDoneSlot &&
+             iter->key() == NameToId(cx->names().done));
+  iter++;
+  MOZ_ASSERT(iter->slot() == GlobalObject::IterResultObjectValueSlot &&
+             iter->key() == NameToId(cx->names().value));
+#endif
+
+  return templateObject;
+}
+
+/*** Iterator objects *******************************************************/
+
+size_t PropertyIteratorObject::sizeOfMisc(
+    mozilla::MallocSizeOf mallocSizeOf) const {
+  return mallocSizeOf(getNativeIterator());
+}
+
+void PropertyIteratorObject::trace(JSTracer* trc, JSObject* obj) {
+  if (NativeIterator* ni =
+          obj->as<PropertyIteratorObject>().getNativeIterator()) {
+    ni->trace(trc);
+  }
+}
+
+void PropertyIteratorObject::finalize(JS::GCContext* gcx, JSObject* obj) {
+  if (NativeIterator* ni =
+          obj->as<PropertyIteratorObject>().getNativeIterator()) {
+    gcx->free_(obj, ni, ni->allocationSize(), MemoryUse::NativeIterator);
+  }
+}
+
+const JSClassOps PropertyIteratorObject::classOps_ = {
+    nullptr,   // addProperty
+    nullptr,   // delProperty
+    nullptr,   // enumerate
+    nullptr,   // newEnumerate
+    nullptr,   // resolve
+    nullptr,   // mayResolve
+    finalize,  // finalize
+    nullptr,   // call
+    nullptr,   // construct
+    trace,     // trace
+};
+
+const JSClass PropertyIteratorObject::class_ = {
+    "Iterator",
+    JSCLASS_HAS_RESERVED_SLOTS(SlotCount) | JSCLASS_BACKGROUND_FINALIZE,
+    &PropertyIteratorObject::classOps_};
+
+static const JSClass ArrayIteratorPrototypeClass = {"Array Iterator", 0};
+
+enum {
+  ArrayIteratorSlotIteratedObject,
+  ArrayIteratorSlotNextIndex,
+  ArrayIteratorSlotItemKind,
+  ArrayIteratorSlotCount
+};
+
+const JSClass ArrayIteratorObject::class_ = {
+    "Array Iterator", JSCLASS_HAS_RESERVED_SLOTS(ArrayIteratorSlotCount)};
+
+ArrayIteratorObject* js::NewArrayIteratorTemplate(JSContext* cx) {
+  RootedObject proto(
+      cx, GlobalObject::getOrCreateArrayIteratorPrototype(cx, cx->global()));
+  if (!proto) {
+    return nullptr;
+  }
+
+  return NewTenuredObjectWithGivenProto<ArrayIteratorObject>(cx, proto);
+}
+
+ArrayIteratorObject* js::NewArrayIterator(JSContext* cx) {
+  RootedObject proto(
+      cx, GlobalObject::getOrCreateArrayIteratorPrototype(cx, cx->global()));
+  if (!proto) {
+    return nullptr;
+  }
+
+  return NewObjectWithGivenProto<ArrayIteratorObject>(cx, proto);
+}
+
+static const JSFunctionSpec array_iterator_methods[] = {
+    JS_SELF_HOSTED_FN("next", "ArrayIteratorNext", 0, 0), JS_FS_END};
+
+static const JSClass StringIteratorPrototypeClass = {"String Iterator", 0};
+
+enum {
+  StringIteratorSlotIteratedObject,
+  StringIteratorSlotNextIndex,
+  StringIteratorSlotCount
+};
+
+const JSClass StringIteratorObject::class_ = {
+    "String Iterator", JSCLASS_HAS_RESERVED_SLOTS(StringIteratorSlotCount)};
+
+static const JSFunctionSpec string_iterator_methods[] = {
+    JS_SELF_HOSTED_FN("next", "StringIteratorNext", 0, 0), JS_FS_END};
+
+StringIteratorObject* js::NewStringIteratorTemplate(JSContext* cx) {
+  RootedObject proto(
+      cx, GlobalObject::getOrCreateStringIteratorPrototype(cx, cx->global()));
+  if (!proto) {
+    return nullptr;
+  }
+
+  return NewTenuredObjectWithGivenProto<StringIteratorObject>(cx, proto);
+}
+
+StringIteratorObject* js::NewStringIterator(JSContext* cx) {
+  RootedObject proto(
+      cx, GlobalObject::getOrCreateStringIteratorPrototype(cx, cx->global()));
+  if (!proto) {
+    return nullptr;
+  }
+
+  return NewObjectWithGivenProto<StringIteratorObject>(cx, proto);
+}
+
+static const JSClass RegExpStringIteratorPrototypeClass = {
+    "RegExp String Iterator", 0};
+
+enum {
+  // The regular expression used for iteration. May hold the original RegExp
+  // object when it is reused instead of a new RegExp object.
