Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

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

66 files changed, 16081 insertions, 0 deletions

diff --git a/dom/streams/BaseQueuingStrategy.h b/dom/streams/BaseQueuingStrategy.h
new file mode 100644
index 0000000000..eef7147c08
--- /dev/null
+++ b/dom/streams/BaseQueuingStrategy.h
@@ -0,0 +1,37 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_BaseQueuingStrategy_h
+#define mozilla_dom_BaseQueuingStrategy_h
+
+#include "nsCycleCollectionParticipant.h"
+#include "nsIGlobalObject.h"
+
+namespace mozilla::dom {
+
+class BaseQueuingStrategy : public nsISupports {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(BaseQueuingStrategy)
+
+  BaseQueuingStrategy(nsISupports* aGlobal, double aHighWaterMark)
+      : mGlobal(do_QueryInterface(aGlobal)), mHighWaterMark(aHighWaterMark) {}
+
+  nsIGlobalObject* GetParentObject() const;
+
+  double HighWaterMark() const { return mHighWaterMark; }
+
+ protected:
+  virtual ~BaseQueuingStrategy() = default;
+
+ protected:
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+  double mHighWaterMark = 0.0;
+};
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/streams/ByteLengthQueuingStrategy.cpp b/dom/streams/ByteLengthQueuingStrategy.cpp
new file mode 100644
index 0000000000..858f6d664b
--- /dev/null
+++ b/dom/streams/ByteLengthQueuingStrategy.cpp
@@ -0,0 +1,100 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/ByteLengthQueuingStrategy.h"
+#include "mozilla/dom/FunctionBinding.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+#include "nsCOMPtr.h"
+#include "nsISupports.h"
+
+#include "js/TypeDecls.h"
+#include "js/PropertyAndElement.h"
+
+namespace mozilla::dom {
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_INHERITED(ByteLengthQueuingStrategy,
+                                                BaseQueuingStrategy)
+NS_IMPL_ADDREF_INHERITED(ByteLengthQueuingStrategy, BaseQueuingStrategy)
+NS_IMPL_RELEASE_INHERITED(ByteLengthQueuingStrategy, BaseQueuingStrategy)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ByteLengthQueuingStrategy)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+NS_INTERFACE_MAP_END_INHERITING(BaseQueuingStrategy)
+
+/* static */
+already_AddRefed<ByteLengthQueuingStrategy>
+ByteLengthQueuingStrategy::Constructor(const GlobalObject& aGlobal,
+                                       const QueuingStrategyInit& aInit) {
+  RefPtr<ByteLengthQueuingStrategy> strategy = new ByteLengthQueuingStrategy(
+      aGlobal.GetAsSupports(), aInit.mHighWaterMark);
+  return strategy.forget();
+}
+
+JSObject* ByteLengthQueuingStrategy::WrapObject(
+    JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
+  return ByteLengthQueuingStrategy_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+static bool ByteLengthQueuingStrategySize(JSContext* cx, unsigned argc,
+                                          JS::Value* vp) {
+  // https://streams.spec.whatwg.org/#blqs-internal-slots
+  JS::CallArgs args = CallArgsFromVp(argc, vp);
+
+  // Step 1: Return ? GetV(chunk, "byteLength").
+  JS::Rooted<JSObject*> chunkObj(cx, JS::ToObject(cx, args.get(0)));
+  if (!chunkObj) {
+    return false;
+  }
+
+  return JS_GetProperty(cx, chunkObj, "byteLength", args.rval());
+}
+
+// https://streams.spec.whatwg.org/#blqs-size
+already_AddRefed<Function> ByteLengthQueuingStrategy::GetSize(
+    ErrorResult& aRv) {
+  // Step 1. Return this's relevant global object's ByteLength queuing strategy
+  // size function.
+  if (RefPtr<Function> fun =
+          mGlobal->GetByteLengthQueuingStrategySizeFunction()) {
+    return fun.forget();
+  }
+
+  // Note: Instead of eagerly allocating a size function for every global object
+  // we do it lazily once in this getter.
+  // After this point the steps refer to:
+  // https://streams.spec.whatwg.org/#byte-length-queuing-strategy-size-function
+
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(mGlobal)) {
+    aRv.ThrowUnknownError("Internal error");
+    return nullptr;
+  }
+  JSContext* cx = jsapi.cx();
+
+  // Step 1. Let steps be the following steps, given chunk
+  // Note: See ByteLengthQueuingStrategySize instead.
+
+  // Step 2. Let F be !CreateBuiltinFunction(steps, 1, "size", « »,
+  // globalObject’s relevant Realm).
+  JS::Rooted<JSFunction*> sizeFunction(
+      cx, JS_NewFunction(cx, ByteLengthQueuingStrategySize, 1, 0, "size"));
+  if (!sizeFunction) {
+    aRv.StealExceptionFromJSContext(cx);
+    return nullptr;
+  }
+
+  // Step 3. Set globalObject’s byte length queuing strategy size function to
+  // a Function that represents a reference to F,
+  // with callback context equal to globalObject’s relevant settings object.
+  JS::Rooted<JSObject*> funObj(cx, JS_GetFunctionObject(sizeFunction));
+  JS::Rooted<JSObject*> global(cx, mGlobal->GetGlobalJSObject());
+  RefPtr<Function> function = new Function(cx, funObj, global, mGlobal);
+  mGlobal->SetByteLengthQueuingStrategySizeFunction(function);
+
+  return function.forget();
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/ByteLengthQueuingStrategy.h b/dom/streams/ByteLengthQueuingStrategy.h
new file mode 100644
index 0000000000..64fe3e4528
--- /dev/null
+++ b/dom/streams/ByteLengthQueuingStrategy.h
@@ -0,0 +1,49 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ByteLengthQueuingStrategy_h
+#define mozilla_dom_ByteLengthQueuingStrategy_h
+
+#include "js/TypeDecls.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/BaseQueuingStrategy.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/FunctionBinding.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsWrapperCache.h"
+#include "nsIGlobalObject.h"
+
+namespace mozilla::dom {
+class ByteLengthQueuingStrategy final : public BaseQueuingStrategy,
+                                        public nsWrapperCache {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS_INHERITED(
+      ByteLengthQueuingStrategy, BaseQueuingStrategy)
+
+ public:
+  explicit ByteLengthQueuingStrategy(nsISupports* aGlobal,
+                                     double aHighWaterMark)
+      : BaseQueuingStrategy(aGlobal, aHighWaterMark) {}
+
+ protected:
+  ~ByteLengthQueuingStrategy() override = default;
+
+ public:
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  static already_AddRefed<ByteLengthQueuingStrategy> Constructor(
+      const GlobalObject& aGlobal, const QueuingStrategyInit& aInit);
+
+  already_AddRefed<Function> GetSize(ErrorResult& aRv);
+};
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/streams/ByteStreamHelpers.cpp b/dom/streams/ByteStreamHelpers.cpp
new file mode 100644
index 0000000000..5568971fed
--- /dev/null
+++ b/dom/streams/ByteStreamHelpers.cpp
@@ -0,0 +1,104 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/ByteStreamHelpers.h"
+#include "mozilla/dom/ReadableByteStreamController.h"
+#include "js/ArrayBuffer.h"
+#include "js/RootingAPI.h"
+#include "js/experimental/TypedData.h"
+#include "mozilla/ErrorResult.h"
+
+namespace mozilla::dom {
+
+// https://streams.spec.whatwg.org/#transfer-array-buffer
+// As some parts of the specifcation want to use the abrupt completion value,
+// this function may leave a pending exception if it returns nullptr.
+JSObject* TransferArrayBuffer(JSContext* aCx, JS::Handle<JSObject*> aObject) {
+  MOZ_ASSERT(JS::IsArrayBufferObject(aObject));
+
+  // Step 1.
+  MOZ_ASSERT(!JS::IsDetachedArrayBufferObject(aObject));
+
+  // Step 3 (Reordered)
+  size_t bufferLength = JS::GetArrayBufferByteLength(aObject);
+
+  // Step 2 (Reordered)
+  void* bufferData = JS::StealArrayBufferContents(aCx, aObject);
+
+  // Step 4.
+  if (!JS::DetachArrayBuffer(aCx, aObject)) {
+    return nullptr;
+  }
+
+  // Step 5.
+  return JS::NewArrayBufferWithContents(aCx, bufferLength, bufferData);
+}
+
+// https://streams.spec.whatwg.org/#can-transfer-array-buffer
+bool CanTransferArrayBuffer(JSContext* aCx, JS::Handle<JSObject*> aObject,
+                            ErrorResult& aRv) {
+  // Step 1. Assert: Type(O) is Object. (Implicit in types)
+  // Step 2. Assert: O has an [[ArrayBufferData]] internal slot.
+  MOZ_ASSERT(JS::IsArrayBufferObject(aObject));
+
+  // Step 3. If ! IsDetachedBuffer(O) is true, return false.
+  if (JS::IsDetachedArrayBufferObject(aObject)) {
+    return false;
+  }
+
+  // Step 4. If SameValue(O.[[ArrayBufferDetachKey]], undefined) is false,
+  // return false.
+  // Step 5. Return true.
+  // Note: WASM memories are the only buffers that would qualify
+  // as having an undefined [[ArrayBufferDetachKey]],
+  bool hasDefinedArrayBufferDetachKey = false;
+  if (!JS::HasDefinedArrayBufferDetachKey(aCx, aObject,
+                                          &hasDefinedArrayBufferDetachKey)) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return false;
+  }
+  return !hasDefinedArrayBufferDetachKey;
+}
+
+// https://streams.spec.whatwg.org/#abstract-opdef-cloneasuint8array
+JSObject* CloneAsUint8Array(JSContext* aCx, JS::Handle<JSObject*> aObject) {
+  // Step 1. Assert: Type(O) is Object. Implicit.
+  // Step 2. Assert: O has an [[ViewedArrayBuffer]] internal slot.
+  MOZ_ASSERT(JS_IsArrayBufferViewObject(aObject));
+
+  // Step 3. Assert: !IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is false.
+  bool isShared;
+  JS::Rooted<JSObject*> viewedArrayBuffer(
+      aCx, JS_GetArrayBufferViewBuffer(aCx, aObject, &isShared));
+  if (!viewedArrayBuffer) {
+    return nullptr;
+  }
+  MOZ_ASSERT(!JS::IsDetachedArrayBufferObject(viewedArrayBuffer));
+
+  // Step 4. Let buffer be ?CloneArrayBuffer(O.[[ViewedArrayBuffer]],
+  //         O.[[ByteOffset]], O.[[ByteLength]], %ArrayBuffer%).
+  size_t byteOffset = JS_GetTypedArrayByteOffset(aObject);
+  size_t byteLength = JS_GetTypedArrayByteLength(aObject);
+  JS::Rooted<JSObject*> buffer(
+      aCx,
+      JS::ArrayBufferClone(aCx, viewedArrayBuffer, byteOffset, byteLength));
+  if (!buffer) {
+    return nullptr;
+  }
+
+  // Step 5. Let array be ! Construct(%Uint8Array%, « buffer »).
+  JS::Rooted<JSObject*> array(
+      aCx, JS_NewUint8ArrayWithBuffer(aCx, buffer, 0,
+                                      static_cast<int64_t>(byteLength)));
+  if (!array) {
+    return nullptr;
+  }
+
+  // Step 6. Return array.
+  return array;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/ByteStreamHelpers.h b/dom/streams/ByteStreamHelpers.h
new file mode 100644
index 0000000000..8557c9b16b
--- /dev/null
+++ b/dom/streams/ByteStreamHelpers.h
@@ -0,0 +1,40 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ByteStreamHelpers_h
+#define mozilla_dom_ByteStreamHelpers_h
+
+#include "js/TypeDecls.h"
+#include "mozilla/ErrorResult.h"
+#include "UnderlyingSourceCallbackHelpers.h"
+
+namespace mozilla::dom {
+
+class ReadableStream;
+class BodyStreamHolder;
+
+// https://streams.spec.whatwg.org/#transfer-array-buffer
+//
+// As some parts of the specifcation want to use the abrupt completion value,
+// this function may leave a pending exception if it returns nullptr.
+JSObject* TransferArrayBuffer(JSContext* aCx, JS::Handle<JSObject*> aObject);
+
+bool CanTransferArrayBuffer(JSContext* aCx, JS::Handle<JSObject*> aObject,
+                            ErrorResult& aRv);
+
+// If this returns null, it will leave a pending exception on aCx which
+// must be handled by the caller (in the spec this is always the case
+// currently).
+JSObject* CloneAsUint8Array(JSContext* aCx, JS::Handle<JSObject*> aObject);
+
+MOZ_CAN_RUN_SCRIPT void
+SetUpReadableByteStreamControllerFromBodyStreamUnderlyingSource(
+    JSContext* aCx, ReadableStream* aStream,
+    BodyStreamHolder* aUnderlyingSource, ErrorResult& aRv);
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/streams/CountQueuingStrategy.cpp b/dom/streams/CountQueuingStrategy.cpp
new file mode 100644
index 0000000000..72f000e604
--- /dev/null
+++ b/dom/streams/CountQueuingStrategy.cpp
@@ -0,0 +1,103 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/CountQueuingStrategy.h"
+#include "mozilla/dom/FunctionBinding.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+#include "nsCOMPtr.h"
+#include "nsISupports.h"
+
+namespace mozilla::dom {
+
+NS_IMPL_CYCLE_COLLECTION(BaseQueuingStrategy, mGlobal)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(BaseQueuingStrategy)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(BaseQueuingStrategy)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(BaseQueuingStrategy)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_INHERITED(CountQueuingStrategy,
+                                                BaseQueuingStrategy)
+NS_IMPL_ADDREF_INHERITED(CountQueuingStrategy, BaseQueuingStrategy)
+NS_IMPL_RELEASE_INHERITED(CountQueuingStrategy, BaseQueuingStrategy)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(CountQueuingStrategy)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+NS_INTERFACE_MAP_END_INHERITING(BaseQueuingStrategy)
+
+/* static */
+already_AddRefed<CountQueuingStrategy> CountQueuingStrategy::Constructor(
+    const GlobalObject& aGlobal, const QueuingStrategyInit& aInit) {
+  RefPtr<CountQueuingStrategy> strategy =
+      new CountQueuingStrategy(aGlobal.GetAsSupports(), aInit.mHighWaterMark);
+  return strategy.forget();
+}
+
+nsIGlobalObject* BaseQueuingStrategy::GetParentObject() const {
+  return mGlobal;
+}
+
+JSObject* CountQueuingStrategy::WrapObject(JSContext* aCx,
+                                           JS::Handle<JSObject*> aGivenProto) {
+  return CountQueuingStrategy_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+// https://streams.spec.whatwg.org/#count-queuing-strategy-size-function
+static bool CountQueuingStrategySize(JSContext* aCx, unsigned aArgc,
+                                     JS::Value* aVp) {
+  JS::CallArgs args = CallArgsFromVp(aArgc, aVp);
+
+  // Step 1.1. Return 1.
+  args.rval().setInt32(1);
+  return true;
+}
+
+// https://streams.spec.whatwg.org/#cqs-size
+already_AddRefed<Function> CountQueuingStrategy::GetSize(ErrorResult& aRv) {
+  // Step 1. Return this's relevant global object's count queuing strategy
+  // size function.
+  if (RefPtr<Function> fun = mGlobal->GetCountQueuingStrategySizeFunction()) {
+    return fun.forget();
+  }
+
+  // Note: Instead of eagerly allocating a size function for every global object
+  // we do it lazily once in this getter.
+  // After this point the steps refer to:
+  // https://streams.spec.whatwg.org/#count-queuing-strategy-size-function.
+
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(mGlobal)) {
+    aRv.ThrowUnknownError("Internal error");
+    return nullptr;
+  }
+  JSContext* cx = jsapi.cx();
+
+  // Step 1. Let steps be the following steps:
+  // Note: See CountQueuingStrategySize instead.
+
+  // Step 2. Let F be
+  // ! CreateBuiltinFunction(steps, 0, "size", « »,
+  //                         globalObject’s relevant Realm).
+  JS::Rooted<JSFunction*> sizeFunction(
+      cx, JS_NewFunction(cx, CountQueuingStrategySize, 0, 0, "size"));
+  if (!sizeFunction) {
+    aRv.StealExceptionFromJSContext(cx);
+    return nullptr;
+  }
+
+  // Step 3. Set globalObject’s count queuing strategy size function to
+  // a Function that represents a reference to F,
+  // with callback context equal to globalObject’s relevant settings object.
+  JS::Rooted<JSObject*> funObj(cx, JS_GetFunctionObject(sizeFunction));
+  JS::Rooted<JSObject*> global(cx, mGlobal->GetGlobalJSObject());
+  RefPtr<Function> function = new Function(cx, funObj, global, mGlobal);
+  mGlobal->SetCountQueuingStrategySizeFunction(function);
+
+  return function.forget();
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/CountQueuingStrategy.h b/dom/streams/CountQueuingStrategy.h
new file mode 100644
index 0000000000..bbf109efbc
--- /dev/null
+++ b/dom/streams/CountQueuingStrategy.h
@@ -0,0 +1,48 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_CountQueuingStrategy_h
+#define mozilla_dom_CountQueuingStrategy_h
+
+#include "js/TypeDecls.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/BaseQueuingStrategy.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsWrapperCache.h"
+#include "nsIGlobalObject.h"
+
+namespace mozilla::dom {
+
+class CountQueuingStrategy final : public BaseQueuingStrategy,
+                                   public nsWrapperCache {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS_INHERITED(CountQueuingStrategy,
+                                                        BaseQueuingStrategy)
+
+ public:
+  explicit CountQueuingStrategy(nsISupports* aGlobal, double aHighWaterMark)
+      : BaseQueuingStrategy(aGlobal, aHighWaterMark) {}
+
+ protected:
+  ~CountQueuingStrategy() override = default;
+
+ public:
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  static already_AddRefed<CountQueuingStrategy> Constructor(
+      const GlobalObject& aGlobal, const QueuingStrategyInit& aInit);
+
+  already_AddRefed<Function> GetSize(ErrorResult& aRv);
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_CountQueuingStrategy_h
diff --git a/dom/streams/QueueWithSizes.h b/dom/streams/QueueWithSizes.h
new file mode 100644
index 0000000000..5a5bb49036
--- /dev/null
+++ b/dom/streams/QueueWithSizes.h
@@ -0,0 +1,140 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_QueueWithSizes_h
+#define mozilla_dom_QueueWithSizes_h
+
+#include <cmath>
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/UniquePtr.h"
+#include "nsTArray.h"
+
+namespace mozilla::dom {
+
+// Note: Consumers of QueueWithSize need to ensure it is traced. See
+// ReadableStreamDefaultController.cpp for an example.
+
+struct ValueWithSize : LinkedListElement<ValueWithSize> {
+  ValueWithSize(JS::Handle<JS::Value> aValue, double aSize)
+      : mValue(aValue), mSize(aSize){};
+
+  JS::Heap<JS::Value> mValue;
+  double mSize = 0.0f;
+};
+
+// This type is a little tricky lifetime wise: Despite the fact that we're
+// talking about linked list of VaueWithSize, what is actually stored in the
+// list are dynamically allocated ValueWithSize*. As a result, these have to be
+// deleted. There are two ways this lifetime is managed:
+//
+// 1. In DequeueValue we pop the first element into a UniquePtr, so that it is
+//    correctly cleaned up at destruction time.
+// 2. We use an AutoCleanLinkedList which will delete elements when destroyed
+//    or `clear`ed.
+using QueueWithSizes = AutoCleanLinkedList<ValueWithSize>;
+
+// https://streams.spec.whatwg.org/#is-non-negative-number
+inline bool IsNonNegativeNumber(double v) {
+  // Step 1. Implicit.
+  // Step 2.
+  if (std::isnan(v)) {
+    return false;
+  }
+
+  // Step 3.
+  return !(v < 0);
+}
+
+// https://streams.spec.whatwg.org/#enqueue-value-with-size
+template <class QueueContainingClass>
+inline void EnqueueValueWithSize(QueueContainingClass aContainer,
+                                 JS::Handle<JS::Value> aValue, double aSize,
+                                 ErrorResult& aRv) {
+  // Step 1. Assert: container has [[queue]] and [[queueTotalSize]] internal
+  // slots. (Implicit by template instantiation.)
+  // Step 2. If ! IsNonNegativeNumber(size) is false, throw a RangeError
+  // exception.
+  if (!IsNonNegativeNumber(aSize)) {
+    aRv.ThrowRangeError("invalid size");
+    return;
+  }
+
+  // Step 3. If size is +∞, throw a RangeError exception.
+  if (std::isinf(aSize)) {
+    aRv.ThrowRangeError("Infinite queue size");
+    return;
+  }
+
+  // Step 4. Append a new value-with-size with value value and size size to
+  // container.[[queue]].
+  // (See the comment on QueueWithSizes for the lifetime reasoning around this
+  // allocation.)
+  ValueWithSize* valueWithSize = new ValueWithSize(aValue, aSize);
+  aContainer->Queue().insertBack(valueWithSize);
+  // Step 5. Set container.[[queueTotalSize]] to container.[[queueTotalSize]] +
+  // size.
+  aContainer->SetQueueTotalSize(aContainer->QueueTotalSize() + aSize);
+}
+
+// https://streams.spec.whatwg.org/#dequeue-value
+template <class QueueContainingClass>
+inline void DequeueValue(QueueContainingClass aContainer,
+                         JS::MutableHandle<JS::Value> aResultValue) {
+  // Step 1. Implicit via template instantiation.
+  // Step 2.
+  MOZ_ASSERT(!aContainer->Queue().isEmpty());
+
+  // Step 3+4
+  // UniquePtr to ensure memory is freed.
+  UniquePtr<ValueWithSize> valueWithSize(aContainer->Queue().popFirst());
+
+  // Step 5.
+  aContainer->SetQueueTotalSize(aContainer->QueueTotalSize() -
+                                valueWithSize->mSize);
+
+  // Step 6.
+  if (aContainer->QueueTotalSize() < 0) {
+    aContainer->SetQueueTotalSize(0);
+  }
+
+  // Step 7.
+  aResultValue.set(valueWithSize->mValue);
+}
+
+// https://streams.spec.whatwg.org/#peek-queue-value
+template <class QueueContainingClass>
+inline void PeekQueueValue(QueueContainingClass aContainer,
+                           JS::MutableHandle<JS::Value> aResultValue) {
+  // Step 1. Assert: container has [[queue]] and [[queueTotalSize]] internal
+  // slots.
+  // Step 2. Assert: container.[[queue]] is not empty.
+  MOZ_ASSERT(!aContainer->Queue().isEmpty());
+
+  // Step 3. Let valueWithSize be container.[[queue]][0].
+  ValueWithSize* valueWithSize = aContainer->Queue().getFirst();
+
+  // Step 4. Return valueWithSize’s value.
+  aResultValue.set(valueWithSize->mValue);
+}
+
+// https://streams.spec.whatwg.org/#reset-queue
+template <class QueueContainingClass>
+inline void ResetQueue(QueueContainingClass aContainer) {
+  // Step 1. Assert: container has [[queue]] and [[queueTotalSize]] internal
+  // slots. (implicit)
+
+  // Step 2. Set container.[[queue]] to a new empty list.
+  aContainer->Queue().clear();
+
+  // Step 3. Set container.[[queueTotalSize]] to 0.
+  aContainer->SetQueueTotalSize(0.0);
+}
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/streams/ReadIntoRequest.h b/dom/streams/ReadIntoRequest.h
new file mode 100644
index 0000000000..9be3d2c027
--- /dev/null
+++ b/dom/streams/ReadIntoRequest.h
@@ -0,0 +1,48 @@
+
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ReadIntoRequest_h
+#define mozilla_dom_ReadIntoRequest_h
+
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/LinkedList.h"
+#include "mozilla/RefPtr.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsISupportsImpl.h"
+
+namespace mozilla::dom {
+
+// This list element needs to be cycle collected by owning structures.
+struct ReadIntoRequest : public nsISupports,
+                         public LinkedListElement<RefPtr<ReadIntoRequest>> {
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(ReadIntoRequest)
+
+  // An algorithm taking a chunk, called when a chunk is available for reading
+  virtual void ChunkSteps(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                          ErrorResult& aRv) = 0;
+
+  // An algorithm taking a chunk or undefined, called when no chunks are
+  // available because the stream is closed
+  MOZ_CAN_RUN_SCRIPT
+  virtual void CloseSteps(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                          ErrorResult& aRv) = 0;
+
+  // An algorithm taking a JavaScript value, called when no chunks are available
+  // because the stream is errored
+  virtual void ErrorSteps(JSContext* aCx, JS::Handle<JS::Value> e,
+                          ErrorResult& aRv) = 0;
+
+ protected:
+  virtual ~ReadIntoRequest() = default;
+};
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/streams/ReadRequest.h b/dom/streams/ReadRequest.h
new file mode 100644
index 0000000000..8de991b4b9
--- /dev/null
+++ b/dom/streams/ReadRequest.h
@@ -0,0 +1,42 @@
+
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ReadRequest_h
+#define mozilla_dom_ReadRequest_h
+
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/LinkedList.h"
+#include "mozilla/RefPtr.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsISupportsImpl.h"
+
+namespace mozilla::dom {
+
+// List Owners must traverse this class.
+struct ReadRequest : public nsISupports,
+                     public LinkedListElement<RefPtr<ReadRequest>> {
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(ReadRequest)
+
+  // PipeToReadRequest::ChunkSteps can run script, for example.
+  MOZ_CAN_RUN_SCRIPT virtual void ChunkSteps(JSContext* aCx,
+                                             JS::Handle<JS::Value> aChunk,
+                                             ErrorResult& aRv) = 0;
+  MOZ_CAN_RUN_SCRIPT virtual void CloseSteps(JSContext* aCx,
+                                             ErrorResult& aRv) = 0;
+  virtual void ErrorSteps(JSContext* aCx, JS::Handle<JS::Value> e,
+                          ErrorResult& aRv) = 0;
+
+ protected:
+  virtual ~ReadRequest() = default;
+};
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/streams/ReadableByteStreamController.cpp b/dom/streams/ReadableByteStreamController.cpp
new file mode 100644
index 0000000000..6a38e0aebe
--- /dev/null
+++ b/dom/streams/ReadableByteStreamController.cpp
@@ -0,0 +1,2108 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/ReadableByteStreamController.h"
+
+#include "ReadIntoRequest.h"
+#include "js/ArrayBuffer.h"
+#include "js/ErrorReport.h"
+#include "js/Exception.h"
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "js/ValueArray.h"
+#include "js/experimental/TypedData.h"
+#include "js/friend/ErrorMessages.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/dom/ByteStreamHelpers.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/Promise-inl.h"
+#include "mozilla/dom/ReadableByteStreamControllerBinding.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/ReadableStreamBYOBReader.h"
+#include "mozilla/dom/ReadableStreamBYOBRequest.h"
+#include "mozilla/dom/ReadableStreamController.h"
+#include "mozilla/dom/ReadableStreamDefaultController.h"
+#include "mozilla/dom/ReadableStreamDefaultReader.h"
+#include "mozilla/dom/ReadableStreamGenericReader.h"
+#include "mozilla/dom/ToJSValue.h"
+#include "mozilla/dom/ScriptSettings.h"
+#include "mozilla/dom/UnderlyingSourceCallbackHelpers.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsIGlobalObject.h"
+#include "nsISupports.h"
+
+#include <algorithm>  // std::min
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-queue-entry
+struct ReadableByteStreamQueueEntry
+    : LinkedListElement<RefPtr<ReadableByteStreamQueueEntry>> {
+  NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(
+      ReadableByteStreamQueueEntry)
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_NATIVE_CLASS(
+      ReadableByteStreamQueueEntry)
+
+  ReadableByteStreamQueueEntry(JS::Handle<JSObject*> aBuffer,
+                               size_t aByteOffset, size_t aByteLength)
+      : mBuffer(aBuffer), mByteOffset(aByteOffset), mByteLength(aByteLength) {
+    mozilla::HoldJSObjects(this);
+  }
+
+  JSObject* Buffer() const { return mBuffer; }
+  void SetBuffer(JS::Handle<JSObject*> aBuffer) { mBuffer = aBuffer; }
+
+  size_t ByteOffset() const { return mByteOffset; }
+  void SetByteOffset(size_t aByteOffset) { mByteOffset = aByteOffset; }
+
+  size_t ByteLength() const { return mByteLength; }
+  void SetByteLength(size_t aByteLength) { mByteLength = aByteLength; }
+
+  void ClearBuffer() { mBuffer = nullptr; }
+
+ private:
+  // An ArrayBuffer, which will be a transferred version of the one originally
+  // supplied by the underlying byte source.
+  JS::Heap<JSObject*> mBuffer;
+
+  // A nonnegative integer number giving the byte offset derived from the view
+  // originally supplied by the underlying byte source
+  size_t mByteOffset = 0;
+
+  // A nonnegative integer number giving the byte length derived from the view
+  // originally supplied by the underlying byte source
+  size_t mByteLength = 0;
+
+  ~ReadableByteStreamQueueEntry() { mozilla::DropJSObjects(this); }
+};
+
+NS_IMPL_CYCLE_COLLECTION_WITH_JS_MEMBERS(ReadableByteStreamQueueEntry, (),
+                                         (mBuffer));
+
+struct PullIntoDescriptor final
+    : LinkedListElement<RefPtr<PullIntoDescriptor>> {
+  NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(PullIntoDescriptor)
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_NATIVE_CLASS(PullIntoDescriptor)
+
+  enum Constructor {
+    DataView,
+#define DEFINE_TYPED_CONSTRUCTOR_ENUM_NAMES(ExternalT, NativeT, Name) Name,
+    JS_FOR_EACH_TYPED_ARRAY(DEFINE_TYPED_CONSTRUCTOR_ENUM_NAMES)
+#undef DEFINE_TYPED_CONSTRUCTOR_ENUM_NAMES
+  };
+
+  static Constructor constructorFromScalar(JS::Scalar::Type type) {
+    switch (type) {
+#define REMAP_PULL_INTO_DESCRIPTOR_TYPE(ExternalT, NativeT, Name) \
+  case JS::Scalar::Name:                                          \
+    return Constructor::Name;
+      JS_FOR_EACH_TYPED_ARRAY(REMAP_PULL_INTO_DESCRIPTOR_TYPE)
+#undef REMAP
+
+      case JS::Scalar::Int64:
+      case JS::Scalar::Simd128:
+      case JS::Scalar::MaxTypedArrayViewType:
+        break;
+    }
+    MOZ_CRASH("Unexpected Scalar::Type");
+  }
+
+  PullIntoDescriptor(JS::Handle<JSObject*> aBuffer, uint64_t aBufferByteLength,
+                     uint64_t aByteOffset, uint64_t aByteLength,
+                     uint64_t aBytesFilled, uint64_t aElementSize,
+                     Constructor aViewConstructor, ReaderType aReaderType)
+      : mBuffer(aBuffer),
+        mBufferByteLength(aBufferByteLength),
+        mByteOffset(aByteOffset),
+        mByteLength(aByteLength),
+        mBytesFilled(aBytesFilled),
+        mElementSize(aElementSize),
+        mViewConstructor(aViewConstructor),
+        mReaderType(aReaderType) {
+    mozilla::HoldJSObjects(this);
+  }
+
+  JSObject* Buffer() const { return mBuffer; }
+  void SetBuffer(JS::Handle<JSObject*> aBuffer) { mBuffer = aBuffer; }
+
+  uint64_t BufferByteLength() const { return mBufferByteLength; }
+  void SetBufferByteLength(const uint64_t aBufferByteLength) {
+    mBufferByteLength = aBufferByteLength;
+  }
+
+  uint64_t ByteOffset() const { return mByteOffset; }
+  void SetByteOffset(const uint64_t aByteOffset) { mByteOffset = aByteOffset; }
+
+  uint64_t ByteLength() const { return mByteLength; }
+  void SetByteLength(const uint64_t aByteLength) { mByteLength = aByteLength; }
+
+  uint64_t BytesFilled() const { return mBytesFilled; }
+  void SetBytesFilled(const uint64_t aBytesFilled) {
+    mBytesFilled = aBytesFilled;
+  }
+
+  uint64_t ElementSize() const { return mElementSize; }
+  void SetElementSize(const uint64_t aElementSize) {
+    mElementSize = aElementSize;
+  }
+
+  Constructor ViewConstructor() const { return mViewConstructor; }
+
+  // Note: Named GetReaderType to avoid name conflict with type.
+  ReaderType GetReaderType() const { return mReaderType; }
+  void SetReaderType(const ReaderType aReaderType) {
+    mReaderType = aReaderType;
+  }
+
+  void ClearBuffer() { mBuffer = nullptr; }
+
+ private:
+  JS::Heap<JSObject*> mBuffer;
+  uint64_t mBufferByteLength = 0;
+  uint64_t mByteOffset = 0;
+  uint64_t mByteLength = 0;
+  uint64_t mBytesFilled = 0;
+  uint64_t mElementSize = 0;
+  Constructor mViewConstructor;
+  ReaderType mReaderType;
+
+  ~PullIntoDescriptor() { mozilla::DropJSObjects(this); }
+};
+
+NS_IMPL_CYCLE_COLLECTION_WITH_JS_MEMBERS(PullIntoDescriptor, (), (mBuffer));
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(ReadableByteStreamController)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(ReadableByteStreamController,
+                                                ReadableStreamController)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mByobRequest, mQueue, mPendingPullIntos)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(ReadableByteStreamController,
+                                                  ReadableStreamController)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mByobRequest, mQueue, mPendingPullIntos)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN_INHERITED(ReadableByteStreamController,
+                                               ReadableStreamController)
+  NS_IMPL_CYCLE_COLLECTION_TRACE_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_TRACE_END
+
+NS_IMPL_ADDREF_INHERITED(ReadableByteStreamController, ReadableStreamController)
+NS_IMPL_RELEASE_INHERITED(ReadableByteStreamController,
+                          ReadableStreamController)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ReadableByteStreamController)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+NS_INTERFACE_MAP_END_INHERITING(ReadableStreamController)
+
+ReadableByteStreamController::ReadableByteStreamController(
+    nsIGlobalObject* aGlobal)
+    : ReadableStreamController(aGlobal) {}
+
+ReadableByteStreamController::~ReadableByteStreamController() {
+  ClearPendingPullIntos();
+  ClearQueue();
+}
+
+void ReadableByteStreamController::ClearQueue() {
+  // Since the pull intos are traced only by the owning
+  // ReadableByteStreamController, when clearning the list we also clear JS
+  // references to avoid dangling JS references.
+  for (auto* queueEntry : mQueue) {
+    queueEntry->ClearBuffer();
+  }
+  mQueue.clear();
+}
+
+void ReadableByteStreamController::ClearPendingPullIntos() {
+  // Since the pull intos are traced only by the owning
+  // ReadableByteStreamController, when clearning the list we also clear JS
+  // references to avoid dangling JS references.
+  for (auto* pullInto : mPendingPullIntos) {
+    pullInto->ClearBuffer();
+  }
+  mPendingPullIntos.clear();
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamcontrollergetbyobrequest
+already_AddRefed<ReadableStreamBYOBRequest>
+ReadableByteStreamControllerGetBYOBRequest(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    ErrorResult& aRv) {
+  // Step 1.
+  if (!aController->GetByobRequest() &&
+      !aController->PendingPullIntos().isEmpty()) {
+    // Step 1.1:
+    PullIntoDescriptor* firstDescriptor =
+        aController->PendingPullIntos().getFirst();
+
+    // Step 1.2:
+    aRv.MightThrowJSException();
+    JS::Rooted<JSObject*> buffer(aCx, firstDescriptor->Buffer());
+    JS::Rooted<JSObject*> view(
+        aCx, JS_NewUint8ArrayWithBuffer(
+                 aCx, buffer,
+                 firstDescriptor->ByteOffset() + firstDescriptor->BytesFilled(),
+                 int64_t(firstDescriptor->ByteLength() -
+                         firstDescriptor->BytesFilled())));
+    if (!view) {
+      aRv.StealExceptionFromJSContext(aCx);
+      return nullptr;
+    }
+
+    // Step 1.3:
+    RefPtr<ReadableStreamBYOBRequest> byobRequest =
+        new ReadableStreamBYOBRequest(aController->GetParentObject());
+
+    // Step 1.4:
+    byobRequest->SetController(aController);
+
+    // Step 1.5:
+    byobRequest->SetView(view);
+
+    // Step 1.6:
+    aController->SetByobRequest(byobRequest);
+  }
+
+  // Step 2.
+  RefPtr<ReadableStreamBYOBRequest> request(aController->GetByobRequest());
+  return request.forget();
+}
+}  // namespace streams_abstract
+
+already_AddRefed<ReadableStreamBYOBRequest>
+ReadableByteStreamController::GetByobRequest(JSContext* aCx, ErrorResult& aRv) {
+  return ReadableByteStreamControllerGetBYOBRequest(aCx, this, aRv);
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-get-desired-size
+Nullable<double> ReadableByteStreamControllerGetDesiredSize(
+    const ReadableByteStreamController* aController) {
+  // Step 1.
+  ReadableStream::ReaderState state = aController->Stream()->State();
+
+  // Step 2.
+  if (state == ReadableStream::ReaderState::Errored) {
+    return nullptr;
+  }
+
+  // Step 3.
+  if (state == ReadableStream::ReaderState::Closed) {
+    return 0.0;
+  }
+
+  // Step 4.
+  return aController->StrategyHWM() - aController->QueueTotalSize();
+}
+
+Nullable<double> ReadableByteStreamController::GetDesiredSize() const {
+  // Step 1.
+  return ReadableByteStreamControllerGetDesiredSize(this);
+}
+
+JSObject* ReadableByteStreamController::WrapObject(
+    JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
+  return ReadableByteStreamController_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+namespace streams_abstract {
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-invalidate-byob-request
+static void ReadableByteStreamControllerInvalidateBYOBRequest(
+    ReadableByteStreamController* aController) {
+  // Step 1.
+  if (!aController->GetByobRequest()) {
+    return;
+  }
+
+  // Step 2.
+  aController->GetByobRequest()->SetController(nullptr);
+
+  // Step 3.
+  aController->GetByobRequest()->SetView(nullptr);
+
+  // Step 4.
+  aController->SetByobRequest(nullptr);
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-clear-pending-pull-intos
+void ReadableByteStreamControllerClearPendingPullIntos(
+    ReadableByteStreamController* aController) {
+  // Step 1.
+  ReadableByteStreamControllerInvalidateBYOBRequest(aController);
+
+  // Step 2.
+  aController->ClearPendingPullIntos();
+}
+
+// https://streams.spec.whatwg.org/#reset-queue
+void ResetQueue(ReadableByteStreamController* aContainer) {
+  // Step 1. Implied by type.
+  // Step 2.
+  aContainer->ClearQueue();
+
+  // Step 3.
+  aContainer->SetQueueTotalSize(0);
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-clear-algorithms
+void ReadableByteStreamControllerClearAlgorithms(
+    ReadableByteStreamController* aController) {
+  // Step 1. Set controller.[[pullAlgorithm]] to undefined.
+  // Step 2. Set controller.[[cancelAlgorithm]] to undefined.
+  aController->ClearAlgorithms();
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-error
+void ReadableByteStreamControllerError(
+    ReadableByteStreamController* aController, JS::Handle<JS::Value> aValue,
+    ErrorResult& aRv) {
+  // Step 1. Let stream be controller.[[stream]].
+  ReadableStream* stream = aController->Stream();
+
+  // Step 2. If stream.[[state]] is not "readable", return.
+  if (stream->State() != ReadableStream::ReaderState::Readable) {
+    return;
+  }
+
+  // Step 3. Perform
+  // !ReadableByteStreamControllerClearPendingPullIntos(controller).
+  ReadableByteStreamControllerClearPendingPullIntos(aController);
+
+  // Step 4. Perform !ResetQueue(controller).
+  ResetQueue(aController);
+
+  // Step 5. Perform !ReadableByteStreamControllerClearAlgorithms(controller).
+  ReadableByteStreamControllerClearAlgorithms(aController);
+
+  // Step 6. Perform !ReadableStreamError(stream, e).
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(aController->GetParentObject())) {
+    return;
+  }
+  ReadableStreamError(jsapi.cx(), stream, aValue, aRv);
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-close
+void ReadableByteStreamControllerClose(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    ErrorResult& aRv) {
+  // Step 1.
+  RefPtr<ReadableStream> stream = aController->Stream();
+
+  // Step 2.
+  if (aController->CloseRequested() ||
+      stream->State() != ReadableStream::ReaderState::Readable) {
+    return;
+  }
+
+  // Step 3.
+  if (aController->QueueTotalSize() > 0) {
+    // Step 3.1
+    aController->SetCloseRequested(true);
+    // Step 3.2
+    return;
+  }
+
+  // Step 4.
+  if (!aController->PendingPullIntos().isEmpty()) {
+    // Step 4.1
+    PullIntoDescriptor* firstPendingPullInto =
+        aController->PendingPullIntos().getFirst();
+    // Step 4.2
+    if (firstPendingPullInto->BytesFilled() > 0) {
+      // Step 4.2.1
+      ErrorResult rv;
+      rv.ThrowTypeError("Leftover Bytes");
+
+      JS::Rooted<JS::Value> exception(aCx);
+      MOZ_ALWAYS_TRUE(ToJSValue(aCx, std::move(rv), &exception));
+
+      // Step 4.2.2
+      ReadableByteStreamControllerError(aController, exception, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+
+      aRv.MightThrowJSException();
+      aRv.ThrowJSException(aCx, exception);
+      return;
+    }
+  }
+
+  // Step 5.
+  ReadableByteStreamControllerClearAlgorithms(aController);
+
+  // Step 6.
+  ReadableStreamClose(aCx, stream, aRv);
+}
+
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#rbs-controller-close
+void ReadableByteStreamController::Close(JSContext* aCx, ErrorResult& aRv) {
+  // Step 1.
+  if (mCloseRequested) {
+    aRv.ThrowTypeError("Close already requested");
+    return;
+  }
+
+  // Step 2.
+  if (Stream()->State() != ReadableStream::ReaderState::Readable) {
+    aRv.ThrowTypeError("Closing un-readable stream controller");
+    return;
+  }
+
+  // Step 3.
+  ReadableByteStreamControllerClose(aCx, this, aRv);
+}
+
+namespace streams_abstract {
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-enqueue-chunk-to-queue
+void ReadableByteStreamControllerEnqueueChunkToQueue(
+    ReadableByteStreamController* aController,
+    JS::Handle<JSObject*> aTransferredBuffer, size_t aByteOffset,
+    size_t aByteLength) {
+  // Step 1.
+  RefPtr<ReadableByteStreamQueueEntry> queueEntry =
+      new ReadableByteStreamQueueEntry(aTransferredBuffer, aByteOffset,
+                                       aByteLength);
+  aController->Queue().insertBack(queueEntry);
+
+  // Step 2.
+  aController->AddToQueueTotalSize(double(aByteLength));
+}
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamcontrollerenqueueclonedchunktoqueue
+void ReadableByteStreamControllerEnqueueClonedChunkToQueue(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    JS::Handle<JSObject*> aBuffer, size_t aByteOffset, size_t aByteLength,
+    ErrorResult& aRv) {
+  // Step 1. Let cloneResult be CloneArrayBuffer(buffer, byteOffset, byteLength,
+  // %ArrayBuffer%).
+  aRv.MightThrowJSException();
+  JS::Rooted<JSObject*> cloneResult(
+      aCx, JS::ArrayBufferClone(aCx, aBuffer, aByteOffset, aByteLength));
+
+  // Step 2. If cloneResult is an abrupt completion,
+  if (!cloneResult) {
+    JS::Rooted<JS::Value> exception(aCx);
+    if (!JS_GetPendingException(aCx, &exception)) {
+      // Uncatchable exception; we should mark aRv and return.
+      aRv.StealExceptionFromJSContext(aCx);
+      return;
+    }
+    JS_ClearPendingException(aCx);
+
+    // Step 2.1. Perform ! ReadableByteStreamControllerError(controller,
+    // cloneResult.[[Value]]).
+    ReadableByteStreamControllerError(aController, exception, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+
+    // Step 2.2. Return cloneResult.
+    aRv.ThrowJSException(aCx, exception);
+    return;
+  }
+
+  // Step 3. Perform !
+  // ReadableByteStreamControllerEnqueueChunkToQueue(controller,
+  // cloneResult.[[Value]], 0, byteLength).
+  ReadableByteStreamControllerEnqueueChunkToQueue(aController, cloneResult, 0,
+                                                  aByteLength);
+}
+
+already_AddRefed<PullIntoDescriptor>
+ReadableByteStreamControllerShiftPendingPullInto(
+    ReadableByteStreamController* aController);
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamcontrollerenqueuedetachedpullintotoqueue
+void ReadableByteStreamControllerEnqueueDetachedPullIntoToQueue(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    PullIntoDescriptor* aPullIntoDescriptor, ErrorResult& aRv) {
+  // Step 1. Assert: pullIntoDescriptor’s reader type is "none".
+  MOZ_ASSERT(aPullIntoDescriptor->GetReaderType() == ReaderType::None);
+
+  // Step 2. If pullIntoDescriptor’s bytes filled > 0,
+  // perform ? ReadableByteStreamControllerEnqueueClonedChunkToQueue(controller,
+  // pullIntoDescriptor’s buffer, pullIntoDescriptor’s byte offset,
+  // pullIntoDescriptor’s bytes filled).
+  if (aPullIntoDescriptor->BytesFilled() > 0) {
+    JS::Rooted<JSObject*> buffer(aCx, aPullIntoDescriptor->Buffer());
+    ReadableByteStreamControllerEnqueueClonedChunkToQueue(
+        aCx, aController, buffer, aPullIntoDescriptor->ByteOffset(),
+        aPullIntoDescriptor->BytesFilled(), aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+  }
+
+  // Step 3. Perform !
+  // ReadableByteStreamControllerShiftPendingPullInto(controller).
+  RefPtr<PullIntoDescriptor> discarded =
+      ReadableByteStreamControllerShiftPendingPullInto(aController);
+  (void)discarded;
+}
+
+// https://streams.spec.whatwg.org/#readable-stream-get-num-read-into-requests
+static size_t ReadableStreamGetNumReadIntoRequests(ReadableStream* aStream) {
+  // Step 1.
+  MOZ_ASSERT(ReadableStreamHasBYOBReader(aStream));
+
+  // Step 2.
+  return aStream->GetReader()->AsBYOB()->ReadIntoRequests().length();
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-should-call-pull
+bool ReadableByteStreamControllerShouldCallPull(
+    ReadableByteStreamController* aController) {
+  // Step 1. Let stream be controller.[[stream]].
+  ReadableStream* stream = aController->Stream();
+
+  // Step 2. If stream.[[state]] is not "readable", return false.
+  if (stream->State() != ReadableStream::ReaderState::Readable) {
+    return false;
+  }
+
+  // Step 3. If controller.[[closeRequested]] is true, return false.
+  if (aController->CloseRequested()) {
+    return false;
+  }
+
+  // Step 4. If controller.[[started]] is false, return false.
+  if (!aController->Started()) {
+    return false;
+  }
+
+  // Step 5. If ! ReadableStreamHasDefaultReader(stream) is true
+  // and ! ReadableStreamGetNumReadRequests(stream) > 0, return true.
+  if (ReadableStreamHasDefaultReader(stream) &&
+      ReadableStreamGetNumReadRequests(stream) > 0) {
+    return true;
+  }
+
+  // Step 6. If ! ReadableStreamHasBYOBReader(stream) is true
+  // and ! ReadableStreamGetNumReadIntoRequests(stream) > 0, return true.
+  if (ReadableStreamHasBYOBReader(stream) &&
+      ReadableStreamGetNumReadIntoRequests(stream) > 0) {
+    return true;
+  }
+
+  // Step 7. Let desiredSize be
+  // ! ReadableByteStreamControllerGetDesiredSize(controller).
+  Nullable<double> desiredSize =
+      ReadableByteStreamControllerGetDesiredSize(aController);
+
+  // Step 8. Assert: desiredSize is not null.
+  MOZ_ASSERT(!desiredSize.IsNull());
+
+  // Step 9. If desiredSize > 0, return true.
+  // Step 10. Return false.
+  return desiredSize.Value() > 0;
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-call-pull-if-needed
+void ReadableByteStreamControllerCallPullIfNeeded(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    ErrorResult& aRv) {
+  // Step 1.
+  bool shouldPull = ReadableByteStreamControllerShouldCallPull(aController);
+
+  // Step 2.
+  if (!shouldPull) {
+    return;
+  }
+
+  // Step 3.
+  if (aController->Pulling()) {
+    aController->SetPullAgain(true);
+    return;
+  }
+
+  // Step 4.
+  MOZ_ASSERT(!aController->PullAgain());
+
+  // Step 5.
+  aController->SetPulling(true);
+
+  // Step 6.
+  RefPtr<ReadableStreamController> controller(aController);
+  RefPtr<UnderlyingSourceAlgorithmsBase> algorithms =
+      aController->GetAlgorithms();
+  RefPtr<Promise> pullPromise = algorithms->PullCallback(aCx, *controller, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Steps 7+8
+  pullPromise->AddCallbacksWithCycleCollectedArgs(
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         ReadableByteStreamController* aController)
+          MOZ_CAN_RUN_SCRIPT_BOUNDARY {
+            // Step 7.1
+            aController->SetPulling(false);
+            // Step 7.2
+            if (aController->PullAgain()) {
+              // Step 7.2.1
+              aController->SetPullAgain(false);
+
+              // Step 7.2.2
+              ReadableByteStreamControllerCallPullIfNeeded(
+                  aCx, MOZ_KnownLive(aController), aRv);
+            }
+          },
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         ReadableByteStreamController* aController) {
+        // Step 8.1
+        ReadableByteStreamControllerError(aController, aValue, aRv);
+      },
+      RefPtr(aController));
+}
+
+bool ReadableByteStreamControllerFillPullIntoDescriptorFromQueue(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    PullIntoDescriptor* aPullIntoDescriptor, ErrorResult& aRv);
+
+JSObject* ReadableByteStreamControllerConvertPullIntoDescriptor(
+    JSContext* aCx, PullIntoDescriptor* pullIntoDescriptor, ErrorResult& aRv);
+
+// https://streams.spec.whatwg.org/#readable-stream-fulfill-read-into-request
+MOZ_CAN_RUN_SCRIPT
+void ReadableStreamFulfillReadIntoRequest(JSContext* aCx,
+                                          ReadableStream* aStream,
+                                          JS::Handle<JS::Value> aChunk,
+                                          bool done, ErrorResult& aRv) {
+  // Step 1. Assert: !ReadableStreamHasBYOBReader(stream) is true.
+  MOZ_ASSERT(ReadableStreamHasBYOBReader(aStream));
+
+  // Step 2. Let reader be stream.[[reader]].
+  ReadableStreamBYOBReader* reader = aStream->GetReader()->AsBYOB();
+
+  // Step 3. Assert: reader.[[readIntoRequests]] is not empty.
+  MOZ_ASSERT(!reader->ReadIntoRequests().isEmpty());
+
+  // Step 4. Let readIntoRequest be reader.[[readIntoRequests]][0].
+  // Step 5. Remove readIntoRequest from reader.[[readIntoRequests]].
+  RefPtr<ReadIntoRequest> readIntoRequest =
+      reader->ReadIntoRequests().popFirst();
+
+  // Step 6. If done is true, perform readIntoRequest’s close steps, given
+  // chunk.