+  RegExpStringIteratorSlotRegExp,
+
+  // The String value being iterated upon.
+  RegExpStringIteratorSlotString,
+
+  // The source string of the original RegExp object. Used to validate we can
+  // reuse the original RegExp object for matching.
+  RegExpStringIteratorSlotSource,
+
+  // The flags of the original RegExp object.
+  RegExpStringIteratorSlotFlags,
+
+  // When non-negative, this slot holds the current lastIndex position when
+  // reusing the original RegExp object for matching. When set to |-1|, the
+  // iterator has finished. When set to any other negative value, the
+  // iterator is not yet exhausted and we're not on the fast path and we're
+  // not reusing the input RegExp object.
+  RegExpStringIteratorSlotLastIndex,
+
+  RegExpStringIteratorSlotCount
+};
+
+static_assert(RegExpStringIteratorSlotRegExp ==
+                  REGEXP_STRING_ITERATOR_REGEXP_SLOT,
+              "RegExpStringIteratorSlotRegExp must match self-hosting define "
+              "for regexp slot.");
+static_assert(RegExpStringIteratorSlotString ==
+                  REGEXP_STRING_ITERATOR_STRING_SLOT,
+              "RegExpStringIteratorSlotString must match self-hosting define "
+              "for string slot.");
+static_assert(RegExpStringIteratorSlotSource ==
+                  REGEXP_STRING_ITERATOR_SOURCE_SLOT,
+              "RegExpStringIteratorSlotString must match self-hosting define "
+              "for source slot.");
+static_assert(RegExpStringIteratorSlotFlags ==
+                  REGEXP_STRING_ITERATOR_FLAGS_SLOT,
+              "RegExpStringIteratorSlotFlags must match self-hosting define "
+              "for flags slot.");
+static_assert(RegExpStringIteratorSlotLastIndex ==
+                  REGEXP_STRING_ITERATOR_LASTINDEX_SLOT,
+              "RegExpStringIteratorSlotLastIndex must match self-hosting "
+              "define for lastIndex slot.");
+
+const JSClass RegExpStringIteratorObject::class_ = {
+    "RegExp String Iterator",
+    JSCLASS_HAS_RESERVED_SLOTS(RegExpStringIteratorSlotCount)};
+
+static const JSFunctionSpec regexp_string_iterator_methods[] = {
+    JS_SELF_HOSTED_FN("next", "RegExpStringIteratorNext", 0, 0),
+
+    JS_FS_END};
+
+RegExpStringIteratorObject* js::NewRegExpStringIteratorTemplate(JSContext* cx) {
+  RootedObject proto(cx, GlobalObject::getOrCreateRegExpStringIteratorPrototype(
+                             cx, cx->global()));
+  if (!proto) {
+    return nullptr;
+  }
+
+  return NewTenuredObjectWithGivenProto<RegExpStringIteratorObject>(cx, proto);
+}
+
+RegExpStringIteratorObject* js::NewRegExpStringIterator(JSContext* cx) {
+  RootedObject proto(cx, GlobalObject::getOrCreateRegExpStringIteratorPrototype(
+                             cx, cx->global()));
+  if (!proto) {
+    return nullptr;
+  }
+
+  return NewObjectWithGivenProto<RegExpStringIteratorObject>(cx, proto);
+}
+
+// static
+PropertyIteratorObject* GlobalObject::getOrCreateEmptyIterator(JSContext* cx) {
+  if (!cx->global()->data().emptyIterator) {
+    RootedIdVector props(cx);  // Empty
+    PropertyIteratorObject* iter =
+        CreatePropertyIterator(cx, nullptr, props, false, nullptr, 0);
+    if (!iter) {
+      return nullptr;
+    }
+    iter->getNativeIterator()->markEmptyIteratorSingleton();
+    cx->global()->data().emptyIterator.init(iter);
+  }
+  return cx->global()->data().emptyIterator;
+}
+
+PropertyIteratorObject* js::ValueToIterator(JSContext* cx, HandleValue vp) {
+  RootedObject obj(cx);
+  if (vp.isObject()) {
+    /* Common case. */
+    obj = &vp.toObject();
+  } else if (vp.isNullOrUndefined()) {
+    /*
+     * Enumerating over null and undefined gives an empty enumerator, so
+     * that |for (var p in <null or undefined>) <loop>;| never executes
+     * <loop>, per ES5 12.6.4.