+  if (done) {
+    readIntoRequest->CloseSteps(aCx, aChunk, aRv);
+    return;
+  }
+
+  // Step 7. Otherwise, perform readIntoRequest’s chunk steps, given chunk.
+  readIntoRequest->ChunkSteps(aCx, aChunk, aRv);
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-commit-pull-into-descriptor
+MOZ_CAN_RUN_SCRIPT
+void ReadableByteStreamControllerCommitPullIntoDescriptor(
+    JSContext* aCx, ReadableStream* aStream,
+    PullIntoDescriptor* pullIntoDescriptor, ErrorResult& aRv) {
+  // Step 1. Assert: stream.[[state]] is not "errored".
+  MOZ_ASSERT(aStream->State() != ReadableStream::ReaderState::Errored);
+
+  // Step 2. Assert: pullIntoDescriptor.reader type is not "none".
+  MOZ_ASSERT(pullIntoDescriptor->GetReaderType() != ReaderType::None);
+
+  // Step 3. Let done be false.
+  bool done = false;
+
+  // Step 4. If stream.[[state]] is "closed",
+  if (aStream->State() == ReadableStream::ReaderState::Closed) {
+    // Step 4.1. Assert: pullIntoDescriptor’s bytes filled is 0.
+    MOZ_ASSERT(pullIntoDescriptor->BytesFilled() == 0);
+
+    // Step 4.2. Set done to true.
+    done = true;
+  }
+
+  // Step 5. Let filledView be !
+  // ReadableByteStreamControllerConvertPullIntoDescriptor(pullIntoDescriptor).
+  JS::Rooted<JSObject*> filledView(
+      aCx, ReadableByteStreamControllerConvertPullIntoDescriptor(
+               aCx, pullIntoDescriptor, aRv));
+  if (aRv.Failed()) {
+    return;
+  }
+  JS::Rooted<JS::Value> filledViewValue(aCx, JS::ObjectValue(*filledView));
+
+  // Step 6. If pullIntoDescriptor’s reader type is "default",
+  if (pullIntoDescriptor->GetReaderType() == ReaderType::Default) {
+    // Step 6.1. Perform !ReadableStreamFulfillReadRequest(stream, filledView,
+    // done).
+    ReadableStreamFulfillReadRequest(aCx, aStream, filledViewValue, done, aRv);
+    return;
+  }
+
+  // Step 7.1. Assert: pullIntoDescriptor’s reader type is "byob".
+  MOZ_ASSERT(pullIntoDescriptor->GetReaderType() == ReaderType::BYOB);
+
+  // Step 7.2 Perform !ReadableStreamFulfillReadIntoRequest(stream, filledView,
+  // done).
+  ReadableStreamFulfillReadIntoRequest(aCx, aStream, filledViewValue, done,
+                                       aRv);
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-process-pull-into-descriptors-using-queue
+MOZ_CAN_RUN_SCRIPT
+void ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    ErrorResult& aRv) {
+  // Step 1. Assert: controller.[[closeRequested]] is false.
+  MOZ_ASSERT(!aController->CloseRequested());
+
+  // Step 2. While controller.[[pendingPullIntos]] is not empty,
+  while (!aController->PendingPullIntos().isEmpty()) {
+    // Step 2.1 If controller.[[queueTotalSize]] is 0, return.
+    if (aController->QueueTotalSize() == 0) {
+      return;
+    }
+
+    // Step 2.2. Let pullIntoDescriptor be controller.[[pendingPullIntos]][0].
+    RefPtr<PullIntoDescriptor> pullIntoDescriptor =
+        aController->PendingPullIntos().getFirst();
+
+    // Step 2.3. If
+    // !ReadableByteStreamControllerFillPullIntoDescriptorFromQueue(controller,
+    // pullIntoDescriptor) is true,
+    bool ready = ReadableByteStreamControllerFillPullIntoDescriptorFromQueue(
+        aCx, aController, pullIntoDescriptor, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+
+    if (ready) {
+      //  Step 2.3.1. Perform
+      //  !ReadableByteStreamControllerShiftPendingPullInto(controller).
+      RefPtr<PullIntoDescriptor> discardedPullIntoDescriptor =
+          ReadableByteStreamControllerShiftPendingPullInto(aController);
+
+      // Step 2.3.2. Perform
+      // !ReadableByteStreamControllerCommitPullIntoDescriptor(controller.[[stream]],
+      //         pullIntoDescriptor).
+      RefPtr<ReadableStream> stream(aController->Stream());
+      ReadableByteStreamControllerCommitPullIntoDescriptor(
+          aCx, stream, pullIntoDescriptor, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+    }
+  }
+}
+
+MOZ_CAN_RUN_SCRIPT
+void ReadableByteStreamControllerHandleQueueDrain(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    ErrorResult& aRv);
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamcontrollerfillreadrequestfromqueue
+MOZ_CAN_RUN_SCRIPT void ReadableByteStreamControllerFillReadRequestFromQueue(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    ReadRequest* aReadRequest, ErrorResult& aRv) {
+  // Step 1. Assert: controller.[[queueTotalSize]] > 0.
+  MOZ_ASSERT(aController->QueueTotalSize() > 0);
+  // Also assert that the queue has a non-zero length;
+  MOZ_ASSERT(aController->Queue().length() > 0);
+
+  // Step 2. Let entry be controller.[[queue]][0].
+  // Step 3. Remove entry from controller.[[queue]].
+  RefPtr<ReadableByteStreamQueueEntry> entry = aController->Queue().popFirst();
+
+  // Assert that we actually got an entry.
+  MOZ_ASSERT(entry);
+
+  // Step 4. Set controller.[[queueTotalSize]] to controller.[[queueTotalSize]]
+  // − entry’s byte length.
+  aController->SetQueueTotalSize(aController->QueueTotalSize() -
+                                 double(entry->ByteLength()));
+
+  // Step 5. Perform ! ReadableByteStreamControllerHandleQueueDrain(controller).
+  ReadableByteStreamControllerHandleQueueDrain(aCx, aController, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 6. Let view be ! Construct(%Uint8Array%, « entry’s buffer, entry’s
+  // byte offset, entry’s byte length »).
+  aRv.MightThrowJSException();
+  JS::Rooted<JSObject*> buffer(aCx, entry->Buffer());
+  JS::Rooted<JSObject*> view(
+      aCx, JS_NewUint8ArrayWithBuffer(aCx, buffer, entry->ByteOffset(),
+                                      int64_t(entry->ByteLength())));
+  if (!view) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+
+  // Step 7. Perform readRequest’s chunk steps, given view.
+  JS::Rooted<JS::Value> viewValue(aCx, JS::ObjectValue(*view));
+  aReadRequest->ChunkSteps(aCx, viewValue, aRv);
+}
+
+MOZ_CAN_RUN_SCRIPT void
+ReadableByteStreamControllerProcessReadRequestsUsingQueue(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    ErrorResult& aRv) {
+  // Step 1. Let reader be controller.[[stream]].[[reader]].
+  // Step 2. Assert: reader implements ReadableStreamDefaultReader.
+  RefPtr<ReadableStreamDefaultReader> reader =
+      aController->Stream()->GetDefaultReader();
+
+  // Step 3. While reader.[[readRequests]] is not empty,
+  while (!reader->ReadRequests().isEmpty()) {
+    // Step 3.1. If controller.[[queueTotalSize]] is 0, return.
+    if (aController->QueueTotalSize() == 0) {
+      return;
+    }
+
+    // Step 3.2. Let readRequest be reader.[[readRequests]][0].
+    // Step 3.3. Remove readRequest from reader.[[readRequests]].
+    RefPtr<ReadRequest> readRequest = reader->ReadRequests().popFirst();
+
+    // Step 3.4. Perform !
+    // ReadableByteStreamControllerFillReadRequestFromQueue(controller,
+    // readRequest).
+    ReadableByteStreamControllerFillReadRequestFromQueue(aCx, aController,
+                                                         readRequest, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+  }
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-enqueue
+void ReadableByteStreamControllerEnqueue(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    JS::Handle<JSObject*> aChunk, ErrorResult& aRv) {
+  aRv.MightThrowJSException();
+
+  // Step 1.
+  RefPtr<ReadableStream> stream = aController->Stream();
+
+  // Step 2.
+  if (aController->CloseRequested() ||
+      stream->State() != ReadableStream::ReaderState::Readable) {
+    return;
+  }
+
+  // Step 3.
+  bool isShared;
+  JS::Rooted<JSObject*> buffer(
+      aCx, JS_GetArrayBufferViewBuffer(aCx, aChunk, &isShared));
+  if (!buffer) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+
+  // Step 4.
+  size_t byteOffset = JS_GetArrayBufferViewByteOffset(aChunk);
+
+  // Step 5.
+  size_t byteLength = JS_GetArrayBufferViewByteLength(aChunk);
+
+  // Step 6.
+  if (JS::IsDetachedArrayBufferObject(buffer)) {
+    aRv.ThrowTypeError("Detatched Array Buffer");
+    return;
+  }
+
+  // Step 7.
+  JS::Rooted<JSObject*> transferredBuffer(aCx,
+                                          TransferArrayBuffer(aCx, buffer));
+  if (!transferredBuffer) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+
+  // Step 8.
+  if (!aController->PendingPullIntos().isEmpty()) {
+    // Step 8.1
+    RefPtr<PullIntoDescriptor> firstPendingPullInto =
+        aController->PendingPullIntos().getFirst();
+
+    // Step 8.2
+    JS::Rooted<JSObject*> pendingBuffer(aCx, firstPendingPullInto->Buffer());
+    if (JS::IsDetachedArrayBufferObject(pendingBuffer)) {
+      aRv.ThrowTypeError("Pending PullInto has detached buffer");
+      return;
+    }
+
+    // Step 8.3. Perform !
+    // ReadableByteStreamControllerInvalidateBYOBRequest(controller).
+    ReadableByteStreamControllerInvalidateBYOBRequest(aController);
+
+    // Step 8.4. Set firstPendingPullInto’s buffer to !
+    // TransferArrayBuffer(firstPendingPullInto’s buffer).
+    pendingBuffer = TransferArrayBuffer(aCx, pendingBuffer);
+    if (!pendingBuffer) {
+      aRv.StealExceptionFromJSContext(aCx);
+      return;
+    }
+    firstPendingPullInto->SetBuffer(pendingBuffer);
+
+    // Step 8.5. If firstPendingPullInto’s reader type is "none", perform ?
+    // ReadableByteStreamControllerEnqueueDetachedPullIntoToQueue(controller,
+    // firstPendingPullInto).
+    if (firstPendingPullInto->GetReaderType() == ReaderType::None) {
+      ReadableByteStreamControllerEnqueueDetachedPullIntoToQueue(
+          aCx, aController, firstPendingPullInto, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+    }
+  }
+
+  // Step 9. If ! ReadableStreamHasDefaultReader(stream) is true,
+  if (ReadableStreamHasDefaultReader(stream)) {
+    // Step 9.1. Perform !
+    // ReadableByteStreamControllerProcessReadRequestsUsingQueue(controller).
+    ReadableByteStreamControllerProcessReadRequestsUsingQueue(aCx, aController,
+                                                              aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+
+    // Step 9.2. If ! ReadableStreamGetNumReadRequests(stream) is 0,
+    if (ReadableStreamGetNumReadRequests(stream) == 0) {
+      // Step 9.2.1 Assert: controller.[[pendingPullIntos]] is empty.
+      MOZ_ASSERT(aController->PendingPullIntos().isEmpty());
+
+      // Step 9.2.2. Perform !
+      // ReadableByteStreamControllerEnqueueChunkToQueue(controller,
+      // transferredBuffer, byteOffset, byteLength).
+      ReadableByteStreamControllerEnqueueChunkToQueue(
+          aController, transferredBuffer, byteOffset, byteLength);
+
+      // Step 9.3. Otherwise,
+    } else {
+      // Step 9.3.1 Assert: controller.[[queue]] is empty.
+      MOZ_ASSERT(aController->Queue().isEmpty());
+
+      // Step 9.3.2. If controller.[[pendingPullIntos]] is not empty,
+      if (!aController->PendingPullIntos().isEmpty()) {
+        // Step 9.3.2.1. Assert: controller.[[pendingPullIntos]][0]'s reader
+        // type is "default".
+        MOZ_ASSERT(
+            aController->PendingPullIntos().getFirst()->GetReaderType() ==
+            ReaderType::Default);
+
+        // Step 9.3.2.2. Perform !
+        // ReadableByteStreamControllerShiftPendingPullInto(controller).
+        RefPtr<PullIntoDescriptor> pullIntoDescriptor =
+            ReadableByteStreamControllerShiftPendingPullInto(aController);
+        (void)pullIntoDescriptor;
+      }
+
+      // Step 9.3.3. Let transferredView be ! Construct(%Uint8Array%, «
+      // transferredBuffer, byteOffset, byteLength »).
+      JS::Rooted<JSObject*> transferredView(
+          aCx, JS_NewUint8ArrayWithBuffer(aCx, transferredBuffer, byteOffset,
+                                          int64_t(byteLength)));
+      if (!transferredView) {
+        aRv.StealExceptionFromJSContext(aCx);
+        return;
+      }
+
+      // Step 9.3.4. Perform ! ReadableStreamFulfillReadRequest(stream,
+      // transferredView, false).
+      JS::Rooted<JS::Value> transferredViewValue(
+          aCx, JS::ObjectValue(*transferredView));
+      ReadableStreamFulfillReadRequest(aCx, stream, transferredViewValue, false,
+                                       aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+    }
+
+    // Step 10. Otherwise, if ! ReadableStreamHasBYOBReader(stream) is true,
+  } else if (ReadableStreamHasBYOBReader(stream)) {
+    // Step 10.1. Perform !
+    // ReadableByteStreamControllerEnqueueChunkToQueue(controller,
+    // transferredBuffer, byteOffset, byteLength).
+    ReadableByteStreamControllerEnqueueChunkToQueue(
+        aController, transferredBuffer, byteOffset, byteLength);
+
+    // Step 10.2 Perform !
+    // ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(controller).
+    ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(
+        aCx, aController, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+
+    // Step 11. Otherwise,
+  } else {
+    // Step 11.1. Assert: ! IsReadableStreamLocked(stream) is false.
+    MOZ_ASSERT(!IsReadableStreamLocked(stream));
+
+    // Step 11.2. Perform !
+    // ReadableByteStreamControllerEnqueueChunkToQueue(controller,
+    // transferredBuffer, byteOffset, byteLength).
+    ReadableByteStreamControllerEnqueueChunkToQueue(
+        aController, transferredBuffer, byteOffset, byteLength);
+  }
+
+  // Step 12. Perform !
+  // ReadableByteStreamControllerCallPullIfNeeded(controller).
+  ReadableByteStreamControllerCallPullIfNeeded(aCx, aController, aRv);
+}
+
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#rbs-controller-enqueue
+void ReadableByteStreamController::Enqueue(JSContext* aCx,
+                                           const ArrayBufferView& aChunk,
+                                           ErrorResult& aRv) {
+  // Step 1.
+  JS::Rooted<JSObject*> chunk(aCx, aChunk.Obj());
+  if (JS_GetArrayBufferViewByteLength(chunk) == 0) {
+    aRv.ThrowTypeError("Zero Length View");
+    return;
+  }
+
+  // Step 2.
+  bool isShared;
+  JS::Rooted<JSObject*> viewedArrayBuffer(
+      aCx, JS_GetArrayBufferViewBuffer(aCx, chunk, &isShared));
+  if (!viewedArrayBuffer) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+
+  if (JS::GetArrayBufferByteLength(viewedArrayBuffer) == 0) {
+    aRv.ThrowTypeError("Zero Length Buffer");
+    return;
+  }
+
+  // Step 3.
+  if (CloseRequested()) {
+    aRv.ThrowTypeError("close requested");
+    return;
+  }
+
+  // Step 4.
+  if (Stream()->State() != ReadableStream::ReaderState::Readable) {
+    aRv.ThrowTypeError("Not Readable");
+    return;
+  }
+
+  // Step 5.
+  ReadableByteStreamControllerEnqueue(aCx, this, chunk, aRv);
+}
+
+// https://streams.spec.whatwg.org/#rbs-controller-error
+void ReadableByteStreamController::Error(JSContext* aCx,
+                                         JS::Handle<JS::Value> aErrorValue,
+                                         ErrorResult& aRv) {
+  // Step 1.
+  ReadableByteStreamControllerError(this, aErrorValue, aRv);
+}
+
+// https://streams.spec.whatwg.org/#rbs-controller-private-cancel
+already_AddRefed<Promise> ReadableByteStreamController::CancelSteps(
+    JSContext* aCx, JS::Handle<JS::Value> aReason, ErrorResult& aRv) {
+  // Step 1.
+  ReadableByteStreamControllerClearPendingPullIntos(this);
+
+  // Step 2.
+  ResetQueue(this);
+
+  // Step 3.
+  Optional<JS::Handle<JS::Value>> reason(aCx, aReason);
+  RefPtr<UnderlyingSourceAlgorithmsBase> algorithms = mAlgorithms;
+  RefPtr<Promise> result = algorithms->CancelCallback(aCx, reason, aRv);
+  if (NS_WARN_IF(aRv.Failed())) {
+    return nullptr;
+  }
+  // Step 4.
+  ReadableByteStreamControllerClearAlgorithms(this);
+
+  // Step 5.
+  return result.forget();
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-handle-queue-drain
+void ReadableByteStreamControllerHandleQueueDrain(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    ErrorResult& aRv) {
+  // Step 1.
+  MOZ_ASSERT(aController->Stream()->State() ==
+             ReadableStream::ReaderState::Readable);
+
+  // Step 2.
+  if (aController->QueueTotalSize() == 0 && aController->CloseRequested()) {
+    // Step 2.1
+    ReadableByteStreamControllerClearAlgorithms(aController);
+
+    // Step 2.2
+    RefPtr<ReadableStream> stream = aController->Stream();
+    ReadableStreamClose(aCx, stream, aRv);
+    return;
+  }
+
+  // Step 3.1
+  ReadableByteStreamControllerCallPullIfNeeded(aCx, aController, aRv);
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#rbs-controller-private-pull
+void ReadableByteStreamController::PullSteps(JSContext* aCx,
+                                             ReadRequest* aReadRequest,
+                                             ErrorResult& aRv) {
+  // Step 1.
+  ReadableStream* stream = Stream();
+
+  // Step 2.
+  MOZ_ASSERT(ReadableStreamHasDefaultReader(stream));
+
+  // Step 3.
+  if (QueueTotalSize() > 0) {
+    // Step 3.1. Assert: ! ReadableStreamGetNumReadRequests ( stream ) is 0.
+    MOZ_ASSERT(ReadableStreamGetNumReadRequests(stream) == 0);
+
+    // Step 3.2. Perform !
+    // ReadableByteStreamControllerFillReadRequestFromQueue(this, readRequest).
+    ReadableByteStreamControllerFillReadRequestFromQueue(aCx, this,
+                                                         aReadRequest, aRv);
+
+    // Step 3.3. Return.
+    return;
+  }
+
+  // Step 4.
+  Maybe<uint64_t> autoAllocateChunkSize = AutoAllocateChunkSize();
+
+  // Step 5.
+  if (autoAllocateChunkSize) {
+    // Step 5.1
+    aRv.MightThrowJSException();
+    JS::Rooted<JSObject*> buffer(
+        aCx, JS::NewArrayBuffer(aCx, *autoAllocateChunkSize));
+    // Step 5.2
+    if (!buffer) {
+      // Step 5.2.1
+      JS::Rooted<JS::Value> bufferError(aCx);
+      if (!JS_GetPendingException(aCx, &bufferError)) {
+        // Uncatchable exception; we should mark aRv and return.
+        aRv.StealExceptionFromJSContext(aCx);
+        return;
+      }
+
+      // It's not expliclitly stated, but I assume the intention here is that
+      // we perform a normal completion here.
+      JS_ClearPendingException(aCx);
+
+      aReadRequest->ErrorSteps(aCx, bufferError, aRv);
+
+      // Step 5.2.2.
+      return;
+    }
+
+    // Step 5.3
+    RefPtr<PullIntoDescriptor> pullIntoDescriptor = new PullIntoDescriptor(
+        buffer, *autoAllocateChunkSize, 0, *autoAllocateChunkSize, 0, 1,
+        PullIntoDescriptor::Constructor::Uint8, ReaderType::Default);
+
+    //  Step 5.4
+    PendingPullIntos().insertBack(pullIntoDescriptor);
+  }
+
+  // Step 6.
+  ReadableStreamAddReadRequest(stream, aReadRequest);
+
+  // Step 7.
+  ReadableByteStreamControllerCallPullIfNeeded(aCx, this, aRv);
+}
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamcontroller-releasesteps
+void ReadableByteStreamController::ReleaseSteps() {
+  // Step 1. If this.[[pendingPullIntos]] is not empty,
+  if (!PendingPullIntos().isEmpty()) {
+    // Step 1.1. Let firstPendingPullInto be this.[[pendingPullIntos]][0].
+    RefPtr<PullIntoDescriptor> firstPendingPullInto =
+        PendingPullIntos().popFirst();
+
+    // Step 1.2. Set firstPendingPullInto’s reader type to "none".
+    firstPendingPullInto->SetReaderType(ReaderType::None);
+
+    // Step 1.3. Set this.[[pendingPullIntos]] to the list «
+    // firstPendingPullInto ».
+    PendingPullIntos().clear();
+    PendingPullIntos().insertBack(firstPendingPullInto);
+  }
+}
+
+namespace streams_abstract {
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-shift-pending-pull-into
+already_AddRefed<PullIntoDescriptor>
+ReadableByteStreamControllerShiftPendingPullInto(
+    ReadableByteStreamController* aController) {
+  // Step 1.
+  MOZ_ASSERT(!aController->GetByobRequest());
+
+  // Step 2 + 3
+  RefPtr<PullIntoDescriptor> descriptor =
+      aController->PendingPullIntos().popFirst();
+
+  // Step 4.
+  return descriptor.forget();
+}
+
+JSObject* ConstructFromPullIntoConstructor(
+    JSContext* aCx, PullIntoDescriptor::Constructor constructor,
+    JS::Handle<JSObject*> buffer, size_t byteOffset, size_t length) {
+  switch (constructor) {
+    case PullIntoDescriptor::Constructor::DataView:
+      return JS_NewDataView(aCx, buffer, byteOffset, length);
+      break;
+
+#define CONSTRUCT_TYPED_ARRAY_TYPE(ExternalT, NativeT, Name)      \
+  case PullIntoDescriptor::Constructor::Name:                     \
+    return JS_New##Name##ArrayWithBuffer(aCx, buffer, byteOffset, \
+                                         int64_t(length));        \
+    break;
+
+      JS_FOR_EACH_TYPED_ARRAY(CONSTRUCT_TYPED_ARRAY_TYPE)
+
+#undef CONSTRUCT_TYPED_ARRAY_TYPE
+
+    default:
+      MOZ_ASSERT_UNREACHABLE("Unknown PullIntoDescriptor::Constructor");
+      return nullptr;
+  }
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-convert-pull-into-descriptor
+JSObject* ReadableByteStreamControllerConvertPullIntoDescriptor(
+    JSContext* aCx, PullIntoDescriptor* pullIntoDescriptor, ErrorResult& aRv) {
+  // Step 1. Let bytesFilled be pullIntoDescriptor’s bytes filled.
+  uint64_t bytesFilled = pullIntoDescriptor->BytesFilled();
+
+  // Step 2. Let elementSize be pullIntoDescriptor’s element size.
+  uint64_t elementSize = pullIntoDescriptor->ElementSize();
+
+  // Step 3. Assert: bytesFilled ≤ pullIntoDescriptor’s byte length.
+  MOZ_ASSERT(bytesFilled <= pullIntoDescriptor->ByteLength());
+
+  // Step 4. Assert: bytesFilled mod elementSize is 0.
+  MOZ_ASSERT(bytesFilled % elementSize == 0);
+
+  // Step 5. Let buffer be ! TransferArrayBuffer(pullIntoDescriptor’s buffer).
+  aRv.MightThrowJSException();
+  JS::Rooted<JSObject*> srcBuffer(aCx, pullIntoDescriptor->Buffer());
+  JS::Rooted<JSObject*> buffer(aCx, TransferArrayBuffer(aCx, srcBuffer));
+  if (!buffer) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return nullptr;
+  }
+
+  // Step 6. Return ! Construct(pullIntoDescriptor’s view constructor,
+  //  « buffer, pullIntoDescriptor’s byte offset, bytesFilled ÷ elementSize »).
+  JS::Rooted<JSObject*> res(
+      aCx, ConstructFromPullIntoConstructor(
+               aCx, pullIntoDescriptor->ViewConstructor(), buffer,
+               pullIntoDescriptor->ByteOffset(), bytesFilled / elementSize));
+  if (!res) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return nullptr;
+  }
+  return res;
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-respond-in-closed-state
+MOZ_CAN_RUN_SCRIPT
+static void ReadableByteStreamControllerRespondInClosedState(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    RefPtr<PullIntoDescriptor>& aFirstDescriptor, ErrorResult& aRv) {
+  // Step 1. Assert: firstDescriptor ’s bytes filled is 0.
+  MOZ_ASSERT(aFirstDescriptor->BytesFilled() == 0);
+
+  // Step 2. If firstDescriptor’s reader type is "none",
+  // perform ! ReadableByteStreamControllerShiftPendingPullInto(controller).
+  if (aFirstDescriptor->GetReaderType() == ReaderType::None) {
+    RefPtr<PullIntoDescriptor> discarded =
+        ReadableByteStreamControllerShiftPendingPullInto(aController);
+    (void)discarded;
+  }
+
+  // Step 3. Let stream be controller.[[stream]].
+  RefPtr<ReadableStream> stream = aController->Stream();
+
+  // Step 4. If ! ReadableStreamHasBYOBReader(stream) is true,
+  if (ReadableStreamHasBYOBReader(stream)) {
+    // Step 4.1. While ! ReadableStreamGetNumReadIntoRequests(stream) > 0,
+    while (ReadableStreamGetNumReadIntoRequests(stream) > 0) {
+      // Step 4.1.1. Let pullIntoDescriptor be !
+      // ReadableByteStreamControllerShiftPendingPullInto(controller).
+      RefPtr<PullIntoDescriptor> pullIntoDescriptor =
+          ReadableByteStreamControllerShiftPendingPullInto(aController);
+
+      // Step 4.1.2. Perform !
+      // ReadableByteStreamControllerCommitPullIntoDescriptor(stream,
+      // pullIntoDescriptor).
+      ReadableByteStreamControllerCommitPullIntoDescriptor(
+          aCx, stream, pullIntoDescriptor, aRv);
+    }
+  }
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-fill-head-pull-into-descriptor
+void ReadableByteStreamControllerFillHeadPullIntoDescriptor(
+    ReadableByteStreamController* aController, size_t aSize,
+    PullIntoDescriptor* aPullIntoDescriptor) {
+  // Step 1. Assert: either controller.[[pendingPullIntos]] is empty, or
+  // controller.[[pendingPullIntos]][0] is pullIntoDescriptor.
+  MOZ_ASSERT(aController->PendingPullIntos().isEmpty() ||
+             aController->PendingPullIntos().getFirst() == aPullIntoDescriptor);
+
+  // Step 2. Assert: controller.[[byobRequest]] is null.
+  MOZ_ASSERT(!aController->GetByobRequest());
+
+  // Step 3. Set pullIntoDescriptor’s bytes filled to bytes filled + size.
+  aPullIntoDescriptor->SetBytesFilled(aPullIntoDescriptor->BytesFilled() +
+                                      aSize);
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-respond-in-readable-state
+MOZ_CAN_RUN_SCRIPT
+static void ReadableByteStreamControllerRespondInReadableState(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    uint64_t aBytesWritten, PullIntoDescriptor* aPullIntoDescriptor,
+    ErrorResult& aRv) {
+  // Step 1. Assert: pullIntoDescriptor’s bytes filled + bytesWritten ≤
+  // pullIntoDescriptor’s byte length.
+  MOZ_ASSERT(aPullIntoDescriptor->BytesFilled() + aBytesWritten <=
+             aPullIntoDescriptor->ByteLength());
+
+  // Step 2. Perform
+  // !ReadableByteStreamControllerFillHeadPullIntoDescriptor(controller,
+  // bytesWritten, pullIntoDescriptor).
+  ReadableByteStreamControllerFillHeadPullIntoDescriptor(
+      aController, aBytesWritten, aPullIntoDescriptor);
+
+  // Step 3. If pullIntoDescriptor’s reader type is "none",
+  if (aPullIntoDescriptor->GetReaderType() == ReaderType::None) {
+    // Step 3.1. Perform ?
+    // ReadableByteStreamControllerEnqueueDetachedPullIntoToQueue(controller,
+    // pullIntoDescriptor).
+    ReadableByteStreamControllerEnqueueDetachedPullIntoToQueue(
+        aCx, aController, aPullIntoDescriptor, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+
+    // Step 3.2. Perform !
+    // ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(controller).
+    ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(
+        aCx, aController, aRv);
+
+    // Step 3.3. Return.
+    return;
+  }
+
+  // Step 4. If pullIntoDescriptor’s bytes filled < pullIntoDescriptor’s element
+  // size, return.
+  if (aPullIntoDescriptor->BytesFilled() < aPullIntoDescriptor->ElementSize()) {
+    return;
+  }
+
+  // Step 5. Perform
+  // !ReadableByteStreamControllerShiftPendingPullInto(controller).
+  RefPtr<PullIntoDescriptor> pullIntoDescriptor =
+      ReadableByteStreamControllerShiftPendingPullInto(aController);
+  (void)pullIntoDescriptor;
+
+  // Step 6. Let remainderSize be pullIntoDescriptor’s bytes filled mod
+  // pullIntoDescriptor’s element size.
+  size_t remainderSize =
+      aPullIntoDescriptor->BytesFilled() % aPullIntoDescriptor->ElementSize();
+
+  // Step 7. If remainderSize > 0,
+  if (remainderSize > 0) {
+    // Step 7.1. Let end be pullIntoDescriptor’s byte offset +
+    // pullIntoDescriptor’s bytes filled.
+    size_t end =
+        aPullIntoDescriptor->ByteOffset() + aPullIntoDescriptor->BytesFilled();
+
+    // Step 7.2. Perform ?
+    // ReadableByteStreamControllerEnqueueClonedChunkToQueue(controller,
+    // pullIntoDescriptor’s buffer, end − remainderSize, remainderSize).
+    JS::Rooted<JSObject*> pullIntoBuffer(aCx, aPullIntoDescriptor->Buffer());
+    ReadableByteStreamControllerEnqueueClonedChunkToQueue(
+        aCx, aController, pullIntoBuffer, end - remainderSize, remainderSize,
+        aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+  }
+
+  // Step 8. Set pullIntoDescriptor’s bytes filled to pullIntoDescriptor’s bytes
+  // filled − remainderSize.
+  aPullIntoDescriptor->SetBytesFilled(aPullIntoDescriptor->BytesFilled() -
+                                      remainderSize);
+
+  // Step 9. Perform
+  // !ReadableByteStreamControllerCommitPullIntoDescriptor(controller.[[stream]],
+  // pullIntoDescriptor).
+  RefPtr<ReadableStream> stream(aController->Stream());
+  ReadableByteStreamControllerCommitPullIntoDescriptor(
+      aCx, stream, aPullIntoDescriptor, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 10. Perform
+  // !ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(controller).
+  ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(
+      aCx, aController, aRv);
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-respond-internal
+void ReadableByteStreamControllerRespondInternal(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    uint64_t aBytesWritten, ErrorResult& aRv) {
+  // Step 1.
+  RefPtr<PullIntoDescriptor> firstDescriptor =
+      aController->PendingPullIntos().getFirst();
+
+  // Step 2.
+  JS::Rooted<JSObject*> buffer(aCx, firstDescriptor->Buffer());
+#ifdef DEBUG
+  bool canTransferBuffer = CanTransferArrayBuffer(aCx, buffer, aRv);
+  MOZ_ASSERT(!aRv.Failed());
+  MOZ_ASSERT(canTransferBuffer);
+#endif
+
+  // Step 3.
+  ReadableByteStreamControllerInvalidateBYOBRequest(aController);
+
+  // Step 4.
+  auto state = aController->Stream()->State();
+
+  // Step 5.
+  if (state == ReadableStream::ReaderState::Closed) {
+    // Step 5.1
+    MOZ_ASSERT(aBytesWritten == 0);
+
+    // Step 5.2
+    ReadableByteStreamControllerRespondInClosedState(aCx, aController,
+                                                     firstDescriptor, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+  } else {
+    // Step 6.1
+    MOZ_ASSERT(state == ReadableStream::ReaderState::Readable);
+
+    // Step 6.2.
+    MOZ_ASSERT(aBytesWritten > 0);
+
+    // Step 6.3
+    ReadableByteStreamControllerRespondInReadableState(
+        aCx, aController, aBytesWritten, firstDescriptor, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+  }
+  // Step 7.
+  ReadableByteStreamControllerCallPullIfNeeded(aCx, aController, aRv);
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-respond
+void ReadableByteStreamControllerRespond(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    uint64_t aBytesWritten, ErrorResult& aRv) {
+  // Step 1.
+  MOZ_ASSERT(!aController->PendingPullIntos().isEmpty());
+
+  // Step 2.
+  PullIntoDescriptor* firstDescriptor =
+      aController->PendingPullIntos().getFirst();
+
+  // Step 3.
+  auto state = aController->Stream()->State();
+
+  // Step 4.
+  if (state == ReadableStream::ReaderState::Closed) {
+    // Step 4.1
+    if (aBytesWritten != 0) {
+      aRv.ThrowTypeError("bytesWritten not zero on closed stream");
+      return;
+    }
+  } else {
+    // Step 5.1
+    MOZ_ASSERT(state == ReadableStream::ReaderState::Readable);
+
+    // Step 5.2
+    if (aBytesWritten == 0) {
+      aRv.ThrowTypeError("bytesWritten 0");
+      return;
+    }
+
+    // Step 5.3
+    if (firstDescriptor->BytesFilled() + aBytesWritten >
+        firstDescriptor->ByteLength()) {
+      aRv.ThrowRangeError("bytesFilled + bytesWritten > byteLength");
+      return;
+    }
+  }
+
+  // Step 6.
+  aRv.MightThrowJSException();
+  JS::Rooted<JSObject*> buffer(aCx, firstDescriptor->Buffer());
+  JS::Rooted<JSObject*> transferredBuffer(aCx,
+                                          TransferArrayBuffer(aCx, buffer));
+  if (!transferredBuffer) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+  firstDescriptor->SetBuffer(transferredBuffer);
+
+  // Step 7.
+  ReadableByteStreamControllerRespondInternal(aCx, aController, aBytesWritten,
+                                              aRv);
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-respond-with-new-view
+void ReadableByteStreamControllerRespondWithNewView(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    JS::Handle<JSObject*> aView, ErrorResult& aRv) {
+  aRv.MightThrowJSException();
+
+  // Step 1.
+  MOZ_ASSERT(!aController->PendingPullIntos().isEmpty());
+
+  // Step 2.
+  bool isSharedMemory;
+  JS::Rooted<JSObject*> viewedArrayBuffer(
+      aCx, JS_GetArrayBufferViewBuffer(aCx, aView, &isSharedMemory));
+  if (!viewedArrayBuffer) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+  MOZ_ASSERT(!JS::IsDetachedArrayBufferObject(viewedArrayBuffer));
+
+  // Step 3.
+  RefPtr<PullIntoDescriptor> firstDescriptor =
+      aController->PendingPullIntos().getFirst();
+
+  // Step 4.
+  ReadableStream::ReaderState state = aController->Stream()->State();
+
+  // Step 5.
+  if (state == ReadableStream::ReaderState::Closed) {
+    // Step 5.1
+    if (JS_GetArrayBufferViewByteLength(aView) != 0) {
+      aRv.ThrowTypeError("View has non-zero length in closed stream");
+      return;
+    }
+  } else {
+    // Step 6.1
+    MOZ_ASSERT(state == ReadableStream::ReaderState::Readable);
+
+    // Step 6.2
+    if (JS_GetArrayBufferViewByteLength(aView) == 0) {
+      aRv.ThrowTypeError("View has zero length in readable stream");
+      return;
+    }
+  }
+
+  // Step 7.
+  if (firstDescriptor->ByteOffset() + firstDescriptor->BytesFilled() !=
+      JS_GetArrayBufferViewByteOffset(aView)) {
+    aRv.ThrowRangeError("Invalid Offset");
+    return;
+  }
+
+  // Step 8.
+  if (firstDescriptor->BufferByteLength() !=
+      JS::GetArrayBufferByteLength(viewedArrayBuffer)) {
+    aRv.ThrowRangeError("Mismatched buffer byte lengths");
+    return;
+  }
+
+  // Step 9.
+  if (firstDescriptor->BytesFilled() + JS_GetArrayBufferViewByteLength(aView) >
+      firstDescriptor->ByteLength()) {
+    aRv.ThrowRangeError("Too many bytes");
+    return;
+  }
+
+  // Step 10. Let viewByteLength be view.[[ByteLength]].
+  size_t viewByteLength = JS_GetArrayBufferViewByteLength(aView);
+
+  // Step 11. Set firstDescriptor’s buffer to ?
+  // TransferArrayBuffer(view.[[ViewedArrayBuffer]]).
+  JS::Rooted<JSObject*> transferedBuffer(
+      aCx, TransferArrayBuffer(aCx, viewedArrayBuffer));
+  if (!transferedBuffer) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+  firstDescriptor->SetBuffer(transferedBuffer);
+
+  // Step 12. Perform ? ReadableByteStreamControllerRespondInternal(controller,
+  // viewByteLength).
+  ReadableByteStreamControllerRespondInternal(aCx, aController, viewByteLength,
+                                              aRv);
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-fill-pull-into-descriptor-from-queue
+bool ReadableByteStreamControllerFillPullIntoDescriptorFromQueue(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    PullIntoDescriptor* aPullIntoDescriptor, ErrorResult& aRv) {
+  // Step 1. Let elementSize be pullIntoDescriptor.[[elementSize]].
+  size_t elementSize = aPullIntoDescriptor->ElementSize();
+
+  // Step 2. Let currentAlignedBytes be pullIntoDescriptor’s bytes filled −
+  // (pullIntoDescriptor’s bytes filled mod elementSize).
+  size_t currentAlignedBytes =
+      aPullIntoDescriptor->BytesFilled() -
+      (aPullIntoDescriptor->BytesFilled() % elementSize);
+
+  // Step 3. Let maxBytesToCopy be min(controller.[[queueTotalSize]],
+  // pullIntoDescriptor’s byte length − pullIntoDescriptor’s bytes filled).
+  size_t maxBytesToCopy =
+      std::min(static_cast<size_t>(aController->QueueTotalSize()),
+               static_cast<size_t>((aPullIntoDescriptor->ByteLength() -
+                                    aPullIntoDescriptor->BytesFilled())));
+
+  // Step 4. Let maxBytesFilled be pullIntoDescriptor’s bytes filled +
+  // maxBytesToCopy.
+  size_t maxBytesFilled = aPullIntoDescriptor->BytesFilled() + maxBytesToCopy;
+
+  // Step 5. Let maxAlignedBytes be maxBytesFilled − (maxBytesFilled mod
+  // elementSize).
+  size_t maxAlignedBytes = maxBytesFilled - (maxBytesFilled % elementSize);
+
+  // Step 6. Let totalBytesToCopyRemaining be maxBytesToCopy.
+  size_t totalBytesToCopyRemaining = maxBytesToCopy;
+
+  // Step 7. Let ready be false.
+  bool ready = false;
+
+  // Step 8. If maxAlignedBytes > currentAlignedBytes,
+  if (maxAlignedBytes > currentAlignedBytes) {
+    // Step 8.1. Set totalBytesToCopyRemaining to maxAlignedBytes −
+    // pullIntoDescriptor’s bytes filled.
+    totalBytesToCopyRemaining =
+        maxAlignedBytes - aPullIntoDescriptor->BytesFilled();
+    // Step 8.2. Set ready to true.
+    ready = true;
+  }
+
+  // Step 9. Let queue be controller.[[queue]].
+  LinkedList<RefPtr<ReadableByteStreamQueueEntry>>& queue =
+      aController->Queue();
+
+  // Step 10. While totalBytesToCopyRemaining > 0,
+  while (totalBytesToCopyRemaining > 0) {
+    // Step 10.1  Let headOfQueue be queue[0].
+    ReadableByteStreamQueueEntry* headOfQueue = queue.getFirst();
+
+    // Step 10.2. Let bytesToCopy be min(totalBytesToCopyRemaining,
+    // headOfQueue’s byte length).
+    size_t bytesToCopy =
+        std::min(totalBytesToCopyRemaining, headOfQueue->ByteLength());
+
+    // Step 10.3. Let destStart be pullIntoDescriptor’s byte offset +
+    // pullIntoDescriptor’s bytes filled.
+    size_t destStart =
+        aPullIntoDescriptor->ByteOffset() + aPullIntoDescriptor->BytesFilled();
+
+    // Step 10.4. Perform !CopyDataBlockBytes(pullIntoDescriptor’s
+    //            buffer.[[ArrayBufferData]], destStart, headOfQueue’s
+    //            buffer.[[ArrayBufferData]], headOfQueue’s byte offset,
+    //            bytesToCopy).
+    JS::Rooted<JSObject*> descriptorBuffer(aCx, aPullIntoDescriptor->Buffer());
+    JS::Rooted<JSObject*> queueBuffer(aCx, headOfQueue->Buffer());
+    if (!JS::ArrayBufferCopyData(aCx, descriptorBuffer, destStart, queueBuffer,
+                                 headOfQueue->ByteOffset(), bytesToCopy)) {
+      aRv.StealExceptionFromJSContext(aCx);
+      return false;
+    }
+
+    // Step 10.5. If headOfQueue’s byte length is bytesToCopy,
+    if (headOfQueue->ByteLength() == bytesToCopy) {
+      // Step 10.5.1. Remove queue[0].
+      queue.popFirst();
+    } else {
+      // Step 10.6.  Otherwise,
+
+      // Step 10.6.1 Set headOfQueue’s byte offset to
+      // headOfQueue’s byte offset +  bytesToCopy.
+      headOfQueue->SetByteOffset(headOfQueue->ByteOffset() + bytesToCopy);
+      // Step 10.6.2  Set headOfQueue’s byte length to
+      // headOfQueue’s byte length − bytesToCopy.
+      headOfQueue->SetByteLength(headOfQueue->ByteLength() - bytesToCopy);
+    }
+
+    // Step 10.7. Set controller.[[queueTotalSize]] to
+    // controller.[[queueTotalSize]] −  bytesToCopy.
+    aController->SetQueueTotalSize(aController->QueueTotalSize() -
+                                   (double)bytesToCopy);
+
+    // Step 10.8, Perform
+    // !ReadableByteStreamControllerFillHeadPullIntoDescriptor(controller,
+    //     bytesToCopy, pullIntoDescriptor).
+    ReadableByteStreamControllerFillHeadPullIntoDescriptor(
+        aController, bytesToCopy, aPullIntoDescriptor);
+
+    // Step 10.9. Set totalBytesToCopyRemaining to totalBytesToCopyRemaining −
+    //     bytesToCopy.
+    totalBytesToCopyRemaining = totalBytesToCopyRemaining - bytesToCopy;
+  }
+
+  // Step 11. If ready is false,
+  if (!ready) {
+    // Step 11.1. Assert: controller.[[queueTotalSize]] is 0.
+    MOZ_ASSERT(aController->QueueTotalSize() == 0);
+
+    // Step 11.2. Assert: pullIntoDescriptor’s bytes filled > 0.
+    MOZ_ASSERT(aPullIntoDescriptor->BytesFilled() > 0);
+
+    // Step 11.3. Assert: pullIntoDescriptor’s bytes filled <
+    // pullIntoDescriptor’s
+    //     element size.
+    MOZ_ASSERT(aPullIntoDescriptor->BytesFilled() <
+               aPullIntoDescriptor->ElementSize());
+  }
+
+  // Step 12. Return ready.
+  return ready;
+}
+
+// https://streams.spec.whatwg.org/#readable-byte-stream-controller-pull-into
+void ReadableByteStreamControllerPullInto(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    JS::Handle<JSObject*> aView, ReadIntoRequest* aReadIntoRequest,
+    ErrorResult& aRv) {
+  aRv.MightThrowJSException();
+
+  // Step 1. Let stream be controller.[[stream]].
+  ReadableStream* stream = aController->Stream();
+
+  // Step 2. Let elementSize be 1.
+  size_t elementSize = 1;
+
+  // Step 3. Let ctor be %DataView%.
+  PullIntoDescriptor::Constructor ctor =
+      PullIntoDescriptor::Constructor::DataView;
+
+  // Step 4. If view has a [[TypedArrayName]] internal slot (i.e., it is not a
+  // DataView),
+  if (JS_IsTypedArrayObject(aView)) {
+    // Step 4.1. Set elementSize to the element size specified in the typed
+    // array constructors table for view.[[TypedArrayName]].
+    JS::Scalar::Type type = JS_GetArrayBufferViewType(aView);
+    elementSize = JS::Scalar::byteSize(type);
+
+    // Step 4.2 Set ctor to the constructor specified in the typed array
+    // constructors table for view.[[TypedArrayName]].
+    ctor = PullIntoDescriptor::constructorFromScalar(type);
+  }
+
+  // Step 5. Let byteOffset be view.[[ByteOffset]].
+  size_t byteOffset = JS_GetArrayBufferViewByteOffset(aView);
+
+  // Step 6. Let byteLength be view.[[ByteLength]].
+  size_t byteLength = JS_GetArrayBufferViewByteLength(aView);
+
+  // Step 7. Let bufferResult be
+  // TransferArrayBuffer(view.[[ViewedArrayBuffer]]).
+  bool isShared;
+  JS::Rooted<JSObject*> viewedArrayBuffer(
+      aCx, JS_GetArrayBufferViewBuffer(aCx, aView, &isShared));
+  if (!viewedArrayBuffer) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+  JS::Rooted<JSObject*> bufferResult(
+      aCx, TransferArrayBuffer(aCx, viewedArrayBuffer));
+
+  // Step 8. If bufferResult is an abrupt completion,
+  if (!bufferResult) {
+    JS::Rooted<JS::Value> pendingException(aCx);
+    if (!JS_GetPendingException(aCx, &pendingException)) {
+      // This means an un-catchable exception. Use StealExceptionFromJSContext
+      // to setup aRv properly.
+      aRv.StealExceptionFromJSContext(aCx);
+      return;
+    }
+
+    // It's not expliclitly stated, but I assume the intention here is that
+    // we perform a normal completion here; we also need to clear the
+    // exception state anyhow to succesfully run ErrorSteps.
+    JS_ClearPendingException(aCx);
+
+    //     Step 8.1. Perform readIntoRequest’s error steps, given
+    //     bufferResult.[[Value]].
+    aReadIntoRequest->ErrorSteps(aCx, pendingException, aRv);
+
+    //     Step 8.2. Return.
+    return;
+  }
+
+  // Step 9. Let buffer be bufferResult.[[Value]].
+  JS::Rooted<JSObject*> buffer(aCx, bufferResult);
+
+  // Step 10. Let pullIntoDescriptor be a new pull-into descriptor with
+  //  buffer: buffer,
+  //  buffer byte length:  buffer.[[ArrayBufferByteLength]],
+  //  byte offset: byteOffset,
+  //  byte length: byteLength,
+  //  bytes filled: 0,
+  //  element size: elementSize,
+  //  view constructor: ctor,
+  //  and reader type: "byob".
+  RefPtr<PullIntoDescriptor> pullIntoDescriptor = new PullIntoDescriptor(
+      buffer, JS::GetArrayBufferByteLength(buffer), byteOffset, byteLength, 0,
+      elementSize, ctor, ReaderType::BYOB);
+
+  // Step 11. If controller.[[pendingPullIntos]] is not empty,
+  if (!aController->PendingPullIntos().isEmpty()) {
+    // Step 11.1. Append pullIntoDescriptor to controller.[[pendingPullIntos]].
+    aController->PendingPullIntos().insertBack(pullIntoDescriptor);
+
+    // Step 11.2. Perform !ReadableStreamAddReadIntoRequest(stream,
+    // readIntoRequest).
+    ReadableStreamAddReadIntoRequest(stream, aReadIntoRequest);
+
+    // Step 11.3. Return.
+    return;
+  }
+
+  // Step 12. If stream.[[state]] is "closed",
+  if (stream->State() == ReadableStream::ReaderState::Closed) {
+    // Step 12.1. Let emptyView be !Construct(ctor, « pullIntoDescriptor’s
+    //            buffer, pullIntoDescriptor’s byte offset, 0 »).
+    JS::Rooted<JSObject*> pullIntoBuffer(aCx, pullIntoDescriptor->Buffer());
+    JS::Rooted<JSObject*> emptyView(
+        aCx,
+        ConstructFromPullIntoConstructor(aCx, ctor, pullIntoBuffer,
+                                         pullIntoDescriptor->ByteOffset(), 0));
+    if (!emptyView) {
+      aRv.StealExceptionFromJSContext(aCx);
+      return;
+    }
+
+    // Step 12.2. Perform readIntoRequest’s close steps, given emptyView.
+    JS::Rooted<JS::Value> emptyViewValue(aCx, JS::ObjectValue(*emptyView));
+    aReadIntoRequest->CloseSteps(aCx, emptyViewValue, aRv);
+
+    // Step 12.3. Return.
+    return;
+  }
+
+  // Step 13,. If controller.[[queueTotalSize]] > 0,
+  if (aController->QueueTotalSize() > 0) {
+    // Step 13.1 If
+    // !ReadableByteStreamControllerFillPullIntoDescriptorFromQueue(controller,
+    //     pullIntoDescriptor) is true,
+    bool ready = ReadableByteStreamControllerFillPullIntoDescriptorFromQueue(
+        aCx, aController, pullIntoDescriptor, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+    if (ready) {
+      // Step 13.1.1  Let filledView be
+      //         !ReadableByteStreamControllerConvertPullIntoDescriptor(pullIntoDescriptor).
+      JS::Rooted<JSObject*> filledView(
+          aCx, ReadableByteStreamControllerConvertPullIntoDescriptor(
+                   aCx, pullIntoDescriptor, aRv));
+      if (aRv.Failed()) {
+        return;
+      }
+      //  Step 13.1.2. Perform
+      //         !ReadableByteStreamControllerHandleQueueDrain(controller).
+      ReadableByteStreamControllerHandleQueueDrain(aCx, aController, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+      // Step 13.1.3.  Perform readIntoRequest’s chunk steps, given filledView.
+      JS::Rooted<JS::Value> filledViewValue(aCx, JS::ObjectValue(*filledView));
+      aReadIntoRequest->ChunkSteps(aCx, filledViewValue, aRv);
+      // Step 13.1.4.   Return.
+      return;
+    }
+
+    // Step 13.2  If controller.[[closeRequested]] is true,
+    if (aController->CloseRequested()) {
+      // Step 13.2.1.   Let e be a TypeError exception.
+      ErrorResult typeError;
+      typeError.ThrowTypeError("Close Requested True during Pull Into");
+
+      JS::Rooted<JS::Value> e(aCx);
+      MOZ_RELEASE_ASSERT(ToJSValue(aCx, std::move(typeError), &e));
+
+      // Step 13.2.2. Perform !ReadableByteStreamControllerError(controller, e).