+     */
+    return GlobalObject::getOrCreateEmptyIterator(cx);
+  } else {
+    obj = ToObject(cx, vp);
+    if (!obj) {
+      return nullptr;
+    }
+  }
+
+  return GetIterator(cx, obj);
+}
+
+void js::CloseIterator(JSObject* obj) {
+  if (!obj->is<PropertyIteratorObject>()) {
+    return;
+  }
+
+  // Remove iterator from the active list, which is a stack. The shared iterator
+  // used for for-in with null/undefined is immutable and unlinked.
+
+  NativeIterator* ni = obj->as<PropertyIteratorObject>().getNativeIterator();
+  if (ni->isEmptyIteratorSingleton()) {
+    return;
+  }
+
+  ni->unlink();
+
+  MOZ_ASSERT(ni->isActive());
+  ni->markInactive();
+
+  ni->clearObjectBeingIterated();
+
+  // Reset the enumerator; it may still be in the cached iterators for
+  // this thread and can be reused.
+  ni->resetPropertyCursorForReuse();
+}
+
+bool js::IteratorCloseForException(JSContext* cx, HandleObject obj) {
+  MOZ_ASSERT(cx->isExceptionPending());
+
+  bool isClosingGenerator = cx->isClosingGenerator();
+  JS::AutoSaveExceptionState savedExc(cx);
+
+  // Implements IteratorClose (ES 7.4.6) for exception unwinding. See
+  // also the bytecode generated by BytecodeEmitter::emitIteratorClose.
+
+  // Step 3.
+  //
+  // Get the "return" method.
+  RootedValue returnMethod(cx);
+  if (!GetProperty(cx, obj, obj, cx->names().return_, &returnMethod)) {
+    return false;
+  }
+
+  // Step 4.
+  //
+  // Do nothing if "return" is null or undefined. Throw a TypeError if the
+  // method is not IsCallable.
+  if (returnMethod.isNullOrUndefined()) {
+    return true;
+  }
+  if (!IsCallable(returnMethod)) {
+    return ReportIsNotFunction(cx, returnMethod);
+  }
+
+  // Step 5, 6, 8.
+  //
+  // Call "return" if it is not null or undefined.
+  RootedValue rval(cx);
+  bool ok = Call(cx, returnMethod, obj, &rval);
+  if (isClosingGenerator) {
+    // Closing an iterator is implemented as an exception, but in spec
+    // terms it is a Completion value with [[Type]] return. In this case
+    // we *do* care if the call threw and if it returned an object.
+    if (!ok) {
+      return false;
+    }
+    if (!rval.isObject()) {
+      return ThrowCheckIsObject(cx, CheckIsObjectKind::IteratorReturn);
+    }
+  } else {
+    // We don't care if the call threw or that it returned an Object, as
+    // Step 6 says if IteratorClose is being called during a throw, the
+    // original throw has primacy.
+    savedExc.restore();
+  }
+
+  return true;
+}
+
+void js::UnwindIteratorForUncatchableException(JSObject* obj) {
+  if (obj->is<PropertyIteratorObject>()) {
+    NativeIterator* ni = obj->as<PropertyIteratorObject>().getNativeIterator();
+    if (ni->isEmptyIteratorSingleton()) {
+      return;
+    }
+    ni->unlink();
+  }
+}
+
+static bool SuppressDeletedProperty(JSContext* cx, NativeIterator* ni,
+                                    HandleObject obj,
+                                    Handle<JSLinearString*> str) {
+  if (ni->objectBeingIterated() != obj) {
+    return true;
+  }
+
+  ni->disableIndices();
+
+  // Optimization for the following common case:
+  //
+  //    for (var p in o) {
+  //        delete o[p];
+  //    }
+  //
+  // Note that usually both strings will be atoms so we only check for pointer
+  // equality here.
+  if (ni->previousPropertyWas(str)) {
+    return true;
+  }
+
+  while (true) {
+    bool restart = false;
+
+    // Check whether id is still to come.
+    GCPtr<JSLinearString*>* const cursor = ni->nextProperty();
+    GCPtr<JSLinearString*>* const end = ni->propertiesEnd();
+    for (GCPtr<JSLinearString*>* idp = cursor; idp < end; ++idp) {
+      // Common case: both strings are atoms.
+      if ((*idp)->isAtom() && str->isAtom()) {
+        if (*idp != str) {
+          continue;
+        }
+      } else {
+        if (!EqualStrings(*idp, str)) {
+          continue;
+        }
+      }
+
+      // Check whether another property along the prototype chain became
+      // visible as a result of this deletion.
+      RootedObject proto(cx);
+      if (!GetPrototype(cx, obj, &proto)) {
+        return false;
+      }
+      if (proto) {
+        RootedId id(cx);
+        RootedValue idv(cx, StringValue(*idp));
+        if (!PrimitiveValueToId<CanGC>(cx, idv, &id)) {
+          return false;
+        }
+
+        Rooted<mozilla::Maybe<PropertyDescriptor>> desc(cx);
+        RootedObject holder(cx);
+        if (!GetPropertyDescriptor(cx, proto, id, &desc, &holder)) {
+          return false;
+        }
+
+        if (desc.isSome() && desc->enumerable()) {
+          continue;
+        }
+      }
+
+      // If GetPropertyDescriptor above removed a property from ni, start
+      // over.
+      if (end != ni->propertiesEnd() || cursor != ni->nextProperty()) {
+        restart = true;
+        break;
+      }
+
+      // No property along the prototype chain stepped in to take the
+      // property's place, so go ahead and delete id from the list.
+      // If it is the next property to be enumerated, just skip it.
+      if (idp == cursor) {
+        ni->incCursor();
+      } else {
+        for (GCPtr<JSLinearString*>* p = idp; p + 1 != end; p++) {
+          *p = *(p + 1);
+        }
+
+        ni->trimLastProperty();
+      }
+
+      ni->markHasUnvisitedPropertyDeletion();
+      return true;
+    }
+
+    if (!restart) {
+      return true;
+    }
+  }
+}
+
+/*
+ * Suppress enumeration of deleted properties. This function must be called
+ * when a property is deleted and there might be active enumerators.
+ *
+ * We maintain a list of active non-escaping for-in enumerators. To suppress
+ * a property, we check whether each active enumerator contains the (obj, id)
+ * pair and has not yet enumerated |id|. If so, and |id| is the next property,
+ * we simply advance the cursor. Otherwise, we delete |id| from the list.
+ *
+ * We do not suppress enumeration of a property deleted along an object's
+ * prototype chain. Only direct deletions on the object are handled.
+ */
+static bool SuppressDeletedPropertyHelper(JSContext* cx, HandleObject obj,
+                                          Handle<JSLinearString*> str) {
+  NativeIteratorListIter iter(obj->compartment()->enumeratorsAddr());
+  while (!iter.done()) {
+    NativeIterator* ni = iter.next();
+    if (!SuppressDeletedProperty(cx, ni, obj, str)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+bool js::SuppressDeletedProperty(JSContext* cx, HandleObject obj, jsid id) {
+  if (MOZ_LIKELY(!obj->compartment()->objectMaybeInIteration(obj))) {
+    return true;
+  }
+
+  if (id.isSymbol()) {
+    return true;
+  }
+
+  Rooted<JSLinearString*> str(cx, IdToString(cx, id));
+  if (!str) {
+    return false;
+  }
+  return SuppressDeletedPropertyHelper(cx, obj, str);
+}
+
+bool js::SuppressDeletedElement(JSContext* cx, HandleObject obj,
+                                uint32_t index) {
+  if (MOZ_LIKELY(!obj->compartment()->objectMaybeInIteration(obj))) {
+    return true;
+  }
+
+  RootedId id(cx);
+  if (!IndexToId(cx, index, &id)) {
+    return false;
+  }
+
+  Rooted<JSLinearString*> str(cx, IdToString(cx, id));
+  if (!str) {
+    return false;
+  }
+  return SuppressDeletedPropertyHelper(cx, obj, str);
+}
+
+#ifdef DEBUG
+void js::AssertDenseElementsNotIterated(NativeObject* obj) {
+  // Search for active iterators for |obj| and assert they don't contain any
+  // property keys that are dense elements. This is used to check correctness
+  // of the MAYBE_IN_ITERATION flag on ObjectElements.