+      ReadableByteStreamControllerError(aController, e, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+
+      // Step 13.2.3. Perform readIntoRequest’s error steps, given e.
+      aReadIntoRequest->ErrorSteps(aCx, e, aRv);
+
+      // Step 13.2.4.  Return.
+      return;
+    }
+  }
+
+  // Step 14. Append pullIntoDescriptor to controller.[[pendingPullIntos]].
+  aController->PendingPullIntos().insertBack(pullIntoDescriptor);
+
+  // Step 15. Perform !ReadableStreamAddReadIntoRequest(stream,
+  // readIntoRequest).
+  ReadableStreamAddReadIntoRequest(stream, aReadIntoRequest);
+
+  // Step 16, Perform
+  // !ReadableByteStreamControllerCallPullIfNeeded(controller).
+  ReadableByteStreamControllerCallPullIfNeeded(aCx, aController, aRv);
+}
+
+// https://streams.spec.whatwg.org/#set-up-readable-byte-stream-controller
+void SetUpReadableByteStreamController(
+    JSContext* aCx, ReadableStream* aStream,
+    ReadableByteStreamController* aController,
+    UnderlyingSourceAlgorithmsBase* aAlgorithms, double aHighWaterMark,
+    Maybe<uint64_t> aAutoAllocateChunkSize, ErrorResult& aRv) {
+  // Step 1. Assert: stream.[[controller]] is undefined.
+  MOZ_ASSERT(!aStream->Controller());
+
+  // Step 2. If autoAllocateChunkSize is not undefined,
+  // Step 2.1. Assert: ! IsInteger(autoAllocateChunkSize) is true. Implicit
+  // Step 2.2. Assert: autoAllocateChunkSize is positive. (Implicit by
+  //           type.)
+
+  // Step 3. Set controller.[[stream]] to stream.
+  aController->SetStream(aStream);
+
+  // Step 4. Set controller.[[pullAgain]] and controller.[[pulling]] to false.
+  aController->SetPullAgain(false);
+  aController->SetPulling(false);
+
+  // Step 5. Set controller.[[byobRequest]] to null.
+  aController->SetByobRequest(nullptr);
+
+  // Step 6. Perform !ResetQueue(controller).
+  ResetQueue(aController);
+
+  // Step 7. Set controller.[[closeRequested]] and controller.[[started]] to
+  // false.
+  aController->SetCloseRequested(false);
+  aController->SetStarted(false);
+
+  // Step 8. Set controller.[[strategyHWM]] to highWaterMark.
+  aController->SetStrategyHWM(aHighWaterMark);
+
+  // Step 9. Set controller.[[pullAlgorithm]] to pullAlgorithm.
+  // Step 10. Set controller.[[cancelAlgorithm]] to cancelAlgorithm.
+  aController->SetAlgorithms(*aAlgorithms);
+
+  // Step 11. Set controller.[[autoAllocateChunkSize]] to autoAllocateChunkSize.
+  aController->SetAutoAllocateChunkSize(aAutoAllocateChunkSize);
+
+  // Step 12. Set controller.[[pendingPullIntos]] to a new empty list.
+  aController->PendingPullIntos().clear();
+
+  // Step 13. Set stream.[[controller]] to controller.
+  aStream->SetController(*aController);
+
+  // Step 14. Let startResult be the result of performing startAlgorithm.
+  JS::Rooted<JS::Value> startResult(aCx, JS::UndefinedValue());
+  RefPtr<ReadableStreamController> controller = aController;
+  aAlgorithms->StartCallback(aCx, *controller, &startResult, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Let startPromise be a promise resolved with startResult.
+  RefPtr<Promise> startPromise =
+      Promise::CreateInfallible(aStream->GetParentObject());
+  startPromise->MaybeResolve(startResult);
+
+  // Step 16+17
+  startPromise->AddCallbacksWithCycleCollectedArgs(
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         ReadableByteStreamController* aController)
+          MOZ_CAN_RUN_SCRIPT_BOUNDARY {
+            MOZ_ASSERT(aController);
+
+            // Step 16.1
+            aController->SetStarted(true);
+
+            // Step 16.2
+            aController->SetPulling(false);
+
+            // Step 16.3
+            aController->SetPullAgain(false);
+
+            // Step 16.4:
+            ReadableByteStreamControllerCallPullIfNeeded(
+                aCx, MOZ_KnownLive(aController), aRv);
+          },
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         ReadableByteStreamController* aController) {
+        // Step 17.1
+        ReadableByteStreamControllerError(aController, aValue, aRv);
+      },
+      RefPtr(aController));
+}
+
+// https://streams.spec.whatwg.org/#set-up-readable-byte-stream-controller-from-underlying-source
+void SetUpReadableByteStreamControllerFromUnderlyingSource(
+    JSContext* aCx, ReadableStream* aStream,
+    JS::Handle<JSObject*> aUnderlyingSource,
+    UnderlyingSource& aUnderlyingSourceDict, double aHighWaterMark,
+    ErrorResult& aRv) {
+  // Step 1. Let controller be a new ReadableByteStreamController.
+  auto controller =
+      MakeRefPtr<ReadableByteStreamController>(aStream->GetParentObject());
+
+  // Step 2 - 7
+  auto algorithms = MakeRefPtr<UnderlyingSourceAlgorithms>(
+      aStream->GetParentObject(), aUnderlyingSource, aUnderlyingSourceDict);
+
+  // Step 8. Let autoAllocateChunkSize be
+  // underlyingSourceDict["autoAllocateChunkSize"], if it exists, or undefined
+  // otherwise.
+  Maybe<uint64_t> autoAllocateChunkSize = mozilla::Nothing();
+  if (aUnderlyingSourceDict.mAutoAllocateChunkSize.WasPassed()) {
+    uint64_t value = aUnderlyingSourceDict.mAutoAllocateChunkSize.Value();
+    // Step 9. If autoAllocateChunkSize is 0, then throw a TypeError
+    // exception.
+    if (value == 0) {
+      aRv.ThrowTypeError("autoAllocateChunkSize can not be zero.");
+      return;
+    }
+    autoAllocateChunkSize = mozilla::Some(value);
+  }
+
+  // Step 10. Perform ? SetUpReadableByteStreamController(stream, controller,
+  // startAlgorithm, pullAlgorithm, cancelAlgorithm, highWaterMark,
+  // autoAllocateChunkSize).
+  SetUpReadableByteStreamController(aCx, aStream, controller, algorithms,
+                                    aHighWaterMark, autoAllocateChunkSize, aRv);
+}
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/ReadableByteStreamController.h b/dom/streams/ReadableByteStreamController.h
new file mode 100644
index 0000000000..45e03ba3b4
--- /dev/null
+++ b/dom/streams/ReadableByteStreamController.h
@@ -0,0 +1,230 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ReadableByteStreamController_h
+#define mozilla_dom_ReadableByteStreamController_h
+
+#include <cstddef>
+#include "UnderlyingSourceCallbackHelpers.h"
+#include "js/RootingAPI.h"
+#include "js/TypeDecls.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+#include "mozilla/dom/QueueWithSizes.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/ReadRequest.h"
+#include "mozilla/dom/ReadableStreamBYOBRequest.h"
+#include "mozilla/dom/ReadableStreamController.h"
+#include "mozilla/dom/TypedArray.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsWrapperCache.h"
+#include "mozilla/dom/Nullable.h"
+#include "nsTArray.h"
+#include "nsISupportsImpl.h"
+
+namespace mozilla::dom {
+
+// https://streams.spec.whatwg.org/#pull-into-descriptor-reader-type
+// Indicates what type of readable stream reader initiated this request,
+// or None if the initiating reader was released.
+enum ReaderType { Default, BYOB, None };
+
+struct PullIntoDescriptor;
+struct ReadableByteStreamQueueEntry;
+struct ReadIntoRequest;
+
+class ReadableByteStreamController final : public ReadableStreamController,
+                                           public nsWrapperCache {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_INHERITED(
+      ReadableByteStreamController, ReadableStreamController)
+
+ public:
+  explicit ReadableByteStreamController(nsIGlobalObject* aGlobal);
+
+ protected:
+  ~ReadableByteStreamController() override;
+
+ public:
+  bool IsDefault() override { return false; }
+  bool IsByte() override { return true; }
+  ReadableStreamDefaultController* AsDefault() override { return nullptr; }
+  ReadableByteStreamController* AsByte() override { return this; }
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  already_AddRefed<ReadableStreamBYOBRequest> GetByobRequest(JSContext* aCx,
+                                                             ErrorResult& aRv);
+
+  Nullable<double> GetDesiredSize() const;
+
+  MOZ_CAN_RUN_SCRIPT void Close(JSContext* aCx, ErrorResult& aRv);
+
+  MOZ_CAN_RUN_SCRIPT void Enqueue(JSContext* aCx, const ArrayBufferView& aChunk,
+                                  ErrorResult& aRv);
+
+  void Error(JSContext* aCx, JS::Handle<JS::Value> aErrorValue,
+             ErrorResult& aRv);
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> CancelSteps(
+      JSContext* aCx, JS::Handle<JS::Value> aReason, ErrorResult& aRv) override;
+  MOZ_CAN_RUN_SCRIPT void PullSteps(JSContext* aCx, ReadRequest* aReadRequest,
+                                    ErrorResult& aRv) override;
+  void ReleaseSteps() override;
+
+  // Internal Slot Accessors
+  Maybe<uint64_t> AutoAllocateChunkSize() { return mAutoAllocateChunkSize; }
+  void SetAutoAllocateChunkSize(Maybe<uint64_t>& aSize) {
+    mAutoAllocateChunkSize = aSize;
+  }
+
+  ReadableStreamBYOBRequest* GetByobRequest() const { return mByobRequest; }
+  void SetByobRequest(ReadableStreamBYOBRequest* aByobRequest) {
+    mByobRequest = aByobRequest;
+  }
+
+  LinkedList<RefPtr<PullIntoDescriptor>>& PendingPullIntos() {
+    return mPendingPullIntos;
+  }
+  void ClearPendingPullIntos();
+
+  double QueueTotalSize() const { return mQueueTotalSize; }
+  void SetQueueTotalSize(double aQueueTotalSize) {
+    mQueueTotalSize = aQueueTotalSize;
+  }
+  void AddToQueueTotalSize(double aLength) { mQueueTotalSize += aLength; }
+
+  double StrategyHWM() const { return mStrategyHWM; }
+  void SetStrategyHWM(double aStrategyHWM) { mStrategyHWM = aStrategyHWM; }
+
+  bool CloseRequested() const { return mCloseRequested; }
+  void SetCloseRequested(bool aCloseRequested) {
+    mCloseRequested = aCloseRequested;
+  }
+
+  LinkedList<RefPtr<ReadableByteStreamQueueEntry>>& Queue() { return mQueue; }
+  void ClearQueue();
+
+  bool Started() const { return mStarted; }
+  void SetStarted(bool aStarted) { mStarted = aStarted; }
+
+  bool Pulling() const { return mPulling; }
+  void SetPulling(bool aPulling) { mPulling = aPulling; }
+
+  bool PullAgain() const { return mPullAgain; }
+  void SetPullAgain(bool aPullAgain) { mPullAgain = aPullAgain; }
+
+ private:
+  // A boolean flag indicating whether the stream has been closed by its
+  // underlying byte source, but still has chunks in its internal queue that
+  // have not yet been read
+  bool mCloseRequested = false;
+
+  // A boolean flag set to true if the stream’s mechanisms requested a call
+  // to the underlying byte source's pull algorithm to pull more data, but the
+  // pull could not yet be done since a previous call is still executing
+  bool mPullAgain = false;
+
+  // A boolean flag indicating whether the underlying byte source has finished
+  // starting
+  bool mStarted = false;
+
+  // A boolean flag set to true while the underlying byte source's pull
+  // algorithm is executing and the returned promise has not yet fulfilled,
+  // used to prevent reentrant calls
+  bool mPulling = false;
+
+  // A positive integer, when the automatic buffer allocation feature is
+  // enabled. In that case, this value specifies the size of buffer to allocate.
+  // It is undefined otherwise.
+  Maybe<uint64_t> mAutoAllocateChunkSize;
+
+  // A ReadableStreamBYOBRequest instance representing the current BYOB pull
+  // request, or null if there are no pending requests
+  RefPtr<ReadableStreamBYOBRequest> mByobRequest;
+
+  // A list of pull-into descriptors
+  LinkedList<RefPtr<PullIntoDescriptor>> mPendingPullIntos;
+
+  // A list of readable byte stream queue entries representing the stream’s
+  // internal queue of chunks
+  //
+  // This is theoretically supposed to be a QueueWithSizes, but it is
+  // mostly not actually manipulated or used like QueueWithSizes, so instead we
+  // use a LinkedList.
+  LinkedList<RefPtr<ReadableByteStreamQueueEntry>> mQueue;
+
+  // The total size, in bytes, of all the chunks stored in [[queue]] (see § 8.1
+  // Queue-with-sizes)
+  double mQueueTotalSize = 0.0;
+
+  // A number supplied to the constructor as part of the stream’s queuing
+  // strategy, indicating the point at which the stream will apply backpressure
+  // to its underlying byte source
+  double mStrategyHWM = 0.0;
+};
+
+namespace streams_abstract {
+
+MOZ_CAN_RUN_SCRIPT void ReadableByteStreamControllerRespond(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    uint64_t aBytesWritten, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void ReadableByteStreamControllerRespondInternal(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    uint64_t aBytesWritten, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void ReadableByteStreamControllerRespondWithNewView(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    JS::Handle<JSObject*> aView, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void ReadableByteStreamControllerPullInto(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    JS::Handle<JSObject*> aView, ReadIntoRequest* aReadIntoRequest,
+    ErrorResult& aRv);
+
+void ReadableByteStreamControllerError(
+    ReadableByteStreamController* aController, JS::Handle<JS::Value> aValue,
+    ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void ReadableByteStreamControllerEnqueue(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    JS::Handle<JSObject*> aChunk, ErrorResult& aRv);
+
+already_AddRefed<ReadableStreamBYOBRequest>
+ReadableByteStreamControllerGetBYOBRequest(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void ReadableByteStreamControllerClose(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void SetUpReadableByteStreamController(
+    JSContext* aCx, ReadableStream* aStream,
+    ReadableByteStreamController* aController,
+    UnderlyingSourceAlgorithmsBase* aAlgorithms, double aHighWaterMark,
+    Maybe<uint64_t> aAutoAllocateChunkSize, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void ReadableByteStreamControllerCallPullIfNeeded(
+    JSContext* aCx, ReadableByteStreamController* aController,
+    ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void SetUpReadableByteStreamControllerFromUnderlyingSource(
+    JSContext* aCx, ReadableStream* aStream,
+    JS::Handle<JSObject*> aUnderlyingSource,
+    UnderlyingSource& aUnderlyingSourceDict, double aHighWaterMark,
+    ErrorResult& aRv);
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/streams/ReadableStream.cpp b/dom/streams/ReadableStream.cpp
new file mode 100644
index 0000000000..7cd84a09ac
--- /dev/null
+++ b/dom/streams/ReadableStream.cpp
@@ -0,0 +1,1279 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/ReadableStream.h"
+
+#include "ReadIntoRequest.h"
+#include "ReadableStreamPipeTo.h"
+#include "ReadableStreamTee.h"
+#include "StreamUtils.h"
+#include "TeeState.h"
+#include "js/Array.h"
+#include "js/Exception.h"
+#include "js/PropertyAndElement.h"
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/CycleCollectedJSContext.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/StaticPrefs_dom.h"
+#include "mozilla/dom/BindingCallContext.h"
+#include "mozilla/dom/ByteStreamHelpers.h"
+#include "mozilla/dom/QueueWithSizes.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+#include "mozilla/dom/ReadRequest.h"
+#include "mozilla/dom/ReadableByteStreamController.h"
+#include "mozilla/dom/ReadableStreamBYOBReader.h"
+#include "mozilla/dom/ReadableStreamBinding.h"
+#include "mozilla/dom/ReadableStreamController.h"
+#include "mozilla/dom/ReadableStreamDefaultController.h"
+#include "mozilla/dom/ReadableStreamDefaultReader.h"
+#include "mozilla/dom/RootedDictionary.h"
+#include "mozilla/dom/ScriptSettings.h"
+#include "mozilla/dom/UnderlyingSourceBinding.h"
+#include "mozilla/dom/UnderlyingSourceCallbackHelpers.h"
+#include "mozilla/dom/WritableStream.h"
+#include "mozilla/dom/WritableStreamDefaultWriter.h"
+#include "nsCOMPtr.h"
+
+#include "mozilla/dom/Promise-inl.h"
+#include "nsIGlobalObject.h"
+#include "nsISupports.h"
+
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    mozilla::Variant<mozilla::Nothing,
+                     RefPtr<mozilla::dom::ReadableStreamDefaultReader>>&
+        aReader,
+    const char* aName, uint32_t aFlags = 0) {
+  if (aReader.is<RefPtr<mozilla::dom::ReadableStreamDefaultReader>>()) {
+    ImplCycleCollectionTraverse(
+        aCallback,
+        aReader.as<RefPtr<mozilla::dom::ReadableStreamDefaultReader>>(), aName,
+        aFlags);
+  }
+}
+
+inline void ImplCycleCollectionUnlink(
+    mozilla::Variant<mozilla::Nothing,
+                     RefPtr<mozilla::dom::ReadableStreamDefaultReader>>&
+        aReader) {
+  aReader = AsVariant(mozilla::Nothing());
+}
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+// Only needed for refcounted objects.
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_WITH_JS_MEMBERS(
+    ReadableStream, (mGlobal, mController, mReader), (mStoredError))
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ReadableStream)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ReadableStream)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ReadableStream)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+ReadableStream::ReadableStream(nsIGlobalObject* aGlobal,
+                               HoldDropJSObjectsCaller aHoldDropCaller)
+    : mGlobal(aGlobal), mReader(nullptr), mHoldDropCaller(aHoldDropCaller) {
+  if (mHoldDropCaller == HoldDropJSObjectsCaller::Implicit) {
+    mozilla::HoldJSObjects(this);
+  }
+}
+
+ReadableStream::ReadableStream(const GlobalObject& aGlobal,
+                               HoldDropJSObjectsCaller aHoldDropCaller)
+    : mGlobal(do_QueryInterface(aGlobal.GetAsSupports())),
+      mReader(nullptr),
+      mHoldDropCaller(aHoldDropCaller) {
+  if (mHoldDropCaller == HoldDropJSObjectsCaller::Implicit) {
+    mozilla::HoldJSObjects(this);
+  }
+}
+
+ReadableStream::~ReadableStream() {
+  if (mHoldDropCaller == HoldDropJSObjectsCaller::Implicit) {
+    mozilla::DropJSObjects(this);
+  }
+}
+
+JSObject* ReadableStream::WrapObject(JSContext* aCx,
+                                     JS::Handle<JSObject*> aGivenProto) {
+  return ReadableStream_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+ReadableStreamDefaultReader* ReadableStream::GetDefaultReader() {
+  return mReader->AsDefault();
+}
+
+void ReadableStream::SetReader(ReadableStreamGenericReader* aReader) {
+  mReader = aReader;
+}
+
+namespace streams_abstract {
+
+// https://streams.spec.whatwg.org/#readable-stream-has-byob-reader
+bool ReadableStreamHasBYOBReader(ReadableStream* aStream) {
+  // Step 1. Let reader be stream.[[reader]].
+  ReadableStreamGenericReader* reader = aStream->GetReader();
+
+  // Step 2. If reader is undefined, return false.
+  if (!reader) {
+    return false;
+  }
+
+  // Step 3. If reader implements ReadableStreamBYOBReader, return true.
+  // Step 4. Return false.
+  return reader->IsBYOB();
+}
+
+// https://streams.spec.whatwg.org/#readable-stream-has-default-reader
+bool ReadableStreamHasDefaultReader(ReadableStream* aStream) {
+  // Step 1. Let reader be stream.[[reader]].
+  ReadableStreamGenericReader* reader = aStream->GetReader();
+
+  // Step 2. If reader is undefined, return false.
+  if (!reader) {
+    return false;
+  }
+
+  // Step 3. If reader implements ReadableStreamDefaultReader, return true.
+  // Step 4. Return false.
+  return reader->IsDefault();
+}
+
+}  // namespace streams_abstract
+
+// Streams Spec: 4.2.4: https://streams.spec.whatwg.org/#rs-prototype
+/* static */
+already_AddRefed<ReadableStream> ReadableStream::Constructor(
+    const GlobalObject& aGlobal,
+    const Optional<JS::Handle<JSObject*>>& aUnderlyingSource,
+    const QueuingStrategy& aStrategy, ErrorResult& aRv) {
+  // Step 1.
+  JS::Rooted<JSObject*> underlyingSourceObj(
+      aGlobal.Context(),
+      aUnderlyingSource.WasPassed() ? aUnderlyingSource.Value() : nullptr);
+
+  // Step 2.
+  RootedDictionary<UnderlyingSource> underlyingSourceDict(aGlobal.Context());
+  if (underlyingSourceObj) {
+    JS::Rooted<JS::Value> objValue(aGlobal.Context(),
+                                   JS::ObjectValue(*underlyingSourceObj));
+    dom::BindingCallContext callCx(aGlobal.Context(),
+                                   "ReadableStream.constructor");
+    aRv.MightThrowJSException();
+    if (!underlyingSourceDict.Init(callCx, objValue)) {
+      aRv.StealExceptionFromJSContext(aGlobal.Context());
+      return nullptr;
+    }
+  }
+
+  // Step 3.
+  RefPtr<ReadableStream> readableStream =
+      new ReadableStream(aGlobal, HoldDropJSObjectsCaller::Implicit);
+
+  // Step 4.
+  if (underlyingSourceDict.mType.WasPassed()) {
+    // Implicit assertion on above check.
+    MOZ_ASSERT(underlyingSourceDict.mType.Value() == ReadableStreamType::Bytes);
+
+    // Step 4.1
+    if (aStrategy.mSize.WasPassed()) {
+      aRv.ThrowRangeError("Implementation preserved member 'size'");
+      return nullptr;
+    }
+
+    // Step 4.2
+    double highWaterMark = ExtractHighWaterMark(aStrategy, 0, aRv);
+    if (aRv.Failed()) {
+      return nullptr;
+    }
+
+    // Step 4.3
+    SetUpReadableByteStreamControllerFromUnderlyingSource(
+        aGlobal.Context(), readableStream, underlyingSourceObj,
+        underlyingSourceDict, highWaterMark, aRv);
+    if (aRv.Failed()) {
+      return nullptr;
+    }
+
+    return readableStream.forget();
+  }
+
+  // Step 5.1 (implicit in above check)
+  // Step 5.2. Extract callback.
+  //
+  // Implementation Note: The specification demands that if the size doesn't
+  // exist, we instead would provide an algorithm that returns 1. Instead, we
+  // will teach callers that a missing callback should simply return 1, rather
+  // than gin up a fake callback here.
+  //
+  // This decision may need to be revisited if the default action ever diverges
+  // within the specification.
+  RefPtr<QueuingStrategySize> sizeAlgorithm =
+      aStrategy.mSize.WasPassed() ? &aStrategy.mSize.Value() : nullptr;
+
+  // Step 5.3
+  double highWaterMark = ExtractHighWaterMark(aStrategy, 1, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 5.4.
+  SetupReadableStreamDefaultControllerFromUnderlyingSource(
+      aGlobal.Context(), readableStream, underlyingSourceObj,
+      underlyingSourceDict, highWaterMark, sizeAlgorithm, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  return readableStream.forget();
+}
+
+// Dealing with const this ptr is a pain, so just re-implement.
+// https://streams.spec.whatwg.org/#is-readable-stream-locked
+bool ReadableStream::Locked() const {
+  // Step 1 + 2.
+  return mReader;
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#initialize-readable-stream
+static void InitializeReadableStream(ReadableStream* aStream) {
+  // Step 1.
+  aStream->SetState(ReadableStream::ReaderState::Readable);
+
+  // Step 2.
+  aStream->SetReader(nullptr);
+  aStream->SetStoredError(JS::UndefinedHandleValue);
+
+  // Step 3.
+  aStream->SetDisturbed(false);
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#create-readable-stream
+MOZ_CAN_RUN_SCRIPT
+already_AddRefed<ReadableStream> ReadableStream::CreateAbstract(
+    JSContext* aCx, nsIGlobalObject* aGlobal,
+    UnderlyingSourceAlgorithmsBase* aAlgorithms,
+    mozilla::Maybe<double> aHighWaterMark, QueuingStrategySize* aSizeAlgorithm,
+    ErrorResult& aRv) {
+  // Step 1. If highWaterMark was not passed, set it to 1.
+  double highWaterMark = aHighWaterMark.valueOr(1.0);
+
+  // Step 2. consumers of sizeAlgorithm
+  //         handle null algorithms correctly.
+  // Step 3.
+  MOZ_ASSERT(IsNonNegativeNumber(highWaterMark));
+  // Step 4.
+  RefPtr<ReadableStream> stream =
+      new ReadableStream(aGlobal, HoldDropJSObjectsCaller::Implicit);
+
+  // Step 5.
+  InitializeReadableStream(stream);
+
+  // Step 6.
+  RefPtr<ReadableStreamDefaultController> controller =
+      new ReadableStreamDefaultController(aGlobal);
+
+  // Step 7.
+  SetUpReadableStreamDefaultController(aCx, stream, controller, aAlgorithms,
+                                       highWaterMark, aSizeAlgorithm, aRv);
+
+  // Step 8.
+  return stream.forget();
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#readable-stream-close
+void ReadableStreamClose(JSContext* aCx, ReadableStream* aStream,
+                         ErrorResult& aRv) {
+  // Step 1.
+  MOZ_ASSERT(aStream->State() == ReadableStream::ReaderState::Readable);
+
+  // Step 2.
+  aStream->SetState(ReadableStream::ReaderState::Closed);
+
+  // Step 3.
+  ReadableStreamGenericReader* reader = aStream->GetReader();
+
+  // Step 4.
+  if (!reader) {
+    return;
+  }
+
+  // Step 5.
+  reader->ClosedPromise()->MaybeResolveWithUndefined();
+
+  // Step 6.
+  if (reader->IsDefault()) {
+    // Step 6.1. Let readRequests be reader.[[readRequests]].
+    // Move LinkedList out of DefaultReader onto stack to avoid the potential
+    // for concurrent modification, which could invalidate the iterator.
+    //
+    // See https://bugs.chromium.org/p/chromium/issues/detail?id=1045874 as an
+    // example of the kind of issue that could occur.
+    LinkedList<RefPtr<ReadRequest>> readRequests =
+        std::move(reader->AsDefault()->ReadRequests());
+
+    // Step 6.2. Set reader.[[readRequests]] to an empty list.
+    // Note: The std::move already cleared this anyway.
+    reader->AsDefault()->ReadRequests().clear();
+
+    // Step 6.3. For each readRequest of readRequests,
+    // Drain the local list and destroy elements along the way.
+    while (RefPtr<ReadRequest> readRequest = readRequests.popFirst()) {
+      // Step 6.3.1. Perform readRequest’s close steps.
+      readRequest->CloseSteps(aCx, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+    }
+  }
+}
+
+// https://streams.spec.whatwg.org/#readable-stream-cancel
+already_AddRefed<Promise> ReadableStreamCancel(JSContext* aCx,
+                                               ReadableStream* aStream,
+                                               JS::Handle<JS::Value> aError,
+                                               ErrorResult& aRv) {
+  // Step 1.
+  aStream->SetDisturbed(true);
+
+  // Step 2.
+  if (aStream->State() == ReadableStream::ReaderState::Closed) {
+    RefPtr<Promise> promise =
+        Promise::CreateInfallible(aStream->GetParentObject());
+    promise->MaybeResolveWithUndefined();
+    return promise.forget();
+  }
+
+  // Step 3.
+  if (aStream->State() == ReadableStream::ReaderState::Errored) {
+    JS::Rooted<JS::Value> storedError(aCx, aStream->StoredError());
+    return Promise::CreateRejected(aStream->GetParentObject(), storedError,
+                                   aRv);
+  }
+
+  // Step 4.
+  ReadableStreamClose(aCx, aStream, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 5.
+  ReadableStreamGenericReader* reader = aStream->GetReader();
+
+  // Step 6.
+  if (reader && reader->IsBYOB()) {
+    // Step 6.1. Let readIntoRequests be reader.[[readIntoRequests]].
+    LinkedList<RefPtr<ReadIntoRequest>> readIntoRequests =
+        std::move(reader->AsBYOB()->ReadIntoRequests());
+
+    // Step 6.2. Set reader.[[readIntoRequests]] to an empty list.
+    // Note: The std::move already cleared this anyway.
+    reader->AsBYOB()->ReadIntoRequests().clear();
+
+    // Step 6.3. For each readIntoRequest of readIntoRequests,
+    while (RefPtr<ReadIntoRequest> readIntoRequest =
+               readIntoRequests.popFirst()) {
+      // Step 6.3.1.Perform readIntoRequest’s close steps, given undefined.
+      readIntoRequest->CloseSteps(aCx, JS::UndefinedHandleValue, aRv);
+      if (aRv.Failed()) {
+        return nullptr;
+      }
+    }
+  }
+
+  // Step 7.
+  RefPtr<ReadableStreamController> controller(aStream->Controller());
+  RefPtr<Promise> sourceCancelPromise =
+      controller->CancelSteps(aCx, aError, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 8.
+  RefPtr<Promise> promise =
+      Promise::CreateInfallible(sourceCancelPromise->GetParentObject());
+
+  // ThenWithCycleCollectedArgs will carry promise, keeping it alive until the
+  // callback executes.
+  Result<RefPtr<Promise>, nsresult> returnResult =
+      sourceCancelPromise->ThenWithCycleCollectedArgs(
+          [](JSContext*, JS::Handle<JS::Value>, ErrorResult&,
+             RefPtr<Promise> newPromise) {
+            newPromise->MaybeResolveWithUndefined();
+            return newPromise.forget();
+          },
+          promise);
+
+  if (returnResult.isErr()) {
+    aRv.Throw(returnResult.unwrapErr());
+    return nullptr;
+  }
+
+  return returnResult.unwrap().forget();
+}
+
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#rs-cancel
+already_AddRefed<Promise> ReadableStream::Cancel(JSContext* aCx,
+                                                 JS::Handle<JS::Value> aReason,
+                                                 ErrorResult& aRv) {
+  // Step 1. If ! IsReadableStreamLocked(this) is true,
+  // return a promise rejected with a TypeError exception.
+  if (Locked()) {
+    aRv.ThrowTypeError("Cannot cancel a stream locked by a reader.");
+    return nullptr;
+  }
+
+  // Step 2. Return ! ReadableStreamCancel(this, reason).
+  RefPtr<ReadableStream> thisRefPtr = this;
+  return ReadableStreamCancel(aCx, thisRefPtr, aReason, aRv);
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#acquire-readable-stream-reader
+already_AddRefed<ReadableStreamDefaultReader>
+AcquireReadableStreamDefaultReader(ReadableStream* aStream, ErrorResult& aRv) {
+  // Step 1.
+  RefPtr<ReadableStreamDefaultReader> reader =
+      new ReadableStreamDefaultReader(aStream->GetParentObject());
+
+  // Step 2.
+  SetUpReadableStreamDefaultReader(reader, aStream, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 3.
+  return reader.forget();
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#rs-get-reader
+void ReadableStream::GetReader(const ReadableStreamGetReaderOptions& aOptions,
+                               OwningReadableStreamReader& resultReader,
+                               ErrorResult& aRv) {
+  // Step 1. If options["mode"] does not exist,
+  // return ? AcquireReadableStreamDefaultReader(this).
+  if (!aOptions.mMode.WasPassed()) {
+    RefPtr<ReadableStreamDefaultReader> defaultReader =
+        AcquireReadableStreamDefaultReader(this, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+    resultReader.SetAsReadableStreamDefaultReader() = defaultReader;
+    return;
+  }
+
+  // Step 2. Assert: options["mode"] is "byob".
+  MOZ_ASSERT(aOptions.mMode.Value() == ReadableStreamReaderMode::Byob);
+
+  // Step 3. Return ? AcquireReadableStreamBYOBReader(this).
+  RefPtr<ReadableStreamBYOBReader> byobReader =
+      AcquireReadableStreamBYOBReader(this, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+  resultReader.SetAsReadableStreamBYOBReader() = byobReader;
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#is-readable-stream-locked
+bool IsReadableStreamLocked(ReadableStream* aStream) {
+  // Step 1 + 2.
+  return aStream->Locked();
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#rs-pipe-through
+MOZ_CAN_RUN_SCRIPT already_AddRefed<ReadableStream> ReadableStream::PipeThrough(
+    const ReadableWritablePair& aTransform, const StreamPipeOptions& aOptions,
+    ErrorResult& aRv) {
+  // Step 1: If ! IsReadableStreamLocked(this) is true, throw a TypeError
+  // exception.
+  if (IsReadableStreamLocked(this)) {
+    aRv.ThrowTypeError("Cannot pipe from a locked stream.");
+    return nullptr;
+  }
+
+  // Step 2: If ! IsWritableStreamLocked(transform["writable"]) is true, throw a
+  // TypeError exception.
+  if (IsWritableStreamLocked(aTransform.mWritable)) {
+    aRv.ThrowTypeError("Cannot pipe to a locked stream.");
+    return nullptr;
+  }
+
+  // Step 3: Let signal be options["signal"] if it exists, or undefined
+  // otherwise.
+  RefPtr<AbortSignal> signal =
+      aOptions.mSignal.WasPassed() ? &aOptions.mSignal.Value() : nullptr;
+
+  // Step 4: Let promise be ! ReadableStreamPipeTo(this, transform["writable"],
+  // options["preventClose"], options["preventAbort"], options["preventCancel"],
+  // signal).
+  RefPtr<WritableStream> writable = aTransform.mWritable;
+  RefPtr<Promise> promise = ReadableStreamPipeTo(
+      this, writable, aOptions.mPreventClose, aOptions.mPreventAbort,
+      aOptions.mPreventCancel, signal, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 5: Set promise.[[PromiseIsHandled]] to true.
+  MOZ_ALWAYS_TRUE(promise->SetAnyPromiseIsHandled());
+
+  // Step 6: Return transform["readable"].
+  return do_AddRef(aTransform.mReadable.get());
+};
+
+namespace streams_abstract {
+
+// https://streams.spec.whatwg.org/#readable-stream-get-num-read-requests
+double ReadableStreamGetNumReadRequests(ReadableStream* aStream) {
+  // Step 1.
+  MOZ_ASSERT(ReadableStreamHasDefaultReader(aStream));
+
+  // Step 2.
+  return double(aStream->GetDefaultReader()->ReadRequests().length());
+}
+
+// https://streams.spec.whatwg.org/#readable-stream-error
+void ReadableStreamError(JSContext* aCx, ReadableStream* aStream,
+                         JS::Handle<JS::Value> aValue, ErrorResult& aRv) {
+  // Step 1.
+  MOZ_ASSERT(aStream->State() == ReadableStream::ReaderState::Readable);
+
+  // Step 2.
+  aStream->SetState(ReadableStream::ReaderState::Errored);
+
+  // Step 3.
+  aStream->SetStoredError(aValue);
+
+  // Step 4.
+  ReadableStreamGenericReader* reader = aStream->GetReader();
+
+  // Step 5.
+  if (!reader) {
+    return;
+  }
+
+  // Step 6.
+  reader->ClosedPromise()->MaybeReject(aValue);
+
+  // Step 7.
+  reader->ClosedPromise()->SetSettledPromiseIsHandled();
+
+  // Step 8.
+  if (reader->IsDefault()) {
+    // Step 8.1. Perform ! ReadableStreamDefaultReaderErrorReadRequests(reader,
+    // e).
+    RefPtr<ReadableStreamDefaultReader> defaultReader = reader->AsDefault();
+    ReadableStreamDefaultReaderErrorReadRequests(aCx, defaultReader, aValue,
+                                                 aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+  } else {
+    // Step 9. Otherwise,
+    // Step 9.1. Assert: reader implements ReadableStreamBYOBReader.
+    MOZ_ASSERT(reader->IsBYOB());
+
+    // Step 9.2. Perform ! ReadableStreamBYOBReaderErrorReadIntoRequests(reader,
+    // e).
+    RefPtr<ReadableStreamBYOBReader> byobReader = reader->AsBYOB();
+    ReadableStreamBYOBReaderErrorReadIntoRequests(aCx, byobReader, aValue, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+  }
+}
+
+// https://streams.spec.whatwg.org/#rs-default-controller-close
+void ReadableStreamFulfillReadRequest(JSContext* aCx, ReadableStream* aStream,
+                                      JS::Handle<JS::Value> aChunk, bool aDone,
+                                      ErrorResult& aRv) {
+  // Step 1.
+  MOZ_ASSERT(ReadableStreamHasDefaultReader(aStream));
+
+  // Step 2.
+  ReadableStreamDefaultReader* reader = aStream->GetDefaultReader();
+
+  // Step 3.
+  MOZ_ASSERT(!reader->ReadRequests().isEmpty());
+
+  // Step 4+5.
+  RefPtr<ReadRequest> readRequest = reader->ReadRequests().popFirst();
+
+  // Step 6.
+  if (aDone) {
+    readRequest->CloseSteps(aCx, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+  }
+
+  // Step 7.
+  readRequest->ChunkSteps(aCx, aChunk, aRv);
+}
+
+// https://streams.spec.whatwg.org/#readable-stream-add-read-request
+void ReadableStreamAddReadRequest(ReadableStream* aStream,
+                                  ReadRequest* aReadRequest) {
+  // Step 1.
+  MOZ_ASSERT(aStream->GetReader()->IsDefault());
+  // Step 2.
+  MOZ_ASSERT(aStream->State() == ReadableStream::ReaderState::Readable);
+  // Step 3.
+  aStream->GetDefaultReader()->ReadRequests().insertBack(aReadRequest);
+}
+
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaulttee
+// Step 14, 15
+MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise>
+ReadableStreamDefaultTeeSourceAlgorithms::CancelCallback(
+    JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+    ErrorResult& aRv) {
+  // Step 1.
+  mTeeState->SetCanceled(mBranch, true);
+
+  // Step 2.
+  mTeeState->SetReason(mBranch, aReason.Value());
+
+  // Step 3.
+
+  if (mTeeState->Canceled(OtherTeeBranch(mBranch))) {
+    // Step 3.1
+
+    JS::Rooted<JSObject*> compositeReason(aCx, JS::NewArrayObject(aCx, 2));
+    if (!compositeReason) {
+      aRv.StealExceptionFromJSContext(aCx);
+      return nullptr;
+    }
+
+    JS::Rooted<JS::Value> reason1(aCx, mTeeState->Reason1());
+    if (!JS_SetElement(aCx, compositeReason, 0, reason1)) {
+      aRv.StealExceptionFromJSContext(aCx);
+      return nullptr;
+    }
+
+    JS::Rooted<JS::Value> reason2(aCx, mTeeState->Reason2());
+    if (!JS_SetElement(aCx, compositeReason, 1, reason2)) {
+      aRv.StealExceptionFromJSContext(aCx);
+      return nullptr;
+    }
+
+    // Step 3.2
+    JS::Rooted<JS::Value> compositeReasonValue(
+        aCx, JS::ObjectValue(*compositeReason));
+    RefPtr<ReadableStream> stream(mTeeState->GetStream());
+    RefPtr<Promise> cancelResult =
+        ReadableStreamCancel(aCx, stream, compositeReasonValue, aRv);
+    if (aRv.Failed()) {
+      return nullptr;
+    }
+
+    // Step 3.3
+    mTeeState->CancelPromise()->MaybeResolve(cancelResult);
+  }
+
+  // Step 4.
+  return do_AddRef(mTeeState->CancelPromise());
+}
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaulttee
+MOZ_CAN_RUN_SCRIPT
+static void ReadableStreamDefaultTee(JSContext* aCx, ReadableStream* aStream,
+                                     bool aCloneForBranch2,
+                                     nsTArray<RefPtr<ReadableStream>>& aResult,
+                                     ErrorResult& aRv) {
+  // Step 1. Implicit.
+  // Step 2. Implicit.
+
+  // Steps 3-12 are contained in the construction of Tee State.
+  RefPtr<TeeState> teeState = TeeState::Create(aStream, aCloneForBranch2, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 13 - 16
+  auto branch1Algorithms = MakeRefPtr<ReadableStreamDefaultTeeSourceAlgorithms>(
+      teeState, TeeBranch::Branch1);
+  auto branch2Algorithms = MakeRefPtr<ReadableStreamDefaultTeeSourceAlgorithms>(
+      teeState, TeeBranch::Branch2);
+
+  // Step 17.
+  nsCOMPtr<nsIGlobalObject> global(
+      do_AddRef(teeState->GetStream()->GetParentObject()));
+  teeState->SetBranch1(ReadableStream::CreateAbstract(
+      aCx, global, branch1Algorithms, mozilla::Nothing(), nullptr, aRv));
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 18.
+  teeState->SetBranch2(ReadableStream::CreateAbstract(
+      aCx, global, branch2Algorithms, mozilla::Nothing(), nullptr, aRv));
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 19.
+  teeState->GetReader()->ClosedPromise()->AddCallbacksWithCycleCollectedArgs(
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         TeeState* aTeeState) {},
+      [](JSContext* aCx, JS::Handle<JS::Value> aReason, ErrorResult& aRv,
+         TeeState* aTeeState) {
+        // Step 19.1.
+        ReadableStreamDefaultControllerError(
+            aCx, aTeeState->Branch1()->DefaultController(), aReason, aRv);
+        if (aRv.Failed()) {
+          return;
+        }
+
+        // Step 19.2
+        ReadableStreamDefaultControllerError(
+            aCx, aTeeState->Branch2()->DefaultController(), aReason, aRv);
+        if (aRv.Failed()) {
+          return;
+        }
+
+        // Step 19.3
+        if (!aTeeState->Canceled1() || !aTeeState->Canceled2()) {
+          aTeeState->CancelPromise()->MaybeResolveWithUndefined();
+        }
+      },
+      RefPtr(teeState));
+
+  // Step 20.
+  aResult.AppendElement(teeState->Branch1());
+  aResult.AppendElement(teeState->Branch2());
+}
+
+// https://streams.spec.whatwg.org/#rs-pipe-to
+already_AddRefed<Promise> ReadableStream::PipeTo(
+    WritableStream& aDestination, const StreamPipeOptions& aOptions,
+    ErrorResult& aRv) {
+  // Step 1. If !IsReadableStreamLocked(this) is true, return a promise rejected
+  // with a TypeError exception.
+  if (IsReadableStreamLocked(this)) {
+    aRv.ThrowTypeError("Cannot pipe from a locked stream.");
+    return nullptr;
+  }
+
+  // Step 2. If !IsWritableStreamLocked(destination) is true, return a promise
+  //         rejected with a TypeError exception.
+  if (IsWritableStreamLocked(&aDestination)) {
+    aRv.ThrowTypeError("Cannot pipe to a locked stream.");
+    return nullptr;
+  }
+
+  // Step 3. Let signal be options["signal"] if it exists, or undefined
+  // otherwise.
+  RefPtr<AbortSignal> signal =
+      aOptions.mSignal.WasPassed() ? &aOptions.mSignal.Value() : nullptr;
+
+  // Step 4. Return ! ReadableStreamPipeTo(this, destination,
+  // options["preventClose"], options["preventAbort"], options["preventCancel"],
+  // signal).
+  return ReadableStreamPipeTo(this, &aDestination, aOptions.mPreventClose,
+                              aOptions.mPreventAbort, aOptions.mPreventCancel,
+                              signal, aRv);
+}
+
+// https://streams.spec.whatwg.org/#readable-stream-tee
+MOZ_CAN_RUN_SCRIPT
+static void ReadableStreamTee(JSContext* aCx, ReadableStream* aStream,
+                              bool aCloneForBranch2,
+                              nsTArray<RefPtr<ReadableStream>>& aResult,
+                              ErrorResult& aRv) {
+  // Step 1. Implicit.
+  // Step 2. Implicit.
+  // Step 3.
+  if (aStream->Controller()->IsByte()) {
+    ReadableByteStreamTee(aCx, aStream, aResult, aRv);
+    return;
+  }
+  // Step 4.
+  ReadableStreamDefaultTee(aCx, aStream, aCloneForBranch2, aResult, aRv);
+}
+
+void ReadableStream::Tee(JSContext* aCx,
+                         nsTArray<RefPtr<ReadableStream>>& aResult,
+                         ErrorResult& aRv) {
+  ReadableStreamTee(aCx, this, false, aResult, aRv);
+}
+
+void ReadableStream::IteratorData::Traverse(
+    nsCycleCollectionTraversalCallback& cb) {
+  ReadableStream::IteratorData* tmp = this;
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mReader);
+}
+void ReadableStream::IteratorData::Unlink() {
+  ReadableStream::IteratorData* tmp = this;
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mReader);
+}
+
+// https://streams.spec.whatwg.org/#rs-get-iterator
+void ReadableStream::InitAsyncIteratorData(
+    IteratorData& aData, Iterator::IteratorType aType,
+    const ReadableStreamIteratorOptions& aOptions, ErrorResult& aRv) {
+  // Step 1. Let reader be ? AcquireReadableStreamDefaultReader(stream).
+  RefPtr<ReadableStreamDefaultReader> reader =
+      AcquireReadableStreamDefaultReader(this, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 2. Set iterator’s reader to reader.
+  aData.mReader = reader;
+
+  // Step 3. Let preventCancel be args[0]["preventCancel"].
+  // Step 4. Set iterator’s prevent cancel to preventCancel.
+  aData.mPreventCancel = aOptions.mPreventCancel;
+}
+
+// https://streams.spec.whatwg.org/#rs-asynciterator-prototype-next
+// Step 4.
+struct IteratorReadRequest : public ReadRequest {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(IteratorReadRequest, ReadRequest)
+
+  RefPtr<Promise> mPromise;
+  RefPtr<ReadableStreamDefaultReader> mReader;
+
+  explicit IteratorReadRequest(Promise* aPromise,
+                               ReadableStreamDefaultReader* aReader)
+      : mPromise(aPromise), mReader(aReader) {}
+
+  // chunk steps, given chunk
+  void ChunkSteps(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                  ErrorResult& aRv) override {
+    // Step 1. Resolve promise with chunk.
+    mPromise->MaybeResolve(aChunk);
+  }
+
+  // close steps
+  void CloseSteps(JSContext* aCx, ErrorResult& aRv) override {
+    // Step 1. Perform ! ReadableStreamDefaultReaderRelease(reader).
+    ReadableStreamDefaultReaderRelease(aCx, mReader, aRv);
+    if (aRv.Failed()) {
+      mPromise->MaybeRejectWithUndefined();
+      return;
+    }
+
+    // Step 2. Resolve promise with end of iteration.
+    iterator_utils::ResolvePromiseForFinished(mPromise);
+  }
+
+  // error steps, given e
+  void ErrorSteps(JSContext* aCx, JS::Handle<JS::Value> aError,
+                  ErrorResult& aRv) override {
+    // Step 1. Perform ! ReadableStreamDefaultReaderRelease(reader).
+    ReadableStreamDefaultReaderRelease(aCx, mReader, aRv);
+    if (aRv.Failed()) {
+      mPromise->MaybeRejectWithUndefined();
+      return;
+    }
+
+    // Step 2. Reject promise with e.
+    mPromise->MaybeReject(aError);
+  }
+
+ protected:
+  virtual ~IteratorReadRequest() = default;
+};
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(IteratorReadRequest, ReadRequest, mPromise,
+                                   mReader)
+
+NS_IMPL_ADDREF_INHERITED(IteratorReadRequest, ReadRequest)
+NS_IMPL_RELEASE_INHERITED(IteratorReadRequest, ReadRequest)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(IteratorReadRequest)
+NS_INTERFACE_MAP_END_INHERITING(ReadRequest)
+
+// https://streams.spec.whatwg.org/#rs-asynciterator-prototype-next
+already_AddRefed<Promise> ReadableStream::GetNextIterationResult(
+    Iterator* aIterator, ErrorResult& aRv) {
+  // Step 1. Let reader be iterator’s reader.
+  RefPtr<ReadableStreamDefaultReader> reader = aIterator->Data().mReader;
+
+  // Step 2. Assert: reader.[[stream]] is not undefined.
+  MOZ_ASSERT(reader->GetStream());
+
+  // Step 3. Let promise be a new promise.
+  RefPtr<Promise> promise = Promise::CreateInfallible(GetParentObject());
+
+  // Step 4. Let readRequest be a new read request with the following items:
+  RefPtr<ReadRequest> request = new IteratorReadRequest(promise, reader);
+
+  // Step 5. Perform ! ReadableStreamDefaultReaderRead(this, readRequest).
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(mGlobal)) {
+    aRv.ThrowUnknownError("Internal error");
+    return nullptr;
+  }
+
+  ReadableStreamDefaultReaderRead(jsapi.cx(), reader, request, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 6. Return promise.
+  return promise.forget();
+}
+
+// https://streams.spec.whatwg.org/#rs-asynciterator-prototype-return
+already_AddRefed<Promise> ReadableStream::IteratorReturn(
+    JSContext* aCx, Iterator* aIterator, JS::Handle<JS::Value> aValue,
+    ErrorResult& aRv) {
+  // Step 1. Let reader be iterator’s reader.
+  RefPtr<ReadableStreamDefaultReader> reader = aIterator->Data().mReader;
+
+  // Step 2. Assert: reader.[[stream]] is not undefined.
+  MOZ_ASSERT(reader->GetStream());
+
+  // Step 3. Assert: reader.[[readRequests]] is empty, as the async iterator
+  // machinery guarantees that any previous calls to next() have settled before
+  // this is called.
+  MOZ_ASSERT(reader->ReadRequests().isEmpty());
+
+  // Step 4. If iterator’s prevent cancel is false:
+  if (!aIterator->Data().mPreventCancel) {
+    // Step 4.1. Let result be ! ReadableStreamReaderGenericCancel(reader, arg).
+    RefPtr<ReadableStream> stream(reader->GetStream());
+    RefPtr<Promise> result = ReadableStreamCancel(aCx, stream, aValue, aRv);
+    if (NS_WARN_IF(aRv.Failed())) {
+      return nullptr;
+    }
+
+    MOZ_DIAGNOSTIC_ASSERT(
+        reader->GetStream(),
+        "We shouldn't have a null stream here (bug 1821169).");
+    if (!reader->GetStream()) {
+      aRv.Throw(NS_ERROR_FAILURE);
+      return nullptr;
+    }
+
+    // Step 4.2. Perform ! ReadableStreamDefaultReaderRelease(reader).
+    ReadableStreamDefaultReaderRelease(aCx, reader, aRv);
+    if (NS_WARN_IF(aRv.Failed())) {
+      return nullptr;
+    }
+
+    // Step 4.3. Return result.
+    return result.forget();
+  }
+
+  // Step 5. Perform ! ReadableStreamDefaultReaderRelease(reader).
+  ReadableStreamDefaultReaderRelease(aCx, reader, aRv);
+  if (NS_WARN_IF(aRv.Failed())) {
+    return nullptr;
+  }
+
+  // Step 6. Return a promise resolved with undefined.