+  //
+  // Ignore iterators that may contain indexed properties from objects on the
+  // prototype chain, as that can result in false positives. See bug 1656744.
+
+  // Limit the number of properties we check to avoid slowing down debug builds
+  // too much.
+  static constexpr uint32_t MaxPropsToCheck = 10;
+  uint32_t propsChecked = 0;
+
+  NativeIteratorListIter iter(obj->compartment()->enumeratorsAddr());
+  while (!iter.done()) {
+    NativeIterator* ni = iter.next();
+    if (ni->objectBeingIterated() == obj &&
+        !ni->maybeHasIndexedPropertiesFromProto()) {
+      for (GCPtr<JSLinearString*>* idp = ni->nextProperty();
+           idp < ni->propertiesEnd(); ++idp) {
+        uint32_t index;
+        if (idp->get()->isIndex(&index)) {
+          MOZ_ASSERT(!obj->containsDenseElement(index));
+        }
+        if (++propsChecked > MaxPropsToCheck) {
+          return;
+        }
+      }
+    }
+  }
+}
+#endif
+
+static const JSFunctionSpec iterator_methods[] = {
+    JS_SELF_HOSTED_SYM_FN(iterator, "IteratorIdentity", 0, 0), JS_FS_END};
+
+static const JSFunctionSpec iterator_static_methods[] = {
+    JS_SELF_HOSTED_FN("from", "IteratorFrom", 1, 0), JS_FS_END};
+
+// These methods are only attached to Iterator.prototype when the
+// Iterator Helpers feature is enabled.
+static const JSFunctionSpec iterator_methods_with_helpers[] = {
+    JS_SELF_HOSTED_FN("map", "IteratorMap", 1, 0),
+    JS_SELF_HOSTED_FN("filter", "IteratorFilter", 1, 0),
+    JS_SELF_HOSTED_FN("take", "IteratorTake", 1, 0),
+    JS_SELF_HOSTED_FN("drop", "IteratorDrop", 1, 0),
+    JS_SELF_HOSTED_FN("asIndexedPairs", "IteratorAsIndexedPairs", 0, 0),
+    JS_SELF_HOSTED_FN("flatMap", "IteratorFlatMap", 1, 0),
+    JS_SELF_HOSTED_FN("reduce", "IteratorReduce", 1, 0),
+    JS_SELF_HOSTED_FN("toArray", "IteratorToArray", 0, 0),
+    JS_SELF_HOSTED_FN("forEach", "IteratorForEach", 1, 0),
+    JS_SELF_HOSTED_FN("some", "IteratorSome", 1, 0),
+    JS_SELF_HOSTED_FN("every", "IteratorEvery", 1, 0),
+    JS_SELF_HOSTED_FN("find", "IteratorFind", 1, 0),
+    JS_SELF_HOSTED_SYM_FN(iterator, "IteratorIdentity", 0, 0),
+    JS_FS_END};
+
+/* static */
+bool GlobalObject::initIteratorProto(JSContext* cx,
+                                     Handle<GlobalObject*> global) {
+  if (global->hasBuiltinProto(ProtoKind::IteratorProto)) {
+    return true;
+  }
+
+  RootedObject proto(
+      cx, GlobalObject::createBlankPrototype<PlainObject>(cx, global));
+  if (!proto) {
+    return false;
+  }
+
+  // %IteratorPrototype%.map.[[Prototype]] is %Generator% and
+  // %Generator%.prototype.[[Prototype]] is %IteratorPrototype%.
+  // Populate the slot early, to prevent runaway mutual recursion.
+  global->initBuiltinProto(ProtoKind::IteratorProto, proto);
+
+  if (!DefinePropertiesAndFunctions(cx, proto, nullptr, iterator_methods)) {
+    // In this case, we leave a partially initialized object in the
+    // slot. There's no obvious way to do better, since this object may already
+    // be in the prototype chain of %GeneratorPrototype%.
+    return false;
+  }
+
+  return true;
+}
+
+/* static */
+template <GlobalObject::ProtoKind Kind, const JSClass* ProtoClass,
+          const JSFunctionSpec* Methods>
+bool GlobalObject::initObjectIteratorProto(JSContext* cx,
+                                           Handle<GlobalObject*> global,
+                                           Handle<JSAtom*> tag) {
+  if (global->hasBuiltinProto(Kind)) {
+    return true;
+  }
+
+  RootedObject iteratorProto(
+      cx, GlobalObject::getOrCreateIteratorPrototype(cx, global));
+  if (!iteratorProto) {
+    return false;
+  }
+
+  RootedObject proto(cx, GlobalObject::createBlankPrototypeInheriting(
+                             cx, ProtoClass, iteratorProto));
+  if (!proto || !DefinePropertiesAndFunctions(cx, proto, nullptr, Methods) ||
+      (tag && !DefineToStringTag(cx, proto, tag))) {
+    return false;
+  }
+
+  global->initBuiltinProto(Kind, proto);
+  return true;
+}
+
+/* static */
+NativeObject* GlobalObject::getOrCreateArrayIteratorPrototype(
+    JSContext* cx, Handle<GlobalObject*> global) {
+  return MaybeNativeObject(getOrCreateBuiltinProto(
+      cx, global, ProtoKind::ArrayIteratorProto,
+      cx->names().ArrayIterator.toHandle(),
+      initObjectIteratorProto<ProtoKind::ArrayIteratorProto,
+                              &ArrayIteratorPrototypeClass,
+                              array_iterator_methods>));
+}
+
+/* static */
+JSObject* GlobalObject::getOrCreateStringIteratorPrototype(
+    JSContext* cx, Handle<GlobalObject*> global) {
+  return getOrCreateBuiltinProto(
+      cx, global, ProtoKind::StringIteratorProto,
+      cx->names().StringIterator.toHandle(),
+      initObjectIteratorProto<ProtoKind::StringIteratorProto,
+                              &StringIteratorPrototypeClass,
+                              string_iterator_methods>);
+}
+
+/* static */
+JSObject* GlobalObject::getOrCreateRegExpStringIteratorPrototype(
+    JSContext* cx, Handle<GlobalObject*> global) {
+  return getOrCreateBuiltinProto(
+      cx, global, ProtoKind::RegExpStringIteratorProto,
+      cx->names().RegExpStringIterator.toHandle(),
+      initObjectIteratorProto<ProtoKind::RegExpStringIteratorProto,
+                              &RegExpStringIteratorPrototypeClass,
+                              regexp_string_iterator_methods>);
+}
+
+// Iterator Helper Proposal 2.1.3.1 Iterator()
+// https://tc39.es/proposal-iterator-helpers/#sec-iterator as of revision
+// ed6e15a
+static bool IteratorConstructor(JSContext* cx, unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+
+  // Step 1.
+  if (!ThrowIfNotConstructing(cx, args, js_Iterator_str)) {
+    return false;
+  }
+  // Throw TypeError if NewTarget is the active function object, preventing the
+  // Iterator constructor from being used directly.
+  if (args.callee() == args.newTarget().toObject()) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_BOGUS_CONSTRUCTOR, js_Iterator_str);
+    return false;
+  }
+
+  // Step 2.
+  RootedObject proto(cx);
+  if (!GetPrototypeFromBuiltinConstructor(cx, args, JSProto_Iterator, &proto)) {
+    return false;
+  }
+
+  JSObject* obj = NewObjectWithClassProto<IteratorObject>(cx, proto);
+  if (!obj) {
+    return false;
+  }
+
+  args.rval().setObject(*obj);
+  return true;
+}
+
+static const ClassSpec IteratorObjectClassSpec = {
+    GenericCreateConstructor<IteratorConstructor, 0, gc::AllocKind::FUNCTION>,
+    GenericCreatePrototype<IteratorObject>,
+    iterator_static_methods,
+    nullptr,
+    iterator_methods_with_helpers,
+    nullptr,
+    nullptr,
+};
+
+const JSClass IteratorObject::class_ = {
+    js_Iterator_str,
+    JSCLASS_HAS_CACHED_PROTO(JSProto_Iterator),
+    JS_NULL_CLASS_OPS,
+    &IteratorObjectClassSpec,
+};
+
+const JSClass IteratorObject::protoClass_ = {
+    "Iterator.prototype",
+    JSCLASS_HAS_CACHED_PROTO(JSProto_Iterator),
+    JS_NULL_CLASS_OPS,
+    &IteratorObjectClassSpec,
+};
+
+// Set up WrapForValidIteratorObject class and its prototype.