+  return Promise::CreateResolvedWithUndefined(GetParentObject(), aRv);
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#readable-stream-add-read-into-request
+void ReadableStreamAddReadIntoRequest(ReadableStream* aStream,
+                                      ReadIntoRequest* aReadIntoRequest) {
+  // Step 1. Assert: stream.[[reader]] implements ReadableStreamBYOBReader.
+  MOZ_ASSERT(aStream->GetReader()->IsBYOB());
+
+  // Step 2. Assert: stream.[[state]] is "readable" or "closed".
+  MOZ_ASSERT(aStream->State() == ReadableStream::ReaderState::Readable ||
+             aStream->State() == ReadableStream::ReaderState::Closed);
+
+  // Step 3. Append readRequest to stream.[[reader]].[[readIntoRequests]].
+  aStream->GetReader()->AsBYOB()->ReadIntoRequests().insertBack(
+      aReadIntoRequest);
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#abstract-opdef-createreadablebytestream
+already_AddRefed<ReadableStream> ReadableStream::CreateByteAbstract(
+    JSContext* aCx, nsIGlobalObject* aGlobal,
+    UnderlyingSourceAlgorithmsBase* aAlgorithms, ErrorResult& aRv) {
+  // Step 1. Let stream be a new ReadableStream.
+  RefPtr<ReadableStream> stream =
+      new ReadableStream(aGlobal, HoldDropJSObjectsCaller::Implicit);
+
+  // Step 2. Perform ! InitializeReadableStream(stream).
+  InitializeReadableStream(stream);
+
+  // Step 3. Let controller be a new ReadableByteStreamController.
+  RefPtr<ReadableByteStreamController> controller =
+      new ReadableByteStreamController(aGlobal);
+
+  // Step 4. Perform ? SetUpReadableByteStreamController(stream, controller,
+  // startAlgorithm, pullAlgorithm, cancelAlgorithm, 0, undefined).
+  SetUpReadableByteStreamController(aCx, stream, controller, aAlgorithms, 0,
+                                    mozilla::Nothing(), aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Return stream.
+  return stream.forget();
+}
+
+// https://streams.spec.whatwg.org/#readablestream-set-up
+// (except this instead creates a new ReadableStream rather than accepting an
+// existing instance)
+// _BOUNDARY because `aAlgorithms->StartCallback` (called by
+// SetUpReadableStreamDefaultController below) should not be able to run script
+// in this case.
+MOZ_CAN_RUN_SCRIPT_BOUNDARY already_AddRefed<ReadableStream>
+ReadableStream::CreateNative(JSContext* aCx, nsIGlobalObject* aGlobal,
+                             UnderlyingSourceAlgorithmsWrapper& aAlgorithms,
+                             mozilla::Maybe<double> aHighWaterMark,
+                             QueuingStrategySize* aSizeAlgorithm,
+                             ErrorResult& aRv) {
+  // an optional number highWaterMark (default 1)
+  double highWaterMark = aHighWaterMark.valueOr(1);
+  // and if given, highWaterMark must be a non-negative, non-NaN number.
+  MOZ_ASSERT(IsNonNegativeNumber(highWaterMark));
+
+  // Step 1: Let startAlgorithm be an algorithm that returns undefined.
+  // Step 2: Let pullAlgorithmWrapper be an algorithm that runs these steps:
+  // Step 3: Let cancelAlgorithmWrapper be an algorithm that runs these steps:
+  // (Done by UnderlyingSourceAlgorithmsWrapper)
+
+  // Step 4: If sizeAlgorithm was not given, then set it to an algorithm that
+  // returns 1. (Callers will treat nullptr as such, see
+  // ReadableStream::Constructor for details)
+
+  // Step 5: Perform ! InitializeReadableStream(stream).
+  RefPtr<ReadableStream> stream =
+      new ReadableStream(aGlobal, HoldDropJSObjectsCaller::Implicit);
+
+  // Step 6: Let controller be a new ReadableStreamDefaultController.
+  auto controller = MakeRefPtr<ReadableStreamDefaultController>(aGlobal);
+
+  // Step 7: Perform ! SetUpReadableStreamDefaultController(stream, controller,
+  // startAlgorithm, pullAlgorithmWrapper, cancelAlgorithmWrapper,
+  // highWaterMark, sizeAlgorithm).
+  SetUpReadableStreamDefaultController(aCx, stream, controller, &aAlgorithms,
+                                       highWaterMark, aSizeAlgorithm, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+  return stream.forget();
+}
+
+// https://streams.spec.whatwg.org/#readablestream-set-up-with-byte-reading-support
+// _BOUNDARY because `aAlgorithms->StartCallback` (called by
+// SetUpReadableByteStreamController below) should not be able to run script in
+// this case.
+MOZ_CAN_RUN_SCRIPT_BOUNDARY void ReadableStream::SetUpByteNative(
+    JSContext* aCx, UnderlyingSourceAlgorithmsWrapper& aAlgorithms,
+    mozilla::Maybe<double> aHighWaterMark, ErrorResult& aRv) {
+  // an optional number highWaterMark (default 0)
+  double highWaterMark = aHighWaterMark.valueOr(0);
+  // and if given, highWaterMark must be a non-negative, non-NaN number.
+  MOZ_ASSERT(IsNonNegativeNumber(highWaterMark));
+
+  // Step 1: Let startAlgorithm be an algorithm that returns undefined.
+  // Step 2: Let pullAlgorithmWrapper be an algorithm that runs these steps:
+  // Step 3: Let cancelAlgorithmWrapper be an algorithm that runs these steps:
+  // (Done by UnderlyingSourceAlgorithmsWrapper)
+
+  // Step 4: Perform ! InitializeReadableStream(stream).
+  // (Covered by constructor)
+
+  // Step 5: Let controller be a new ReadableByteStreamController.
+  auto controller = MakeRefPtr<ReadableByteStreamController>(GetParentObject());
+
+  // Step 6: Perform ! SetUpReadableByteStreamController(stream, controller,
+  // startAlgorithm, pullAlgorithmWrapper, cancelAlgorithmWrapper,
+  // highWaterMark, undefined).
+  SetUpReadableByteStreamController(aCx, this, controller, &aAlgorithms,
+                                    highWaterMark, Nothing(), aRv);
+}
+
+already_AddRefed<ReadableStream> ReadableStream::CreateByteNative(
+    JSContext* aCx, nsIGlobalObject* aGlobal,
+    UnderlyingSourceAlgorithmsWrapper& aAlgorithms,
+    mozilla::Maybe<double> aHighWaterMark, ErrorResult& aRv) {
+  RefPtr<ReadableStream> stream =
+      new ReadableStream(aGlobal, HoldDropJSObjectsCaller::Implicit);
+  stream->SetUpByteNative(aCx, aAlgorithms, aHighWaterMark, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+  return stream.forget();
+}
+
+// https://streams.spec.whatwg.org/#readablestream-close
+void ReadableStream::CloseNative(JSContext* aCx, ErrorResult& aRv) {
+  MOZ_ASSERT(mController->GetAlgorithms()->IsNative());
+
+  // Step 1: If stream.[[controller]] implements ReadableByteStreamController,
+  if (mController->IsByte()) {
+    RefPtr<ReadableByteStreamController> controller = mController->AsByte();
+
+    // Step 1.1: Perform !
+    // ReadableByteStreamControllerClose(stream.[[controller]]).
+    ReadableByteStreamControllerClose(aCx, controller, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+
+    // Step 1.2: If stream.[[controller]].[[pendingPullIntos]] is not empty,
+    // perform ! ReadableByteStreamControllerRespond(stream.[[controller]], 0).
+    if (!controller->PendingPullIntos().isEmpty()) {
+      ReadableByteStreamControllerRespond(aCx, controller, 0, aRv);
+    }
+    return;
+  }
+
+  // Step 2: Otherwise, perform !
+  // ReadableStreamDefaultControllerClose(stream.[[controller]]).
+  RefPtr<ReadableStreamDefaultController> controller = mController->AsDefault();
+  ReadableStreamDefaultControllerClose(aCx, controller, aRv);
+}
+
+// https://streams.spec.whatwg.org/#readablestream-error
+void ReadableStream::ErrorNative(JSContext* aCx, JS::Handle<JS::Value> aError,
+                                 ErrorResult& aRv) {
+  // Step 1: If stream.[[controller]] implements ReadableByteStreamController,
+  // then perform ! ReadableByteStreamControllerError(stream.[[controller]], e).
+  if (mController->IsByte()) {
+    ReadableByteStreamControllerError(mController->AsByte(), aError, aRv);
+    return;
+  }
+  // Step 2: Otherwise, perform !
+  // ReadableStreamDefaultControllerError(stream.[[controller]], e).
+  ReadableStreamDefaultControllerError(aCx, mController->AsDefault(), aError,
+                                       aRv);
+}
+
+// https://streams.spec.whatwg.org/#readablestream-current-byob-request-view
+static void CurrentBYOBRequestView(JSContext* aCx,
+                                   ReadableByteStreamController& aController,
+                                   JS::MutableHandle<JSObject*> aRetVal,
+                                   ErrorResult& aRv) {
+  // Step 1. Assert: stream.[[controller]] implements
+  // ReadableByteStreamController. (implicit)
+
+  // Step 2: Let byobRequest be !
+  // ReadableByteStreamControllerGetBYOBRequest(stream.[[controller]]).
+  RefPtr<ReadableStreamBYOBRequest> byobRequest =
+      ReadableByteStreamControllerGetBYOBRequest(aCx, &aController, aRv);
+  // Step 3: If byobRequest is null, then return null.
+  if (!byobRequest) {
+    aRetVal.set(nullptr);
+    return;
+  }
+  // Step 4: Return byobRequest.[[view]].
+  byobRequest->GetView(aCx, aRetVal);
+}
+
+static bool HasSameBufferView(JSContext* aCx, JS::Handle<JSObject*> aX,
+                              JS::Handle<JSObject*> aY, ErrorResult& aRv) {
+  bool isShared;
+  JS::Rooted<JSObject*> viewedBufferX(
+      aCx, JS_GetArrayBufferViewBuffer(aCx, aX, &isShared));
+  if (!viewedBufferX) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return false;
+  }
+
+  JS::Rooted<JSObject*> viewedBufferY(
+      aCx, JS_GetArrayBufferViewBuffer(aCx, aY, &isShared));
+  if (!viewedBufferY) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return false;
+  }
+
+  return viewedBufferX == viewedBufferY;
+}
+
+// https://streams.spec.whatwg.org/#readablestream-enqueue
+void ReadableStream::EnqueueNative(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                                   ErrorResult& aRv) {
+  MOZ_ASSERT(mController->GetAlgorithms()->IsNative());
+
+  // Step 1: If stream.[[controller]] implements
+  // ReadableStreamDefaultController,
+  if (mController->IsDefault()) {
+    // Step 1.1: Perform !
+    // ReadableStreamDefaultControllerEnqueue(stream.[[controller]], chunk).
+    RefPtr<ReadableStreamDefaultController> controller =
+        mController->AsDefault();
+    ReadableStreamDefaultControllerEnqueue(aCx, controller, aChunk, aRv);
+    return;
+  }
+
+  // Step 2.1: Assert: stream.[[controller]] implements
+  // ReadableByteStreamController.
+  MOZ_ASSERT(mController->IsByte());
+  RefPtr<ReadableByteStreamController> controller = mController->AsByte();
+
+  // Step 2.2: Assert: chunk is an ArrayBufferView.
+  MOZ_ASSERT(aChunk.isObject() &&
+             JS_IsArrayBufferViewObject(&aChunk.toObject()));
+  JS::Rooted<JSObject*> chunk(aCx, &aChunk.toObject());
+
+  // Step 3: Let byobView be the current BYOB request view for stream.
+  JS::Rooted<JSObject*> byobView(aCx);
+  CurrentBYOBRequestView(aCx, *controller, &byobView, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 4: If byobView is non-null, and chunk.[[ViewedArrayBuffer]] is
+  // byobView.[[ViewedArrayBuffer]], then:
+  if (byobView && HasSameBufferView(aCx, chunk, byobView, aRv)) {
+    // Step 4.1: Assert: chunk.[[ByteOffset]] is byobView.[[ByteOffset]].
+    MOZ_ASSERT(JS_GetArrayBufferViewByteOffset(chunk) ==
+               JS_GetArrayBufferViewByteOffset(byobView));
+    // Step 4.2: Assert: chunk.[[ByteLength]] ≤ byobView.[[ByteLength]].
+    MOZ_ASSERT(JS_GetArrayBufferViewByteLength(chunk) ==
+               JS_GetArrayBufferViewByteLength(byobView));
+    // Step 4.3: Perform ?
+    // ReadableByteStreamControllerRespond(stream.[[controller]],
+    // chunk.[[ByteLength]]).
+    ReadableByteStreamControllerRespond(
+        aCx, controller, JS_GetArrayBufferViewByteLength(chunk), aRv);
+    return;
+  }
+
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 5: Otherwise, perform ?
+  // ReadableByteStreamControllerEnqueue(stream.[[controller]], chunk).
+  ReadableByteStreamControllerEnqueue(aCx, controller, chunk, aRv);
+}
+
+// https://streams.spec.whatwg.org/#readablestream-get-a-reader
+// To get a reader for a ReadableStream stream, return ?
+// AcquireReadableStreamDefaultReader(stream). The result will be a
+// ReadableStreamDefaultReader.
+already_AddRefed<mozilla::dom::ReadableStreamDefaultReader>
+ReadableStream::GetReader(ErrorResult& aRv) {
+  return AcquireReadableStreamDefaultReader(this, aRv);
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/ReadableStream.h b/dom/streams/ReadableStream.h
new file mode 100644
index 0000000000..95e29d9fc0
--- /dev/null
+++ b/dom/streams/ReadableStream.h
@@ -0,0 +1,282 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ReadableStream_h
+#define mozilla_dom_ReadableStream_h
+
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/IterableIterator.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+#include "mozilla/dom/ReadableStreamController.h"
+#include "mozilla/dom/ReadableStreamDefaultController.h"
+#include "mozilla/dom/UnderlyingSourceCallbackHelpers.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsWrapperCache.h"
+
+namespace mozilla::dom {
+
+class Promise;
+class ReadableStreamDefaultReader;
+class ReadableStreamGenericReader;
+struct ReadableStreamGetReaderOptions;
+struct ReadableStreamIteratorOptions;
+struct ReadIntoRequest;
+class WritableStream;
+struct ReadableWritablePair;
+struct StreamPipeOptions;
+
+using ReadableStreamReader =
+    ReadableStreamDefaultReaderOrReadableStreamBYOBReader;
+using OwningReadableStreamReader =
+    OwningReadableStreamDefaultReaderOrReadableStreamBYOBReader;
+class NativeUnderlyingSource;
+class BodyStreamHolder;
+class UniqueMessagePortId;
+class MessagePort;
+
+class ReadableStream : public nsISupports, public nsWrapperCache {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(ReadableStream)
+
+  friend class WritableStream;
+
+ protected:
+  virtual ~ReadableStream();
+
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+
+  // If one extends ReadableStream with another cycle collectable class,
+  // calling HoldJSObjects and DropJSObjects should happen using 'this' of
+  // that extending class. And in that case Explicit should be passed to the
+  // constructor of ReadableStream so that it doesn't make those calls.
+  // See also https://bugzilla.mozilla.org/show_bug.cgi?id=1801214.
+  enum class HoldDropJSObjectsCaller { Implicit, Explicit };
+
+  explicit ReadableStream(const GlobalObject& aGlobal,
+                          HoldDropJSObjectsCaller aHoldDropCaller);
+  explicit ReadableStream(nsIGlobalObject* aGlobal,
+                          HoldDropJSObjectsCaller aHoldDropCaller);
+
+ public:
+  // Abstract algorithms
+  MOZ_CAN_RUN_SCRIPT static already_AddRefed<ReadableStream> CreateAbstract(
+      JSContext* aCx, nsIGlobalObject* aGlobal,
+      UnderlyingSourceAlgorithmsBase* aAlgorithms,
+      mozilla::Maybe<double> aHighWaterMark,
+      QueuingStrategySize* aSizeAlgorithm, ErrorResult& aRv);
+  MOZ_CAN_RUN_SCRIPT static already_AddRefed<ReadableStream> CreateByteAbstract(
+      JSContext* aCx, nsIGlobalObject* aGlobal,
+      UnderlyingSourceAlgorithmsBase* aAlgorithms, ErrorResult& aRv);
+
+  // Slot Getter/Setters:
+  MOZ_KNOWN_LIVE ReadableStreamController* Controller() { return mController; }
+  ReadableStreamDefaultController* DefaultController() {
+    MOZ_ASSERT(mController && mController->IsDefault());
+    return mController->AsDefault();
+  }
+  void SetController(ReadableStreamController& aController) {
+    MOZ_ASSERT(!mController);
+    mController = &aController;
+  }
+
+  bool Disturbed() const { return mDisturbed; }
+  void SetDisturbed(bool aDisturbed) { mDisturbed = aDisturbed; }
+
+  ReadableStreamGenericReader* GetReader() { return mReader; }
+  void SetReader(ReadableStreamGenericReader* aReader);
+
+  ReadableStreamDefaultReader* GetDefaultReader();
+
+  enum class ReaderState { Readable, Closed, Errored };
+
+  ReaderState State() const { return mState; }
+  void SetState(const ReaderState& aState) { mState = aState; }
+
+  JS::Value StoredError() const { return mStoredError; }
+  void SetStoredError(JS::Handle<JS::Value> aStoredError) {
+    mStoredError = aStoredError;
+  }
+
+  nsIInputStream* MaybeGetInputStreamIfUnread() {
+    MOZ_ASSERT(!Disturbed());
+    if (UnderlyingSourceAlgorithmsBase* algorithms =
+            Controller()->GetAlgorithms()) {
+      return algorithms->MaybeGetInputStreamIfUnread();
+    }
+    return nullptr;
+  }
+
+  // [Transferable]
+  // https://html.spec.whatwg.org/multipage/structured-data.html#transfer-steps
+  MOZ_CAN_RUN_SCRIPT bool Transfer(JSContext* aCx,
+                                   UniqueMessagePortId& aPortId);
+  MOZ_CAN_RUN_SCRIPT static already_AddRefed<ReadableStream>
+  ReceiveTransferImpl(JSContext* aCx, nsIGlobalObject* aGlobal,
+                      MessagePort& aPort);
+  // https://html.spec.whatwg.org/multipage/structured-data.html#transfer-receiving-steps
+  MOZ_CAN_RUN_SCRIPT static bool ReceiveTransfer(
+      JSContext* aCx, nsIGlobalObject* aGlobal, MessagePort& aPort,
+      JS::MutableHandle<JSObject*> aReturnObject);
+
+  // Public functions to implement other specs
+  // https://streams.spec.whatwg.org/#other-specs-rs
+
+  // https://streams.spec.whatwg.org/#readablestream-set-up
+  static already_AddRefed<ReadableStream> CreateNative(
+      JSContext* aCx, nsIGlobalObject* aGlobal,
+      UnderlyingSourceAlgorithmsWrapper& aAlgorithms,
+      mozilla::Maybe<double> aHighWaterMark,
+      QueuingStrategySize* aSizeAlgorithm, ErrorResult& aRv);
+
+  // https://streams.spec.whatwg.org/#readablestream-set-up-with-byte-reading-support
+
+ protected:
+  // Sets up the ReadableStream with byte reading support. Intended for
+  // subclasses.
+  void SetUpByteNative(JSContext* aCx,
+                       UnderlyingSourceAlgorithmsWrapper& aAlgorithms,
+                       mozilla::Maybe<double> aHighWaterMark, ErrorResult& aRv);
+
+ public:
+  // Creates and sets up a ReadableStream with byte reading support. Use
+  // SetUpByteNative for this purpose in subclasses.
+  static already_AddRefed<ReadableStream> CreateByteNative(
+      JSContext* aCx, nsIGlobalObject* aGlobal,
+      UnderlyingSourceAlgorithmsWrapper& aAlgorithms,
+      mozilla::Maybe<double> aHighWaterMark, ErrorResult& aRv);
+
+  // The following algorithms must only be used on ReadableStream instances
+  // initialized via the above set up or set up with byte reading support
+  // algorithms (not, e.g., on web-developer-created instances):
+
+  // https://streams.spec.whatwg.org/#readablestream-close
+  MOZ_CAN_RUN_SCRIPT void CloseNative(JSContext* aCx, ErrorResult& aRv);
+
+  // https://streams.spec.whatwg.org/#readablestream-error
+  void ErrorNative(JSContext* aCx, JS::Handle<JS::Value> aError,
+                   ErrorResult& aRv);
+
+  // https://streams.spec.whatwg.org/#readablestream-enqueue
+  MOZ_CAN_RUN_SCRIPT void EnqueueNative(JSContext* aCx,
+                                        JS::Handle<JS::Value> aChunk,
+                                        ErrorResult& aRv);
+
+  // The following algorithms can be used on arbitrary ReadableStream instances,
+  // including ones that are created by web developers. They can all fail in
+  // various operation-specific ways, and these failures should be handled by
+  // the calling specification.
+
+  // https://streams.spec.whatwg.org/#readablestream-get-a-reader
+  already_AddRefed<mozilla::dom::ReadableStreamDefaultReader> GetReader(
+      ErrorResult& aRv);
+
+  // IDL layer functions
+
+  nsIGlobalObject* GetParentObject() const { return mGlobal; }
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  // IDL methods
+
+  // TODO: Use MOZ_CAN_RUN_SCRIPT when IDL constructors can use it (bug 1749042)
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY static already_AddRefed<ReadableStream>
+  Constructor(const GlobalObject& aGlobal,
+              const Optional<JS::Handle<JSObject*>>& aUnderlyingSource,
+              const QueuingStrategy& aStrategy, ErrorResult& aRv);
+
+  bool Locked() const;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> Cancel(
+      JSContext* cx, JS::Handle<JS::Value> aReason, ErrorResult& aRv);
+
+  void GetReader(const ReadableStreamGetReaderOptions& aOptions,
+                 OwningReadableStreamReader& resultReader, ErrorResult& aRv);
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<ReadableStream> PipeThrough(
+      const ReadableWritablePair& aTransform, const StreamPipeOptions& aOptions,
+      ErrorResult& aRv);
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> PipeTo(
+      WritableStream& aDestination, const StreamPipeOptions& aOptions,
+      ErrorResult& aRv);
+
+  MOZ_CAN_RUN_SCRIPT void Tee(JSContext* aCx,
+                              nsTArray<RefPtr<ReadableStream>>& aResult,
+                              ErrorResult& aRv);
+
+  struct IteratorData {
+    void Traverse(nsCycleCollectionTraversalCallback& cb);
+    void Unlink();
+
+    RefPtr<ReadableStreamDefaultReader> mReader;
+    bool mPreventCancel;
+  };
+
+  using Iterator = AsyncIterableIterator<ReadableStream>;
+
+  void InitAsyncIteratorData(IteratorData& aData, Iterator::IteratorType aType,
+                             const ReadableStreamIteratorOptions& aOptions,
+                             ErrorResult& aRv);
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> GetNextIterationResult(
+      Iterator* aIterator, ErrorResult& aRv);
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> IteratorReturn(
+      JSContext* aCx, Iterator* aIterator, JS::Handle<JS::Value> aValue,
+      ErrorResult& aRv);
+
+  // Internal Slots:
+ private:
+  RefPtr<ReadableStreamController> mController;
+  bool mDisturbed = false;
+  RefPtr<ReadableStreamGenericReader> mReader;
+  ReaderState mState = ReaderState::Readable;
+  JS::Heap<JS::Value> mStoredError;
+
+  HoldDropJSObjectsCaller mHoldDropCaller;
+};
+
+namespace streams_abstract {
+
+bool IsReadableStreamLocked(ReadableStream* aStream);
+
+double ReadableStreamGetNumReadRequests(ReadableStream* aStream);
+
+void ReadableStreamError(JSContext* aCx, ReadableStream* aStream,
+                         JS::Handle<JS::Value> aValue, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void ReadableStreamClose(JSContext* aCx,
+                                            ReadableStream* aStream,
+                                            ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void ReadableStreamFulfillReadRequest(
+    JSContext* aCx, ReadableStream* aStream, JS::Handle<JS::Value> aChunk,
+    bool done, ErrorResult& aRv);
+
+void ReadableStreamAddReadRequest(ReadableStream* aStream,
+                                  ReadRequest* aReadRequest);
+void ReadableStreamAddReadIntoRequest(ReadableStream* aStream,
+                                      ReadIntoRequest* aReadIntoRequest);
+
+MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> ReadableStreamCancel(
+    JSContext* aCx, ReadableStream* aStream, JS::Handle<JS::Value> aError,
+    ErrorResult& aRv);
+
+already_AddRefed<ReadableStreamDefaultReader>
+AcquireReadableStreamDefaultReader(ReadableStream* aStream, ErrorResult& aRv);
+
+bool ReadableStreamHasBYOBReader(ReadableStream* aStream);
+bool ReadableStreamHasDefaultReader(ReadableStream* aStream);
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_ReadableStream_h
diff --git a/dom/streams/ReadableStreamBYOBReader.cpp b/dom/streams/ReadableStreamBYOBReader.cpp
new file mode 100644
index 0000000000..13ba608566
--- /dev/null
+++ b/dom/streams/ReadableStreamBYOBReader.cpp
@@ -0,0 +1,360 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/ReadableStreamBYOBReader.h"
+
+#include "ReadIntoRequest.h"
+#include "js/ArrayBuffer.h"
+#include "js/experimental/TypedData.h"
+#include "mozilla/dom/ReadableStreamBYOBReader.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/ReadableStreamBYOBReaderBinding.h"
+#include "mozilla/dom/ReadableStreamGenericReader.h"
+#include "mozilla/dom/RootedDictionary.h"
+#include "nsCOMPtr.h"
+#include "nsISupportsImpl.h"
+
+// Temporary Includes
+#include "mozilla/dom/ReadableByteStreamController.h"
+#include "mozilla/dom/ReadableStreamBYOBRequest.h"
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_INHERITED(ReadableStreamBYOBReader,
+                                                ReadableStreamGenericReader,
+                                                mReadIntoRequests)
+NS_IMPL_ADDREF_INHERITED(ReadableStreamBYOBReader, ReadableStreamGenericReader)
+NS_IMPL_RELEASE_INHERITED(ReadableStreamBYOBReader, ReadableStreamGenericReader)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ReadableStreamBYOBReader)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+NS_INTERFACE_MAP_END_INHERITING(ReadableStreamGenericReader)
+
+ReadableStreamBYOBReader::ReadableStreamBYOBReader(nsISupports* aGlobal)
+    : ReadableStreamGenericReader(do_QueryInterface(aGlobal)),
+      mReadIntoRequests({}) {}
+
+JSObject* ReadableStreamBYOBReader::WrapObject(
+    JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
+  return ReadableStreamBYOBReader_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+// https://streams.spec.whatwg.org/#set-up-readable-stream-byob-reader
+void SetUpReadableStreamBYOBReader(ReadableStreamBYOBReader* reader,
+                                   ReadableStream& stream, ErrorResult& rv) {
+  // Step 1. If !IsReadableStreamLocked(stream) is true, throw a TypeError
+  // exception.
+  if (IsReadableStreamLocked(&stream)) {
+    rv.ThrowTypeError("Trying to read locked stream");
+    return;
+  }
+
+  // Step 2. If stream.[[controller]] does not implement
+  // ReadableByteStreamController, throw a TypeError exception.
+  if (!stream.Controller()->IsByte()) {
+    rv.ThrowTypeError("Trying to read with incompatible controller");
+    return;
+  }
+
+  // Step 3. Perform ! ReadableStreamReaderGenericInitialize(reader, stream).
+  ReadableStreamReaderGenericInitialize(reader, &stream);
+
+  // Step 4. Set reader.[[readIntoRequests]] to a new empty list.
+  reader->ReadIntoRequests().clear();
+}
+
+// https://streams.spec.whatwg.org/#byob-reader-constructor
+/* static */ already_AddRefed<ReadableStreamBYOBReader>
+ReadableStreamBYOBReader::Constructor(const GlobalObject& global,
+                                      ReadableStream& stream, ErrorResult& rv) {
+  nsCOMPtr<nsIGlobalObject> globalObject =
+      do_QueryInterface(global.GetAsSupports());
+  RefPtr<ReadableStreamBYOBReader> reader =
+      new ReadableStreamBYOBReader(globalObject);
+
+  // Step 1.
+  SetUpReadableStreamBYOBReader(reader, stream, rv);
+  if (rv.Failed()) {
+    return nullptr;
+  }
+
+  return reader.forget();
+}
+
+struct Read_ReadIntoRequest final : public ReadIntoRequest {
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(Read_ReadIntoRequest,
+                                           ReadIntoRequest)
+
+  RefPtr<Promise> mPromise;
+
+  explicit Read_ReadIntoRequest(Promise* aPromise) : mPromise(aPromise) {}
+
+  void ChunkSteps(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                  ErrorResult& aRv) override {
+    MOZ_ASSERT(aChunk.isObject());
+    // https://streams.spec.whatwg.org/#byob-reader-read Step 6.
+    //
+    // chunk steps, given chunk:
+    //         Resolve promise with «[ "value" → chunk, "done" → false ]».
+
+    // We need to wrap this as the chunk could have come from
+    // another compartment.
+    JS::Rooted<JSObject*> chunk(aCx, &aChunk.toObject());
+    if (!JS_WrapObject(aCx, &chunk)) {
+      aRv.StealExceptionFromJSContext(aCx);
+      return;
+    }
+
+    RootedDictionary<ReadableStreamReadResult> result(aCx);
+    result.mValue = aChunk;
+    result.mDone.Construct(false);
+
+    mPromise->MaybeResolve(result);
+  }
+
+  void CloseSteps(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                  ErrorResult& aRv) override {
+    MOZ_ASSERT(aChunk.isObject() || aChunk.isUndefined());
+    // https://streams.spec.whatwg.org/#byob-reader-read Step 6.
+    //
+    // close steps, given chunk:
+    // Resolve promise with «[ "value" → chunk, "done" → true ]».
+    RootedDictionary<ReadableStreamReadResult> result(aCx);
+    if (aChunk.isObject()) {
+      // We need to wrap this as the chunk could have come from
+      // another compartment.
+      JS::Rooted<JSObject*> chunk(aCx, &aChunk.toObject());
+      if (!JS_WrapObject(aCx, &chunk)) {
+        aRv.StealExceptionFromJSContext(aCx);
+        return;
+      }
+
+      result.mValue = aChunk;
+    }
+    result.mDone.Construct(true);
+
+    mPromise->MaybeResolve(result);
+  }
+
+  void ErrorSteps(JSContext* aCx, JS::Handle<JS::Value> e,
+                  ErrorResult& aRv) override {
+    // https://streams.spec.whatwg.org/#byob-reader-read Step 6.
+    //
+    // error steps, given e:
+    // Reject promise with e.
+    mPromise->MaybeReject(e);
+  }
+
+ protected:
+  ~Read_ReadIntoRequest() override = default;
+};
+
+NS_IMPL_CYCLE_COLLECTION(ReadIntoRequest)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ReadIntoRequest)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ReadIntoRequest)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ReadIntoRequest)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(Read_ReadIntoRequest, ReadIntoRequest,
+                                   mPromise)
+NS_IMPL_ADDREF_INHERITED(Read_ReadIntoRequest, ReadIntoRequest)
+NS_IMPL_RELEASE_INHERITED(Read_ReadIntoRequest, ReadIntoRequest)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(Read_ReadIntoRequest)
+NS_INTERFACE_MAP_END_INHERITING(ReadIntoRequest)
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#readable-stream-byob-reader-read
+void ReadableStreamBYOBReaderRead(JSContext* aCx,
+                                  ReadableStreamBYOBReader* aReader,
+                                  JS::Handle<JSObject*> aView,
+                                  ReadIntoRequest* aReadIntoRequest,
+                                  ErrorResult& aRv) {
+  // Step 1.Let stream be reader.[[stream]].
+  ReadableStream* stream = aReader->GetStream();
+
+  // Step 2. Assert: stream is not undefined.
+  MOZ_ASSERT(stream);
+
+  // Step 3. Set stream.[[disturbed]] to true.
+  stream->SetDisturbed(true);
+
+  // Step 4. If stream.[[state]] is "errored", perform readIntoRequest’s error
+  // steps given stream.[[storedError]].
+  if (stream->State() == ReadableStream::ReaderState::Errored) {
+    JS::Rooted<JS::Value> error(aCx, stream->StoredError());
+
+    aReadIntoRequest->ErrorSteps(aCx, error, aRv);
+    return;
+  }
+
+  // Step 5. Otherwise, perform
+  // !ReadableByteStreamControllerPullInto(stream.[[controller]], view,
+  // readIntoRequest).
+  MOZ_ASSERT(stream->Controller()->IsByte());
+  RefPtr<ReadableByteStreamController> controller(
+      stream->Controller()->AsByte());
+  ReadableByteStreamControllerPullInto(aCx, controller, aView, aReadIntoRequest,
+                                       aRv);
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#byob-reader-read
+already_AddRefed<Promise> ReadableStreamBYOBReader::Read(
+    const ArrayBufferView& aArray, ErrorResult& aRv) {
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(GetParentObject())) {
+    aRv.ThrowUnknownError("Internal error");
+    return nullptr;
+  }
+  JSContext* cx = jsapi.cx();
+
+  JS::Rooted<JSObject*> view(cx, aArray.Obj());
+
+  // Step 1. If view.[[ByteLength]] is 0, return a promise rejected with a
+  // TypeError exception.
+  if (JS_GetArrayBufferViewByteLength(view) == 0) {
+    // Binding code should convert this thrown value into a rejected promise.
+    aRv.ThrowTypeError("Zero Length View");
+    return nullptr;
+  }
+
+  // Step 2. If view.[[ViewedArrayBuffer]].[[ArrayBufferByteLength]] is 0,
+  // return a promise rejected with a TypeError exception.
+  bool isSharedMemory;
+  JS::Rooted<JSObject*> viewedArrayBuffer(
+      cx, JS_GetArrayBufferViewBuffer(cx, view, &isSharedMemory));
+  if (!viewedArrayBuffer) {
+    aRv.StealExceptionFromJSContext(cx);
+    return nullptr;
+  }
+
+  if (JS::GetArrayBufferByteLength(viewedArrayBuffer) == 0) {
+    aRv.ThrowTypeError("zero length viewed buffer");
+    return nullptr;
+  }
+
+  // Step 3. If ! IsDetachedBuffer(view.[[ViewedArrayBuffer]]) is true, return a
+  // promise rejected with a TypeError exception.
+  if (JS::IsDetachedArrayBufferObject(viewedArrayBuffer)) {
+    aRv.ThrowTypeError("Detached Buffer");
+    return nullptr;
+  }
+
+  // Step 4. If this.[[stream]] is undefined, return a promise rejected with a
+  // TypeError exception.
+  if (!GetStream()) {
+    aRv.ThrowTypeError("Reader has undefined stream");
+    return nullptr;
+  }
+
+  // Step 5.
+  RefPtr<Promise> promise = Promise::CreateInfallible(GetParentObject());
+
+  // Step 6. Let readIntoRequest be a new read-into request with the following
+  // items:
+  RefPtr<ReadIntoRequest> readIntoRequest = new Read_ReadIntoRequest(promise);
+
+  // Step 7. Perform ! ReadableStreamBYOBReaderRead(this, view,
+  // readIntoRequest).
+  ReadableStreamBYOBReaderRead(cx, this, view, readIntoRequest, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 8. Return promise.
+  return promise.forget();
+}
+
+namespace streams_abstract {
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreambyobreadererrorreadintorequests
+void ReadableStreamBYOBReaderErrorReadIntoRequests(
+    JSContext* aCx, ReadableStreamBYOBReader* aReader,
+    JS::Handle<JS::Value> aError, ErrorResult& aRv) {
+  // Step 1. Let readIntoRequests be reader.[[readIntoRequests]].
+  LinkedList<RefPtr<ReadIntoRequest>> readIntoRequests =
+      std::move(aReader->ReadIntoRequests());
+
+  // Step 2. Set reader.[[readIntoRequests]] to a new empty list.
+  // Note: The std::move already cleared this anyway.
+  aReader->ReadIntoRequests().clear();
+
+  // Step 3. For each readIntoRequest of readIntoRequests,
+  while (RefPtr<ReadIntoRequest> readIntoRequest =
+             readIntoRequests.popFirst()) {
+    // Step 3.1. Perform readIntoRequest’s error steps, given e.
+    readIntoRequest->ErrorSteps(aCx, aError, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+  }
+}
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreambyobreaderrelease
+void ReadableStreamBYOBReaderRelease(JSContext* aCx,
+                                     ReadableStreamBYOBReader* aReader,
+                                     ErrorResult& aRv) {
+  // Step 1. Perform ! ReadableStreamReaderGenericRelease(reader).
+  ReadableStreamReaderGenericRelease(aReader, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 2. Let e be a new TypeError exception.
+  ErrorResult rv;
+  rv.ThrowTypeError("Releasing lock");
+  JS::Rooted<JS::Value> error(aCx);
+  MOZ_ALWAYS_TRUE(ToJSValue(aCx, std::move(rv), &error));
+
+  // Step 3. Perform ! ReadableStreamBYOBReaderErrorReadIntoRequests(reader, e).
+  ReadableStreamBYOBReaderErrorReadIntoRequests(aCx, aReader, error, aRv);
+}
+
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#byob-reader-release-lock
+void ReadableStreamBYOBReader::ReleaseLock(ErrorResult& aRv) {
+  // Step 1. If this.[[stream]] is undefined, return.
+  if (!mStream) {
+    return;
+  }
+
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(mGlobal)) {
+    return aRv.ThrowUnknownError("Internal error");
+  }
+  JSContext* cx = jsapi.cx();
+
+  // Step 2. Perform ! ReadableStreamBYOBReaderRelease(this).
+  RefPtr<ReadableStreamBYOBReader> thisRefPtr = this;
+  ReadableStreamBYOBReaderRelease(cx, thisRefPtr, aRv);
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#acquire-readable-stream-byob-reader
+already_AddRefed<ReadableStreamBYOBReader> AcquireReadableStreamBYOBReader(
+    ReadableStream* aStream, ErrorResult& aRv) {
+  // Step 1. Let reader be a new ReadableStreamBYOBReader.
+  RefPtr<ReadableStreamBYOBReader> reader =
+      new ReadableStreamBYOBReader(aStream->GetParentObject());
+
+  // Step 2. Perform ? SetUpReadableStreamBYOBReader(reader, stream).
+  SetUpReadableStreamBYOBReader(reader, *aStream, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 3. Return reader.
+  return reader.forget();
+}
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/ReadableStreamBYOBReader.h b/dom/streams/ReadableStreamBYOBReader.h
new file mode 100644
index 0000000000..896f1acbf9
--- /dev/null
+++ b/dom/streams/ReadableStreamBYOBReader.h
@@ -0,0 +1,91 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ReadableStreamBYOBReader_h
+#define mozilla_dom_ReadableStreamBYOBReader_h
+
+#include "js/TypeDecls.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/ReadableStreamGenericReader.h"
+#include "mozilla/dom/TypedArray.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsWrapperCache.h"
+#include "mozilla/LinkedList.h"
+
+namespace mozilla::dom {
+
+class Promise;
+struct ReadIntoRequest;
+class ReadableStream;
+
+}  // namespace mozilla::dom
+
+namespace mozilla::dom {
+
+class ReadableStreamBYOBReader final : public ReadableStreamGenericReader,
+                                       public nsWrapperCache {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS_INHERITED(
+      ReadableStreamBYOBReader, ReadableStreamGenericReader)
+
+ public:
+  explicit ReadableStreamBYOBReader(nsISupports* aGlobal);
+
+  bool IsDefault() override { return false; };
+  bool IsBYOB() override { return true; }
+  ReadableStreamDefaultReader* AsDefault() override {
+    MOZ_CRASH("Should have verified IsDefault first");
+    return nullptr;
+  }
+  ReadableStreamBYOBReader* AsBYOB() override { return this; }
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  static already_AddRefed<ReadableStreamBYOBReader> Constructor(
+      const GlobalObject& global, ReadableStream& stream, ErrorResult& rv);
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> Read(
+      const ArrayBufferView& aArray, ErrorResult& rv);
+
+  void ReleaseLock(ErrorResult& rv);
+
+  LinkedList<RefPtr<ReadIntoRequest>>& ReadIntoRequests() {
+    return mReadIntoRequests;
+  }
+
+ private:
+  ~ReadableStreamBYOBReader() override = default;
+
+  LinkedList<RefPtr<ReadIntoRequest>> mReadIntoRequests;
+};
+
+namespace streams_abstract {
+
+already_AddRefed<ReadableStreamBYOBReader> AcquireReadableStreamBYOBReader(
+    ReadableStream* aStream, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void ReadableStreamBYOBReaderRead(
+    JSContext* aCx, ReadableStreamBYOBReader* aReader,
+    JS::Handle<JSObject*> aView, ReadIntoRequest* aReadIntoRequest,
+    ErrorResult& aRv);
+
+void ReadableStreamBYOBReaderErrorReadIntoRequests(
+    JSContext* aCx, ReadableStreamBYOBReader* aReader,
+    JS::Handle<JS::Value> aError, ErrorResult& aRv);
+
+void ReadableStreamBYOBReaderRelease(JSContext* aCx,
+                                     ReadableStreamBYOBReader* aReader,
+                                     ErrorResult& aRv);
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_ReadableStreamBYOBReader_h
diff --git a/dom/streams/ReadableStreamBYOBRequest.cpp b/dom/streams/ReadableStreamBYOBRequest.cpp
new file mode 100644
index 0000000000..316b76484f
--- /dev/null
+++ b/dom/streams/ReadableStreamBYOBRequest.cpp
@@ -0,0 +1,129 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/ReadableStreamBYOBRequest.h"
+
+#include "mozilla/dom/ByteStreamHelpers.h"
+#include "js/ArrayBuffer.h"
+#include "js/TypeDecls.h"
+#include "mozilla/dom/ReadableByteStreamController.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/ReadableStreamBYOBRequestBinding.h"
+#include "js/experimental/TypedData.h"
+#include "mozilla/dom/ReadableStreamController.h"
+#include "nsCOMPtr.h"
+#include "nsIGlobalObject.h"
+#include "nsWrapperCache.h"
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+ReadableStreamBYOBRequest::ReadableStreamBYOBRequest(nsIGlobalObject* aGlobal)
+    : mGlobal(aGlobal) {
+  mozilla::HoldJSObjects(this);
+}
+
+ReadableStreamBYOBRequest::~ReadableStreamBYOBRequest() {
+  mozilla::DropJSObjects(this);
+}
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_WITH_JS_MEMBERS(ReadableStreamBYOBRequest,
+                                                      (mGlobal, mController),
+                                                      (mView))
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ReadableStreamBYOBRequest)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ReadableStreamBYOBRequest)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ReadableStreamBYOBRequest)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+JSObject* ReadableStreamBYOBRequest::WrapObject(
+    JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
+  return ReadableStreamBYOBRequest_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+// https://streams.spec.whatwg.org/#rs-byob-request-view
+void ReadableStreamBYOBRequest::GetView(
+    JSContext* cx, JS::MutableHandle<JSObject*> aRetVal) const {
+  // Step 1.
+  aRetVal.set(mView);
+}
+
+// https://streams.spec.whatwg.org/#rs-byob-request-respond
+void ReadableStreamBYOBRequest::Respond(JSContext* aCx, uint64_t bytesWritten,
+                                        ErrorResult& aRv) {
+  // Step 1.
+  if (!mController) {
+    aRv.ThrowTypeError("Undefined Controller");
+    return;
+  }
+
+  // Step 2.
+  bool isSharedMemory;
+  JS::Rooted<JSObject*> view(aCx, mView);
+  JS::Rooted<JSObject*> arrayBuffer(
+      aCx, JS_GetArrayBufferViewBuffer(aCx, view, &isSharedMemory));
+  if (!arrayBuffer) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+
+  if (JS::IsDetachedArrayBufferObject(arrayBuffer)) {
+    aRv.ThrowTypeError("View of Detached buffer");
+    return;
+  }
+
+  // Step 3.
+  MOZ_ASSERT(JS_GetArrayBufferViewByteLength(view) > 0);
+
+  // Step 4.
+  MOZ_ASSERT(JS::GetArrayBufferByteLength(arrayBuffer) > 0);
+
+  // Step 5.
+  RefPtr<ReadableByteStreamController> controller(mController);
+  ReadableByteStreamControllerRespond(aCx, controller, bytesWritten, aRv);
+}
+
+// https://streams.spec.whatwg.org/#rs-byob-request-respond-with-new-view
+void ReadableStreamBYOBRequest::RespondWithNewView(JSContext* aCx,
+                                                   const ArrayBufferView& view,
+                                                   ErrorResult& aRv) {
+  // Step 1.
+  if (!mController) {
+    aRv.ThrowTypeError("Undefined Controller");
+    return;
+  }
+
+  // Step 2.
+  bool isSharedMemory;
+  JS::Rooted<JSObject*> rootedViewObj(aCx, view.Obj());
+  JS::Rooted<JSObject*> viewedArrayBuffer(
+      aCx, JS_GetArrayBufferViewBuffer(aCx, rootedViewObj, &isSharedMemory));
+  if (!viewedArrayBuffer) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+
+  if (JS::IsDetachedArrayBufferObject(viewedArrayBuffer)) {
+    aRv.ThrowTypeError("View of Detatched Array Buffer");
+    return;
+  }
+
+  // Step 3.
+  RefPtr<ReadableByteStreamController> controller(mController);
+  ReadableByteStreamControllerRespondWithNewView(aCx, controller, rootedViewObj,
+                                                 aRv);
+}
+
+void ReadableStreamBYOBRequest::SetController(
+    ReadableByteStreamController* aController) {
+  mController = aController;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/ReadableStreamBYOBRequest.h b/dom/streams/ReadableStreamBYOBRequest.h
new file mode 100644
index 0000000000..fb8ad62979
--- /dev/null
+++ b/dom/streams/ReadableStreamBYOBRequest.h
@@ -0,0 +1,68 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ReadableStreamBYOBRequest_h
+#define mozilla_dom_ReadableStreamBYOBRequest_h
+
+#include "js/RootingAPI.h"
+#include "js/TypeDecls.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/TypedArray.h"
+#include "nsCOMPtr.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsWrapperCache.h"
+#include "nsIGlobalObject.h"
+
+namespace mozilla::dom {
+
+class ReadableByteStreamController;
+
+class ReadableStreamBYOBRequest final : public nsISupports,
+                                        public nsWrapperCache {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(ReadableStreamBYOBRequest)
+
+ public:
+  explicit ReadableStreamBYOBRequest(nsIGlobalObject* aGlobal);
+
+ protected:
+  ~ReadableStreamBYOBRequest();
+
+ public:
+  nsIGlobalObject* GetParentObject() const { return mGlobal; }
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  void GetView(JSContext* cx, JS::MutableHandle<JSObject*> aRetVal) const;
+
+  MOZ_CAN_RUN_SCRIPT void Respond(JSContext* aCx, uint64_t bytesWritten,
+                                  ErrorResult& aRv);
+
+  MOZ_CAN_RUN_SCRIPT void RespondWithNewView(JSContext* aCx,
+                                             const ArrayBufferView& view,
+                                             ErrorResult& aRv);
+
+  ReadableByteStreamController* Controller() { return mController; }
+  void SetController(ReadableByteStreamController* aController);
+
+  JSObject* View() { return mView; }
+  void SetView(JS::Handle<JSObject*> aView) { mView = aView; }
+
+ private:
+  // Internal Slots
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+  RefPtr<ReadableByteStreamController> mController;
+  JS::Heap<JSObject*> mView;
+};
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/streams/ReadableStreamController.h b/dom/streams/ReadableStreamController.h
new file mode 100644
index 0000000000..766c933017
--- /dev/null
+++ b/dom/streams/ReadableStreamController.h
@@ -0,0 +1,73 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ReadableStreamController_h
+#define mozilla_dom_ReadableStreamController_h
+
+#include "mozilla/ErrorResult.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsIGlobalObject.h"
+#include "nsISupports.h"
+#include "UnderlyingSourceCallbackHelpers.h"
+
+namespace mozilla::dom {
+struct ReadRequest;
+class ReadableStream;
+class ReadableStreamDefaultController;
+class ReadableByteStreamController;
+
+class ReadableStreamController : public nsISupports {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(ReadableStreamController)
+
+  ReadableStreamController(nsIGlobalObject* aGlobal);
+
+  nsIGlobalObject* GetParentObject() const { return mGlobal; }
+
+  virtual bool IsDefault() = 0;
+  virtual bool IsByte() = 0;
+  virtual ReadableStreamDefaultController* AsDefault() = 0;
+  virtual ReadableByteStreamController* AsByte() = 0;
+
+  MOZ_CAN_RUN_SCRIPT
+  virtual already_AddRefed<Promise> CancelSteps(JSContext* aCx,
+                                                JS::Handle<JS::Value> aReason,
+                                                ErrorResult& aRv) = 0;
+  MOZ_CAN_RUN_SCRIPT
+  virtual void PullSteps(JSContext* aCx, ReadRequest* aReadRequest,
+                         ErrorResult& aRv) = 0;
+
+  // No JS implementable UnderlyingSource callback exists for this.
+  virtual void ReleaseSteps() = 0;
+
+  UnderlyingSourceAlgorithmsBase* GetAlgorithms() const { return mAlgorithms; }
+  void SetAlgorithms(UnderlyingSourceAlgorithmsBase& aAlgorithms) {
+    mAlgorithms = &aAlgorithms;
+  }
+  void ClearAlgorithms() {
+    MOZ_ASSERT(mAlgorithms);
+    mAlgorithms->ReleaseObjects();
+    mAlgorithms = nullptr;
+  }
+
+  ReadableStream* Stream() const { return mStream; }
+  void SetStream(ReadableStream* aStream);
+
+ protected:
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+
+  // The algorithms for the underlying source
+  RefPtr<UnderlyingSourceAlgorithmsBase> mAlgorithms;
+
+  RefPtr<ReadableStream> mStream;
+
+  virtual ~ReadableStreamController() = default;
+};
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/streams/ReadableStreamDefaultController.cpp b/dom/streams/ReadableStreamDefaultController.cpp
new file mode 100644
index 0000000000..9540bb8e6c
--- /dev/null
+++ b/dom/streams/ReadableStreamDefaultController.cpp
@@ -0,0 +1,648 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/ReadableStreamDefaultController.h"
+
+#include "js/Exception.h"
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/Promise-inl.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/ReadableStreamController.h"
+#include "mozilla/dom/ReadableStreamDefaultControllerBinding.h"
+#include "mozilla/dom/ReadableStreamDefaultReaderBinding.h"
+#include "mozilla/dom/UnderlyingSourceBinding.h"
+#include "mozilla/dom/UnderlyingSourceCallbackHelpers.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsISupports.h"
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+NS_IMPL_CYCLE_COLLECTION(ReadableStreamController, mGlobal, mAlgorithms,
+                         mStream)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ReadableStreamController)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ReadableStreamController)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ReadableStreamController)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+ReadableStreamController::ReadableStreamController(nsIGlobalObject* aGlobal)
+    : mGlobal(aGlobal) {}
+
+void ReadableStreamController::SetStream(ReadableStream* aStream) {
+  mStream = aStream;
+}
+
+// Note: Using the individual macros vs NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE
+// because I need to specify a manual implementation of
+// NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN.
+NS_IMPL_CYCLE_COLLECTION_CLASS(ReadableStreamDefaultController)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(ReadableStreamDefaultController,
+                                                ReadableStreamController)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mStrategySizeAlgorithm)
+  tmp->mQueue.clear();
+  NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(
+    ReadableStreamDefaultController, ReadableStreamController)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mStrategySizeAlgorithm)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN_INHERITED(ReadableStreamDefaultController,
+                                               ReadableStreamController)
+  NS_IMPL_CYCLE_COLLECTION_TRACE_PRESERVED_WRAPPER
+  // Trace the associated queue.
+  for (const auto& queueEntry : tmp->mQueue) {
+    aCallbacks.Trace(&queueEntry->mValue, "mQueue.mValue", aClosure);
+  }
+NS_IMPL_CYCLE_COLLECTION_TRACE_END
+
+NS_IMPL_ADDREF_INHERITED(ReadableStreamDefaultController,
+                         ReadableStreamController)
+NS_IMPL_RELEASE_INHERITED(ReadableStreamDefaultController,
+                          ReadableStreamController)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ReadableStreamDefaultController)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+NS_INTERFACE_MAP_END_INHERITING(ReadableStreamController)
+
+ReadableStreamDefaultController::ReadableStreamDefaultController(
+    nsIGlobalObject* aGlobal)
+    : ReadableStreamController(aGlobal) {
+  mozilla::HoldJSObjects(this);
+}
+
+ReadableStreamDefaultController::~ReadableStreamDefaultController() {
+  // MG:XXX: LinkedLists are required to be empty at destruction, but it seems
+  //         it is possible to have a controller be destructed while still
+  //         having entries in its queue.
+  //
+  //         This needs to be verified as not indicating some other issue.
+  mozilla::DropJSObjects(this);
+  mQueue.clear();
+}
+
+JSObject* ReadableStreamDefaultController::WrapObject(
+    JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
+  return ReadableStreamDefaultController_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+namespace streams_abstract {
+
+// https://streams.spec.whatwg.org/#readable-stream-default-controller-can-close-or-enqueue
+static bool ReadableStreamDefaultControllerCanCloseOrEnqueue(
+    ReadableStreamDefaultController* aController) {
+  // Step 1. Let state be controller.[[stream]].[[state]].
+  ReadableStream::ReaderState state = aController->Stream()->State();
+
+  // Step 2. If controller.[[closeRequested]] is false and state is "readable",
+  // return true.
+  // Step 3. Return false.
+  return !aController->CloseRequested() &&
+         state == ReadableStream::ReaderState::Readable;
+}
+
+// https://streams.spec.whatwg.org/#readable-stream-default-controller-can-close-or-enqueue
+// This is a variant of ReadableStreamDefaultControllerCanCloseOrEnqueue
+// that also throws when the function would return false to improve error
+// messages.
+bool ReadableStreamDefaultControllerCanCloseOrEnqueueAndThrow(
+    ReadableStreamDefaultController* aController,
+    CloseOrEnqueue aCloseOrEnqueue, ErrorResult& aRv) {
+  // Step 1. Let state be controller.[[stream]].[[state]].
+  ReadableStream::ReaderState state = aController->Stream()->State();
+
+  nsCString prefix;
+  if (aCloseOrEnqueue == CloseOrEnqueue::Close) {
+    prefix = "Cannot close a stream that "_ns;
+  } else {
+    prefix = "Cannot enqueue into a stream that "_ns;
+  }
+
+  switch (state) {
+    case ReadableStream::ReaderState::Readable:
+      // Step 2. If controller.[[closeRequested]] is false and
+      // state is "readable", return true.
+      // Note: We don't error/check for [[closeRequest]] first, because
+      // [[closedRequest]] is still true even after the state is "closed".
+      // This doesn't cause any spec observable difference.
+      if (!aController->CloseRequested()) {
+        return true;
+      }
+
+      // Step 3. Return false.
+      aRv.ThrowTypeError(prefix + "has already been requested to close."_ns);
+      return false;
+
+    case ReadableStream::ReaderState::Closed:
+      aRv.ThrowTypeError(prefix + "is already closed."_ns);
+      return false;
+
+    case ReadableStream::ReaderState::Errored:
+      aRv.ThrowTypeError(prefix + "has errored."_ns);
+      return false;
+
+    default:
+      MOZ_ASSERT_UNREACHABLE("Unknown ReaderState");
+      return false;
+  }
+}
+
+Nullable<double> ReadableStreamDefaultControllerGetDesiredSize(
+    ReadableStreamDefaultController* aController) {
+  ReadableStream::ReaderState state = aController->Stream()->State();
+  if (state == ReadableStream::ReaderState::Errored) {
+    return nullptr;
+  }
+
+  if (state == ReadableStream::ReaderState::Closed) {
+    return 0.0;
+  }
+
+  return aController->StrategyHWM() - aController->QueueTotalSize();
+}
+
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#rs-default-controller-desired-size
+Nullable<double> ReadableStreamDefaultController::GetDesiredSize() {
+  // Step 1.
+  return ReadableStreamDefaultControllerGetDesiredSize(this);
+}
+
+namespace streams_abstract {
+
+// https://streams.spec.whatwg.org/#readable-stream-default-controller-clear-algorithms
+//
+// Note: nullptr is used to indicate we run the default algorithm at the
+//       moment,
+//       so the below doesn't quite match the spec, but serves the correct
+//       purpose for disconnecting the algorithms from the object graph to allow
+//       collection.
+//
+//       As far as I know, this isn't currently visible, but we need to keep
+//       this in mind. This is a weakness of this current implementation, and
+//       I'd prefer to have a better answer here eventually.
+void ReadableStreamDefaultControllerClearAlgorithms(
+    ReadableStreamDefaultController* aController) {
+  // Step 1.
+  // Step 2.
+  aController->ClearAlgorithms();
+
+  // Step 3.
+  aController->setStrategySizeAlgorithm(nullptr);
+}
+
+// https://streams.spec.whatwg.org/#readable-stream-default-controller-close
+void ReadableStreamDefaultControllerClose(
+    JSContext* aCx, ReadableStreamDefaultController* aController,
+    ErrorResult& aRv) {
+  // Step 1.
+  if (!ReadableStreamDefaultControllerCanCloseOrEnqueue(aController)) {
+    return;
+  }
+
+  // Step 2.
+  RefPtr<ReadableStream> stream = aController->Stream();
+
+  // Step 3.
+  aController->SetCloseRequested(true);
+
+  // Step 4.
+  if (aController->Queue().isEmpty()) {
+    // Step 4.1
+    ReadableStreamDefaultControllerClearAlgorithms(aController);
+
+    // Step 4.2
+    ReadableStreamClose(aCx, stream, aRv);
+  }
+}
+
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#rs-default-controller-close
+void ReadableStreamDefaultController::Close(JSContext* aCx, ErrorResult& aRv) {
+  // Step 1.
+  if (!ReadableStreamDefaultControllerCanCloseOrEnqueueAndThrow(
+          this, CloseOrEnqueue::Close, aRv)) {
+    return;
+  }
+
+  // Step 2.
+  ReadableStreamDefaultControllerClose(aCx, this, aRv);
+}
+
+namespace streams_abstract {
+
+MOZ_CAN_RUN_SCRIPT static void ReadableStreamDefaultControllerCallPullIfNeeded(
+    JSContext* aCx, ReadableStreamDefaultController* aController,
+    ErrorResult& aRv);
+
+// https://streams.spec.whatwg.org/#readable-stream-default-controller-enqueue
+void ReadableStreamDefaultControllerEnqueue(
+    JSContext* aCx, ReadableStreamDefaultController* aController,
+    JS::Handle<JS::Value> aChunk, ErrorResult& aRv) {
+  // Step 1.
+  if (!ReadableStreamDefaultControllerCanCloseOrEnqueue(aController)) {
+    return;
+  }
+
+  // Step 2.
+  RefPtr<ReadableStream> stream = aController->Stream();
+
+  // Step 3.
+  if (IsReadableStreamLocked(stream) &&
+      ReadableStreamGetNumReadRequests(stream) > 0) {
+    ReadableStreamFulfillReadRequest(aCx, stream, aChunk, false, aRv);
+  } else {
+    // Step 4.1
+    Optional<JS::Handle<JS::Value>> optionalChunk(aCx, aChunk);
+
+    // Step 4.3 (Re-ordered);
+    RefPtr<QueuingStrategySize> sizeAlgorithm(
+        aController->StrategySizeAlgorithm());
+
+    // If !sizeAlgorithm, we return 1, which is inlined from
+    // https://streams.spec.whatwg.org/#make-size-algorithm-from-size-function
+    double chunkSize =
+        sizeAlgorithm
+            ? sizeAlgorithm->Call(
+                  optionalChunk, aRv,
+                  "ReadableStreamDefaultController.[[strategySizeAlgorithm]]",
+                  CallbackObject::eRethrowExceptions)
+            : 1.0;
+
+    // If this is an uncatchable exception we can't continue.
+    if (aRv.IsUncatchableException()) {
+      return;
+    }
+
+    // Step 4.2:
+    if (aRv.MaybeSetPendingException(
+            aCx, "ReadableStreamDefaultController.enqueue")) {
+      JS::Rooted<JS::Value> errorValue(aCx);
+
+      JS_GetPendingException(aCx, &errorValue);
+
+      // Step 4.2.1
+
+      ReadableStreamDefaultControllerError(aCx, aController, errorValue, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+
+      // Step 4.2.2 Caller must treat aRv as if it were a completion
+      // value
+      aRv.MightThrowJSException();
+      aRv.ThrowJSException(aCx, errorValue);
+      return;
+    }
+
+    // Step 4.4
+    EnqueueValueWithSize(aController, aChunk, chunkSize, aRv);
+
+    // Step 4.5
+    // Note we convert the pending exception to a JS value here, and then
+    // re-throw it because we save this exception and re-expose it elsewhere
+    // and there are tests to ensure the identity of these errors are the same.
+    if (aRv.MaybeSetPendingException(
+            aCx, "ReadableStreamDefaultController.enqueue")) {
+      JS::Rooted<JS::Value> errorValue(aCx);
+
+      JS_GetPendingException(aCx, &errorValue);
+
+      // Step 4.5.1
+      ReadableStreamDefaultControllerError(aCx, aController, errorValue, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+
+      // Step 4.5.2 Caller must treat aRv as if it were a completion
+      // value
+      aRv.MightThrowJSException();
+      aRv.ThrowJSException(aCx, errorValue);
+      return;
+    }
+  }
+
+  // Step 5.
+  ReadableStreamDefaultControllerCallPullIfNeeded(aCx, aController, aRv);
+}
+
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#rs-default-controller-close
+void ReadableStreamDefaultController::Enqueue(JSContext* aCx,
+                                              JS::Handle<JS::Value> aChunk,
+                                              ErrorResult& aRv) {
+  // Step 1.
+  if (!ReadableStreamDefaultControllerCanCloseOrEnqueueAndThrow(
+          this, CloseOrEnqueue::Enqueue, aRv)) {
+    return;
+  }
+
+  // Step 2.
+  ReadableStreamDefaultControllerEnqueue(aCx, this, aChunk, aRv);
+}
+
+void ReadableStreamDefaultController::Error(JSContext* aCx,
+                                            JS::Handle<JS::Value> aError,
+                                            ErrorResult& aRv) {
+  ReadableStreamDefaultControllerError(aCx, this, aError, aRv);
+}
+
+namespace streams_abstract {
+
+// https://streams.spec.whatwg.org/#readable-stream-default-controller-should-call-pull
+bool ReadableStreamDefaultControllerShouldCallPull(
+    ReadableStreamDefaultController* aController) {
+  // Step 1.
+  ReadableStream* stream = aController->Stream();
+
+  // Step 2.
+  if (!ReadableStreamDefaultControllerCanCloseOrEnqueue(aController)) {
+    return false;
+  }
+
+  // Step 3.
+  if (!aController->Started()) {
+    return false;
+  }
+
+  // Step 4.
+  if (IsReadableStreamLocked(stream) &&
+      ReadableStreamGetNumReadRequests(stream) > 0) {
+    return true;
+  }
+
+  // Step 5.
+  Nullable<double> desiredSize =
+      ReadableStreamDefaultControllerGetDesiredSize(aController);
+
+  // Step 6.
+  MOZ_ASSERT(!desiredSize.IsNull());
+
+  // Step 7 + 8
+  return desiredSize.Value() > 0;
+}
+
+// https://streams.spec.whatwg.org/#readable-stream-default-controller-error
+void ReadableStreamDefaultControllerError(
+    JSContext* aCx, ReadableStreamDefaultController* aController,
+    JS::Handle<JS::Value> aValue, ErrorResult& aRv) {
+  // Step 1.
+  ReadableStream* stream = aController->Stream();
+
+  // Step 2.
+  if (stream->State() != ReadableStream::ReaderState::Readable) {
+    return;
+  }
+
+  // Step 3.
+  ResetQueue(aController);
+
+  // Step 4.
+  ReadableStreamDefaultControllerClearAlgorithms(aController);
+
+  // Step 5.
+  ReadableStreamError(aCx, stream, aValue, aRv);
+}
+
+// https://streams.spec.whatwg.org/#readable-stream-default-controller-call-pull-if-needed
+static void ReadableStreamDefaultControllerCallPullIfNeeded(
+    JSContext* aCx, ReadableStreamDefaultController* aController,
+    ErrorResult& aRv) {
+  // Step 1.
+  bool shouldPull = ReadableStreamDefaultControllerShouldCallPull(aController);
+
+  // Step 2.
+  if (!shouldPull) {
+    return;
+  }
+
+  // Step 3.
+  if (aController->Pulling()) {
+    // Step 3.1
+    aController->SetPullAgain(true);
+    // Step 3.2
+    return;
+  }
+
+  // Step 4.
+  MOZ_ASSERT(!aController->PullAgain());
+
+  // Step 5.
+  aController->SetPulling(true);
+
+  // Step 6.
+  RefPtr<UnderlyingSourceAlgorithmsBase> algorithms =
+      aController->GetAlgorithms();
+  RefPtr<Promise> pullPromise =
+      algorithms->PullCallback(aCx, *aController, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 7 + 8:
+  pullPromise->AddCallbacksWithCycleCollectedArgs(
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         ReadableStreamDefaultController* mController)
+          MOZ_CAN_RUN_SCRIPT_BOUNDARY {
+            // Step 7.1
+            mController->SetPulling(false);
+            // Step 7.2
+            if (mController->PullAgain()) {
+              // Step 7.2.1
+              mController->SetPullAgain(false);
+
+              // Step 7.2.2
+              ErrorResult rv;
+              ReadableStreamDefaultControllerCallPullIfNeeded(
+                  aCx, MOZ_KnownLive(mController), aRv);
+            }
+          },
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         ReadableStreamDefaultController* mController) {
+        // Step 8.1
+        ReadableStreamDefaultControllerError(aCx, mController, aValue, aRv);
+      },
+      RefPtr(aController));
+}
+
+// https://streams.spec.whatwg.org/#set-up-readable-stream-default-controller
+void SetUpReadableStreamDefaultController(
+    JSContext* aCx, ReadableStream* aStream,
+    ReadableStreamDefaultController* aController,
+    UnderlyingSourceAlgorithmsBase* aAlgorithms, double aHighWaterMark,
+    QueuingStrategySize* aSizeAlgorithm, ErrorResult& aRv) {
+  // Step 1.
+  MOZ_ASSERT(!aStream->Controller());
+
+  // Step 2.
+  aController->SetStream(aStream);
+
+  // Step 3.
+  ResetQueue(aController);
+
+  // Step 4.
+  aController->SetStarted(false);
+  aController->SetCloseRequested(false);
+  aController->SetPullAgain(false);
+  aController->SetPulling(false);
+
+  // Step 5.
+  aController->setStrategySizeAlgorithm(aSizeAlgorithm);
+  aController->SetStrategyHWM(aHighWaterMark);
+
+  // Step 6.
+  // Step 7.
+  aController->SetAlgorithms(*aAlgorithms);
+
+  // Step 8.
+  aStream->SetController(*aController);
+
+  // Step 9. Default algorithm returns undefined. See Step 2 of
+  // https://streams.spec.whatwg.org/#set-up-readable-stream-default-controller
+  JS::Rooted<JS::Value> startResult(aCx, JS::UndefinedValue());
+  RefPtr<ReadableStreamDefaultController> controller = aController;
+  aAlgorithms->StartCallback(aCx, *controller, &startResult, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 10.
+  RefPtr<Promise> startPromise =
+      Promise::CreateInfallible(aStream->GetParentObject());
+  startPromise->MaybeResolve(startResult);
+
+  // Step 11 & 12:
+  startPromise->AddCallbacksWithCycleCollectedArgs(
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         ReadableStreamDefaultController* aController)
+          MOZ_CAN_RUN_SCRIPT_BOUNDARY {
+            MOZ_ASSERT(aController);
+
+            // Step 11.1
+            aController->SetStarted(true);
+
+            // Step 11.2
+            aController->SetPulling(false);
+
+            // Step 11.3
+            aController->SetPullAgain(false);
+
+            // Step 11.4:
+            ReadableStreamDefaultControllerCallPullIfNeeded(
+                aCx, MOZ_KnownLive(aController), aRv);
+          },
+
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         ReadableStreamDefaultController* aController) {
+        // Step 12.1
+        ReadableStreamDefaultControllerError(aCx, aController, aValue, aRv);
+      },
+      RefPtr(aController));
+}
+
+// https://streams.spec.whatwg.org/#set-up-readable-stream-default-controller-from-underlying-source
+void SetupReadableStreamDefaultControllerFromUnderlyingSource(
+    JSContext* aCx, ReadableStream* aStream,
+    JS::Handle<JSObject*> aUnderlyingSource,
+    UnderlyingSource& aUnderlyingSourceDict, double aHighWaterMark,
+    QueuingStrategySize* aSizeAlgorithm, ErrorResult& aRv) {
+  // Step 1.
+  RefPtr<ReadableStreamDefaultController> controller =
+      new ReadableStreamDefaultController(aStream->GetParentObject());
+
+  // Step 2 - 7
+  RefPtr<UnderlyingSourceAlgorithms> algorithms =
+      new UnderlyingSourceAlgorithms(aStream->GetParentObject(),
+                                     aUnderlyingSource, aUnderlyingSourceDict);
+
+  // Step 8:
+  SetUpReadableStreamDefaultController(aCx, aStream, controller, algorithms,
+                                       aHighWaterMark, aSizeAlgorithm, aRv);
+}
+
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#rs-default-controller-private-cancel
+already_AddRefed<Promise> ReadableStreamDefaultController::CancelSteps(
+    JSContext* aCx, JS::Handle<JS::Value> aReason, ErrorResult& aRv) {
+  // Step 1.
+  ResetQueue(this);
+
+  // Step 2.
+  Optional<JS::Handle<JS::Value>> errorOption(aCx, aReason);
+  RefPtr<UnderlyingSourceAlgorithmsBase> algorithms = mAlgorithms;
+  RefPtr<Promise> result = algorithms->CancelCallback(aCx, errorOption, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 3.
+  ReadableStreamDefaultControllerClearAlgorithms(this);
+
+  // Step 4.
+  return result.forget();
+}
+
+// https://streams.spec.whatwg.org/#rs-default-controller-private-pull
+void ReadableStreamDefaultController::PullSteps(JSContext* aCx,
+                                                ReadRequest* aReadRequest,
+                                                ErrorResult& aRv) {
+  // Step 1.
+  RefPtr<ReadableStream> stream = mStream;
+
+  // Step 2.
+  if (!mQueue.isEmpty()) {
+    // Step 2.1
+    JS::Rooted<JS::Value> chunk(aCx);
+    DequeueValue(this, &chunk);
+
+    // Step 2.2
+    if (CloseRequested() && mQueue.isEmpty()) {
+      // Step 2.2.1
+      ReadableStreamDefaultControllerClearAlgorithms(this);
+      // Step 2.2.2
+      ReadableStreamClose(aCx, stream, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+    } else {
+      // Step 2.3
+      ReadableStreamDefaultControllerCallPullIfNeeded(aCx, this, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+    }
+
+    // Step 2.4
+    aReadRequest->ChunkSteps(aCx, chunk, aRv);
+  } else {
+    // Step 3.
+    // Step 3.1
+    ReadableStreamAddReadRequest(stream, aReadRequest);
+    // Step 3.2
+    ReadableStreamDefaultControllerCallPullIfNeeded(aCx, this, aRv);
+  }
+}
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaultcontroller-releasesteps
+void ReadableStreamDefaultController::ReleaseSteps() {
+  // Step 1. Return.
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/ReadableStreamDefaultController.h b/dom/streams/ReadableStreamDefaultController.h
new file mode 100644
index 0000000000..f94a692f15
--- /dev/null
+++ b/dom/streams/ReadableStreamDefaultController.h
@@ -0,0 +1,163 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ReadableStreamDefaultController_h
+#define mozilla_dom_ReadableStreamDefaultController_h
+
+#include "js/TypeDecls.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+#include "mozilla/dom/QueueWithSizes.h"
+#include "mozilla/dom/ReadableStreamController.h"
+#include "mozilla/dom/ReadRequest.h"
+#include "mozilla/dom/UnderlyingSourceCallbackHelpers.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsIGlobalObject.h"
+#include "nsISupports.h"
+#include "nsWrapperCache.h"
+#include "mozilla/dom/Nullable.h"
+#include "nsTArray.h"
+
+namespace mozilla::dom {
+
+class ReadableStream;
+class ReadableStreamDefaultReader;
+struct UnderlyingSource;
+class ReadableStreamGenericReader;
+
+class ReadableStreamDefaultController final : public ReadableStreamController,
+                                              public nsWrapperCache {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_INHERITED(
+      ReadableStreamDefaultController, ReadableStreamController)
+
+ public:
+  explicit ReadableStreamDefaultController(nsIGlobalObject* aGlobal);
+
+ protected:
+  ~ReadableStreamDefaultController() override;
+
+ public:
+  bool IsDefault() override { return true; }
+  bool IsByte() override { return false; }
+  ReadableStreamDefaultController* AsDefault() override { return this; }
+  ReadableByteStreamController* AsByte() override { return nullptr; }
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  Nullable<double> GetDesiredSize();
+
+  MOZ_CAN_RUN_SCRIPT void Close(JSContext* aCx, ErrorResult& aRv);
+
+  MOZ_CAN_RUN_SCRIPT void Enqueue(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                                  ErrorResult& aRv);
+
+  void Error(JSContext* aCx, JS::Handle<JS::Value> aError, ErrorResult& aRv);
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> CancelSteps(
+      JSContext* aCx, JS::Handle<JS::Value> aReason, ErrorResult& aRv) override;
+  MOZ_CAN_RUN_SCRIPT void PullSteps(JSContext* aCx, ReadRequest* aReadRequest,
+                                    ErrorResult& aRv) override;
+
+  void ReleaseSteps() override;
+
+  // Internal Slot Accessors
+  bool CloseRequested() const { return mCloseRequested; }
+  void SetCloseRequested(bool aCloseRequested) {
+    mCloseRequested = aCloseRequested;
+  }
+
+  bool PullAgain() const { return mPullAgain; }
+  void SetPullAgain(bool aPullAgain) { mPullAgain = aPullAgain; }
+
+  bool Pulling() const { return mPulling; }
+  void SetPulling(bool aPulling) { mPulling = aPulling; }
+
+  QueueWithSizes& Queue() { return mQueue; }
+
+  double QueueTotalSize() const { return mQueueTotalSize; }
+  void SetQueueTotalSize(double aQueueTotalSize) {
+    mQueueTotalSize = aQueueTotalSize;
+  }
+
+  bool Started() const { return mStarted; }
+  void SetStarted(bool aStarted) { mStarted = aStarted; }
+
+  double StrategyHWM() const { return mStrategyHWM; }
+  void SetStrategyHWM(double aStrategyHWM) { mStrategyHWM = aStrategyHWM; }
+
+  QueuingStrategySize* StrategySizeAlgorithm() const {
+    return mStrategySizeAlgorithm;
+  }
+  void setStrategySizeAlgorithm(QueuingStrategySize* aStrategySizeAlgorithm) {
+    mStrategySizeAlgorithm = aStrategySizeAlgorithm;
+  }
+
+ private:
+  // Internal Slots:
+  bool mCloseRequested = false;
+  bool mPullAgain = false;
+  bool mPulling = false;
+  QueueWithSizes mQueue = {};
+  double mQueueTotalSize = 0.0;
+  bool mStarted = false;
+  double mStrategyHWM = false;
+  RefPtr<QueuingStrategySize> mStrategySizeAlgorithm;
+};
+
+namespace streams_abstract {
+
+MOZ_CAN_RUN_SCRIPT void SetUpReadableStreamDefaultController(
+    JSContext* aCx, ReadableStream* aStream,
+    ReadableStreamDefaultController* aController,
+    UnderlyingSourceAlgorithmsBase* aAlgorithms, double aHighWaterMark,
+    QueuingStrategySize* aSizeAlgorithm, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void
+SetupReadableStreamDefaultControllerFromUnderlyingSource(
+    JSContext* aCx, ReadableStream* aStream,
+    JS::Handle<JSObject*> aUnderlyingSource,
+    UnderlyingSource& aUnderlyingSourceDict, double aHighWaterMark,
+    QueuingStrategySize* aSizeAlgorithm, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void ReadableStreamDefaultControllerEnqueue(
+    JSContext* aCx, ReadableStreamDefaultController* aController,
+    JS::Handle<JS::Value> aChunk, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void ReadableStreamDefaultControllerClose(
+    JSContext* aCx, ReadableStreamDefaultController* aController,
+    ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void ReadableStreamDefaultReaderRead(
+    JSContext* aCx, ReadableStreamGenericReader* reader, ReadRequest* aRequest,
+    ErrorResult& aRv);
+
+void ReadableStreamDefaultControllerError(
+    JSContext* aCx, ReadableStreamDefaultController* aController,
+    JS::Handle<JS::Value> aValue, ErrorResult& aRv);
+
+Nullable<double> ReadableStreamDefaultControllerGetDesiredSize(
+    ReadableStreamDefaultController* aController);
+
+enum class CloseOrEnqueue { Close, Enqueue };
+
+bool ReadableStreamDefaultControllerCanCloseOrEnqueueAndThrow(
+    ReadableStreamDefaultController* aController,
+    CloseOrEnqueue aCloseOrEnqueue, ErrorResult& aRv);
+
+bool ReadableStreamDefaultControllerShouldCallPull(
+    ReadableStreamDefaultController* aController);
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_ReadableStreamDefaultController_h
diff --git a/dom/streams/ReadableStreamDefaultReader.cpp b/dom/streams/ReadableStreamDefaultReader.cpp
new file mode 100644
index 0000000000..ff95fb37a8
--- /dev/null
+++ b/dom/streams/ReadableStreamDefaultReader.cpp
@@ -0,0 +1,440 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/ReadableStreamDefaultReader.h"
+
+#include "mozilla/dom/AutoEntryScript.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/RootedDictionary.h"
+#include "js/PropertyAndElement.h"
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "jsapi.h"
+#include "mozilla/dom/ReadableStreamDefaultReaderBinding.h"
+#include "mozilla/dom/UnderlyingSourceBinding.h"
+#include "nsCOMPtr.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsISupports.h"
+#include "nsWrapperCache.h"
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+NS_IMPL_CYCLE_COLLECTION(ReadableStreamGenericReader, mClosedPromise, mStream,
+                         mGlobal)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ReadableStreamGenericReader)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ReadableStreamGenericReader)
+NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(ReadableStreamGenericReader)
+NS_IMPL_CYCLE_COLLECTION_TRACE_END
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ReadableStreamGenericReader)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_INHERITED(ReadableStreamDefaultReader,
+                                                ReadableStreamGenericReader,
+                                                mReadRequests)
+NS_IMPL_ADDREF_INHERITED(ReadableStreamDefaultReader,
+                         ReadableStreamGenericReader)
+NS_IMPL_RELEASE_INHERITED(ReadableStreamDefaultReader,
+                          ReadableStreamGenericReader)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ReadableStreamDefaultReader)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+NS_INTERFACE_MAP_END_INHERITING(ReadableStreamGenericReader)
+
+ReadableStreamDefaultReader::ReadableStreamDefaultReader(nsISupports* aGlobal)
+    : ReadableStreamGenericReader(do_QueryInterface(aGlobal)),
+      nsWrapperCache() {}
+
+ReadableStreamDefaultReader::~ReadableStreamDefaultReader() {
+  mReadRequests.clear();
+}
+
+JSObject* ReadableStreamDefaultReader::WrapObject(
+    JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
+  return ReadableStreamDefaultReader_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#readable-stream-reader-generic-initialize
+bool ReadableStreamReaderGenericInitialize(ReadableStreamGenericReader* aReader,
+                                           ReadableStream* aStream) {
+  // Step 1.
+  aReader->SetStream(aStream);
+
+  // Step 2.
+  aStream->SetReader(aReader);
+
+  aReader->SetClosedPromise(
+      Promise::CreateInfallible(aReader->GetParentObject()));
+
+  switch (aStream->State()) {
+      // Step 3.
+    case ReadableStream::ReaderState::Readable:
+      // Step 3.1
+      // Promise created above.
+      return true;
+    // Step 4.
+    case ReadableStream::ReaderState::Closed:
+      // Step 4.1.
+      aReader->ClosedPromise()->MaybeResolve(JS::UndefinedHandleValue);
+
+      return true;
+    // Step 5.
+    case ReadableStream::ReaderState::Errored: {
+      // Step 5.1 Implicit
+      // Step 5.2
+      JS::RootingContext* rcx = RootingCx();
+      JS::Rooted<JS::Value> rootedError(rcx, aStream->StoredError());
+      aReader->ClosedPromise()->MaybeReject(rootedError);
+
+      // Step 5.3
+      aReader->ClosedPromise()->SetSettledPromiseIsHandled();
+      return true;
+    }
+    default:
+      MOZ_ASSERT_UNREACHABLE("Unknown ReaderState");
+      return false;
+  }
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#default-reader-constructor &&
+// https://streams.spec.whatwg.org/#set-up-readable-stream-default-reader
+/* static */
+already_AddRefed<ReadableStreamDefaultReader>
+ReadableStreamDefaultReader::Constructor(const GlobalObject& aGlobal,
+                                         ReadableStream& aStream,
+                                         ErrorResult& aRv) {
+  RefPtr<ReadableStreamDefaultReader> reader =
+      new ReadableStreamDefaultReader(aGlobal.GetAsSupports());
+
+  // https://streams.spec.whatwg.org/#set-up-readable-stream-default-reader
+  // Step 1.
+  if (aStream.Locked()) {
+    aRv.ThrowTypeError(
+        "Cannot create a new reader for a readable stream already locked by "
+        "another reader.");
+    return nullptr;
+  }
+
+  // Step 2.
+  RefPtr<ReadableStream> streamPtr = &aStream;
+  if (!ReadableStreamReaderGenericInitialize(reader, streamPtr)) {
+    return nullptr;
+  }
+
+  // Step 3.
+  reader->mReadRequests.clear();
+
+  return reader.forget();
+}
+
+void Read_ReadRequest::ChunkSteps(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                                  ErrorResult& aRv) {
+  // https://streams.spec.whatwg.org/#default-reader-read Step 3.
+  // chunk steps, given chunk:
+  //  Step 1. Resolve promise with «[ "value" → chunk, "done" → false ]».
+
+  // Value may need to be wrapped if stream and reader are in different
+  // compartments.
+  JS::Rooted<JS::Value> chunk(aCx, aChunk);
+  if (!JS_WrapValue(aCx, &chunk)) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+
+  RootedDictionary<ReadableStreamReadResult> result(aCx);
+  result.mValue = chunk;
+  result.mDone.Construct(false);
+
+  // Ensure that the object is created with the current global.
+  JS::Rooted<JS::Value> value(aCx);
+  if (!ToJSValue(aCx, std::move(result), &value)) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+
+  mPromise->MaybeResolve(value);
+}
+
+void Read_ReadRequest::CloseSteps(JSContext* aCx, ErrorResult& aRv) {
+  // https://streams.spec.whatwg.org/#default-reader-read Step 3.
+  // close steps:
+  //  Step 1. Resolve promise with «[ "value" → undefined, "done" → true ]».
+  RootedDictionary<ReadableStreamReadResult> result(aCx);
+  result.mValue.setUndefined();
+  result.mDone.Construct(true);
+
+  JS::Rooted<JS::Value> value(aCx);
+  if (!ToJSValue(aCx, std::move(result), &value)) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+
+  mPromise->MaybeResolve(value);
+}
+
+void Read_ReadRequest::ErrorSteps(JSContext* aCx, JS::Handle<JS::Value> e,
+                                  ErrorResult& aRv) {
+  // https://streams.spec.whatwg.org/#default-reader-read Step 3.
+  // error steps:
+  //  Step 1. Reject promise with e.
+  mPromise->MaybeReject(e);
+}
+
+NS_IMPL_CYCLE_COLLECTION(ReadRequest)
+NS_IMPL_CYCLE_COLLECTION_INHERITED(Read_ReadRequest, ReadRequest, mPromise)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ReadRequest)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ReadRequest)
+
+NS_IMPL_ADDREF_INHERITED(Read_ReadRequest, ReadRequest)
+NS_IMPL_RELEASE_INHERITED(Read_ReadRequest, ReadRequest)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ReadRequest)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(Read_ReadRequest)
+NS_INTERFACE_MAP_END_INHERITING(ReadRequest)
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#readable-stream-default-reader-read
+void ReadableStreamDefaultReaderRead(JSContext* aCx,
+                                     ReadableStreamGenericReader* aReader,
+                                     ReadRequest* aRequest, ErrorResult& aRv) {
+  // Step 1.
+  ReadableStream* stream = aReader->GetStream();
+
+  // Step 2.
+  MOZ_ASSERT(stream);
+
+  // Step 3.
+  stream->SetDisturbed(true);
+
+  switch (stream->State()) {
+    // Step 4.
+    case ReadableStream::ReaderState::Closed: {
+      aRequest->CloseSteps(aCx, aRv);
+      return;
+    }
+
+    case ReadableStream::ReaderState::Errored: {
+      JS::Rooted<JS::Value> storedError(aCx, stream->StoredError());
+      aRequest->ErrorSteps(aCx, storedError, aRv);
+      return;
+    }
+
+    case ReadableStream::ReaderState::Readable: {
+      RefPtr<ReadableStreamController> controller(stream->Controller());
+      MOZ_ASSERT(controller);
+      controller->PullSteps(aCx, aRequest, aRv);
+      return;
+    }
+  }
+}
+}  // namespace streams_abstract
+
+// Return a raw pointer here to avoid refcounting, but make sure it's safe
+// (the object should be kept alive by the callee).
+// https://streams.spec.whatwg.org/#default-reader-read
+already_AddRefed<Promise> ReadableStreamDefaultReader::Read(ErrorResult& aRv) {
+  // Step 1.
+  if (!mStream) {
+    aRv.ThrowTypeError("Reading is not possible after calling releaseLock.");
+    return nullptr;
+  }
+
+  // Step 2.
+  RefPtr<Promise> promise = Promise::CreateInfallible(GetParentObject());
+
+  // Step 3.
+  RefPtr<ReadRequest> request = new Read_ReadRequest(promise);
+
+  // Step 4.
+  AutoEntryScript aes(mGlobal, "ReadableStreamDefaultReader::Read");
+  JSContext* cx = aes.cx();
+
+  ReadableStreamDefaultReaderRead(cx, this, request, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 5.
+  return promise.forget();
+}
+
+namespace streams_abstract {
+
+// https://streams.spec.whatwg.org/#readable-stream-reader-generic-release
+void ReadableStreamReaderGenericRelease(ReadableStreamGenericReader* aReader,
+                                        ErrorResult& aRv) {
+  // Step 1. Let stream be reader.[[stream]].
+  RefPtr<ReadableStream> stream = aReader->GetStream();
+
+  // Step 2. Assert: stream is not undefined.
+  MOZ_ASSERT(stream);
+
+  // Step 3. Assert: stream.[[reader]] is reader.
+  MOZ_ASSERT(stream->GetReader() == aReader);
+
+  // Step 4. If stream.[[state]] is "readable", reject reader.[[closedPromise]]
+  // with a TypeError exception.
+  if (stream->State() == ReadableStream::ReaderState::Readable) {
+    aReader->ClosedPromise()->MaybeRejectWithTypeError(
+        "Releasing lock on readable stream");
+  } else {
+    // Step 5. Otherwise, set reader.[[closedPromise]] to a promise rejected
+    // with a TypeError exception.
+    RefPtr<Promise> promise = Promise::CreateRejectedWithTypeError(
+        aReader->GetParentObject(), "Lock Released"_ns, aRv);
+    aReader->SetClosedPromise(promise.forget());
+  }
+
+  // Step 6. Set reader.[[closedPromise]].[[PromiseIsHandled]] to true.
+  aReader->ClosedPromise()->SetSettledPromiseIsHandled();
+
+  // Step 7. Perform ! stream.[[controller]].[[ReleaseSteps]]().
+  stream->Controller()->ReleaseSteps();
+
+  // Step 8. Set stream.[[reader]] to undefined.
+  stream->SetReader(nullptr);
+
+  // Step 9. Set reader.[[stream]] to undefined.
+  aReader->SetStream(nullptr);
+}
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaultreadererrorreadrequests
+void ReadableStreamDefaultReaderErrorReadRequests(
+    JSContext* aCx, ReadableStreamDefaultReader* aReader,
+    JS::Handle<JS::Value> aError, ErrorResult& aRv) {
+  // Step 1. Let readRequests be reader.[[readRequests]].
+  LinkedList<RefPtr<ReadRequest>> readRequests =
+      std::move(aReader->ReadRequests());
+
+  // Step 2. Set reader.[[readRequests]] to a new empty list.
+  // Note: The std::move already cleared this anyway.
+  aReader->ReadRequests().clear();
+
+  // Step 3. For each readRequest of readRequests,
+  while (RefPtr<ReadRequest> readRequest = readRequests.popFirst()) {
+    // Step 3.1. Perform readRequest’s error steps, given e.
+    readRequest->ErrorSteps(aCx, aError, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+  }
+}
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaultreaderrelease
+void ReadableStreamDefaultReaderRelease(JSContext* aCx,
+                                        ReadableStreamDefaultReader* aReader,
+                                        ErrorResult& aRv) {
+  // Step 1. Perform ! ReadableStreamReaderGenericRelease(reader).
+  ReadableStreamReaderGenericRelease(aReader, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 2. Let e be a new TypeError exception.
+  ErrorResult rv;
+  rv.ThrowTypeError("Releasing lock");
+  JS::Rooted<JS::Value> error(aCx);
+  MOZ_ALWAYS_TRUE(ToJSValue(aCx, std::move(rv), &error));
+
+  // Step 3. Perform ! ReadableStreamDefaultReaderErrorReadRequests(reader, e).
+  ReadableStreamDefaultReaderErrorReadRequests(aCx, aReader, error, aRv);
+}
+
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#default-reader-release-lock
+void ReadableStreamDefaultReader::ReleaseLock(ErrorResult& aRv) {
+  // Step 1. If this.[[stream]] is undefined, return.
+  if (!mStream) {
+    return;
+  }
+
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(mGlobal)) {
+    return aRv.ThrowUnknownError("Internal error");
+  }
+  JSContext* cx = jsapi.cx();
+
+  // Step 2. Perform ! ReadableStreamDefaultReaderRelease(this).
+  RefPtr<ReadableStreamDefaultReader> thisRefPtr = this;
+  ReadableStreamDefaultReaderRelease(cx, thisRefPtr, aRv);
+}
+
+// https://streams.spec.whatwg.org/#generic-reader-closed
+already_AddRefed<Promise> ReadableStreamGenericReader::Closed() const {
+  // Step 1.
+  return do_AddRef(mClosedPromise);
+}
+
+// https://streams.spec.whatwg.org/#readable-stream-reader-generic-cancel
+MOZ_CAN_RUN_SCRIPT
+static already_AddRefed<Promise> ReadableStreamGenericReaderCancel(
+    JSContext* aCx, ReadableStreamGenericReader* aReader,
+    JS::Handle<JS::Value> aReason, ErrorResult& aRv) {
+  // Step 1 (Strong ref for below call).
+  RefPtr<ReadableStream> stream = aReader->GetStream();
+
+  // Step 2.
+  MOZ_ASSERT(stream);
+
+  // Step 3.
+  return ReadableStreamCancel(aCx, stream, aReason, aRv);
+}
+
+already_AddRefed<Promise> ReadableStreamGenericReader::Cancel(
+    JSContext* aCx, JS::Handle<JS::Value> aReason, ErrorResult& aRv) {
+  // Step 1. If this.[[stream]] is undefined,
+  // return a promise rejected with a TypeError exception.
+  if (!mStream) {
+    aRv.ThrowTypeError("Canceling is not possible after calling releaseLock.");
+    return nullptr;
+  }
+
+  // Step 2. Return ! ReadableStreamReaderGenericCancel(this, reason).
+  return ReadableStreamGenericReaderCancel(aCx, this, aReason, aRv);
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#set-up-readable-stream-default-reader
+void SetUpReadableStreamDefaultReader(ReadableStreamDefaultReader* aReader,
+                                      ReadableStream* aStream,
+                                      ErrorResult& aRv) {
+  // Step 1.
+  if (IsReadableStreamLocked(aStream)) {
+    return aRv.ThrowTypeError(
+        "Cannot get a new reader for a readable stream already locked by "
+        "another reader.");
+  }
+
+  // Step 2.
+  if (!ReadableStreamReaderGenericInitialize(aReader, aStream)) {
+    return;
+  }
+
+  // Step 3.
+  aReader->ReadRequests().clear();
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#readablestreamdefaultreader-read-a-chunk
+// To read a chunk from a ReadableStreamDefaultReader reader, given a read
+// request readRequest, perform ! ReadableStreamDefaultReaderRead(reader,
+// readRequest).
+void ReadableStreamDefaultReader::ReadChunk(JSContext* aCx,
+                                            ReadRequest& aRequest,
+                                            ErrorResult& aRv) {
+  ReadableStreamDefaultReaderRead(aCx, this, &aRequest, aRv);
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/ReadableStreamDefaultReader.h b/dom/streams/ReadableStreamDefaultReader.h
new file mode 100644
index 0000000000..d037b98cb7
--- /dev/null
+++ b/dom/streams/ReadableStreamDefaultReader.h
@@ -0,0 +1,122 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ReadableStreamDefaultReader_h
+#define mozilla_dom_ReadableStreamDefaultReader_h
+
+#include "js/TypeDecls.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/ReadableStreamGenericReader.h"
+#include "mozilla/dom/ReadRequest.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsISupports.h"
+#include "nsWrapperCache.h"
+#include "mozilla/LinkedList.h"
+
+namespace mozilla::dom {
+
+class Promise;
+class ReadableStream;
+
+// https://streams.spec.whatwg.org/#default-reader-read
+struct Read_ReadRequest : public ReadRequest {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(Read_ReadRequest, ReadRequest)
+
+  RefPtr<Promise> mPromise;
+
+  explicit Read_ReadRequest(Promise* aPromise) : mPromise(aPromise) {}
+
+  void ChunkSteps(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                  ErrorResult& aRv) override;
+
+  void CloseSteps(JSContext* aCx, ErrorResult& aRv) override;
+
+  void ErrorSteps(JSContext* aCx, JS::Handle<JS::Value> e,
+                  ErrorResult& aRv) override;
+
+ protected:
+  ~Read_ReadRequest() override = default;
+};
+
+class ReadableStreamDefaultReader final : public ReadableStreamGenericReader,
+                                          public nsWrapperCache
+
+{
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS_INHERITED(
+      ReadableStreamDefaultReader, ReadableStreamGenericReader)
+
+ public:
+  explicit ReadableStreamDefaultReader(nsISupports* aGlobal);
+
+ protected:
+  ~ReadableStreamDefaultReader() override;
+
+ public:
+  bool IsDefault() override { return true; }
+  bool IsBYOB() override { return false; }
+  ReadableStreamDefaultReader* AsDefault() override { return this; }
+  ReadableStreamBYOBReader* AsBYOB() override {
+    MOZ_CRASH();
+    return nullptr;
+  }
+
+  // Public functions to implement other specs
+  // https://streams.spec.whatwg.org/#other-specs-rs-create
+
+  // The following algorithms can be used on arbitrary ReadableStream instances,
+  // including ones that are created by web developers. They can all fail in
+  // various operation-specific ways, and these failures should be handled by
+  // the calling specification.
+
+  // https://streams.spec.whatwg.org/#readablestreamdefaultreader-read-a-chunk
+  MOZ_CAN_RUN_SCRIPT void ReadChunk(JSContext* aCx, ReadRequest& aRequest,
+                                    ErrorResult& aRv);
+
+  // IDL layer functions
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  // IDL methods
+
+  static already_AddRefed<ReadableStreamDefaultReader> Constructor(
+      const GlobalObject& aGlobal, ReadableStream& stream, ErrorResult& aRv);
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> Read(ErrorResult& aRv);
+
+  void ReleaseLock(ErrorResult& aRv);
+
+  LinkedList<RefPtr<ReadRequest>>& ReadRequests() { return mReadRequests; }
+
+ private:
+  LinkedList<RefPtr<ReadRequest>> mReadRequests = {};
+};
+
+namespace streams_abstract {
+
+void SetUpReadableStreamDefaultReader(ReadableStreamDefaultReader* aReader,
+                                      ReadableStream* aStream,
+                                      ErrorResult& aRv);
+
+void ReadableStreamDefaultReaderErrorReadRequests(
+    JSContext* aCx, ReadableStreamDefaultReader* aReader,
+    JS::Handle<JS::Value> aError, ErrorResult& aRv);
+
+void ReadableStreamDefaultReaderRelease(JSContext* aCx,
+                                        ReadableStreamDefaultReader* aReader,
+                                        ErrorResult& aRv);
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_ReadableStreamDefaultReader_h
diff --git a/dom/streams/ReadableStreamGenericReader.h b/dom/streams/ReadableStreamGenericReader.h
new file mode 100644
index 0000000000..ccc966bdc4
--- /dev/null
+++ b/dom/streams/ReadableStreamGenericReader.h
@@ -0,0 +1,77 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ReadableStreamGenericReader_h
+#define mozilla_dom_ReadableStreamGenericReader_h
+
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/ReadableStreamDefaultReaderBinding.h"
+#include "nsISupports.h"
+#include "nsCycleCollectionParticipant.h"
+
+namespace mozilla::dom {
+
+class ReadableStream;
+class ReadableStreamDefaultReader;
+class ReadableStreamBYOBReader;
+
+// Base class for internal slots of readable stream readers
+class ReadableStreamGenericReader : public nsISupports {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(ReadableStreamGenericReader)
+
+  explicit ReadableStreamGenericReader(nsCOMPtr<nsIGlobalObject> aGlobal)
+      : mGlobal(std::move(aGlobal)) {}
+
+  nsIGlobalObject* GetParentObject() const { return mGlobal; }
+
+  virtual bool IsDefault() = 0;
+  virtual bool IsBYOB() = 0;
+  virtual ReadableStreamDefaultReader* AsDefault() = 0;
+  virtual ReadableStreamBYOBReader* AsBYOB() = 0;
+
+  Promise* ClosedPromise() { return mClosedPromise; }
+  void SetClosedPromise(already_AddRefed<Promise>&& aClosedPromise) {
+    mClosedPromise = aClosedPromise;
+  }
+
+  ReadableStream* GetStream() { return mStream; }
+  void SetStream(already_AddRefed<ReadableStream>&& aStream) {
+    mStream = aStream;
+  }
+  void SetStream(ReadableStream* aStream) {
+    RefPtr<ReadableStream> stream(aStream);
+    SetStream(stream.forget());
+  }
+
+  // IDL Methods
+  already_AddRefed<Promise> Closed() const;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> Cancel(
+      JSContext* aCx, JS::Handle<JS::Value> aReason, ErrorResult& aRv);
+
+ protected:
+  virtual ~ReadableStreamGenericReader() = default;
+
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+  RefPtr<Promise> mClosedPromise;
+  RefPtr<ReadableStream> mStream;
+};
+
+namespace streams_abstract {
+
+bool ReadableStreamReaderGenericInitialize(ReadableStreamGenericReader* aReader,
+                                           ReadableStream* aStream);
+
+void ReadableStreamReaderGenericRelease(ReadableStreamGenericReader* aReader,
+                                        ErrorResult& aRv);
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/streams/ReadableStreamPipeTo.cpp b/dom/streams/ReadableStreamPipeTo.cpp
new file mode 100644
index 0000000000..45cc79b0b2
--- /dev/null
+++ b/dom/streams/ReadableStreamPipeTo.cpp
@@ -0,0 +1,977 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "ReadableStreamPipeTo.h"
+
+#include "mozilla/dom/AbortFollower.h"
+#include "mozilla/dom/AbortSignal.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/ReadableStreamDefaultReader.h"
+#include "mozilla/dom/WritableStream.h"
+#include "mozilla/dom/WritableStreamDefaultWriter.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/Promise-inl.h"
+#include "mozilla/dom/PromiseNativeHandler.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/ErrorResult.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsISupportsImpl.h"
+
+#include "js/Exception.h"
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+struct PipeToReadRequest;
+class WriteFinishedPromiseHandler;
+class ShutdownActionFinishedPromiseHandler;
+
+// https://streams.spec.whatwg.org/#readable-stream-pipe-to (Steps 14-15.)
+//
+// This class implements everything that is required to read all chunks from
+// the reader (source) and write them to writer (destination), while
+// following the constraints given in the spec using our implementation-defined
+// behavior.