+static const JSFunctionSpec wrap_for_valid_iterator_methods[] = {
+    JS_SELF_HOSTED_FN("next", "WrapForValidIteratorNext", 1, 0),
+    JS_SELF_HOSTED_FN("return", "WrapForValidIteratorReturn", 1, 0),
+    JS_SELF_HOSTED_FN("throw", "WrapForValidIteratorThrow", 1, 0),
+    JS_FS_END,
+};
+
+static const JSClass WrapForValidIteratorPrototypeClass = {
+    "Wrap For Valid Iterator", 0};
+
+const JSClass WrapForValidIteratorObject::class_ = {
+    "Wrap For Valid Iterator",
+    JSCLASS_HAS_RESERVED_SLOTS(WrapForValidIteratorObject::SlotCount),
+};
+
+/* static */
+NativeObject* GlobalObject::getOrCreateWrapForValidIteratorPrototype(
+    JSContext* cx, Handle<GlobalObject*> global) {
+  return MaybeNativeObject(getOrCreateBuiltinProto(
+      cx, global, ProtoKind::WrapForValidIteratorProto,
+      Handle<JSAtom*>(nullptr),
+      initObjectIteratorProto<ProtoKind::WrapForValidIteratorProto,
+                              &WrapForValidIteratorPrototypeClass,
+                              wrap_for_valid_iterator_methods>));
+}
+
+WrapForValidIteratorObject* js::NewWrapForValidIterator(JSContext* cx) {
+  RootedObject proto(cx, GlobalObject::getOrCreateWrapForValidIteratorPrototype(
+                             cx, cx->global()));
+  if (!proto) {
+    return nullptr;
+  }
+  return NewObjectWithGivenProto<WrapForValidIteratorObject>(cx, proto);
+}
+
+// Common iterator object returned by Iterator Helper methods.
+static const JSFunctionSpec iterator_helper_methods[] = {
+    JS_SELF_HOSTED_FN("next", "IteratorHelperNext", 1, 0),
+    JS_SELF_HOSTED_FN("return", "IteratorHelperReturn", 1, 0),
+    JS_SELF_HOSTED_FN("throw", "IteratorHelperThrow", 1, 0), JS_FS_END};
+
+static const JSClass IteratorHelperPrototypeClass = {"Iterator Helper", 0};
+
+const JSClass IteratorHelperObject::class_ = {
+    "Iterator Helper",
+    JSCLASS_HAS_RESERVED_SLOTS(IteratorHelperObject::SlotCount),
+};
+
+/* static */
+NativeObject* GlobalObject::getOrCreateIteratorHelperPrototype(
+    JSContext* cx, Handle<GlobalObject*> global) {
+  return MaybeNativeObject(getOrCreateBuiltinProto(
+      cx, global, ProtoKind::IteratorHelperProto, Handle<JSAtom*>(nullptr),
+      initObjectIteratorProto<ProtoKind::IteratorHelperProto,
+                              &IteratorHelperPrototypeClass,
+                              iterator_helper_methods>));
+}
+
+IteratorHelperObject* js::NewIteratorHelper(JSContext* cx) {
+  RootedObject proto(
+      cx, GlobalObject::getOrCreateIteratorHelperPrototype(cx, cx->global()));
+  if (!proto) {
+    return nullptr;
+  }
+  return NewObjectWithGivenProto<IteratorHelperObject>(cx, proto);
+}
+
+bool js::IterableToArray(JSContext* cx, HandleValue iterable,
+                         MutableHandle<ArrayObject*> array) {
+  JS::ForOfIterator iterator(cx);
+  if (!iterator.init(iterable, JS::ForOfIterator::ThrowOnNonIterable)) {
+    return false;
+  }
+
+  array.set(NewDenseEmptyArray(cx));
+  if (!array) {
+    return false;
+  }
+
+  RootedValue nextValue(cx);
+  while (true) {
+    bool done;
+    if (!iterator.next(&nextValue, &done)) {
+      return false;
+    }
+    if (done) {
+      break;
+    }
+
+    if (!NewbornArrayPush(cx, array, nextValue)) {
+      return false;
+    }
+  }
+  return true;
+}