+//
+// The cycle-collected references look roughly like this:
+// clang-format off
+//
+// Closed promise <-- ReadableStreamDefaultReader <--> ReadableStream
+//         |                  ^              |
+//         |(PromiseHandler)  |(mReader)     |(ReadRequest)
+//         |                  |              |
+//         |-------------> PipeToPump <-------
+//                         ^  |   |
+//         |---------------|  |   |
+//         |                  |   |-------(mLastWrite) -------->
+//         |(PromiseHandler)  |   |< ---- (PromiseHandler) ---- Promise
+//         |                  |                                   ^
+//         |                  |(mWriter)                          |(mWriteRequests)
+//         |                  v                                   |
+// Closed promise <-- WritableStreamDefaultWriter <--------> WritableStream
+//
+// clang-format on
+class PipeToPump final : public AbortFollower {
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(PipeToPump)
+
+  friend struct PipeToReadRequest;
+  friend class WriteFinishedPromiseHandler;
+  friend class ShutdownActionFinishedPromiseHandler;
+
+  PipeToPump(Promise* aPromise, ReadableStreamDefaultReader* aReader,
+             WritableStreamDefaultWriter* aWriter, bool aPreventClose,
+             bool aPreventAbort, bool aPreventCancel)
+      : mPromise(aPromise),
+        mReader(aReader),
+        mWriter(aWriter),
+        mPreventClose(aPreventClose),
+        mPreventAbort(aPreventAbort),
+        mPreventCancel(aPreventCancel) {}
+
+  MOZ_CAN_RUN_SCRIPT void Start(JSContext* aCx, AbortSignal* aSignal);
+
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY void RunAbortAlgorithm() override;
+
+ private:
+  ~PipeToPump() override = default;
+
+  MOZ_CAN_RUN_SCRIPT void PerformAbortAlgorithm(JSContext* aCx,
+                                                AbortSignalImpl* aSignal);
+
+  MOZ_CAN_RUN_SCRIPT bool SourceOrDestErroredOrClosed(JSContext* aCx);
+
+  using ShutdownAction = already_AddRefed<Promise> (*)(
+      JSContext*, PipeToPump*, JS::Handle<mozilla::Maybe<JS::Value>>,
+      ErrorResult&);
+
+  MOZ_CAN_RUN_SCRIPT void ShutdownWithAction(
+      JSContext* aCx, ShutdownAction aAction,
+      JS::Handle<mozilla::Maybe<JS::Value>> aError);
+  MOZ_CAN_RUN_SCRIPT void ShutdownWithActionAfterFinishedWrite(
+      JSContext* aCx, ShutdownAction aAction,
+      JS::Handle<mozilla::Maybe<JS::Value>> aError);
+
+  MOZ_CAN_RUN_SCRIPT void Shutdown(
+      JSContext* aCx, JS::Handle<mozilla::Maybe<JS::Value>> aError);
+
+  void Finalize(JSContext* aCx, JS::Handle<mozilla::Maybe<JS::Value>> aError);
+
+  MOZ_CAN_RUN_SCRIPT void OnReadFulfilled(JSContext* aCx,
+                                          JS::Handle<JS::Value> aChunk,
+                                          ErrorResult& aRv);
+  MOZ_CAN_RUN_SCRIPT void OnWriterReady(JSContext* aCx, JS::Handle<JS::Value>);
+  MOZ_CAN_RUN_SCRIPT void Read(JSContext* aCx);
+
+  MOZ_CAN_RUN_SCRIPT void OnSourceClosed(JSContext* aCx, JS::Handle<JS::Value>);
+  MOZ_CAN_RUN_SCRIPT void OnSourceErrored(
+      JSContext* aCx, JS::Handle<JS::Value> aSourceStoredError);
+
+  MOZ_CAN_RUN_SCRIPT void OnDestClosed(JSContext* aCx, JS::Handle<JS::Value>);
+  MOZ_CAN_RUN_SCRIPT void OnDestErrored(JSContext* aCx,
+                                        JS::Handle<JS::Value> aDestStoredError);
+
+  RefPtr<Promise> mPromise;
+  RefPtr<ReadableStreamDefaultReader> mReader;
+  RefPtr<WritableStreamDefaultWriter> mWriter;
+  RefPtr<Promise> mLastWritePromise;
+  const bool mPreventClose;
+  const bool mPreventAbort;
+  const bool mPreventCancel;
+  bool mShuttingDown = false;
+#ifdef DEBUG
+  bool mReadChunk = false;
+#endif
+};
+
+// This is a helper class for PipeToPump that allows it to attach
+// member functions as promise handlers.
+class PipeToPumpHandler final : public PromiseNativeHandler {
+  virtual ~PipeToPumpHandler() = default;
+
+  using FunPtr = void (PipeToPump::*)(JSContext*, JS::Handle<JS::Value>);
+
+  RefPtr<PipeToPump> mPipeToPump;
+  FunPtr mResolved;
+  FunPtr mRejected;
+
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(PipeToPumpHandler)
+
+  explicit PipeToPumpHandler(PipeToPump* aPipeToPump, FunPtr aResolved,
+                             FunPtr aRejected)
+      : mPipeToPump(aPipeToPump), mResolved(aResolved), mRejected(aRejected) {}
+
+  void ResolvedCallback(JSContext* aCx, JS::Handle<JS::Value> aValue,
+                        ErrorResult&) override {
+    if (mResolved) {
+      (mPipeToPump->*mResolved)(aCx, aValue);
+    }
+  }
+
+  void RejectedCallback(JSContext* aCx, JS::Handle<JS::Value> aReason,
+                        ErrorResult&) override {
+    if (mRejected) {
+      (mPipeToPump->*mRejected)(aCx, aReason);
+    }
+  }
+};
+
+NS_IMPL_CYCLE_COLLECTION(PipeToPumpHandler, mPipeToPump)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(PipeToPumpHandler)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(PipeToPumpHandler)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(PipeToPumpHandler)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+void PipeToPump::RunAbortAlgorithm() {
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(mReader->GetStream()->GetParentObject())) {
+    NS_WARNING(
+        "Failed to initialize AutoJSAPI in PipeToPump::RunAbortAlgorithm");
+    return;
+  }
+  JSContext* cx = jsapi.cx();
+
+  RefPtr<AbortSignalImpl> signal = Signal();
+  PerformAbortAlgorithm(cx, signal);
+}
+
+void PipeToPump::PerformAbortAlgorithm(JSContext* aCx,
+                                       AbortSignalImpl* aSignal) {
+  MOZ_ASSERT(aSignal->Aborted());
+
+  // https://streams.spec.whatwg.org/#readable-stream-pipe-to
+  // Step 14.1. Let abortAlgorithm be the following steps:
+  // Note: All the following steps are 14.1.xx
+
+  // Step 1. Let error be signal’s abort reason.
+  JS::Rooted<JS::Value> error(aCx);
+  aSignal->GetReason(aCx, &error);
+
+  auto action = [](JSContext* aCx, PipeToPump* aPipeToPump,
+                   JS::Handle<mozilla::Maybe<JS::Value>> aError,
+                   ErrorResult& aRv) MOZ_CAN_RUN_SCRIPT {
+    JS::Rooted<JS::Value> error(aCx, *aError);
+
+    // Step 2. Let actions be an empty ordered set.
+    nsTArray<RefPtr<Promise>> actions;
+
+    // Step 3. If preventAbort is false, append the following action to actions:
+    if (!aPipeToPump->mPreventAbort) {
+      RefPtr<WritableStream> dest = aPipeToPump->mWriter->GetStream();
+
+      // Step 3.1. If dest.[[state]] is "writable", return !
+      // WritableStreamAbort(dest, error).
+      if (dest->State() == WritableStream::WriterState::Writable) {
+        RefPtr<Promise> p = WritableStreamAbort(aCx, dest, error, aRv);
+        if (aRv.Failed()) {
+          return already_AddRefed<Promise>();
+        }
+        actions.AppendElement(p);
+      }
+
+      // Step 3.2. Otherwise, return a promise resolved with undefined.
+      // Note: This is basically a no-op.
+    }
+
+    // Step 4. If preventCancel is false, append the following action action to
+    // actions:
+    if (!aPipeToPump->mPreventCancel) {
+      RefPtr<ReadableStream> source = aPipeToPump->mReader->GetStream();
+
+      // Step 4.1. If source.[[state]] is "readable", return !
+      // ReadableStreamCancel(source, error).
+      if (source->State() == ReadableStream::ReaderState::Readable) {
+        RefPtr<Promise> p = ReadableStreamCancel(aCx, source, error, aRv);
+        if (aRv.Failed()) {
+          return already_AddRefed<Promise>();
+        }
+        actions.AppendElement(p);
+      }
+
+      // Step 4.2. Otherwise, return a promise resolved with undefined.
+      // No-op again.
+    }
+
+    // Step 5. .. action consisting of getting a promise to wait for
+    // all of the actions in actions ...
+    return Promise::All(aCx, actions, aRv);
+  };
+
+  // Step 5. Shutdown with an action consisting of getting a promise to wait for
+  // all of the actions in actions, and with error.
+  JS::Rooted<Maybe<JS::Value>> someError(aCx, Some(error.get()));
+  ShutdownWithAction(aCx, action, someError);
+}
+
+bool PipeToPump::SourceOrDestErroredOrClosed(JSContext* aCx) {
+  // (Constraint) Error and close states must be propagated:
+  // the following conditions must be applied in order.
+  RefPtr<ReadableStream> source = mReader->GetStream();
+  RefPtr<WritableStream> dest = mWriter->GetStream();
+
+  // Step 1. Errors must be propagated forward: if source.[[state]] is or
+  // becomes "errored", then
+  if (source->State() == ReadableStream::ReaderState::Errored) {
+    JS::Rooted<JS::Value> storedError(aCx, source->StoredError());
+    OnSourceErrored(aCx, storedError);
+    return true;
+  }
+
+  // Step 2. Errors must be propagated backward: if dest.[[state]] is or becomes
+  // "errored", then
+  if (dest->State() == WritableStream::WriterState::Errored) {
+    JS::Rooted<JS::Value> storedError(aCx, dest->StoredError());
+    OnDestErrored(aCx, storedError);
+    return true;
+  }
+
+  // Step 3. Closing must be propagated forward: if source.[[state]] is or
+  // becomes "closed", then
+  if (source->State() == ReadableStream::ReaderState::Closed) {
+    OnSourceClosed(aCx, JS::UndefinedHandleValue);
+    return true;
+  }
+
+  // Step 4. Closing must be propagated backward:
+  // if ! WritableStreamCloseQueuedOrInFlight(dest) is true
+  // or dest.[[state]] is "closed", then
+  if (dest->CloseQueuedOrInFlight() ||
+      dest->State() == WritableStream::WriterState::Closed) {
+    OnDestClosed(aCx, JS::UndefinedHandleValue);
+    return true;
+  }
+
+  return false;
+}
+
+// https://streams.spec.whatwg.org/#readable-stream-pipe-to
+// Steps 14-15.
+void PipeToPump::Start(JSContext* aCx, AbortSignal* aSignal) {
+  // Step 14. If signal is not undefined,
+  if (aSignal) {
+    // Step 14.1. Let abortAlgorithm be the following steps:
+    // ... This is implemented by RunAbortAlgorithm.
+
+    // Step 14.2. If signal is aborted, perform abortAlgorithm and
+    // return promise.
+    if (aSignal->Aborted()) {
+      PerformAbortAlgorithm(aCx, aSignal);
+      return;
+    }
+
+    // Step 14.3. Add abortAlgorithm to signal.
+    Follow(aSignal);
+  }
+
+  // Step 15. In parallel but not really; see #905, using reader and writer,
+  // read all chunks from source and write them to dest.
+  // Due to the locking provided by the reader and writer,
+  // the exact manner in which this happens is not observable to author code,
+  // and so there is flexibility in how this is done.
+
+  // (Constraint) Error and close states must be propagated
+
+  // Before piping has started, we have to check for source/destination being
+  // errored/closed manually.
+  if (SourceOrDestErroredOrClosed(aCx)) {
+    return;
+  }
+
+  // We use the following two promises to propagate error and close states
+  // during piping.
+  RefPtr<Promise> readerClosed = mReader->ClosedPromise();
+  readerClosed->AppendNativeHandler(new PipeToPumpHandler(
+      this, &PipeToPump::OnSourceClosed, &PipeToPump::OnSourceErrored));
+
+  // Note: Because we control the destination/writer it should never be closed
+  // after we did the initial check above with SourceOrDestErroredOrClosed.
+  RefPtr<Promise> writerClosed = mWriter->ClosedPromise();
+  writerClosed->AppendNativeHandler(new PipeToPumpHandler(
+      this, &PipeToPump::OnDestClosed, &PipeToPump::OnDestErrored));
+
+  Read(aCx);
+}
+
+class WriteFinishedPromiseHandler final : public PromiseNativeHandler {
+  RefPtr<PipeToPump> mPipeToPump;
+  PipeToPump::ShutdownAction mAction;
+  bool mHasError;
+  JS::Heap<JS::Value> mError;
+
+  virtual ~WriteFinishedPromiseHandler() { mozilla::DropJSObjects(this); };
+
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(WriteFinishedPromiseHandler)
+
+  explicit WriteFinishedPromiseHandler(
+      JSContext* aCx, PipeToPump* aPipeToPump,
+      PipeToPump::ShutdownAction aAction,
+      JS::Handle<mozilla::Maybe<JS::Value>> aError)
+      : mPipeToPump(aPipeToPump), mAction(aAction) {
+    mHasError = aError.isSome();
+    if (mHasError) {
+      mError = *aError;
+    }
+    mozilla::HoldJSObjects(this);
+  }
+
+  MOZ_CAN_RUN_SCRIPT void WriteFinished(JSContext* aCx) {
+    RefPtr<PipeToPump> pipeToPump = mPipeToPump;  // XXX known-live?
+    JS::Rooted<Maybe<JS::Value>> error(aCx);
+    if (mHasError) {
+      error = Some(mError);
+    }
+    pipeToPump->ShutdownWithActionAfterFinishedWrite(aCx, mAction, error);
+  }
+
+  MOZ_CAN_RUN_SCRIPT void ResolvedCallback(JSContext* aCx,
+                                           JS::Handle<JS::Value> aValue,
+                                           ErrorResult&) override {
+    WriteFinished(aCx);
+  }
+
+  MOZ_CAN_RUN_SCRIPT void RejectedCallback(JSContext* aCx,
+                                           JS::Handle<JS::Value> aReason,
+                                           ErrorResult&) override {
+    WriteFinished(aCx);
+  }
+};
+
+NS_IMPL_CYCLE_COLLECTION_WITH_JS_MEMBERS(WriteFinishedPromiseHandler,
+                                         (mPipeToPump), (mError))
+NS_IMPL_CYCLE_COLLECTING_ADDREF(WriteFinishedPromiseHandler)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(WriteFinishedPromiseHandler)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(WriteFinishedPromiseHandler)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+// https://streams.spec.whatwg.org/#rs-pipeTo-shutdown-with-action
+// Shutdown with an action: if any of the above requirements ask to shutdown
+// with an action action, optionally with an error originalError, then:
+void PipeToPump::ShutdownWithAction(
+    JSContext* aCx, ShutdownAction aAction,
+    JS::Handle<mozilla::Maybe<JS::Value>> aError) {
+  // Step 1. If shuttingDown is true, abort these substeps.
+  if (mShuttingDown) {
+    return;
+  }
+
+  // Step 2. Set shuttingDown to true.
+  mShuttingDown = true;
+
+  // Step 3. If dest.[[state]] is "writable" and !
+  // WritableStreamCloseQueuedOrInFlight(dest) is false,
+  RefPtr<WritableStream> dest = mWriter->GetStream();
+  if (dest->State() == WritableStream::WriterState::Writable &&
+      !dest->CloseQueuedOrInFlight()) {
+    // Step 3.1. If any chunks have been read but not yet written, write them to
+    // dest.
+    // Step 3.2. Wait until every chunk that has been read has been
+    // written (i.e. the corresponding promises have settled).
+    //
+    // Note: Write requests are processed in order, so when the promise
+    // for the last written chunk is settled all previous chunks have been
+    // written as well.
+    if (mLastWritePromise) {
+      mLastWritePromise->AppendNativeHandler(
+          new WriteFinishedPromiseHandler(aCx, this, aAction, aError));
+      return;
+    }
+  }
+
+  // Don't have to wait for last write, immediately continue.
+  ShutdownWithActionAfterFinishedWrite(aCx, aAction, aError);
+}
+
+class ShutdownActionFinishedPromiseHandler final : public PromiseNativeHandler {
+  RefPtr<PipeToPump> mPipeToPump;
+  bool mHasError;
+  JS::Heap<JS::Value> mError;
+
+  virtual ~ShutdownActionFinishedPromiseHandler() {
+    mozilla::DropJSObjects(this);
+  }
+
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(
+      ShutdownActionFinishedPromiseHandler)
+
+  explicit ShutdownActionFinishedPromiseHandler(
+      JSContext* aCx, PipeToPump* aPipeToPump,
+      JS::Handle<mozilla::Maybe<JS::Value>> aError)
+      : mPipeToPump(aPipeToPump) {
+    mHasError = aError.isSome();
+    if (mHasError) {
+      mError = *aError;
+    }
+    mozilla::HoldJSObjects(this);
+  }
+
+  void ResolvedCallback(JSContext* aCx, JS::Handle<JS::Value> aValue,
+                        ErrorResult&) override {
+    // https://streams.spec.whatwg.org/#rs-pipeTo-shutdown-with-action
+    // Step 5. Upon fulfillment of p, finalize, passing along originalError if
+    // it was given.
+    JS::Rooted<Maybe<JS::Value>> error(aCx);
+    if (mHasError) {
+      error = Some(mError);
+    }
+    mPipeToPump->Finalize(aCx, error);
+  }
+
+  void RejectedCallback(JSContext* aCx, JS::Handle<JS::Value> aReason,
+                        ErrorResult&) override {
+    // https://streams.spec.whatwg.org/#rs-pipeTo-shutdown-with-action
+    // Step 6. Upon rejection of p with reason newError, finalize with
+    // newError.
+    JS::Rooted<Maybe<JS::Value>> error(aCx, Some(aReason));
+    mPipeToPump->Finalize(aCx, error);
+  }
+};
+
+NS_IMPL_CYCLE_COLLECTION_WITH_JS_MEMBERS(ShutdownActionFinishedPromiseHandler,
+                                         (mPipeToPump), (mError))
+NS_IMPL_CYCLE_COLLECTING_ADDREF(ShutdownActionFinishedPromiseHandler)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(ShutdownActionFinishedPromiseHandler)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ShutdownActionFinishedPromiseHandler)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+// https://streams.spec.whatwg.org/#rs-pipeTo-shutdown-with-action
+// Continuation after Step 3. triggered a promise resolution.
+void PipeToPump::ShutdownWithActionAfterFinishedWrite(
+    JSContext* aCx, ShutdownAction aAction,
+    JS::Handle<mozilla::Maybe<JS::Value>> aError) {
+  if (!aAction) {
+    // Used to implement shutdown without action. Finalize immediately.
+    Finalize(aCx, aError);
+    return;
+  }
+
+  // Step 4. Let p be the result of performing action.
+  RefPtr<PipeToPump> thisRefPtr = this;
+  ErrorResult rv;
+  RefPtr<Promise> p = aAction(aCx, thisRefPtr, aError, rv);
+
+  // Error while calling actions above, continue immediately with finalization.
+  if (rv.MaybeSetPendingException(aCx)) {
+    JS::Rooted<Maybe<JS::Value>> someError(aCx);
+
+    JS::Rooted<JS::Value> error(aCx);
+    if (JS_GetPendingException(aCx, &error)) {
+      someError = Some(error.get());
+    }
+
+    JS_ClearPendingException(aCx);
+
+    Finalize(aCx, someError);
+    return;
+  }
+
+  // Steps 5-6.
+  p->AppendNativeHandler(
+      new ShutdownActionFinishedPromiseHandler(aCx, this, aError));
+}
+
+// https://streams.spec.whatwg.org/#rs-pipeTo-shutdown
+// Shutdown: if any of the above requirements or steps ask to shutdown,
+// optionally with an error error, then:
+void PipeToPump::Shutdown(JSContext* aCx,
+                          JS::Handle<mozilla::Maybe<JS::Value>> aError) {
+  // Note: We implement "shutdown" in terms of "shutdown with action".
+  // We can observe that when passing along an action that always succeeds
+  // shutdown with action and shutdown have the same behavior, when
+  // Ignoring the potential micro task for the promise that we skip anyway.
+  ShutdownWithAction(aCx, nullptr, aError);
+}
+
+// https://streams.spec.whatwg.org/#rs-pipeTo-finalize
+// Finalize: both forms of shutdown will eventually ask to finalize,
+// optionally with an error error, which means to perform the following steps:
+void PipeToPump::Finalize(JSContext* aCx,
+                          JS::Handle<mozilla::Maybe<JS::Value>> aError) {
+  IgnoredErrorResult rv;
+  // Step 1. Perform ! WritableStreamDefaultWriterRelease(writer).
+  WritableStreamDefaultWriterRelease(aCx, mWriter);
+
+  // Step 2. If reader implements ReadableStreamBYOBReader,
+  // perform ! ReadableStreamBYOBReaderRelease(reader).
+  // Note: We always use a default reader.
+  MOZ_ASSERT(!mReader->IsBYOB());
+
+  // Step 3. Otherwise, perform ! ReadableStreamDefaultReaderRelease(reader).
+  ReadableStreamDefaultReaderRelease(aCx, mReader, rv);
+  NS_WARNING_ASSERTION(!rv.Failed(),
+                       "ReadableStreamReaderGenericRelease should not fail.");
+
+  // Step 3. If signal is not undefined, remove abortAlgorithm from signal.
+  if (IsFollowing()) {
+    Unfollow();
+  }
+
+  // Step 4. If error was given, reject promise with error.
+  if (aError.isSome()) {
+    JS::Rooted<JS::Value> error(aCx, *aError);
+    mPromise->MaybeReject(error);
+  } else {
+    // Step 5. Otherwise, resolve promise with undefined.
+    mPromise->MaybeResolveWithUndefined();
+  }
+
+  // Remove all references.
+  mPromise = nullptr;
+  mReader = nullptr;
+  mWriter = nullptr;
+  mLastWritePromise = nullptr;
+  Unfollow();
+}
+
+void PipeToPump::OnReadFulfilled(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                                 ErrorResult& aRv) {
+  // (Constraint) Shutdown must stop activity:
+  // if shuttingDown becomes true, the user agent must not initiate further
+  // reads from reader, and must only perform writes of already-read chunks ...
+  //
+  // We may reach this point after |On{Source,Dest}{Clos,Error}ed| has responded
+  // to an out-of-band change.  Per the comment in |OnSourceErrored|, we want to
+  // allow the implicated shutdown to proceed, and we don't want to interfere
+  // with or additionally alter its operation.  Particularly, we don't want to
+  // queue up the successfully-read chunk (if there was one, and this isn't just
+  // reporting "done") to be written: it wasn't "already-read" when that
+  // error/closure happened.
+  //
+  // All specified reactions to a closure/error invoke either the shutdown, or
+  // shutdown with an action, algorithms.  Those algorithms each abort if either
+  // shutdown algorithm has already been invoked.  So we check for shutdown here
+  // in case of asynchronous closure/error and abort if shutdown has already
+  // started (and possibly finished).
+  //
+  // TODO: Implement the eventual resolution from
+  // https://github.com/whatwg/streams/issues/1207
+  if (mShuttingDown) {
+    return;
+  }
+
+  // Write asynchronously. Roughly this is like:
+  // `Promise.resolve().then(() => stream.write(chunk));`
+  // XXX: The spec currently does not require asynchronicity, but this still
+  // matches other engines' behavior. See
+  // https://github.com/whatwg/streams/issues/1243.
+  RefPtr<Promise> promise =
+      Promise::CreateInfallible(mWriter->GetParentObject());
+  promise->MaybeResolveWithUndefined();
+  auto result = promise->ThenWithCycleCollectedArgsJS(
+      [](JSContext* aCx, JS::Handle<JS::Value>, ErrorResult& aRv,
+         const RefPtr<PipeToPump>& aSelf,
+         const RefPtr<WritableStreamDefaultWriter>& aWriter,
+         JS::Handle<JS::Value> aChunk)
+          MOZ_CAN_RUN_SCRIPT_FOR_DEFINITION -> already_AddRefed<Promise> {
+            RefPtr<Promise> promise =
+                WritableStreamDefaultWriterWrite(aCx, aWriter, aChunk, aRv);
+
+            // Last read has finished, so it's time to start reading again.
+            aSelf->Read(aCx);
+
+            return promise.forget();
+          },
+      std::make_tuple(RefPtr{this}, mWriter), std::make_tuple(aChunk));
+  if (result.isErr()) {
+    mLastWritePromise = nullptr;
+    return;
+  }
+  mLastWritePromise = result.unwrap();
+
+  mLastWritePromise->AppendNativeHandler(
+      new PipeToPumpHandler(this, nullptr, &PipeToPump::OnDestErrored));
+}
+
+void PipeToPump::OnWriterReady(JSContext* aCx, JS::Handle<JS::Value>) {
+  // Writer is ready again (i.e. backpressure was resolved), so read.
+  Read(aCx);
+}
+
+struct PipeToReadRequest : public ReadRequest {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(PipeToReadRequest, ReadRequest)
+
+  RefPtr<PipeToPump> mPipeToPump;
+
+  explicit PipeToReadRequest(PipeToPump* aPipeToPump)
+      : mPipeToPump(aPipeToPump) {}
+
+  MOZ_CAN_RUN_SCRIPT void ChunkSteps(JSContext* aCx,
+                                     JS::Handle<JS::Value> aChunk,
+                                     ErrorResult& aRv) override {
+    RefPtr<PipeToPump> pipeToPump = mPipeToPump;  // XXX known live?
+    pipeToPump->OnReadFulfilled(aCx, aChunk, aRv);
+  }
+
+  // The reader's closed promise handlers will already call OnSourceClosed/
+  // OnSourceErrored, so these steps can just be ignored.
+  void CloseSteps(JSContext* aCx, ErrorResult& aRv) override {}
+  void ErrorSteps(JSContext* aCx, JS::Handle<JS::Value> aError,
+                  ErrorResult& aRv) override {}
+
+ protected:
+  virtual ~PipeToReadRequest() = default;
+};
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(PipeToReadRequest, ReadRequest, mPipeToPump)
+
+NS_IMPL_ADDREF_INHERITED(PipeToReadRequest, ReadRequest)
+NS_IMPL_RELEASE_INHERITED(PipeToReadRequest, ReadRequest)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(PipeToReadRequest)
+NS_INTERFACE_MAP_END_INHERITING(ReadRequest)
+
+void PipeToPump::Read(JSContext* aCx) {
+#ifdef DEBUG
+  mReadChunk = true;
+#endif
+
+  // (Constraint) Shutdown must stop activity:
+  // If shuttingDown becomes true, the user agent must not initiate
+  // further reads from reader
+  if (mShuttingDown) {
+    return;
+  }
+
+  // (Constraint) Backpressure must be enforced:
+  // While WritableStreamDefaultWriterGetDesiredSize(writer) is ≤ 0 or is null,
+  // the user agent must not read from reader.
+  Nullable<double> desiredSize =
+      WritableStreamDefaultWriterGetDesiredSize(mWriter);
+  if (desiredSize.IsNull()) {
+    // This means the writer has errored. This is going to be handled
+    // by the writer closed promise.
+    return;
+  }
+
+  if (desiredSize.Value() <= 0) {
+    // Wait for the writer to become ready before reading more data from
+    // the reader. We don't care about rejections here, because those are
+    // already handled by the writer closed promise.
+    RefPtr<Promise> readyPromise = mWriter->Ready();
+    readyPromise->AppendNativeHandler(
+        new PipeToPumpHandler(this, &PipeToPump::OnWriterReady, nullptr));
+    return;
+  }
+
+  RefPtr<ReadableStreamDefaultReader> reader = mReader;
+  RefPtr<ReadRequest> request = new PipeToReadRequest(this);
+  ErrorResult rv;
+  ReadableStreamDefaultReaderRead(aCx, reader, request, rv);
+  if (rv.MaybeSetPendingException(aCx)) {
+    // XXX It's actually not quite obvious what we should do here.
+    // We've got an error during reading, so on the surface it seems logical
+    // to invoke `OnSourceErrored`. However in certain cases the required
+    // condition > source.[[state]] is or becomes "errored" < won't actually
+    // happen i.e. when `WritableStreamDefaultWriterWrite` called from
+    // `OnReadFulfilled` (via PipeToReadRequest::ChunkSteps) fails in
+    // a synchronous fashion.
+    JS::Rooted<JS::Value> error(aCx);
+    JS::Rooted<Maybe<JS::Value>> someError(aCx);
+
+    // The error was moved to the JSContext by MaybeSetPendingException.
+    if (JS_GetPendingException(aCx, &error)) {
+      someError = Some(error.get());
+    }
+
+    JS_ClearPendingException(aCx);
+
+    Shutdown(aCx, someError);
+  }
+}
+
+// Step 3. Closing must be propagated forward: if source.[[state]] is or
+// becomes "closed", then
+void PipeToPump::OnSourceClosed(JSContext* aCx, JS::Handle<JS::Value>) {
+  // Step 3.1. If preventClose is false, shutdown with an action of
+  // ! WritableStreamDefaultWriterCloseWithErrorPropagation(writer).
+  if (!mPreventClose) {
+    ShutdownWithAction(
+        aCx,
+        [](JSContext* aCx, PipeToPump* aPipeToPump,
+           JS::Handle<mozilla::Maybe<JS::Value>> aError, ErrorResult& aRv)
+            MOZ_CAN_RUN_SCRIPT {
+              RefPtr<WritableStreamDefaultWriter> writer = aPipeToPump->mWriter;
+              return WritableStreamDefaultWriterCloseWithErrorPropagation(
+                  aCx, writer, aRv);
+            },
+        JS::NothingHandleValue);
+  } else {
+    // Step 3.2 Otherwise, shutdown.
+    Shutdown(aCx, JS::NothingHandleValue);
+  }
+}
+
+// Step 1. Errors must be propagated forward: if source.[[state]] is or
+// becomes "errored", then
+void PipeToPump::OnSourceErrored(JSContext* aCx,
+                                 JS::Handle<JS::Value> aSourceStoredError) {
+  // If |source| becomes errored not during a pending read, it's clear we must
+  // react immediately.
+  //
+  // But what if |source| becomes errored *during* a pending read?  Should this
+  // first error, or the pending-read second error, predominate?  Two semantics
+  // are possible when |source|/|dest| become closed or errored while there's a
+  // pending read:
+  //
+  //   1. Wait until the read fulfills or rejects, then respond to the
+  //      closure/error without regard to the read having fulfilled or rejected.
+  //      (This will simply not react to the read being rejected, or it will
+  //      queue up the read chunk to be written during shutdown.)
+  //   2. React to the closure/error immediately per "Error and close states
+  //      must be propagated".  Then when the read fulfills or rejects later, do
+  //      nothing.
+  //
+  // The spec doesn't clearly require either semantics.  It requires that
+  // *already-read* chunks be written (at least if |dest| didn't become errored
+  // or closed such that no further writes can occur).  But it's silent as to
+  // not-fully-read chunks.  (These semantic differences may only be observable
+  // with very carefully constructed readable/writable streams.)
+  //
+  // It seems best, generally, to react to the temporally-earliest problem that
+  // arises, so we implement option #2.  (Blink, in contrast, currently
+  // implements option #1.)
+  //
+  // All specified reactions to a closure/error invoke either the shutdown, or
+  // shutdown with an action, algorithms.  Those algorithms each abort if either
+  // shutdown algorithm has already been invoked.  So we don't need to do
+  // anything special here to deal with a pending read.
+  //
+  // TODO: Implement the eventual resolution from
+  // https://github.com/whatwg/streams/issues/1207
+
+  // Step 1.1 If preventAbort is false, shutdown with an action of
+  // ! WritableStreamAbort(dest, source.[[storedError]])
+  // and with source.[[storedError]].
+  JS::Rooted<Maybe<JS::Value>> error(aCx, Some(aSourceStoredError));
+  if (!mPreventAbort) {
+    ShutdownWithAction(
+        aCx,
+        [](JSContext* aCx, PipeToPump* aPipeToPump,
+           JS::Handle<mozilla::Maybe<JS::Value>> aError, ErrorResult& aRv)
+            MOZ_CAN_RUN_SCRIPT {
+              JS::Rooted<JS::Value> error(aCx, *aError);
+              RefPtr<WritableStream> dest = aPipeToPump->mWriter->GetStream();
+              return WritableStreamAbort(aCx, dest, error, aRv);
+            },
+        error);
+  } else {
+    // Step 1.1. Otherwise, shutdown with source.[[storedError]].
+    Shutdown(aCx, error);
+  }
+}
+
+// Step 4. Closing must be propagated backward:
+// if ! WritableStreamCloseQueuedOrInFlight(dest) is true
+// or dest.[[state]] is "closed", then
+void PipeToPump::OnDestClosed(JSContext* aCx, JS::Handle<JS::Value>) {
+  // Step 4.1. Assert: no chunks have been read or written.
+  // Note: No reading automatically implies no writing.
+  // In a perfect world OnDestClosed would only be called before we start
+  // piping, because afterwards the writer has an exclusive lock on the stream.
+  // In reality the closed promise can still be resolved after we release
+  // the lock on the writer in Finalize.
+  if (mShuttingDown) {
+    return;
+  }
+  MOZ_ASSERT(!mReadChunk);
+
+  // Step 4.2. Let destClosed be a new TypeError.
+  JS::Rooted<Maybe<JS::Value>> destClosed(aCx, Nothing());
+  {
+    ErrorResult rv;
+    rv.ThrowTypeError("Cannot pipe to closed stream");
+    JS::Rooted<JS::Value> error(aCx);
+    bool ok = ToJSValue(aCx, std::move(rv), &error);
+    MOZ_RELEASE_ASSERT(ok, "must be ok");
+    destClosed = Some(error.get());
+  }
+
+  // Step 4.3. If preventCancel is false, shutdown with an action of
+  // ! ReadableStreamCancel(source, destClosed) and with destClosed.
+  if (!mPreventCancel) {
+    ShutdownWithAction(
+        aCx,
+        [](JSContext* aCx, PipeToPump* aPipeToPump,
+           JS::Handle<mozilla::Maybe<JS::Value>> aError, ErrorResult& aRv)
+            MOZ_CAN_RUN_SCRIPT {
+              JS::Rooted<JS::Value> error(aCx, *aError);
+              RefPtr<ReadableStream> dest = aPipeToPump->mReader->GetStream();
+              return ReadableStreamCancel(aCx, dest, error, aRv);
+            },
+        destClosed);
+  } else {
+    // Step 4.4. Otherwise, shutdown with destClosed.
+    Shutdown(aCx, destClosed);
+  }
+}
+
+// Step 2. Errors must be propagated backward: if dest.[[state]] is or becomes
+// "errored", then
+void PipeToPump::OnDestErrored(JSContext* aCx,
+                               JS::Handle<JS::Value> aDestStoredError) {
+  // Step 2.1. If preventCancel is false, shutdown with an action of
+  // ! ReadableStreamCancel(source, dest.[[storedError]])
+  // and with dest.[[storedError]].
+  JS::Rooted<Maybe<JS::Value>> error(aCx, Some(aDestStoredError));
+  if (!mPreventCancel) {
+    ShutdownWithAction(
+        aCx,
+        [](JSContext* aCx, PipeToPump* aPipeToPump,
+           JS::Handle<mozilla::Maybe<JS::Value>> aError, ErrorResult& aRv)
+            MOZ_CAN_RUN_SCRIPT {
+              JS::Rooted<JS::Value> error(aCx, *aError);
+              RefPtr<ReadableStream> dest = aPipeToPump->mReader->GetStream();
+              return ReadableStreamCancel(aCx, dest, error, aRv);
+            },
+        error);
+  } else {
+    // Step 2.1. Otherwise, shutdown with dest.[[storedError]].
+    Shutdown(aCx, error);
+  }
+}
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(PipeToPump)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(PipeToPump)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(PipeToPump)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(PipeToPump)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(PipeToPump)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mPromise)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mReader)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mWriter)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mLastWritePromise)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(PipeToPump)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mPromise)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mReader)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mWriter)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mLastWritePromise)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#readable-stream-pipe-to
+already_AddRefed<Promise> ReadableStreamPipeTo(
+    ReadableStream* aSource, WritableStream* aDest, bool aPreventClose,
+    bool aPreventAbort, bool aPreventCancel, AbortSignal* aSignal,
+    mozilla::ErrorResult& aRv) {
+  // Step 1. Assert: source implements ReadableStream. (Implicit)
+  // Step 2. Assert: dest implements WritableStream. (Implicit)
+  // Step 3. Assert: preventClose, preventAbort, and preventCancel are all
+  //         booleans (Implicit)
+  // Step 4. If signal was not given, let signal be
+  //         undefined. (Implicit)
+  // Step 5. Assert: either signal is undefined, or signal
+  //         implements AbortSignal. (Implicit)
+  // Step 6. Assert: !IsReadableStreamLocked(source) is false.
+  MOZ_ASSERT(!IsReadableStreamLocked(aSource));
+
+  // Step 7. Assert: !IsWritableStreamLocked(dest) is false.
+  MOZ_ASSERT(!IsWritableStreamLocked(aDest));
+
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(aSource->GetParentObject())) {
+    aRv.ThrowUnknownError("Internal error");
+    return nullptr;
+  }
+  JSContext* cx = jsapi.cx();
+
+  // Step 8. If source.[[controller]] implements ReadableByteStreamController,
+  //         let reader be either !AcquireReadableStreamBYOBReader(source) or
+  //         !AcquireReadableStreamDefaultReader(source), at the user agent’s
+  //         discretion.
+  // Step 9. Otherwise, let reader be
+  //         !AcquireReadableStreamDefaultReader(source).
+
+  // Note: In the interests of simplicity, we choose here to always acquire
+  // a default reader.
+  RefPtr<ReadableStreamDefaultReader> reader =
+      AcquireReadableStreamDefaultReader(aSource, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 10. Let writer be ! AcquireWritableStreamDefaultWriter(dest).
+  RefPtr<WritableStreamDefaultWriter> writer =
+      AcquireWritableStreamDefaultWriter(aDest, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 11. Set source.[[disturbed]] to true.
+  aSource->SetDisturbed(true);
+
+  // Step 12. Let shuttingDown be false.
+  // Note: PipeToPump ensures this by construction.
+
+  // Step 13. Let promise be a new promise.
+  RefPtr<Promise> promise =
+      Promise::CreateInfallible(aSource->GetParentObject());
+
+  // Steps 14-15.
+  RefPtr<PipeToPump> pump = new PipeToPump(
+      promise, reader, writer, aPreventClose, aPreventAbort, aPreventCancel);
+  pump->Start(cx, aSignal);
+
+  // Step 16. Return promise.
+  return promise.forget();
+}
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/ReadableStreamPipeTo.h b/dom/streams/ReadableStreamPipeTo.h
new file mode 100644
index 0000000000..6c92cb8c32
--- /dev/null
+++ b/dom/streams/ReadableStreamPipeTo.h
@@ -0,0 +1,34 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ReadableStreamPipeTo_h
+#define mozilla_dom_ReadableStreamPipeTo_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/AlreadyAddRefed.h"
+
+namespace mozilla {
+
+class ErrorResult;
+
+namespace dom {
+
+class AbortSignal;
+class Promise;
+class ReadableStream;
+class WritableStream;
+
+namespace streams_abstract {
+MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> ReadableStreamPipeTo(
+    ReadableStream* aSource, WritableStream* aDest, bool aPreventClose,
+    bool aPreventAbort, bool aPreventCancel, AbortSignal* aSignal,
+    ErrorResult& aRv);
+}
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_ReadableStreamPipeTo_h
diff --git a/dom/streams/ReadableStreamTee.cpp b/dom/streams/ReadableStreamTee.cpp
new file mode 100644
index 0000000000..9bb30d8859
--- /dev/null
+++ b/dom/streams/ReadableStreamTee.cpp
@@ -0,0 +1,1011 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "ReadableStreamTee.h"
+
+#include "ReadIntoRequest.h"
+#include "TeeState.h"
+#include "js/Exception.h"
+#include "js/TypeDecls.h"
+#include "js/experimental/TypedData.h"
+#include "mozilla/dom/ByteStreamHelpers.h"
+#include "mozilla/dom/Promise-inl.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/ReadableStreamBYOBReader.h"
+#include "mozilla/dom/ReadableStreamDefaultController.h"
+#include "mozilla/dom/ReadableStreamGenericReader.h"
+#include "mozilla/dom/ReadableStreamDefaultReader.h"
+#include "mozilla/dom/ReadableByteStreamController.h"
+#include "mozilla/dom/UnderlyingSourceBinding.h"
+#include "mozilla/dom/UnderlyingSourceCallbackHelpers.h"
+#include "nsCycleCollectionParticipant.h"
+#include "mozilla/CycleCollectedJSContext.h"
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(ReadableStreamDefaultTeeSourceAlgorithms,
+                                   UnderlyingSourceAlgorithmsBase, mTeeState)
+NS_IMPL_ADDREF_INHERITED(ReadableStreamDefaultTeeSourceAlgorithms,
+                         UnderlyingSourceAlgorithmsBase)
+NS_IMPL_RELEASE_INHERITED(ReadableStreamDefaultTeeSourceAlgorithms,
+                          UnderlyingSourceAlgorithmsBase)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(
+    ReadableStreamDefaultTeeSourceAlgorithms)
+NS_INTERFACE_MAP_END_INHERITING(UnderlyingSourceAlgorithmsBase)
+
+already_AddRefed<Promise>
+ReadableStreamDefaultTeeSourceAlgorithms::PullCallback(
+    JSContext* aCx, ReadableStreamController& aController, ErrorResult& aRv) {
+  nsCOMPtr<nsIGlobalObject> global = aController.GetParentObject();
+  mTeeState->PullCallback(aCx, global, aRv);
+  if (!aRv.Failed()) {
+    return Promise::CreateResolvedWithUndefined(global, aRv);
+  }
+  return nullptr;
+}
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(ReadableStreamDefaultTeeReadRequest)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN_INHERITED(
+    ReadableStreamDefaultTeeReadRequest, ReadRequest)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mTeeState)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN_INHERITED(
+    ReadableStreamDefaultTeeReadRequest, ReadRequest)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mTeeState)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_ADDREF_INHERITED(ReadableStreamDefaultTeeReadRequest, ReadRequest)
+NS_IMPL_RELEASE_INHERITED(ReadableStreamDefaultTeeReadRequest, ReadRequest)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ReadableStreamDefaultTeeReadRequest)
+NS_INTERFACE_MAP_END_INHERITING(ReadRequest)
+
+void ReadableStreamDefaultTeeReadRequest::ChunkSteps(
+    JSContext* aCx, JS::Handle<JS::Value> aChunk, ErrorResult& aRv) {
+  // Step 1.
+  class ReadableStreamDefaultTeeReadRequestChunkSteps
+      : public MicroTaskRunnable {
+    // Virtually const, but is cycle collected
+    MOZ_KNOWN_LIVE RefPtr<TeeState> mTeeState;
+    JS::PersistentRooted<JS::Value> mChunk;
+
+   public:
+    ReadableStreamDefaultTeeReadRequestChunkSteps(JSContext* aCx,
+                                                  TeeState* aTeeState,
+                                                  JS::Handle<JS::Value> aChunk)
+        : mTeeState(aTeeState), mChunk(aCx, aChunk) {}
+
+    MOZ_CAN_RUN_SCRIPT
+    void Run(AutoSlowOperation& aAso) override {
+      AutoJSAPI jsapi;
+      if (NS_WARN_IF(!jsapi.Init(mTeeState->GetStream()->GetParentObject()))) {
+        return;
+      }
+      JSContext* cx = jsapi.cx();
+      // Step Numbering below is relative to Chunk steps Microtask:
+      //
+      // Step 1.
+      mTeeState->SetReadAgain(false);
+
+      // Step 2.
+      JS::Rooted<JS::Value> chunk1(cx, mChunk);
+      JS::Rooted<JS::Value> chunk2(cx, mChunk);
+
+      // Step 3. Skipped until we implement cloneForBranch2 path.
+      MOZ_RELEASE_ASSERT(!mTeeState->CloneForBranch2());
+
+      // Step 4.
+      if (!mTeeState->Canceled1()) {
+        IgnoredErrorResult rv;
+        // Since we controlled the creation of the two stream branches, we know
+        // they both have default controllers.
+        RefPtr<ReadableStreamDefaultController> controller(
+            mTeeState->Branch1()->DefaultController());
+        ReadableStreamDefaultControllerEnqueue(cx, controller, chunk1, rv);
+        (void)NS_WARN_IF(rv.Failed());
+      }
+
+      // Step 5.
+      if (!mTeeState->Canceled2()) {
+        IgnoredErrorResult rv;
+        RefPtr<ReadableStreamDefaultController> controller(
+            mTeeState->Branch2()->DefaultController());
+        ReadableStreamDefaultControllerEnqueue(cx, controller, chunk2, rv);
+        (void)NS_WARN_IF(rv.Failed());
+      }
+
+      // Step 6.
+      mTeeState->SetReading(false);
+
+      // Step 7. If |readAgain| is true, perform |pullAlgorithm|.
+      if (mTeeState->ReadAgain()) {
+        IgnoredErrorResult rv;
+        nsCOMPtr<nsIGlobalObject> global(
+            mTeeState->GetStream()->GetParentObject());
+        mTeeState->PullCallback(cx, global, rv);
+        (void)NS_WARN_IF(rv.Failed());
+      }
+    }
+
+    bool Suppressed() override {
+      nsIGlobalObject* global = mTeeState->GetStream()->GetParentObject();
+      return global && global->IsInSyncOperation();
+    }
+  };
+
+  RefPtr<ReadableStreamDefaultTeeReadRequestChunkSteps> task =
+      MakeRefPtr<ReadableStreamDefaultTeeReadRequestChunkSteps>(aCx, mTeeState,
+                                                                aChunk);
+  CycleCollectedJSContext::Get()->DispatchToMicroTask(task.forget());
+}
+
+void ReadableStreamDefaultTeeReadRequest::CloseSteps(JSContext* aCx,
+                                                     ErrorResult& aRv) {
+  // Step Numbering below is relative to 'close steps' of
+  // https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaulttee
+  //
+  // Step 1.
+  mTeeState->SetReading(false);
+
+  // Step 2.
+  if (!mTeeState->Canceled1()) {
+    RefPtr<ReadableStreamDefaultController> controller(
+        mTeeState->Branch1()->DefaultController());
+    ReadableStreamDefaultControllerClose(aCx, controller, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+  }
+
+  // Step 3.
+  if (!mTeeState->Canceled2()) {
+    RefPtr<ReadableStreamDefaultController> controller(
+        mTeeState->Branch2()->DefaultController());
+    ReadableStreamDefaultControllerClose(aCx, controller, aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+  }
+
+  // Step 4.
+  if (!mTeeState->Canceled1() || !mTeeState->Canceled2()) {
+    mTeeState->CancelPromise()->MaybeResolveWithUndefined();
+  }
+}
+
+void ReadableStreamDefaultTeeReadRequest::ErrorSteps(
+    JSContext* aCx, JS::Handle<JS::Value> aError, ErrorResult& aRv) {
+  mTeeState->SetReading(false);
+}
+
+MOZ_CAN_RUN_SCRIPT void PullWithDefaultReader(JSContext* aCx,
+                                              TeeState* aTeeState,
+                                              ErrorResult& aRv);
+MOZ_CAN_RUN_SCRIPT void PullWithBYOBReader(JSContext* aCx, TeeState* aTeeState,
+                                           JS::Handle<JSObject*> aView,
+                                           TeeBranch aForBranch,
+                                           ErrorResult& aRv);
+
+// Algorithm described in
+// https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamtee, Steps
+// 17 and Steps 18, genericized across branch numbers:
+//
+// Note: As specified this algorithm always returns a promise resolved with
+// undefined, however as some places immediately discard said promise, we
+// provide this version which doesn't return a promise.
+//
+// NativeByteStreamTeePullAlgorithm, which implements
+// UnderlyingSourcePullCallbackHelper is the version which provies the return
+// promise.
+MOZ_CAN_RUN_SCRIPT void ByteStreamTeePullAlgorithm(JSContext* aCx,
+                                                   TeeBranch aForBranch,
+                                                   TeeState* aTeeState,
+                                                   ErrorResult& aRv) {
+  // Step {17,18}.1: If reading is true,
+  if (aTeeState->Reading()) {
+    // Step {17,18}.1.1: Set readAgainForBranch{1,2} to true.
+    aTeeState->SetReadAgainForBranch(aForBranch, true);
+
+    // Step {17,18}.1.1: Return a promise resolved with undefined.
+    return;
+  }
+
+  // Step {17,18}.2: Set reading to true.
+  aTeeState->SetReading(true);
+
+  // Step {17,18}.3: Let byobRequest be
+  // !ReadableByteStreamControllerGetBYOBRequest(branch{1,2}.[[controller]]).
+  RefPtr<ReadableStreamBYOBRequest> byobRequest =
+      ReadableByteStreamControllerGetBYOBRequest(
+          aCx, aTeeState->Branch(aForBranch)->Controller()->AsByte(), aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step {17,18}.4: If byobRequest is null, perform pullWithDefaultReader.
+  if (!byobRequest) {
+    PullWithDefaultReader(aCx, aTeeState, aRv);
+  } else {
+    // Step {17,18}.5: Otherwise, perform pullWithBYOBReader, given
+    // byobRequest.[[view]] and {false, true}.
+    JS::Rooted<JSObject*> view(aCx, byobRequest->View());
+    PullWithBYOBReader(aCx, aTeeState, view, aForBranch, aRv);
+  }
+
+  // Step {17,18}.6: Return a promise resolved with undefined.
+}
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamtee
+class ByteStreamTeeSourceAlgorithms final
+    : public UnderlyingSourceAlgorithmsBase {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ByteStreamTeeSourceAlgorithms,
+                                           UnderlyingSourceAlgorithmsBase)
+
+  ByteStreamTeeSourceAlgorithms(TeeState* aTeeState, TeeBranch aBranch)
+      : mTeeState(aTeeState), mBranch(aBranch) {}
+
+  MOZ_CAN_RUN_SCRIPT void StartCallback(JSContext* aCx,
+                                        ReadableStreamController& aController,
+                                        JS::MutableHandle<JS::Value> aRetVal,
+                                        ErrorResult& aRv) override {
+    // Step 21: Let startAlgorithm be an algorithm that returns undefined.
+    aRetVal.setUndefined();
+  }
+
+  // Step 17, 18
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> PullCallback(
+      JSContext* aCx, ReadableStreamController& aController,
+      ErrorResult& aRv) override {
+    // Step 1 - 5
+    ByteStreamTeePullAlgorithm(aCx, mBranch, MOZ_KnownLive(mTeeState), aRv);
+
+    // Step 6: Return a promise resolved with undefined.
+    return Promise::CreateResolvedWithUndefined(
+        mTeeState->GetStream()->GetParentObject(), aRv);
+  }
+
+  // Step 19, 20
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> CancelCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) override {
+    // Step 1.
+    mTeeState->SetCanceled(mBranch, true);
+
+    // Step 2.
+    mTeeState->SetReason(mBranch, aReason.Value());
+
+    // Step 3.
+    if (mTeeState->Canceled(otherStream())) {
+      // Step 3.1
+      JS::Rooted<JSObject*> compositeReason(aCx, JS::NewArrayObject(aCx, 2));
+      if (!compositeReason) {
+        aRv.StealExceptionFromJSContext(aCx);
+        return nullptr;
+      }
+
+      JS::Rooted<JS::Value> reason1(aCx, mTeeState->Reason1());
+      if (!JS_SetElement(aCx, compositeReason, 0, reason1)) {
+        aRv.StealExceptionFromJSContext(aCx);
+        return nullptr;
+      }
+
+      JS::Rooted<JS::Value> reason2(aCx, mTeeState->Reason2());
+      if (!JS_SetElement(aCx, compositeReason, 1, reason2)) {
+        aRv.StealExceptionFromJSContext(aCx);
+        return nullptr;
+      }
+
+      // Step 3.2
+      JS::Rooted<JS::Value> compositeReasonValue(
+          aCx, JS::ObjectValue(*compositeReason));
+      RefPtr<ReadableStream> stream(mTeeState->GetStream());
+      RefPtr<Promise> cancelResult =
+          ReadableStreamCancel(aCx, stream, compositeReasonValue, aRv);
+      if (aRv.Failed()) {
+        return nullptr;
+      }
+
+      // Step 3.3
+      mTeeState->CancelPromise()->MaybeResolve(cancelResult);
+    }
+
+    // Step 4.
+    return do_AddRef(mTeeState->CancelPromise());
+  };
+
+ protected:
+  ~ByteStreamTeeSourceAlgorithms() override = default;
+
+ private:
+  TeeBranch otherStream() { return OtherTeeBranch(mBranch); }
+
+  RefPtr<TeeState> mTeeState;
+  TeeBranch mBranch;
+};
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(ByteStreamTeeSourceAlgorithms,
+                                   UnderlyingSourceAlgorithmsBase, mTeeState)
+NS_IMPL_ADDREF_INHERITED(ByteStreamTeeSourceAlgorithms,
+                         UnderlyingSourceAlgorithmsBase)
+NS_IMPL_RELEASE_INHERITED(ByteStreamTeeSourceAlgorithms,
+                          UnderlyingSourceAlgorithmsBase)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ByteStreamTeeSourceAlgorithms)
+NS_INTERFACE_MAP_END_INHERITING(UnderlyingSourceAlgorithmsBase)
+
+struct PullWithDefaultReaderReadRequest final : public ReadRequest {
+  RefPtr<TeeState> mTeeState;
+
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(PullWithDefaultReaderReadRequest,
+                                           ReadRequest)
+
+  explicit PullWithDefaultReaderReadRequest(TeeState* aTeeState)
+      : mTeeState(aTeeState) {}
+
+  void ChunkSteps(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                  ErrorResult& aRv) override {
+    // https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamtee
+    // Step 15.2.1
+    class PullWithDefaultReaderChunkStepMicrotask : public MicroTaskRunnable {
+      RefPtr<TeeState> mTeeState;
+      JS::PersistentRooted<JSObject*> mChunk;
+
+     public:
+      PullWithDefaultReaderChunkStepMicrotask(JSContext* aCx,
+                                              TeeState* aTeeState,
+                                              JS::Handle<JSObject*> aChunk)
+          : mTeeState(aTeeState), mChunk(aCx, aChunk) {}
+
+      MOZ_CAN_RUN_SCRIPT
+      void Run(AutoSlowOperation& aAso) override {
+        // Step Numbering in this function is relative to the Queue a microtask
+        // of the Chunk steps of 15.2.1 of
+        // https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamtee
+        AutoJSAPI jsapi;
+        if (NS_WARN_IF(
+                !jsapi.Init(mTeeState->GetStream()->GetParentObject()))) {
+          return;
+        }
+        JSContext* cx = jsapi.cx();
+
+        // Step 1. Set readAgainForBranch1 to false.
+        mTeeState->SetReadAgainForBranch1(false);
+
+        // Step 2. Set readAgainForBranch2 to false.
+        mTeeState->SetReadAgainForBranch2(false);
+
+        // Step 3. Let chunk1 and chunk2 be chunk.
+        JS::Rooted<JSObject*> chunk1(cx, mChunk);
+        JS::Rooted<JSObject*> chunk2(cx, mChunk);
+
+        // Step 4. If canceled1 is false and canceled2 is false,
+        ErrorResult rv;
+        if (!mTeeState->Canceled1() && !mTeeState->Canceled2()) {
+          // Step 4.1. Let cloneResult be CloneAsUint8Array(chunk).
+          JS::Rooted<JSObject*> cloneResult(cx, CloneAsUint8Array(cx, mChunk));
+
+          // Step 4.2. If cloneResult is an abrupt completion,
+          if (!cloneResult) {
+            // Step 4.2.1 Perform
+            // !ReadableByteStreamControllerError(branch1.[[controller]],
+            // cloneResult.[[Value]]).
+            JS::Rooted<JS::Value> exceptionValue(cx);
+            if (!JS_GetPendingException(cx, &exceptionValue)) {
+              // Uncatchable exception, simply return.
+              return;
+            }
+            JS_ClearPendingException(cx);
+
+            ErrorResult rv;
+            ReadableByteStreamControllerError(
+                mTeeState->Branch1()->Controller()->AsByte(), exceptionValue,
+                rv);
+            if (rv.MaybeSetPendingException(
+                    cx, "Error during ReadableByteStreamControllerError")) {
+              return;
+            }
+
+            // Step 4.2.2. Perform !
+            // ReadableByteStreamControllerError(branch2.[[controller]],
+            // cloneResult.[[Value]]).
+            ReadableByteStreamControllerError(
+                mTeeState->Branch2()->Controller()->AsByte(), exceptionValue,
+                rv);
+            if (rv.MaybeSetPendingException(
+                    cx, "Error during ReadableByteStreamControllerError")) {
+              return;
+            }
+
+            // Step 4.2.3. Resolve cancelPromise with !
+            // ReadableStreamCancel(stream, cloneResult.[[Value]]).
+            RefPtr<ReadableStream> stream(mTeeState->GetStream());
+            RefPtr<Promise> promise =
+                ReadableStreamCancel(cx, stream, exceptionValue, rv);
+            if (rv.MaybeSetPendingException(
+                    cx, "Error during ReadableByteStreamControllerError")) {
+              return;
+            }
+            mTeeState->CancelPromise()->MaybeResolve(promise);
+
+            // Step 4.2.4. Return.
+            return;
+          }
+
+          // Step 4.3. Otherwise, set chunk2 to cloneResult.[[Value]].
+          chunk2 = cloneResult;
+        }
+
+        // Step 5. If canceled1 is false,
+        // perform ! ReadableByteStreamControllerEnqueue(branch1.[[controller]],
+        // chunk1).
+        if (!mTeeState->Canceled1()) {
+          ErrorResult rv;
+          RefPtr<ReadableByteStreamController> controller(
+              mTeeState->Branch1()->Controller()->AsByte());
+          ReadableByteStreamControllerEnqueue(cx, controller, chunk1, rv);
+          if (rv.MaybeSetPendingException(
+                  cx, "Error during ReadableByteStreamControllerEnqueue")) {
+            return;
+          }
+        }
+
+        // Step 6. If canceled2 is false,
+        // perform ! ReadableByteStreamControllerEnqueue(branch2.[[controller]],
+        // chunk2).
+        if (!mTeeState->Canceled2()) {
+          ErrorResult rv;
+          RefPtr<ReadableByteStreamController> controller(
+              mTeeState->Branch2()->Controller()->AsByte());
+          ReadableByteStreamControllerEnqueue(cx, controller, chunk2, rv);
+          if (rv.MaybeSetPendingException(
+                  cx, "Error during ReadableByteStreamControllerEnqueue")) {
+            return;
+          }
+        }
+
+        // Step 7. Set reading to false.
+        mTeeState->SetReading(false);
+
+        // Step 8. If readAgainForBranch1 is true, perform pull1Algorithm.
+        if (mTeeState->ReadAgainForBranch1()) {
+          ByteStreamTeePullAlgorithm(cx, TeeBranch::Branch1,
+                                     MOZ_KnownLive(mTeeState), rv);
+        } else if (mTeeState->ReadAgainForBranch2()) {
+          // Step 9. Otherwise, if readAgainForBranch2 is true, perform
+          // pull2Algorithm.
+          ByteStreamTeePullAlgorithm(cx, TeeBranch::Branch2,
+                                     MOZ_KnownLive(mTeeState), rv);
+        }
+      }
+
+      bool Suppressed() override {
+        nsIGlobalObject* global = mTeeState->GetStream()->GetParentObject();
+        return global && global->IsInSyncOperation();
+      }
+    };
+
+    MOZ_ASSERT(aChunk.isObjectOrNull());
+    MOZ_ASSERT(aChunk.toObjectOrNull() != nullptr);
+    JS::Rooted<JSObject*> chunk(aCx, &aChunk.toObject());
+    RefPtr<PullWithDefaultReaderChunkStepMicrotask> task =
+        MakeRefPtr<PullWithDefaultReaderChunkStepMicrotask>(aCx, mTeeState,
+                                                            chunk);
+    CycleCollectedJSContext::Get()->DispatchToMicroTask(task.forget());
+  }
+
+  MOZ_CAN_RUN_SCRIPT void CloseSteps(JSContext* aCx,
+                                     ErrorResult& aRv) override {
+    // Step numbering below is relative to Step 15.2. 'close steps' of
+    // https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamtee
+
+    // Step 1. Set reading to false.
+    mTeeState->SetReading(false);
+
+    // Step 2. If canceled1 is false, perform !
+    // ReadableByteStreamControllerClose(branch1.[[controller]]).
+    RefPtr<ReadableByteStreamController> branch1Controller =
+        mTeeState->Branch1()->Controller()->AsByte();
+    if (!mTeeState->Canceled1()) {
+      ReadableByteStreamControllerClose(aCx, branch1Controller, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+    }
+
+    // Step 3. If canceled2 is false, perform !
+    // ReadableByteStreamControllerClose(branch2.[[controller]]).
+    RefPtr<ReadableByteStreamController> branch2Controller =
+        mTeeState->Branch2()->Controller()->AsByte();
+    if (!mTeeState->Canceled2()) {
+      ReadableByteStreamControllerClose(aCx, branch2Controller, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+    }
+
+    // Step 4. If branch1.[[controller]].[[pendingPullIntos]] is not empty,
+    // perform ! ReadableByteStreamControllerRespond(branch1.[[controller]], 0).
+    if (!branch1Controller->PendingPullIntos().isEmpty()) {
+      ReadableByteStreamControllerRespond(aCx, branch1Controller, 0, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+    }
+
+    // Step 5. If branch2.[[controller]].[[pendingPullIntos]] is not empty,
+    // perform ! ReadableByteStreamControllerRespond(branch2.[[controller]], 0).
+    if (!branch2Controller->PendingPullIntos().isEmpty()) {
+      ReadableByteStreamControllerRespond(aCx, branch2Controller, 0, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+    }
+
+    // Step 6. If canceled1 is false or canceled2 is false, resolve
+    // cancelPromise with undefined.
+    if (!mTeeState->Canceled1() || !mTeeState->Canceled2()) {
+      mTeeState->CancelPromise()->MaybeResolveWithUndefined();
+    }
+  }
+
+  void ErrorSteps(JSContext* aCx, JS::Handle<JS::Value> aError,
+                  ErrorResult& aRv) override {
+    mTeeState->SetReading(false);
+  }
+
+ protected:
+  ~PullWithDefaultReaderReadRequest() override = default;
+};
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(PullWithDefaultReaderReadRequest,
+                                   ReadRequest, mTeeState)
+NS_IMPL_ADDREF_INHERITED(PullWithDefaultReaderReadRequest, ReadRequest)
+NS_IMPL_RELEASE_INHERITED(PullWithDefaultReaderReadRequest, ReadRequest)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(PullWithDefaultReaderReadRequest)
+NS_INTERFACE_MAP_END_INHERITING(ReadRequest)
+
+void ForwardReaderError(TeeState* aTeeState,
+                        ReadableStreamGenericReader* aThisReader);
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamtee:
+// Step 15.
+void PullWithDefaultReader(JSContext* aCx, TeeState* aTeeState,
+                           ErrorResult& aRv) {
+  RefPtr<ReadableStreamGenericReader> reader = aTeeState->GetReader();
+
+  // Step 15.1. If reader implements ReadableStreamBYOBReader,
+  if (reader->IsBYOB()) {
+    // Step 15.1.1. Assert: reader.[[readIntoRequests]] is empty.
+    MOZ_ASSERT(reader->AsBYOB()->ReadIntoRequests().length() == 0);
+
+    // Step 15.1.2. Perform ! ReadableStreamBYOBReaderRelease(reader).
+    ReadableStreamBYOBReaderRelease(aCx, reader->AsBYOB(), aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+
+    // Step 15.1.3. Set reader to ! AcquireReadableStreamDefaultReader(stream).
+    reader = AcquireReadableStreamDefaultReader(aTeeState->GetStream(), aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+    aTeeState->SetReader(reader);
+
+    // Step 16.1.4. Perform forwardReaderError, given reader.
+    ForwardReaderError(aTeeState, reader);
+  }
+
+  // Step 15.2
+  RefPtr<ReadRequest> readRequest =
+      new PullWithDefaultReaderReadRequest(aTeeState);
+
+  // Step 15.3
+  ReadableStreamDefaultReaderRead(aCx, reader, readRequest, aRv);
+}
+
+class PullWithBYOBReader_ReadIntoRequest final : public ReadIntoRequest {
+  RefPtr<TeeState> mTeeState;
+  const TeeBranch mForBranch;
+  ~PullWithBYOBReader_ReadIntoRequest() override = default;
+
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(PullWithBYOBReader_ReadIntoRequest,
+                                           ReadIntoRequest)
+
+  explicit PullWithBYOBReader_ReadIntoRequest(TeeState* aTeeState,
+                                              TeeBranch aForBranch)
+      : mTeeState(aTeeState), mForBranch(aForBranch) {}
+
+  void ChunkSteps(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                  ErrorResult& aRv) override {
+    // https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamtee
+    // Step 16.4 chunk steps, Step 1.
+    class PullWithBYOBReaderChunkMicrotask : public MicroTaskRunnable {
+      RefPtr<TeeState> mTeeState;
+      JS::PersistentRooted<JSObject*> mChunk;
+      const TeeBranch mForBranch;
+
+     public:
+      PullWithBYOBReaderChunkMicrotask(JSContext* aCx, TeeState* aTeeState,
+                                       JS::Handle<JSObject*> aChunk,
+                                       TeeBranch aForBranch)
+          : mTeeState(aTeeState), mChunk(aCx, aChunk), mForBranch(aForBranch) {}
+
+      MOZ_CAN_RUN_SCRIPT
+      void Run(AutoSlowOperation& aAso) override {
+        AutoJSAPI jsapi;
+        if (NS_WARN_IF(
+                !jsapi.Init(mTeeState->GetStream()->GetParentObject()))) {
+          return;
+        }
+        JSContext* cx = jsapi.cx();
+        ErrorResult rv;
+        // https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamtee
+        //
+        // Step Numbering below is relative to Chunk steps Microtask at
+        // Step 16.4 chunk steps, Step 1.
+
+        // Step 1.
+        mTeeState->SetReadAgainForBranch1(false);
+
+        // Step 2.
+        mTeeState->SetReadAgainForBranch2(false);
+
+        // Step 3.
+        bool byobCanceled = mTeeState->Canceled(mForBranch);
+        // Step 4.
+        bool otherCanceled = mTeeState->Canceled(OtherTeeBranch(mForBranch));
+
+        // Rather than store byobBranch / otherBranch, we re-derive the pointers
+        // below, as borrowed from steps 16.2/16.3
+        ReadableStream* byobBranch = mTeeState->Branch(mForBranch);
+        ReadableStream* otherBranch =
+            mTeeState->Branch(OtherTeeBranch(mForBranch));
+
+        // Step 5.
+        if (!otherCanceled) {
+          // Step 5.1 (using the name clonedChunk because we don't want to name
+          // the completion record explicitly)
+          JS::Rooted<JSObject*> clonedChunk(cx, CloneAsUint8Array(cx, mChunk));
+
+          // Step 5.2. If cloneResult is an abrupt completion,
+          if (!clonedChunk) {
+            JS::Rooted<JS::Value> exception(cx);
+            if (!JS_GetPendingException(cx, &exception)) {
+              // Uncatchable exception. Return with pending
+              // exception still on context.
+              return;
+            }
+
+            // It's not expliclitly stated, but I assume the intention here is
+            // that we perform a normal completion here, so we clear the
+            // exception.
+            JS_ClearPendingException(cx);
+
+            // Step 5.2.1
+
+            ReadableByteStreamControllerError(
+                byobBranch->Controller()->AsByte(), exception, rv);
+            if (rv.MaybeSetPendingException(cx)) {
+              return;
+            }
+
+            // Step 5.2.2.
+            ReadableByteStreamControllerError(
+                otherBranch->Controller()->AsByte(), exception, rv);
+            if (rv.MaybeSetPendingException(cx)) {
+              return;
+            }
+
+            // Step 5.2.3.
+            RefPtr<ReadableStream> stream = mTeeState->GetStream();
+            RefPtr<Promise> cancelPromise =
+                ReadableStreamCancel(cx, stream, exception, rv);
+            if (rv.MaybeSetPendingException(cx)) {
+              return;
+            }
+            mTeeState->CancelPromise()->MaybeResolve(cancelPromise);
+
+            // Step 5.2.4.
+            return;
+          }
+
+          // Step 5.3 (implicitly handled above by name selection)
+          // Step 5.4.
+          if (!byobCanceled) {
+            RefPtr<ReadableByteStreamController> controller(
+                byobBranch->Controller()->AsByte());
+            ReadableByteStreamControllerRespondWithNewView(cx, controller,
+                                                           mChunk, rv);
+            if (rv.MaybeSetPendingException(cx)) {
+              return;
+            }
+          }
+
+          // Step 5.4.
+          RefPtr<ReadableByteStreamController> otherController =
+              otherBranch->Controller()->AsByte();
+          ReadableByteStreamControllerEnqueue(cx, otherController, clonedChunk,
+                                              rv);
+          if (rv.MaybeSetPendingException(cx)) {
+            return;
+          }
+          // Step 6.
+        } else if (!byobCanceled) {
+          RefPtr<ReadableByteStreamController> byobController =
+              byobBranch->Controller()->AsByte();
+          ReadableByteStreamControllerRespondWithNewView(cx, byobController,
+                                                         mChunk, rv);
+          if (rv.MaybeSetPendingException(cx)) {
+            return;
+          }
+        }
+
+        // Step 7.
+        mTeeState->SetReading(false);
+
+        // Step 8.
+        if (mTeeState->ReadAgainForBranch1()) {
+          ByteStreamTeePullAlgorithm(cx, TeeBranch::Branch1,
+                                     MOZ_KnownLive(mTeeState), rv);
+          if (rv.MaybeSetPendingException(cx)) {
+            return;
+          }
+        } else if (mTeeState->ReadAgainForBranch2()) {
+          ByteStreamTeePullAlgorithm(cx, TeeBranch::Branch2,
+                                     MOZ_KnownLive(mTeeState), rv);
+          if (rv.MaybeSetPendingException(cx)) {
+            return;
+          }
+        }
+      }
+
+      bool Suppressed() override {
+        nsIGlobalObject* global = mTeeState->GetStream()->GetParentObject();
+        return global && global->IsInSyncOperation();
+      }
+    };
+
+    MOZ_ASSERT(aChunk.isObjectOrNull());
+    MOZ_ASSERT(aChunk.toObjectOrNull());
+    JS::Rooted<JSObject*> chunk(aCx, aChunk.toObjectOrNull());
+    RefPtr<PullWithBYOBReaderChunkMicrotask> task =
+        MakeRefPtr<PullWithBYOBReaderChunkMicrotask>(aCx, mTeeState, chunk,
+                                                     mForBranch);
+    CycleCollectedJSContext::Get()->DispatchToMicroTask(task.forget());
+  }
+
+  MOZ_CAN_RUN_SCRIPT
+  void CloseSteps(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                  ErrorResult& aRv) override {
+    // Step 1.
+    mTeeState->SetReading(false);
+
+    // Step 2.
+    bool byobCanceled = mTeeState->Canceled(mForBranch);
+
+    // Step 3.
+    bool otherCanceled = mTeeState->Canceled(OtherTeeBranch(mForBranch));
+
+    // Rather than store byobBranch / otherBranch, we re-derive the pointers
+    // below, as borrowed from steps 16.2/16.3
+    ReadableStream* byobBranch = mTeeState->Branch(mForBranch);
+    ReadableStream* otherBranch = mTeeState->Branch(OtherTeeBranch(mForBranch));
+
+    // Step 4.
+    if (!byobCanceled) {
+      RefPtr<ReadableByteStreamController> controller =
+          byobBranch->Controller()->AsByte();
+      ReadableByteStreamControllerClose(aCx, controller, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+    }
+    // Step 5.
+    if (!otherCanceled) {
+      RefPtr<ReadableByteStreamController> controller =
+          otherBranch->Controller()->AsByte();
+      ReadableByteStreamControllerClose(aCx, controller, aRv);
+      if (aRv.Failed()) {
+        return;
+      }
+    }
+
+    // Step 6.
+    if (!aChunk.isUndefined()) {
+      MOZ_ASSERT(aChunk.isObject());
+      MOZ_ASSERT(aChunk.toObjectOrNull());
+
+      JS::Rooted<JSObject*> chunkObject(aCx, &aChunk.toObject());
+      MOZ_ASSERT(JS_IsArrayBufferViewObject(chunkObject));
+      // Step 6.1.
+      MOZ_ASSERT(JS_GetArrayBufferViewByteLength(chunkObject) == 0);
+
+      // Step 6.2.
+      if (!byobCanceled) {
+        RefPtr<ReadableByteStreamController> byobController(
+            byobBranch->Controller()->AsByte());
+        ReadableByteStreamControllerRespondWithNewView(aCx, byobController,
+                                                       chunkObject, aRv);
+        if (aRv.Failed()) {
+          return;
+        }
+      }
+
+      // Step 6.3
+      if (!otherCanceled &&
+          !otherBranch->Controller()->AsByte()->PendingPullIntos().isEmpty()) {
+        RefPtr<ReadableByteStreamController> otherController(
+            otherBranch->Controller()->AsByte());
+        ReadableByteStreamControllerRespond(aCx, otherController, 0, aRv);
+        if (aRv.Failed()) {
+          return;
+        }
+      }
+    }
+
+    // Step 7.
+    if (!byobCanceled || !otherCanceled) {
+      mTeeState->CancelPromise()->MaybeResolveWithUndefined();
+    }
+  }
+
+  void ErrorSteps(JSContext* aCx, JS::Handle<JS::Value> e,
+                  ErrorResult& aRv) override {
+    // Step 1.
+    mTeeState->SetReading(false);
+  }
+};
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(PullWithBYOBReader_ReadIntoRequest,
+                                   ReadIntoRequest, mTeeState)
+NS_IMPL_ADDREF_INHERITED(PullWithBYOBReader_ReadIntoRequest, ReadIntoRequest)
+NS_IMPL_RELEASE_INHERITED(PullWithBYOBReader_ReadIntoRequest, ReadIntoRequest)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(PullWithBYOBReader_ReadIntoRequest)
+NS_INTERFACE_MAP_END_INHERITING(ReadIntoRequest)
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamtee
+// Step 16.
+void PullWithBYOBReader(JSContext* aCx, TeeState* aTeeState,
+                        JS::Handle<JSObject*> aView, TeeBranch aForBranch,
+                        ErrorResult& aRv) {
+  // Step 16.1
+  if (aTeeState->GetReader()->IsDefault()) {
+    // Step 16.1.1
+    MOZ_ASSERT(aTeeState->GetDefaultReader()->ReadRequests().isEmpty());
+
+    // Step 16.1.2. Perform ! ReadableStreamDefaultReaderRelease(reader).
+    ReadableStreamDefaultReaderRelease(aCx, aTeeState->GetDefaultReader(), aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+
+    // Step 16.1.3. Set reader to !AcquireReadableStreamBYOBReader(stream).
+    RefPtr<ReadableStreamBYOBReader> reader =
+        AcquireReadableStreamBYOBReader(aTeeState->GetStream(), aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+    aTeeState->SetReader(reader);
+
+    // Step 16.1.4. Perform forwardReaderError, given reader.
+    ForwardReaderError(aTeeState, reader);
+  }
+
+  // Step 16.2. Unused in this function, moved to consumers.
+  // Step 16.3. Unused in this function, moved to consumers.
+
+  // Step 16.4.
+  RefPtr<ReadIntoRequest> readIntoRequest =
+      new PullWithBYOBReader_ReadIntoRequest(aTeeState, aForBranch);
+
+  // Step 16.5.
+  RefPtr<ReadableStreamBYOBReader> byobReader =
+      aTeeState->GetReader()->AsBYOB();
+  ReadableStreamBYOBReaderRead(aCx, byobReader, aView, readIntoRequest, aRv);
+}
+
+// See https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamtee
+// Step 14.
+void ForwardReaderError(TeeState* aTeeState,
+                        ReadableStreamGenericReader* aThisReader) {
+  aThisReader->ClosedPromise()->AddCallbacksWithCycleCollectedArgs(
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         TeeState* aTeeState, ReadableStreamGenericReader* aThisReader) {},
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         TeeState* aTeeState, ReadableStreamGenericReader* aReader) {
+        // Step 14.1.1
+        if (aTeeState->GetReader() != aReader) {
+          return;
+        }
+
+        ErrorResult rv;
+        // Step 14.1.2: Perform
+        // !ReadableByteStreamControllerError(branch1.[[controller]], r).
+        MOZ_ASSERT(aTeeState->Branch1()->Controller()->IsByte());
+        ReadableByteStreamControllerError(
+            aTeeState->Branch1()->Controller()->AsByte(), aValue, aRv);
+        if (aRv.Failed()) {
+          return;
+        }
+
+        // Step 14.1.3: Perform
+        // !ReadableByteStreamControllerError(branch2.[[controller]], r).
+        MOZ_ASSERT(aTeeState->Branch2()->Controller()->IsByte());
+        ReadableByteStreamControllerError(
+            aTeeState->Branch2()->Controller()->AsByte(), aValue, aRv);
+        if (aRv.Failed()) {
+          return;
+        }
+
+        // Step 14.1.4: If canceled1 is false or canceled2 is false, resolve
+        // cancelPromise with undefined.
+        if (!aTeeState->Canceled1() || !aTeeState->Canceled2()) {
+          aTeeState->CancelPromise()->MaybeResolveWithUndefined();
+        }
+      },
+      RefPtr(aTeeState), RefPtr(aThisReader));
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamtee
+void ReadableByteStreamTee(JSContext* aCx, ReadableStream* aStream,
+                           nsTArray<RefPtr<ReadableStream>>& aResult,
+                           ErrorResult& aRv) {
+  // Step 1. Implicit
+  // Step 2.
+  MOZ_ASSERT(aStream->Controller()->IsByte());
+
+  // Step 3-13 captured as part of TeeState allocation
+  RefPtr<TeeState> teeState = TeeState::Create(aStream, false, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 14: See ForwardReaderError
+  // Step 15. See PullWithDefaultReader
+  // Step 16. See PullWithBYOBReader
+  // Step 17,18. See {Native,}ByteStreamTeePullAlgorithm
+  // Step 19,20. See ReadableByteStreamTeeCancelAlgorithm
+  // Step 21. Elided because consumers know how to handle nullptr correctly.
+  // Step 22.
+  nsCOMPtr<nsIGlobalObject> global = aStream->GetParentObject();
+  auto branch1Algorithms =
+      MakeRefPtr<ByteStreamTeeSourceAlgorithms>(teeState, TeeBranch::Branch1);
+  teeState->SetBranch1(
+      ReadableStream::CreateByteAbstract(aCx, global, branch1Algorithms, aRv));
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 23.
+  auto branch2Algorithms =
+      MakeRefPtr<ByteStreamTeeSourceAlgorithms>(teeState, TeeBranch::Branch2);
+  teeState->SetBranch2(
+      ReadableStream::CreateByteAbstract(aCx, global, branch2Algorithms, aRv));
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 24.
+  ForwardReaderError(teeState, teeState->GetReader());
+
+  // Step 25.
+  aResult.AppendElement(teeState->Branch1());
+  aResult.AppendElement(teeState->Branch2());
+}
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/ReadableStreamTee.h b/dom/streams/ReadableStreamTee.h
new file mode 100644
index 0000000000..855fcf4286
--- /dev/null
+++ b/dom/streams/ReadableStreamTee.h
@@ -0,0 +1,91 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_ReadableStreamTee_h
+#define mozilla_dom_ReadableStreamTee_h
+
+#include "mozilla/dom/ReadRequest.h"
+#include "mozilla/dom/ReadableStreamController.h"
+#include "mozilla/dom/UnderlyingSourceCallbackHelpers.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsISupportsImpl.h"
+
+namespace mozilla::dom {
+struct TeeState;
+enum class TeeBranch : bool;
+class ReadableStream;
+
+// Implementation of the Pull algorithm steps for ReadableStreamDefaultTee,
+// from
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaulttee
+// Step 12.
+class ReadableStreamDefaultTeeSourceAlgorithms final
+    : public UnderlyingSourceAlgorithmsBase {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(
+      ReadableStreamDefaultTeeSourceAlgorithms, UnderlyingSourceAlgorithmsBase)
+
+  ReadableStreamDefaultTeeSourceAlgorithms(TeeState* aTeeState,
+                                           TeeBranch aBranch)
+      : mTeeState(aTeeState), mBranch(aBranch) {}
+
+  MOZ_CAN_RUN_SCRIPT void StartCallback(JSContext* aCx,
+                                        ReadableStreamController& aController,
+                                        JS::MutableHandle<JS::Value> aRetVal,
+                                        ErrorResult& aRv) override {
+    aRetVal.setUndefined();
+  }
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> PullCallback(
+      JSContext* aCx, ReadableStreamController& aController,
+      ErrorResult& aRv) override;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> CancelCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) override;
+
+ protected:
+  ~ReadableStreamDefaultTeeSourceAlgorithms() override = default;
+
+ private:
+  // Virtually const, but is cycle collected
+  MOZ_KNOWN_LIVE RefPtr<TeeState> mTeeState;
+  TeeBranch mBranch;
+};
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaulttee
+// Step 12.3
+struct ReadableStreamDefaultTeeReadRequest final : public ReadRequest {
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ReadableStreamDefaultTeeReadRequest,
+                                           ReadRequest)
+
+  RefPtr<TeeState> mTeeState;
+
+  explicit ReadableStreamDefaultTeeReadRequest(TeeState* aTeeState)
+      : mTeeState(aTeeState) {}
+
+  void ChunkSteps(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                  ErrorResult& aRv) override;
+
+  MOZ_CAN_RUN_SCRIPT void CloseSteps(JSContext* aCx, ErrorResult& aRv) override;
+
+  void ErrorSteps(JSContext* aCx, JS::Handle<JS::Value> aError,
+                  ErrorResult& aRv) override;
+
+ protected:
+  ~ReadableStreamDefaultTeeReadRequest() override = default;
+};
+
+namespace streams_abstract {
+MOZ_CAN_RUN_SCRIPT void ReadableByteStreamTee(
+    JSContext* aCx, ReadableStream* aStream,
+    nsTArray<RefPtr<ReadableStream>>& aResult, ErrorResult& aRv);
+}
+
+}  // namespace mozilla::dom
+#endif
diff --git a/dom/streams/StreamUtils.cpp b/dom/streams/StreamUtils.cpp
new file mode 100644
index 0000000000..15ad80867d
--- /dev/null
+++ b/dom/streams/StreamUtils.cpp
@@ -0,0 +1,34 @@
+/* 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 "StreamUtils.h"
+
+#include <cmath>
+#include "mozilla/dom/QueuingStrategyBinding.h"
+
+namespace mozilla::dom {
+
+// Streams Spec: 7.4
+// https://streams.spec.whatwg.org/#validate-and-normalize-high-water-mark
+double ExtractHighWaterMark(const QueuingStrategy& aStrategy,
+                            double aDefaultHWM, mozilla::ErrorResult& aRv) {
+  // Step 1.
+  if (!aStrategy.mHighWaterMark.WasPassed()) {
+    return aDefaultHWM;
+  }
+
+  // Step 2.
+  double highWaterMark = aStrategy.mHighWaterMark.Value();
+
+  // Step 3.
+  if (std::isnan(highWaterMark) || highWaterMark < 0) {
+    aRv.ThrowRangeError("Invalid highWaterMark");
+    return 0.0;
+  }
+
+  // Step 4.
+  return highWaterMark;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/StreamUtils.h b/dom/streams/StreamUtils.h
new file mode 100644
index 0000000000..f0b09c63b8
--- /dev/null
+++ b/dom/streams/StreamUtils.h
@@ -0,0 +1,57 @@
+/* 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 mozilla_dom_StreamUtils_h
+#define mozilla_dom_StreamUtils_h
+
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/Promise.h"
+
+class nsIGlobalObject;
+
+namespace mozilla::dom {
+
+struct QueuingStrategy;
+
+double ExtractHighWaterMark(const QueuingStrategy& aStrategy,
+                            double aDefaultHWM, ErrorResult& aRv);
+
+// Promisification algorithm, shared among:
+// Step 2 and 3 of https://streams.spec.whatwg.org/#readablestream-set-up
+// Step 2 and 3 of
+// https://streams.spec.whatwg.org/#readablestream-set-up-with-byte-reading-support
+// Step 2 and 3 of https://streams.spec.whatwg.org/#writablestream-set-up
+// Step 5 and 6 of https://streams.spec.whatwg.org/#transformstream-set-up
+template <typename T>
+MOZ_CAN_RUN_SCRIPT static already_AddRefed<Promise> PromisifyAlgorithm(
+    nsIGlobalObject* aGlobal, T aFunc, mozilla::ErrorResult& aRv) {
+  // Step 1. Let result be the result of running (algorithm). If this throws an
+  // exception e, return a promise rejected with e.
+  RefPtr<Promise> result;
+  if constexpr (!std::is_same<decltype(aFunc(aRv)), void>::value) {
+    result = aFunc(aRv);
+  } else {
+    aFunc(aRv);
+  }
+
+  if (aRv.IsUncatchableException()) {
+    return nullptr;
+  }
+
+  if (aRv.Failed()) {
+    return Promise::CreateRejectedWithErrorResult(aGlobal, aRv);
+  }
+
+  // Step 2. If result is a Promise, then return result.
+  if (result) {
+    return result.forget();
+  }
+
+  // Step 3. Return a promise resolved with undefined.
+  return Promise::CreateResolvedWithUndefined(aGlobal, aRv);
+}
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_StreamUtils_h
diff --git a/dom/streams/TeeState.cpp b/dom/streams/TeeState.cpp
new file mode 100644
index 0000000000..8600ea1205
--- /dev/null
+++ b/dom/streams/TeeState.cpp
@@ -0,0 +1,87 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "TeeState.h"
+
+#include "ReadableStreamTee.h"
+#include "js/Value.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/Promise.h"
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+NS_IMPL_CYCLE_COLLECTION_WITH_JS_MEMBERS(TeeState,
+                                         (mStream, mReader, mBranch1, mBranch2,
+                                          mCancelPromise),
+                                         (mReason1, mReason2))
+NS_IMPL_CYCLE_COLLECTING_ADDREF(TeeState)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(TeeState)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(TeeState)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+TeeState::TeeState(ReadableStream* aStream, bool aCloneForBranch2)
+    : mStream(aStream),
+      mReason1(JS::NullValue()),
+      mReason2(JS::NullValue()),
+      mCloneForBranch2(aCloneForBranch2) {
+  mozilla::HoldJSObjects(this);
+  MOZ_RELEASE_ASSERT(!aCloneForBranch2,
+                     "cloneForBranch2 path is not implemented.");
+}
+
+already_AddRefed<TeeState> TeeState::Create(ReadableStream* aStream,
+                                            bool aCloneForBranch2,
+                                            ErrorResult& aRv) {
+  RefPtr<TeeState> teeState = new TeeState(aStream, aCloneForBranch2);
+
+  RefPtr<ReadableStreamDefaultReader> reader =
+      AcquireReadableStreamDefaultReader(teeState->GetStream(), aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+  teeState->SetReader(reader);
+
+  RefPtr<Promise> promise =
+      Promise::CreateInfallible(teeState->GetStream()->GetParentObject());
+  teeState->SetCancelPromise(promise);
+
+  return teeState.forget();
+}
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaulttee
+// Pull Algorithm Steps:
+void TeeState::PullCallback(JSContext* aCx, nsIGlobalObject* aGlobal,
+                            ErrorResult& aRv) {
+  // Step 13.1: If reading is true,
+  if (Reading()) {
+    // Step 13.1.1: Set readAgain to true.
+    SetReadAgain(true);
+
+    // Step 13.1.2: Return a promise resolved with undefined.
+    // (The caller will create it if necessary)
+    return;
+  }
+
+  // Step 13.2: Set reading to true.
+  SetReading(true);
+
+  // Step 13.3: Let readRequest be a read request with the following items:
+  RefPtr<ReadRequest> readRequest =
+      new ReadableStreamDefaultTeeReadRequest(this);
+
+  // Step 13.4: Perform ! ReadableStreamDefaultReaderRead(reader, readRequest).
+  RefPtr<ReadableStreamGenericReader> reader = GetReader();
+  ReadableStreamDefaultReaderRead(aCx, reader, readRequest, aRv);
+
+  // Step 13.5: Return a promise resolved with undefined.
+  // (The caller will create it if necessary)
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/TeeState.h b/dom/streams/TeeState.h
new file mode 100644
index 0000000000..9a688be662
--- /dev/null
+++ b/dom/streams/TeeState.h
@@ -0,0 +1,179 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_TeeState_h
+#define mozilla_dom_TeeState_h
+
+#include "mozilla/HoldDropJSObjects.h"
+#include "nsISupports.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/ReadableStreamDefaultReader.h"
+#include "mozilla/dom/Promise.h"
+
+namespace mozilla::dom {
+
+class ReadableStreamDefaultTeeSourceAlgorithms;
+
+enum class TeeBranch : bool {
+  Branch1,
+  Branch2,
+};
+
+inline TeeBranch OtherTeeBranch(TeeBranch aBranch) {
+  if (aBranch == TeeBranch::Branch1) {
+    return TeeBranch::Branch2;
+  }
+  return TeeBranch::Branch1;
+}
+
+// A closure capturing the free variables in the ReadableStreamTee family of
+// algorithms.
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaulttee
+// https://streams.spec.whatwg.org/#abstract-opdef-readablebytestreamtee
+struct TeeState : public nsISupports {
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(TeeState)
+
+  static already_AddRefed<TeeState> Create(ReadableStream* aStream,
+                                           bool aCloneForBranch2,
+                                           ErrorResult& aRv);
+
+  ReadableStream* GetStream() const { return mStream; }
+  void SetStream(ReadableStream* aStream) { mStream = aStream; }
+
+  ReadableStreamGenericReader* GetReader() const { return mReader; }
+  void SetReader(ReadableStreamGenericReader* aReader) { mReader = aReader; }
+
+  ReadableStreamDefaultReader* GetDefaultReader() const {
+    return mReader->AsDefault();
+  }
+
+  bool ReadAgain() const { return mReadAgain; }
+  void SetReadAgain(bool aReadAgain) { mReadAgain = aReadAgain; }
+
+  // ReadableByteStreamTee uses ReadAgainForBranch{1,2};
+  bool ReadAgainForBranch1() const { return ReadAgain(); }
+  void SetReadAgainForBranch1(bool aReadAgainForBranch1) {
+    SetReadAgain(aReadAgainForBranch1);
+  }
+
+  bool ReadAgainForBranch2() const { return mReadAgainForBranch2; }
+  void SetReadAgainForBranch2(bool aReadAgainForBranch2) {
+    mReadAgainForBranch2 = aReadAgainForBranch2;
+  }
+
+  bool Reading() const { return mReading; }
+  void SetReading(bool aReading) { mReading = aReading; }
+
+  bool Canceled1() const { return mCanceled1; }
+  void SetCanceled1(bool aCanceled1) { mCanceled1 = aCanceled1; }
+
+  bool Canceled2() const { return mCanceled2; }
+  void SetCanceled2(bool aCanceled2) { mCanceled2 = aCanceled2; }
+
+  void SetCanceled(TeeBranch aBranch, bool aCanceled) {
+    aBranch == TeeBranch::Branch1 ? SetCanceled1(aCanceled)
+                                  : SetCanceled2(aCanceled);
+  }
+  bool Canceled(TeeBranch aBranch) {
+    return aBranch == TeeBranch::Branch1 ? Canceled1() : Canceled2();
+  }
+
+  JS::Value Reason1() const { return mReason1; }
+  void SetReason1(JS::Handle<JS::Value> aReason1) { mReason1 = aReason1; }
+
+  JS::Value Reason2() const { return mReason2; }
+  void SetReason2(JS::Handle<JS::Value> aReason2) { mReason2 = aReason2; }
+
+  void SetReason(TeeBranch aBranch, JS::Handle<JS::Value> aReason) {
+    aBranch == TeeBranch::Branch1 ? SetReason1(aReason) : SetReason2(aReason);
+  }
+
+  ReadableStream* Branch1() const { return mBranch1; }
+  void SetBranch1(already_AddRefed<ReadableStream> aBranch1) {
+    mBranch1 = aBranch1;
+  }
+
+  ReadableStream* Branch2() const { return mBranch2; }
+  void SetBranch2(already_AddRefed<ReadableStream> aBranch2) {
+    mBranch2 = aBranch2;
+  }
+
+  Promise* CancelPromise() const { return mCancelPromise; }
+  void SetCancelPromise(Promise* aCancelPromise) {
+    mCancelPromise = aCancelPromise;
+  }
+
+  bool CloneForBranch2() const { return mCloneForBranch2; }
+  void setCloneForBranch2(bool aCloneForBranch2) {
+    mCloneForBranch2 = aCloneForBranch2;
+  }
+
+  // Some code is better served by using an enum into various internal slots to
+  // avoid duplication: Here we provide alternative accessors for that case.
+  ReadableStream* Branch(TeeBranch aBranch) const {
+    return aBranch == TeeBranch::Branch1 ? Branch1() : Branch2();
+  }
+
+  void SetReadAgainForBranch(TeeBranch aBranch, bool aValue) {
+    if (aBranch == TeeBranch::Branch1) {
+      SetReadAgainForBranch1(aValue);
+      return;
+    }
+    SetReadAgainForBranch2(aValue);
+  }
+
+  MOZ_CAN_RUN_SCRIPT void PullCallback(JSContext* aCx, nsIGlobalObject* aGlobal,
+                                       ErrorResult& aRv);
+
+ private:
+  TeeState(ReadableStream* aStream, bool aCloneForBranch2);
+
+  // Implicit:
+  RefPtr<ReadableStream> mStream;
+
+  // Step 3. (Step Numbering is based on ReadableStreamDefaultTee)
+  RefPtr<ReadableStreamGenericReader> mReader;
+
+  // Step 4.
+  bool mReading = false;
+
+  // Step 5.
+  // (Aliased to readAgainForBranch1, for the purpose of ReadableByteStreamTee)
+  bool mReadAgain = false;
+
+  // ReadableByteStreamTee
+  bool mReadAgainForBranch2 = false;
+
+  // Step 6.
+  bool mCanceled1 = false;
+
+  // Step 7.
+  bool mCanceled2 = false;
+
+  // Step 8.
+  JS::Heap<JS::Value> mReason1;
+
+  // Step 9.
+  JS::Heap<JS::Value> mReason2;
+
+  // Step 10.
+  RefPtr<ReadableStream> mBranch1;
+
+  // Step 11.
+  RefPtr<ReadableStream> mBranch2;
+
+  // Step 12.
+  RefPtr<Promise> mCancelPromise;
+
+  // Implicit:
+  bool mCloneForBranch2 = false;
+
+  virtual ~TeeState() { mozilla::DropJSObjects(this); }
+};
+
+}  // namespace mozilla::dom
+#endif
diff --git a/dom/streams/Transferable.cpp b/dom/streams/Transferable.cpp
new file mode 100644
index 0000000000..f3e9504d86
--- /dev/null
+++ b/dom/streams/Transferable.cpp
@@ -0,0 +1,1068 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "ErrorList.h"
+#include "ReadableStreamPipeTo.h"
+#include "js/RootingAPI.h"
+#include "js/String.h"
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/dom/DOMExceptionBinding.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsIDOMEventListener.h"
+#include "nsIGlobalObject.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/ResultVariant.h"
+#include "mozilla/dom/DOMException.h"
+#include "mozilla/dom/MessageEvent.h"
+#include "mozilla/dom/MessageChannel.h"
+#include "mozilla/dom/MessagePort.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/Promise-inl.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/WritableStream.h"
+#include "mozilla/dom/TransformStream.h"
+#include "nsISupportsImpl.h"
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+static void PackAndPostMessage(JSContext* aCx, MessagePort* aPort,
+                               const nsAString& aType,
+                               JS::Handle<JS::Value> aValue, ErrorResult& aRv) {
+  JS::Rooted<JSObject*> obj(aCx,
+                            JS_NewObjectWithGivenProto(aCx, nullptr, nullptr));
+  if (!obj) {
+    // XXX: Should we crash here and there? See also bug 1762233.
+    JS_ClearPendingException(aCx);
+    aRv.Throw(NS_ERROR_UNEXPECTED);
+    return;
+  }
+
+  JS::Rooted<JS::Value> type(aCx);
+  if (!xpc::NonVoidStringToJsval(aCx, aType, &type)) {
+    JS_ClearPendingException(aCx);
+    aRv.Throw(NS_ERROR_UNEXPECTED);
+    return;
+  }
+  if (!JS_DefineProperty(aCx, obj, "type", type, JSPROP_ENUMERATE)) {
+    JS_ClearPendingException(aCx);
+    aRv.Throw(NS_ERROR_UNEXPECTED);
+    return;
+  }
+  JS::Rooted<JS::Value> value(aCx, aValue);
+  if (!JS_WrapValue(aCx, &value)) {
+    JS_ClearPendingException(aCx);
+    aRv.Throw(NS_ERROR_UNEXPECTED);
+    return;
+  }
+  if (!JS_DefineProperty(aCx, obj, "value", value, JSPROP_ENUMERATE)) {
+    JS_ClearPendingException(aCx);
+    aRv.Throw(NS_ERROR_UNEXPECTED);
+    return;
+  }
+
+  Sequence<JSObject*> transferables;  // none in this case
+  JS::Rooted<JS::Value> objValue(aCx, JS::ObjectValue(*obj));
+  aPort->PostMessage(aCx, objValue, transferables, aRv);
+}
+
+// https://streams.spec.whatwg.org/#abstract-opdef-crossrealmtransformsenderror
+static void CrossRealmTransformSendError(JSContext* aCx, MessagePort* aPort,
+                                         JS::Handle<JS::Value> aError) {
+  // Step 1: Perform PackAndPostMessage(port, "error", error), discarding the
+  // result.
+  PackAndPostMessage(aCx, aPort, u"error"_ns, aError, IgnoreErrors());
+}
+
+class SetUpTransformWritableMessageEventListener final
+    : public nsIDOMEventListener {
+ public:
+  SetUpTransformWritableMessageEventListener(
+      WritableStreamDefaultController* aController, Promise* aPromise)
+      : mController(aController), mBackpressurePromise(aPromise) {}
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(SetUpTransformWritableMessageEventListener)
+
+  // https://streams.spec.whatwg.org/#abstract-opdef-setupcrossrealmtransformwritable
+  // The handler steps of Step 4.
+  MOZ_CAN_RUN_SCRIPT NS_IMETHOD HandleEvent(Event* aEvent) override {
+    AutoJSAPI jsapi;
+    if (!jsapi.Init(mController->GetParentObject())) {
+      return NS_OK;
+    }
+    JSContext* cx = jsapi.cx();
+    MessageEvent* messageEvent = aEvent->AsMessageEvent();
+    if (NS_WARN_IF(!messageEvent || !messageEvent->IsTrusted())) {
+      return NS_OK;
+    }
+
+    // Step 1: Let data be the data of the message.
+    JS::Rooted<JS::Value> dataValue(cx);
+    IgnoredErrorResult rv;
+    messageEvent->GetData(cx, &dataValue, rv);
+    if (rv.Failed()) {
+      return NS_OK;
+    }
+
+    // Step 2: Assert: Type(data) is Object.
+    // (But we check in runtime instead to avoid potential malicious events from
+    // a compromised process. Same below.)
+    if (NS_WARN_IF(!dataValue.isObject())) {
+      return NS_OK;
+    }
+    JS::Rooted<JSObject*> data(cx, &dataValue.toObject());
+
+    // Step 3: Let type be ! Get(data, "type").
+    JS::Rooted<JS::Value> type(cx);
+    if (!JS_GetProperty(cx, data, "type", &type)) {
+      // XXX: See bug 1762233
+      JS_ClearPendingException(cx);
+      return NS_OK;
+    }
+
+    // Step 4: Let value be ! Get(data, "value").
+    JS::Rooted<JS::Value> value(cx);
+    if (!JS_GetProperty(cx, data, "value", &value)) {
+      JS_ClearPendingException(cx);
+      return NS_OK;
+    }
+
+    // Step 5: Assert: Type(type) is String.
+    if (NS_WARN_IF(!type.isString())) {
+      return NS_OK;
+    }
+
+    // Step 6: If type is "pull",
+    bool equals = false;
+    if (!JS_StringEqualsLiteral(cx, type.toString(), "pull", &equals)) {
+      JS_ClearPendingException(cx);
+      return NS_OK;
+    }
+    if (equals) {
+      // Step 6.1: If backpressurePromise is not undefined,
+      MaybeResolveAndClearBackpressurePromise();
+      return NS_OK;  // implicit
+    }
+
+    // Step 7: If type is "error",
+    if (!JS_StringEqualsLiteral(cx, type.toString(), "error", &equals)) {
+      JS_ClearPendingException(cx);
+      return NS_OK;
+    }
+    if (equals) {
+      // Step 7.1: Perform !
+      // WritableStreamDefaultControllerErrorIfNeeded(controller, value).
+      WritableStreamDefaultControllerErrorIfNeeded(cx, mController, value, rv);
+      if (rv.Failed()) {
+        return NS_OK;
+      }
+
+      // Step 7.2: If backpressurePromise is not undefined,
+      MaybeResolveAndClearBackpressurePromise();
+      return NS_OK;  // implicit
+    }
+
+    // Logically it should be unreachable here, but we should expect random
+    // malicious messages.
+    NS_WARNING("Got an unexpected type other than pull/error.");
+    return NS_OK;
+  }
+
+  void MaybeResolveAndClearBackpressurePromise() {
+    if (mBackpressurePromise) {
+      mBackpressurePromise->MaybeResolveWithUndefined();
+      mBackpressurePromise = nullptr;
+    }
+  }
+
+  // Note: This promise field is shared with the sink algorithms.
+  Promise* BackpressurePromise() { return mBackpressurePromise; }
+
+  void CreateBackpressurePromise() {
+    mBackpressurePromise =
+        Promise::CreateInfallible(mController->GetParentObject());
+  }
+
+ private:
+  ~SetUpTransformWritableMessageEventListener() = default;
+
+  // mController never changes before CC
+  // TODO: MOZ_IMMUTABLE_OUTSIDE_CC
+  MOZ_KNOWN_LIVE RefPtr<WritableStreamDefaultController> mController;
+  RefPtr<Promise> mBackpressurePromise;
+};
+
+NS_IMPL_CYCLE_COLLECTION(SetUpTransformWritableMessageEventListener,
+                         mController, mBackpressurePromise)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(SetUpTransformWritableMessageEventListener)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(SetUpTransformWritableMessageEventListener)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(
+    SetUpTransformWritableMessageEventListener)
+  NS_INTERFACE_MAP_ENTRY(nsIDOMEventListener)
+NS_INTERFACE_MAP_END
+
+class SetUpTransformWritableMessageErrorEventListener final
+    : public nsIDOMEventListener {
+ public:
+  SetUpTransformWritableMessageErrorEventListener(
+      WritableStreamDefaultController* aController, MessagePort* aPort)
+      : mController(aController), mPort(aPort) {}
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(
+      SetUpTransformWritableMessageErrorEventListener)
+
+  // https://streams.spec.whatwg.org/#abstract-opdef-setupcrossrealmtransformwritable
+  // The handler steps of Step 5.
+  MOZ_CAN_RUN_SCRIPT NS_IMETHOD HandleEvent(Event* aEvent) override {
+    auto cleanupPort =
+        MakeScopeExit([port = RefPtr<MessagePort>(mPort)]() { port->Close(); });
+
+    if (NS_WARN_IF(!aEvent->AsMessageEvent() || !aEvent->IsTrusted())) {
+      return NS_OK;
+    }
+
+    // Step 1: Let error be a new "DataCloneError" DOMException.
+    RefPtr<DOMException> exception =
+        DOMException::Create(NS_ERROR_DOM_DATA_CLONE_ERR);
+
+    AutoJSAPI jsapi;
+    if (!jsapi.Init(mPort->GetParentObject())) {
+      return NS_OK;
+    }
+    JSContext* cx = jsapi.cx();
+    JS::Rooted<JS::Value> error(cx);
+    if (!ToJSValue(cx, *exception, &error)) {
+      return NS_OK;
+    }
+
+    // Step 2: Perform ! CrossRealmTransformSendError(port, error).
+    CrossRealmTransformSendError(cx, mPort, error);
+
+    // Step 3: Perform
+    // ! WritableStreamDefaultControllerErrorIfNeeded(controller, error).
+    WritableStreamDefaultControllerErrorIfNeeded(cx, mController, error,
+                                                 IgnoreErrors());
+
+    // Step 4: Disentangle port.
+    // (Close() does it)
+    mPort->Close();
+    cleanupPort.release();
+
+    return NS_OK;
+  }
+
+ private:
+  ~SetUpTransformWritableMessageErrorEventListener() = default;
+
+  // mController never changes before CC
+  // TODO: MOZ_IMMUTABLE_OUTSIDE_CC
+  MOZ_KNOWN_LIVE RefPtr<WritableStreamDefaultController> mController;
+  RefPtr<MessagePort> mPort;
+};
+
+NS_IMPL_CYCLE_COLLECTION(SetUpTransformWritableMessageErrorEventListener,
+                         mController, mPort)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(SetUpTransformWritableMessageErrorEventListener)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(
+    SetUpTransformWritableMessageErrorEventListener)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(
+    SetUpTransformWritableMessageErrorEventListener)
+  NS_INTERFACE_MAP_ENTRY(nsIDOMEventListener)
+NS_INTERFACE_MAP_END
+
+// https://streams.spec.whatwg.org/#abstract-opdef-packandpostmessagehandlingerror
+static bool PackAndPostMessageHandlingError(
+    JSContext* aCx, mozilla::dom::MessagePort* aPort, const nsAString& aType,
+    JS::Handle<JS::Value> aValue, JS::MutableHandle<JS::Value> aError) {
+  // Step 1: Let result be PackAndPostMessage(port, type, value).
+  ErrorResult rv;
+  PackAndPostMessage(aCx, aPort, aType, aValue, rv);
+
+  // Step 2: If result is an abrupt completion,
+  if (rv.Failed()) {
+    // Step 2.2: Perform ! CrossRealmTransformSendError(port, result.[[Value]]).
+    MOZ_ALWAYS_TRUE(ToJSValue(aCx, std::move(rv), aError));
+    CrossRealmTransformSendError(aCx, aPort, aError);
+    return false;
+  }
+
+  // Step 3: Return result as a completion record.
+  return true;
+}
+
+// https://streams.spec.whatwg.org/#abstract-opdef-setupcrossrealmtransformwritable
+class CrossRealmWritableUnderlyingSinkAlgorithms final
+    : public UnderlyingSinkAlgorithmsBase {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(
+      CrossRealmWritableUnderlyingSinkAlgorithms, UnderlyingSinkAlgorithmsBase)
+
+  CrossRealmWritableUnderlyingSinkAlgorithms(
+      SetUpTransformWritableMessageEventListener* aListener, MessagePort* aPort)
+      : mListener(aListener), mPort(aPort) {}
+
+  void StartCallback(JSContext* aCx,
+                     WritableStreamDefaultController& aController,
+                     JS::MutableHandle<JS::Value> aRetVal,
+                     ErrorResult& aRv) override {
+    // Step 7. Let startAlgorithm be an algorithm that returns undefined.
+    aRetVal.setUndefined();
+  }
+
+  already_AddRefed<Promise> WriteCallback(
+      JSContext* aCx, JS::Handle<JS::Value> aChunk,
+      WritableStreamDefaultController& aController, ErrorResult& aRv) override {
+    // Step 1: If backpressurePromise is undefined, set backpressurePromise to a
+    // promise resolved with undefined.
+    // Note: This promise field is shared with the message event listener.
+    if (!mListener->BackpressurePromise()) {
+      mListener->CreateBackpressurePromise();
+      mListener->BackpressurePromise()->MaybeResolveWithUndefined();
+    }
+
+    // Step 2: Return the result of reacting to backpressurePromise with the
+    // following fulfillment steps:
+    auto result =
+        mListener->BackpressurePromise()->ThenWithCycleCollectedArgsJS(
+            [](JSContext* aCx, JS::Handle<JS::Value>, ErrorResult& aRv,
+               SetUpTransformWritableMessageEventListener* aListener,
+               MessagePort* aPort,
+               JS::Handle<JS::Value> aChunk) -> already_AddRefed<Promise> {
+              // Step 2.1: Set backpressurePromise to a new promise.
+              aListener->CreateBackpressurePromise();
+
+              // Step 2.2: Let result be PackAndPostMessageHandlingError(port,
+              // "chunk", chunk).
+              JS::Rooted<JS::Value> error(aCx);
+              bool result = PackAndPostMessageHandlingError(
+                  aCx, aPort, u"chunk"_ns, aChunk, &error);
+
+              // Step 2.3: If result is an abrupt completion,
+              if (!result) {
+                // Step 2.3.1: Disentangle port.
+                // (Close() does it)
+                aPort->Close();
+
+                // Step 2.3.2: Return a promise rejected with result.[[Value]].
+                return Promise::CreateRejected(aPort->GetParentObject(), error,
+                                               aRv);
+              }
+
+              // Step 2.4: Otherwise, return a promise resolved with undefined.
+              return Promise::CreateResolvedWithUndefined(
+                  aPort->GetParentObject(), aRv);
+            },
+            std::make_tuple(mListener, mPort), std::make_tuple(aChunk));
+    if (result.isErr()) {
+      aRv.Throw(result.unwrapErr());
+      return nullptr;
+    }
+    return result.unwrap().forget();
+  }
+
+  already_AddRefed<Promise> CloseCallback(JSContext* aCx,
+                                          ErrorResult& aRv) override {
+    // Step 1: Perform ! PackAndPostMessage(port, "close", undefined).
+    PackAndPostMessage(aCx, mPort, u"close"_ns, JS::UndefinedHandleValue, aRv);
+    // (We'll check the result after step 2)
+
+    // Step 2: Disentangle port.
+    // (Close() will do this)
+    mPort->Close();
+
+    if (aRv.Failed()) {
+      return nullptr;
+    }
+
+    // Step 3: Return a promise resolved with undefined.
+    return Promise::CreateResolvedWithUndefined(mPort->GetParentObject(), aRv);
+  }
+
+  already_AddRefed<Promise> AbortCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) override {
+    // Step 1: Let result be PackAndPostMessageHandlingError(port, "error",
+    // reason).
+    JS::Rooted<JS::Value> error(aCx);
+    bool result = PackAndPostMessageHandlingError(
+        aCx, mPort, u"error"_ns,
+        aReason.WasPassed() ? aReason.Value() : JS::UndefinedHandleValue,
+        &error);
+
+    // Step 2: Disentangle port.
+    // (Close() will do this)
+    mPort->Close();
+
+    // Step 3: If result is an abrupt completion, return a promise rejected with
+    // result.[[Value]].
+    if (!result) {
+      return Promise::CreateRejected(mPort->GetParentObject(), error, aRv);
+    }
+
+    // Step 4: Otherwise, return a promise resolved with undefined.
+    return Promise::CreateResolvedWithUndefined(mPort->GetParentObject(), aRv);
+  }
+
+ protected:
+  ~CrossRealmWritableUnderlyingSinkAlgorithms() override = default;
+
+ private:
+  RefPtr<SetUpTransformWritableMessageEventListener> mListener;
+  RefPtr<MessagePort> mPort;
+};
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(CrossRealmWritableUnderlyingSinkAlgorithms,
+                                   UnderlyingSinkAlgorithmsBase, mListener,
+                                   mPort)
+NS_IMPL_ADDREF_INHERITED(CrossRealmWritableUnderlyingSinkAlgorithms,
+                         UnderlyingSinkAlgorithmsBase)
+NS_IMPL_RELEASE_INHERITED(CrossRealmWritableUnderlyingSinkAlgorithms,
+                          UnderlyingSinkAlgorithmsBase)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(
+    CrossRealmWritableUnderlyingSinkAlgorithms)
+NS_INTERFACE_MAP_END_INHERITING(UnderlyingSinkAlgorithmsBase)
+
+// https://streams.spec.whatwg.org/#abstract-opdef-setupcrossrealmtransformwritable
+MOZ_CAN_RUN_SCRIPT static void SetUpCrossRealmTransformWritable(
+    WritableStream* aWritable, MessagePort* aPort, ErrorResult& aRv) {
+  // (This is only needed for step 12, but let's do this early to fail early
+  // enough)
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(aWritable->GetParentObject())) {
+    return;
+  }
+  JSContext* cx = jsapi.cx();
+
+  // Step 1: Perform ! InitializeWritableStream(stream).
+  // (Done by the constructor)
+
+  // Step 2: Let controller be a new WritableStreamDefaultController.
+  auto controller = MakeRefPtr<WritableStreamDefaultController>(
+      aWritable->GetParentObject(), *aWritable);
+
+  // Step 3: Let backpressurePromise be a new promise.
+  RefPtr<Promise> backpressurePromise =
+      Promise::CreateInfallible(aWritable->GetParentObject());
+
+  // Step 4: Add a handler for port’s message event with the following steps:
+  auto listener = MakeRefPtr<SetUpTransformWritableMessageEventListener>(
+      controller, backpressurePromise);
+  aPort->AddEventListener(u"message"_ns, listener, false);
+
+  // Step 5: Add a handler for port’s messageerror event with the following
+  // steps:
+  auto errorListener =
+      MakeRefPtr<SetUpTransformWritableMessageErrorEventListener>(controller,
+                                                                  aPort);
+  aPort->AddEventListener(u"messageerror"_ns, errorListener, false);
+
+  // Step 6: Enable port’s port message queue.
+  // (Start() does it)
+  aPort->Start();
+
+  // Step 7 - 10:
+  auto algorithms =
+      MakeRefPtr<CrossRealmWritableUnderlyingSinkAlgorithms>(listener, aPort);
+
+  // Step 11: Let sizeAlgorithm be an algorithm that returns 1.
+  // (nullptr should serve this purpose. See also WritableStream::Constructor)
+
+  // Step 12: Perform ! SetUpWritableStreamDefaultController(stream, controller,
+  // startAlgorithm, writeAlgorithm, closeAlgorithm, abortAlgorithm, 1,
+  // sizeAlgorithm).
+  SetUpWritableStreamDefaultController(cx, aWritable, controller, algorithms, 1,
+                                       /* aSizeAlgorithm */ nullptr, aRv);
+}
+
+class SetUpTransformReadableMessageEventListener final
+    : public nsIDOMEventListener {
+ public:
+  SetUpTransformReadableMessageEventListener(
+      ReadableStreamDefaultController* aController, MessagePort* aPort)
+      : mController(aController), mPort(aPort) {}
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(SetUpTransformReadableMessageEventListener)
+
+  // https://streams.spec.whatwg.org/#abstract-opdef-setupcrossrealmtransformreadable
+  // The handler steps of Step 3.
+  MOZ_CAN_RUN_SCRIPT NS_IMETHOD HandleEvent(Event* aEvent) override {
+    auto cleanupPort =
+        MakeScopeExit([port = RefPtr<MessagePort>(mPort)]() { port->Close(); });
+
+    AutoJSAPI jsapi;
+    if (!jsapi.Init(mPort->GetParentObject())) {
+      return NS_OK;
+    }
+    JSContext* cx = jsapi.cx();
+    MessageEvent* messageEvent = aEvent->AsMessageEvent();
+    if (NS_WARN_IF(!messageEvent || !messageEvent->IsTrusted())) {
+      return NS_OK;
+    }
+
+    // Step 1: Let data be the data of the message.
+    JS::Rooted<JS::Value> dataValue(cx);
+    IgnoredErrorResult rv;
+    messageEvent->GetData(cx, &dataValue, rv);
+    if (rv.Failed()) {
+      return NS_OK;
+    }
+
+    // Step 2: Assert: Type(data) is Object.
+    // (But we check in runtime instead to avoid potential malicious events from
+    // a compromised process. Same below.)
+    if (NS_WARN_IF(!dataValue.isObject())) {
+      return NS_OK;
+    }
+    JS::Rooted<JSObject*> data(cx, JS::ToObject(cx, dataValue));
+
+    // Step 3: Let type be ! Get(data, "type").
+    JS::Rooted<JS::Value> type(cx);
+    if (!JS_GetProperty(cx, data, "type", &type)) {
+      // XXX: See bug 1762233
+      JS_ClearPendingException(cx);
+      return NS_OK;
+    }
+
+    // Step 4: Let value be ! Get(data, "value").
+    JS::Rooted<JS::Value> value(cx);
+    if (!JS_GetProperty(cx, data, "value", &value)) {
+      JS_ClearPendingException(cx);
+      return NS_OK;
+    }
+
+    // Step 5: Assert: Type(type) is String.
+    if (NS_WARN_IF(!type.isString())) {
+      return NS_OK;
+    }
+
+    // Step 6: If type is "chunk",
+    bool equals = false;
+    if (!JS_StringEqualsLiteral(cx, type.toString(), "chunk", &equals)) {
+      JS_ClearPendingException(cx);
+      return NS_OK;
+    }
+    if (equals) {
+      // Step 6.1: Perform ! ReadableStreamDefaultControllerEnqueue(controller,
+      // value).
+      ReadableStreamDefaultControllerEnqueue(cx, mController, value,
+                                             IgnoreErrors());
+      cleanupPort.release();
+      return NS_OK;  // implicit
+    }
+
+    // Step 7: Otherwise, if type is "close",
+    if (!JS_StringEqualsLiteral(cx, type.toString(), "close", &equals)) {
+      JS_ClearPendingException(cx);
+      return NS_OK;
+    }
+    if (equals) {
+      // Step 7.1: Perform ! ReadableStreamDefaultControllerClose(controller).
+      ReadableStreamDefaultControllerClose(cx, mController, IgnoreErrors());
+      // Step 7.2: Disentangle port.
+      // (Close() does it)
+      mPort->Close();
+      cleanupPort.release();
+      return NS_OK;  // implicit
+    }
+
+    // Step 8: Otherwise, if type is "error",
+    if (!JS_StringEqualsLiteral(cx, type.toString(), "error", &equals)) {
+      JS_ClearPendingException(cx);
+      return NS_OK;
+    }
+    if (equals) {
+      // Step 8.1: Perform ! ReadableStreamDefaultControllerError(controller,
+      // value).
+      ReadableStreamDefaultControllerError(cx, mController, value,
+                                           IgnoreErrors());
+
+      // Step 8.2: Disentangle port.
+      // (Close() does it)
+      mPort->Close();
+      cleanupPort.release();
+      return NS_OK;  // implicit
+    }
+
+    // Logically it should be unreachable here, but we should expect random
+    // malicious messages.
+    NS_WARNING("Got an unexpected type other than chunk/close/error.");
+    return NS_OK;
+  }
+
+ private:
+  ~SetUpTransformReadableMessageEventListener() = default;
+
+  // mController never changes before CC
+  // TODO: MOZ_IMMUTABLE_OUTSIDE_CC
+  MOZ_KNOWN_LIVE RefPtr<ReadableStreamDefaultController> mController;
+  RefPtr<MessagePort> mPort;
+};
+
+NS_IMPL_CYCLE_COLLECTION(SetUpTransformReadableMessageEventListener,
+                         mController, mPort)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(SetUpTransformReadableMessageEventListener)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(SetUpTransformReadableMessageEventListener)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(
+    SetUpTransformReadableMessageEventListener)
+  NS_INTERFACE_MAP_ENTRY(nsIDOMEventListener)
+NS_INTERFACE_MAP_END
+
+class SetUpTransformReadableMessageErrorEventListener final
+    : public nsIDOMEventListener {
+ public:
+  SetUpTransformReadableMessageErrorEventListener(
+      ReadableStreamDefaultController* aController, MessagePort* aPort)
+      : mController(aController), mPort(aPort) {}
+
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(
+      SetUpTransformReadableMessageErrorEventListener)
+
+  // https://streams.spec.whatwg.org/#abstract-opdef-setupcrossrealmtransformreadable
+  // The handler steps of Step 4.
+  MOZ_CAN_RUN_SCRIPT NS_IMETHOD HandleEvent(Event* aEvent) override {
+    auto cleanupPort =
+        MakeScopeExit([port = RefPtr<MessagePort>(mPort)]() { port->Close(); });
+
+    if (NS_WARN_IF(!aEvent->AsMessageEvent() || !aEvent->IsTrusted())) {
+      return NS_OK;
+    }
+
+    // Step 1: Let error be a new "DataCloneError" DOMException.
+    RefPtr<DOMException> exception =
+        DOMException::Create(NS_ERROR_DOM_DATA_CLONE_ERR);
+
+    AutoJSAPI jsapi;
+    if (!jsapi.Init(mPort->GetParentObject())) {
+      return NS_OK;
+    }
+    JSContext* cx = jsapi.cx();
+    JS::Rooted<JS::Value> error(cx);
+    if (!ToJSValue(cx, *exception, &error)) {
+      return NS_OK;
+    }
+
+    // Step 2: Perform ! CrossRealmTransformSendError(port, error).
+    CrossRealmTransformSendError(cx, mPort, error);
+
+    // Step 3: Perform ! ReadableStreamDefaultControllerError(controller,
+    // error).
+    ReadableStreamDefaultControllerError(cx, mController, error,
+                                         IgnoreErrors());
+
+    // Step 4: Disentangle port.
+    // (Close() does it)
+    mPort->Close();
+    cleanupPort.release();
+
+    return NS_OK;
+  }
+
+ private:
+  ~SetUpTransformReadableMessageErrorEventListener() = default;
+
+  RefPtr<ReadableStreamDefaultController> mController;
+  RefPtr<MessagePort> mPort;
+};
+
+NS_IMPL_CYCLE_COLLECTION(SetUpTransformReadableMessageErrorEventListener,
+                         mController, mPort)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(SetUpTransformReadableMessageErrorEventListener)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(
+    SetUpTransformReadableMessageErrorEventListener)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(
+    SetUpTransformReadableMessageErrorEventListener)
+  NS_INTERFACE_MAP_ENTRY(nsIDOMEventListener)
+NS_INTERFACE_MAP_END
+
+// https://streams.spec.whatwg.org/#abstract-opdef-setupcrossrealmtransformreadable
+class CrossRealmReadableUnderlyingSourceAlgorithms final
+    : public UnderlyingSourceAlgorithmsBase {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(
+      CrossRealmReadableUnderlyingSourceAlgorithms,
+      UnderlyingSourceAlgorithmsBase)
+
+  explicit CrossRealmReadableUnderlyingSourceAlgorithms(MessagePort* aPort)
+      : mPort(aPort) {}
+
+  void StartCallback(JSContext* aCx, ReadableStreamController& aController,
+                     JS::MutableHandle<JS::Value> aRetVal,
+                     ErrorResult& aRv) override {
+    // Step 6. Let startAlgorithm be an algorithm that returns undefined.
+    aRetVal.setUndefined();
+  }
+
+  already_AddRefed<Promise> PullCallback(JSContext* aCx,
+                                         ReadableStreamController& aController,
+                                         ErrorResult& aRv) override {
+    // Step 7: Let pullAlgorithm be the following steps:
+
+    // Step 7.1: Perform ! PackAndPostMessage(port, "pull", undefined).
+    PackAndPostMessage(aCx, mPort, u"pull"_ns, JS::UndefinedHandleValue, aRv);
+    if (aRv.Failed()) {
+      return nullptr;
+    }
+
+    // Step 7.2: Return a promise resolved with undefined.
+    return Promise::CreateResolvedWithUndefined(mPort->GetParentObject(), aRv);
+  }
+
+  already_AddRefed<Promise> CancelCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) override {
+    // Step 8: Let cancelAlgorithm be the following steps, taking a reason
+    // argument:
+
+    // Step 8.1: Let result be PackAndPostMessageHandlingError(port, "error",
+    // reason).
+    JS::Rooted<JS::Value> error(aCx);
+    bool result = PackAndPostMessageHandlingError(
+        aCx, mPort, u"error"_ns,
+        aReason.WasPassed() ? aReason.Value() : JS::UndefinedHandleValue,
+        &error);
+
+    // Step 8.2: Disentangle port.
+    // (Close() does it)
+    mPort->Close();
+
+    // Step 8.3: If result is an abrupt completion, return a promise rejected
+    // with result.[[Value]].
+    if (!result) {
+      return Promise::CreateRejected(mPort->GetParentObject(), error, aRv);
+    }
+
+    // Step 8.4: Otherwise, return a promise resolved with undefined.
+    return Promise::CreateResolvedWithUndefined(mPort->GetParentObject(), aRv);
+  }
+
+ protected:
+  ~CrossRealmReadableUnderlyingSourceAlgorithms() override = default;
+
+ private:
+  RefPtr<MessagePort> mPort;
+};
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(CrossRealmReadableUnderlyingSourceAlgorithms,
+                                   UnderlyingSourceAlgorithmsBase, mPort)
+NS_IMPL_ADDREF_INHERITED(CrossRealmReadableUnderlyingSourceAlgorithms,
+                         UnderlyingSourceAlgorithmsBase)
+NS_IMPL_RELEASE_INHERITED(CrossRealmReadableUnderlyingSourceAlgorithms,
+                          UnderlyingSourceAlgorithmsBase)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(
+    CrossRealmReadableUnderlyingSourceAlgorithms)
+NS_INTERFACE_MAP_END_INHERITING(UnderlyingSourceAlgorithmsBase)
+
+// https://streams.spec.whatwg.org/#abstract-opdef-setupcrossrealmtransformreadable
+MOZ_CAN_RUN_SCRIPT static void SetUpCrossRealmTransformReadable(
+    ReadableStream* aReadable, MessagePort* aPort, ErrorResult& aRv) {
+  // (This is only needed for step 10, but let's do this early to fail early
+  // enough)
+  AutoJSAPI jsapi;
+  if (!jsapi.Init(aReadable->GetParentObject())) {
+    return;
+  }
+  JSContext* cx = jsapi.cx();
+
+  // Step 1: Perform ! InitializeReadableStream(stream).
+  // (This is implicitly done by the constructor)
+
+  // Step 2: Let controller be a new ReadableStreamDefaultController.
+  auto controller =
+      MakeRefPtr<ReadableStreamDefaultController>(aReadable->GetParentObject());
+
+  // Step 3: Add a handler for port’s message event with the following steps:
+  auto listener =
+      MakeRefPtr<SetUpTransformReadableMessageEventListener>(controller, aPort);
+  aPort->AddEventListener(u"message"_ns, listener, false);
+
+  // Step 4: Add a handler for port’s messageerror event with the following
+  // steps:
+  auto errorListener =
+      MakeRefPtr<SetUpTransformReadableMessageErrorEventListener>(controller,
+                                                                  aPort);
+  aPort->AddEventListener(u"messageerror"_ns, errorListener, false);
+
+  // Step 5: Enable port’s port message queue.
+  // (Start() does it)
+  aPort->Start();
+
+  // Step 6-8:
+  auto algorithms =
+      MakeRefPtr<CrossRealmReadableUnderlyingSourceAlgorithms>(aPort);
+
+  // Step 9: Let sizeAlgorithm be an algorithm that returns 1.
+  // (nullptr should serve this purpose. See also ReadableStream::Constructor)
+
+  // Step 10: Perform ! SetUpReadableStreamDefaultController(stream, controller,
+  // startAlgorithm, pullAlgorithm, cancelAlgorithm, 0, sizeAlgorithm).
+  SetUpReadableStreamDefaultController(cx, aReadable, controller, algorithms, 0,
+                                       /* aSizeAlgorithm */ nullptr, aRv);
+}
+
+// https://streams.spec.whatwg.org/#ref-for-transfer-steps
+bool ReadableStream::Transfer(JSContext* aCx, UniqueMessagePortId& aPortId) {
+  // Step 1: If ! IsReadableStreamLocked(value) is true, throw a
+  // "DataCloneError" DOMException.
+  // (Implemented in StructuredCloneHolder::CustomCanTransferHandler, but double
+  // check here as the state might have changed in case this ReadableStream is
+  // created by a TransferStream and being transferred together with the
+  // parent.)
+  if (IsReadableStreamLocked(this)) {
+    return false;
+  }
+
+  // Step 2: Let port1 be a new MessagePort in the current Realm.
+  // Step 3: Let port2 be a new MessagePort in the current Realm.
+  // Step 4: Entangle port1 and port2.
+  // (The MessageChannel constructor does exactly that.)
+  // https://html.spec.whatwg.org/multipage/web-messaging.html#dom-messagechannel
+  ErrorResult rv;
+  RefPtr<dom::MessageChannel> channel =
+      dom::MessageChannel::Constructor(mGlobal, rv);
+  if (rv.MaybeSetPendingException(aCx)) {
+    return false;
+  }
+
+  // Step 5: Let writable be a new WritableStream in the current Realm.
+  RefPtr<WritableStream> writable = new WritableStream(
+      mGlobal, WritableStream::HoldDropJSObjectsCaller::Implicit);
+
+  // Step 6: Perform ! SetUpCrossRealmTransformWritable(writable, port1).
+  // MOZ_KnownLive because Port1 never changes before CC
+  SetUpCrossRealmTransformWritable(writable, MOZ_KnownLive(channel->Port1()),
+                                   rv);
+  if (rv.MaybeSetPendingException(aCx)) {
+    return false;
+  }
+
+  // Step 7: Let promise be ! ReadableStreamPipeTo(value, writable, false,
+  // false, false).
+  RefPtr<Promise> promise =
+      ReadableStreamPipeTo(this, writable, false, false, false, nullptr, rv);
+  if (rv.MaybeSetPendingException(aCx)) {
+    return false;
+  }
+
+  // Step 8: Set promise.[[PromiseIsHandled]] to true.
+  MOZ_ALWAYS_TRUE(promise->SetAnyPromiseIsHandled());
+
+  // Step 9: Set dataHolder.[[port]] to ! StructuredSerializeWithTransfer(port2,
+  // « port2 »).
+  channel->Port2()->CloneAndDisentangle(aPortId);
+
+  return true;
+}
+
+// https://streams.spec.whatwg.org/#ref-for-transfer-receiving-steps
+MOZ_CAN_RUN_SCRIPT already_AddRefed<ReadableStream>
+ReadableStream::ReceiveTransferImpl(JSContext* aCx, nsIGlobalObject* aGlobal,
+                                    MessagePort& aPort) {
+  // Step 1: Let deserializedRecord be
+  // ! StructuredDeserializeWithTransfer(dataHolder.[[port]], the current
+  // Realm).
+  // Step 2: Let port be deserializedRecord.[[Deserialized]].
+
+  // Step 3: Perform ! SetUpCrossRealmTransformReadable(value, port).
+  RefPtr<ReadableStream> readable =
+      new ReadableStream(aGlobal, HoldDropJSObjectsCaller::Implicit);
+  ErrorResult rv;
+  SetUpCrossRealmTransformReadable(readable, &aPort, rv);
+  if (rv.MaybeSetPendingException(aCx)) {
+    return nullptr;
+  }
+  return readable.forget();
+}
+
+bool ReadableStream::ReceiveTransfer(
+    JSContext* aCx, nsIGlobalObject* aGlobal, MessagePort& aPort,
+    JS::MutableHandle<JSObject*> aReturnObject) {
+  RefPtr<ReadableStream> readable =
+      ReadableStream::ReceiveTransferImpl(aCx, aGlobal, aPort);
+  if (!readable) {
+    return false;
+  }
+
+  JS::Rooted<JS::Value> value(aCx);
+  if (!GetOrCreateDOMReflector(aCx, readable, &value)) {
+    JS_ClearPendingException(aCx);
+    return false;
+  }
+  aReturnObject.set(&value.toObject());
+
+  return true;
+}
+
+// https://streams.spec.whatwg.org/#ref-for-transfer-steps①
+bool WritableStream::Transfer(JSContext* aCx, UniqueMessagePortId& aPortId) {
+  // Step 1: If ! IsWritableStreamLocked(value) is true, throw a
+  // "DataCloneError" DOMException.
+  // (Implemented in StructuredCloneHolder::CustomCanTransferHandler, but double
+  // check here as the state might have changed in case this WritableStream is
+  // created by a TransferStream and being transferred together with the
+  // parent.)
+  if (IsWritableStreamLocked(this)) {
+    return false;
+  }
+
+  // Step 2: Let port1 be a new MessagePort in the current Realm.
+  // Step 3: Let port2 be a new MessagePort in the current Realm.
+  // Step 4: Entangle port1 and port2.
+  // (The MessageChannel constructor does exactly that.)
+  // https://html.spec.whatwg.org/multipage/web-messaging.html#dom-messagechannel
+  ErrorResult rv;
+  RefPtr<dom::MessageChannel> channel =
+      dom::MessageChannel::Constructor(mGlobal, rv);
+  if (rv.MaybeSetPendingException(aCx)) {
+    return false;
+  }
+
+  // Step 5: Let readable be a new ReadableStream in the current Realm.
+  RefPtr<ReadableStream> readable = new ReadableStream(
+      mGlobal, ReadableStream::HoldDropJSObjectsCaller::Implicit);
+
+  // Step 6: Perform ! SetUpCrossRealmTransformReadable(readable, port1).
+  // MOZ_KnownLive because Port1 never changes before CC
+  SetUpCrossRealmTransformReadable(readable, MOZ_KnownLive(channel->Port1()),
+                                   rv);
+  if (rv.MaybeSetPendingException(aCx)) {
+    return false;
+  }
+
+  // Step 7: Let promise be ! ReadableStreamPipeTo(readable, value, false,
+  // false, false).
+  RefPtr<Promise> promise =
+      ReadableStreamPipeTo(readable, this, false, false, false, nullptr, rv);
+  if (rv.Failed()) {
+    return false;
+  }
+
+  // Step 8: Set promise.[[PromiseIsHandled]] to true.
+  MOZ_ALWAYS_TRUE(promise->SetAnyPromiseIsHandled());
+
+  // Step 9: Set dataHolder.[[port]] to ! StructuredSerializeWithTransfer(port2,
+  // « port2 »).
+  channel->Port2()->CloneAndDisentangle(aPortId);
+
+  return true;
+}
+
+// https://streams.spec.whatwg.org/#ref-for-transfer-receiving-steps①
+MOZ_CAN_RUN_SCRIPT already_AddRefed<WritableStream>
+WritableStream::ReceiveTransferImpl(JSContext* aCx, nsIGlobalObject* aGlobal,
+                                    MessagePort& aPort) {
+  // Step 1: Let deserializedRecord be !
+  // StructuredDeserializeWithTransfer(dataHolder.[[port]], the current Realm).
+  // Step 2: Let port be a deserializedRecord.[[Deserialized]].
+
+  // Step 3: Perform ! SetUpCrossRealmTransformWritable(value, port).
+  RefPtr<WritableStream> writable = new WritableStream(
+      aGlobal, WritableStream::HoldDropJSObjectsCaller::Implicit);
+  ErrorResult rv;
+  SetUpCrossRealmTransformWritable(writable, &aPort, rv);
+  if (rv.MaybeSetPendingException(aCx)) {
+    return nullptr;
+  }
+  return writable.forget();
+}
+
+// https://streams.spec.whatwg.org/#ref-for-transfer-receiving-steps①
+bool WritableStream::ReceiveTransfer(
+    JSContext* aCx, nsIGlobalObject* aGlobal, MessagePort& aPort,
+    JS::MutableHandle<JSObject*> aReturnObject) {
+  RefPtr<WritableStream> writable =
+      WritableStream::ReceiveTransferImpl(aCx, aGlobal, aPort);
+  if (!writable) {
+    return false;
+  }
+
+  JS::Rooted<JS::Value> value(aCx);
+  if (!GetOrCreateDOMReflector(aCx, writable, &value)) {
+    JS_ClearPendingException(aCx);
+    return false;
+  }
+  aReturnObject.set(&value.toObject());
+
+  return true;
+}
+
+// https://streams.spec.whatwg.org/#ref-for-transfer-steps②
+bool TransformStream::Transfer(JSContext* aCx, UniqueMessagePortId& aPortId1,
+                               UniqueMessagePortId& aPortId2) {
+  // Step 1: Let readable be value.[[readable]].
+  // Step 2: Let writable be value.[[writable]].
+  // Step 3: If ! IsReadableStreamLocked(readable) is true, throw a
+  // "DataCloneError" DOMException.
+  // Step 4: If ! IsWritableStreamLocked(writable) is true, throw a
+  // "DataCloneError" DOMException.
+  // (Implemented in StructuredCloneHolder::CustomCanTransferHandler, but double
+  // check here as the state might have changed by
+  // Readable/WritableStream::Transfer in case the stream members of this
+  // TransformStream are being transferred together.)
+  if (IsReadableStreamLocked(mReadable) || IsWritableStreamLocked(mWritable)) {
+    return false;
+  }
+
+  // Step 5: Set dataHolder.[[readable]] to !
+  // StructuredSerializeWithTransfer(readable, « readable »).
+  // TODO: Mark mReadable as MOZ_KNOWN_LIVE again (bug 1769854)
+  if (!MOZ_KnownLive(mReadable)->Transfer(aCx, aPortId1)) {
+    return false;
+  }
+
+  // Step 6: Set dataHolder.[[writable]] to !
+  // StructuredSerializeWithTransfer(writable, « writable »).
+  // TODO: Mark mReadable as MOZ_KNOWN_LIVE again (bug 1769854)
+  return MOZ_KnownLive(mWritable)->Transfer(aCx, aPortId2);
+}
+
+// https://streams.spec.whatwg.org/#ref-for-transfer-receiving-steps②
+bool TransformStream::ReceiveTransfer(
+    JSContext* aCx, nsIGlobalObject* aGlobal, MessagePort& aPort1,
+    MessagePort& aPort2, JS::MutableHandle<JSObject*> aReturnObject) {
+  // Step 1: Let readableRecord be !
+  // StructuredDeserializeWithTransfer(dataHolder.[[readable]], the current
+  // Realm).
+  RefPtr<ReadableStream> readable =
+      ReadableStream::ReceiveTransferImpl(aCx, aGlobal, aPort1);
+  if (!readable) {
+    return false;
+  }
+
+  // Step 2: Let writableRecord be !
+  // StructuredDeserializeWithTransfer(dataHolder.[[writable]], the current
+  // Realm).
+  RefPtr<WritableStream> writable =
+      WritableStream::ReceiveTransferImpl(aCx, aGlobal, aPort2);
+  if (!writable) {
+    return false;
+  }
+
+  // Step 3: Set value.[[readable]] to readableRecord.[[Deserialized]].
+  // Step 4: Set value.[[writable]] to writableRecord.[[Deserialized]].
+  // Step 5: Set value.[[backpressure]], value.[[backpressureChangePromise]],
+  // and value.[[controller]] to undefined.
+  RefPtr<TransformStream> stream =
+      new TransformStream(aGlobal, readable, writable);
+  JS::Rooted<JS::Value> value(aCx);
+  if (!GetOrCreateDOMReflector(aCx, stream, &value)) {
+    JS_ClearPendingException(aCx);
+    return false;
+  }
+  aReturnObject.set(&value.toObject());
+
+  return true;
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/TransformStream.cpp b/dom/streams/TransformStream.cpp
new file mode 100644
index 0000000000..e517ecda89
--- /dev/null
+++ b/dom/streams/TransformStream.cpp
@@ -0,0 +1,706 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/TransformStream.h"
+
+#include "StreamUtils.h"
+#include "TransformerCallbackHelpers.h"
+#include "UnderlyingSourceCallbackHelpers.h"
+#include "js/TypeDecls.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/Promise-inl.h"
+#include "mozilla/dom/WritableStream.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/RootedDictionary.h"
+#include "mozilla/dom/TransformStreamBinding.h"
+#include "mozilla/dom/TransformerBinding.h"
+#include "nsWrapperCache.h"
+
+// XXX: GCC somehow does not allow attributes before lambda return types, while
+// clang requires so. See also bug 1627007.
+#ifdef __clang__
+#  define MOZ_CAN_RUN_SCRIPT_BOUNDARY_LAMBDA MOZ_CAN_RUN_SCRIPT_BOUNDARY
+#else
+#  define MOZ_CAN_RUN_SCRIPT_BOUNDARY_LAMBDA
+#endif
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(TransformStream, mGlobal,
+                                      mBackpressureChangePromise, mController,
+                                      mReadable, mWritable)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(TransformStream)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(TransformStream)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(TransformStream)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+// https://streams.spec.whatwg.org/#transformstream-set-up
+// (except this instead creates a new TransformStream rather than accepting an
+// existing instance)
+already_AddRefed<TransformStream> TransformStream::CreateGeneric(
+    const GlobalObject& aGlobal, TransformerAlgorithmsWrapper& aAlgorithms,
+    ErrorResult& aRv) {
+  // Step 1. Let writableHighWaterMark be 1.
+  double writableHighWaterMark = 1;
+
+  // Step 2. Let writableSizeAlgorithm be an algorithm that returns 1.
+  // Note: Callers should recognize nullptr as a callback that returns 1. See
+  // also WritableStream::Constructor for this design decision.
+  RefPtr<QueuingStrategySize> writableSizeAlgorithm;
+
+  // Step 3. Let readableHighWaterMark be 0.
+  double readableHighWaterMark = 0;
+
+  // Step 4. Let readableSizeAlgorithm be an algorithm that returns 1.
+  // Note: Callers should recognize nullptr as a callback that returns 1. See
+  // also ReadableStream::Constructor for this design decision.
+  RefPtr<QueuingStrategySize> readableSizeAlgorithm;
+
+  // Step 5. Let transformAlgorithmWrapper be an algorithm that runs these steps
+  // given a value chunk:
+  // Step 6. Let flushAlgorithmWrapper be an algorithm that runs these steps:
+  // (Done by TransformerAlgorithmsWrapper)
+
+  // Step 7. Let startPromise be a promise resolved with undefined.
+  nsCOMPtr<nsIGlobalObject> global = do_QueryInterface(aGlobal.GetAsSupports());
+  RefPtr<Promise> startPromise =
+      Promise::CreateResolvedWithUndefined(global, aRv);
+  if (!startPromise) {
+    return nullptr;
+  }
+
+  // Step 8. Perform ! InitializeTransformStream(stream, startPromise,
+  // writableHighWaterMark, writableSizeAlgorithm, readableHighWaterMark,
+  // readableSizeAlgorithm).
+  RefPtr<TransformStream> stream =
+      new TransformStream(global, nullptr, nullptr);
+  stream->Initialize(aGlobal.Context(), startPromise, writableHighWaterMark,
+                     writableSizeAlgorithm, readableHighWaterMark,
+                     readableSizeAlgorithm, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 9. Let controller be a new TransformStreamDefaultController.
+  auto controller = MakeRefPtr<TransformStreamDefaultController>(global);
+
+  // Step 10. Perform ! SetUpTransformStreamDefaultController(stream,
+  // controller, transformAlgorithmWrapper, flushAlgorithmWrapper).
+  SetUpTransformStreamDefaultController(aGlobal.Context(), *stream, *controller,
+                                        aAlgorithms);
+
+  return stream.forget();
+}
+
+TransformStream::TransformStream(nsIGlobalObject* aGlobal) : mGlobal(aGlobal) {
+  mozilla::HoldJSObjects(this);
+}
+
+TransformStream::TransformStream(nsIGlobalObject* aGlobal,
+                                 ReadableStream* aReadable,
+                                 WritableStream* aWritable)
+    : mGlobal(aGlobal), mReadable(aReadable), mWritable(aWritable) {
+  mozilla::HoldJSObjects(this);
+}
+
+TransformStream::~TransformStream() { mozilla::DropJSObjects(this); }
+
+JSObject* TransformStream::WrapObject(JSContext* aCx,
+                                      JS::Handle<JSObject*> aGivenProto) {
+  return TransformStream_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+namespace streams_abstract {
+
+// https://streams.spec.whatwg.org/#transform-stream-error-writable-and-unblock-write
+void TransformStreamErrorWritableAndUnblockWrite(JSContext* aCx,
+                                                 TransformStream* aStream,
+                                                 JS::Handle<JS::Value> aError,
+                                                 ErrorResult& aRv) {
+  // Step 1: Perform !
+  // TransformStreamDefaultControllerClearAlgorithms(stream.[[controller]]).
+  aStream->Controller()->SetAlgorithms(nullptr);
+
+  // Step 2: Perform !
+  // WritableStreamDefaultControllerErrorIfNeeded(stream.[[writable]].[[controller]],
+  // e).
+  // TODO: Remove MOZ_KnownLive (bug 1761577)
+  WritableStreamDefaultControllerErrorIfNeeded(
+      aCx, MOZ_KnownLive(aStream->Writable()->Controller()), aError, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 3: If stream.[[backpressure]] is true, perform !
+  // TransformStreamSetBackpressure(stream, false).
+  if (aStream->Backpressure()) {
+    aStream->SetBackpressure(false);
+  }
+}
+
+// https://streams.spec.whatwg.org/#transform-stream-error
+void TransformStreamError(JSContext* aCx, TransformStream* aStream,
+                          JS::Handle<JS::Value> aError, ErrorResult& aRv) {
+  // Step 1: Perform !
+  // ReadableStreamDefaultControllerError(stream.[[readable]].[[controller]],
+  // e).
+  ReadableStreamDefaultControllerError(
+      aCx, aStream->Readable()->Controller()->AsDefault(), aError, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 2: Perform ! TransformStreamErrorWritableAndUnblockWrite(stream, e).
+  TransformStreamErrorWritableAndUnblockWrite(aCx, aStream, aError, aRv);
+}
+
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#transform-stream-default-controller-perform-transform
+MOZ_CAN_RUN_SCRIPT static already_AddRefed<Promise>
+TransformStreamDefaultControllerPerformTransform(
+    JSContext* aCx, TransformStreamDefaultController* aController,
+    JS::Handle<JS::Value> aChunk, ErrorResult& aRv) {
+  // Step 1: Let transformPromise be the result of performing
+  // controller.[[transformAlgorithm]], passing chunk.
+  RefPtr<TransformerAlgorithmsBase> algorithms = aController->Algorithms();
+  RefPtr<Promise> transformPromise =
+      algorithms->TransformCallback(aCx, aChunk, *aController, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 2: Return the result of reacting to transformPromise with the
+  // following rejection steps given the argument r:
+  auto result = transformPromise->CatchWithCycleCollectedArgs(
+      [](JSContext* aCx, JS::Handle<JS::Value> aError, ErrorResult& aRv,
+         const RefPtr<TransformStreamDefaultController>& aController)
+          MOZ_CAN_RUN_SCRIPT_BOUNDARY_LAMBDA -> already_AddRefed<Promise> {
+            // Step 2.1: Perform ! TransformStreamError(controller.[[stream]],
+            // r).
+            // TODO: Remove MOZ_KnownLive (bug 1761577)
+            TransformStreamError(aCx, MOZ_KnownLive(aController->Stream()),
+                                 aError, aRv);
+            if (aRv.Failed()) {
+              return nullptr;
+            }
+
+            // Step 2.2: Throw r.
+            JS::Rooted<JS::Value> r(aCx, aError);
+            aRv.MightThrowJSException();
+            aRv.ThrowJSException(aCx, r);
+            return nullptr;
+          },
+      RefPtr(aController));
+  if (result.isErr()) {
+    aRv.Throw(result.unwrapErr());
+    return nullptr;
+  }
+  return result.unwrap().forget();
+}
+
+// https://streams.spec.whatwg.org/#initialize-transform-stream
+class TransformStreamUnderlyingSinkAlgorithms final
+    : public UnderlyingSinkAlgorithmsBase {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(
+      TransformStreamUnderlyingSinkAlgorithms, UnderlyingSinkAlgorithmsBase)
+
+  TransformStreamUnderlyingSinkAlgorithms(Promise* aStartPromise,
+                                          TransformStream* aStream)
+      : mStartPromise(aStartPromise), mStream(aStream) {}
+
+  void StartCallback(JSContext* aCx,
+                     WritableStreamDefaultController& aController,
+                     JS::MutableHandle<JS::Value> aRetVal,
+                     ErrorResult& aRv) override {
+    // Step 1. Let startAlgorithm be an algorithm that returns startPromise.
+    // (Same as TransformStreamUnderlyingSourceAlgorithms::StartCallback)
+    aRetVal.setObject(*mStartPromise->PromiseObj());
+  }
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> WriteCallback(
+      JSContext* aCx, JS::Handle<JS::Value> aChunk,
+      WritableStreamDefaultController& aController, ErrorResult& aRv) override {
+    // Step 2. Let writeAlgorithm be the following steps, taking a chunk
+    // argument:
+    // Step 2.1. Return ! TransformStreamDefaultSinkWriteAlgorithm(stream,
+    // chunk).
+
+    // Inlining TransformStreamDefaultSinkWriteAlgorithm here:
+    // https://streams.spec.whatwg.org/#transform-stream-default-sink-write-algorithm
+
+    // Step 1: Assert: stream.[[writable]].[[state]] is "writable".
+    MOZ_ASSERT(mStream->Writable()->State() ==
+               WritableStream::WriterState::Writable);
+
+    // Step 2: Let controller be stream.[[controller]].
+    RefPtr<TransformStreamDefaultController> controller = mStream->Controller();
+
+    // Step 3: If stream.[[backpressure]] is true,
+    if (mStream->Backpressure()) {
+      // Step 3.1: Let backpressureChangePromise be
+      // stream.[[backpressureChangePromise]].
+      RefPtr<Promise> backpressureChangePromise =
+          mStream->BackpressureChangePromise();
+
+      // Step 3.2: Assert: backpressureChangePromise is not undefined.
+      MOZ_ASSERT(backpressureChangePromise);
+
+      // Step 3.3: Return the result of reacting to backpressureChangePromise
+      // with the following fulfillment steps:
+      auto result = backpressureChangePromise->ThenWithCycleCollectedArgsJS(
+          [](JSContext* aCx, JS::Handle<JS::Value>, ErrorResult& aRv,
+             const RefPtr<TransformStream>& aStream,
+             const RefPtr<TransformStreamDefaultController>& aController,
+             JS::Handle<JS::Value> aChunk)
+              MOZ_CAN_RUN_SCRIPT_BOUNDARY_LAMBDA -> already_AddRefed<Promise> {
+                // Step 1: Let writable be stream.[[writable]].
+                RefPtr<WritableStream> writable = aStream->Writable();
+
+                // Step 2: Let state be writable.[[state]].
+                WritableStream::WriterState state = writable->State();
+
+                // Step 3: If state is "erroring", throw
+                // writable.[[storedError]].
+                if (state == WritableStream::WriterState::Erroring) {
+                  JS::Rooted<JS::Value> storedError(aCx,
+                                                    writable->StoredError());
+                  aRv.MightThrowJSException();
+                  aRv.ThrowJSException(aCx, storedError);
+                  return nullptr;
+                }
+
+                // Step 4: Assert: state is "writable".
+                MOZ_ASSERT(state == WritableStream::WriterState::Writable);
+
+                // Step 5: Return !
+                // TransformStreamDefaultControllerPerformTransform(controller,
+                // chunk).
+                return TransformStreamDefaultControllerPerformTransform(
+                    aCx, aController, aChunk, aRv);
+              },
+          std::make_tuple(mStream, controller), std::make_tuple(aChunk));
+
+      if (result.isErr()) {
+        aRv.Throw(result.unwrapErr());
+        return nullptr;
+      }
+      return result.unwrap().forget();
+    }
+
+    // Step 4: Return !
+    // TransformStreamDefaultControllerPerformTransform(controller, chunk).
+    return TransformStreamDefaultControllerPerformTransform(aCx, controller,
+                                                            aChunk, aRv);
+  }
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> AbortCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) override {
+    // Step 3. Let abortAlgorithm be the following steps, taking a reason
+    // argument:
+    // Step 3.1. Return ! TransformStreamDefaultSinkAbortAlgorithm(stream,
+    // reason).
+
+    // Inlining TransformStreamDefaultSinkAbortAlgorithm here:
+    // https://streams.spec.whatwg.org/#transform-stream-default-sink-abort-algorithm
+
+    // Step 1:Perform ! TransformStreamError(stream, reason).
+    TransformStreamError(
+        aCx, mStream,
+        aReason.WasPassed() ? aReason.Value() : JS::UndefinedHandleValue, aRv);
+    if (aRv.Failed()) {
+      return nullptr;
+    }
+
+    // Step 2: Return a promise resolved with undefined.
+    return Promise::CreateResolvedWithUndefined(mStream->GetParentObject(),
+                                                aRv);
+  }
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> CloseCallback(
+      JSContext* aCx, ErrorResult& aRv) override {
+    // Step 4. Let closeAlgorithm be the following steps:
+    // Step 4.1. Return ! TransformStreamDefaultSinkCloseAlgorithm(stream).
+
+    // Inlining TransformStreamDefaultSinkCloseAlgorithm here:
+    // https://streams.spec.whatwg.org/#transform-stream-default-sink-close-algorithm
+
+    // Step 1: Let readable be stream.[[readable]].
+    RefPtr<ReadableStream> readable = mStream->Readable();
+
+    // Step 2: Let controller be stream.[[controller]].
+    RefPtr<TransformStreamDefaultController> controller = mStream->Controller();
+
+    // Step 3: Let flushPromise be the result of performing
+    // controller.[[flushAlgorithm]].
+    RefPtr<TransformerAlgorithmsBase> algorithms = controller->Algorithms();
+    RefPtr<Promise> flushPromise =
+        algorithms->FlushCallback(aCx, *controller, aRv);
+    if (aRv.Failed()) {
+      return nullptr;
+    }
+
+    // Step 4: Perform !
+    // TransformStreamDefaultControllerClearAlgorithms(controller).
+    controller->SetAlgorithms(nullptr);
+
+    // Step 5: Return the result of reacting to flushPromise:
+    Result<RefPtr<Promise>, nsresult> result =
+        flushPromise->ThenCatchWithCycleCollectedArgs(
+            [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+               const RefPtr<ReadableStream>& aReadable,
+               const RefPtr<TransformStream>& aStream)
+                MOZ_CAN_RUN_SCRIPT_BOUNDARY_LAMBDA
+            -> already_AddRefed<Promise> {
+                  // Step 5.1: If flushPromise was fulfilled, then:
+
+                  // Step 5.1.1: If readable.[[state]] is "errored", throw
+                  // readable.[[storedError]].
+                  if (aReadable->State() ==
+                      ReadableStream::ReaderState::Errored) {
+                    JS::Rooted<JS::Value> storedError(aCx,
+                                                      aReadable->StoredError());
+                    aRv.MightThrowJSException();
+                    aRv.ThrowJSException(aCx, storedError);
+                    return nullptr;
+                  }
+
+                  // Step 5.1.2: Perform !
+                  // ReadableStreamDefaultControllerClose(readable.[[controller]]).
+                  ReadableStreamDefaultControllerClose(
+                      aCx, MOZ_KnownLive(aReadable->Controller()->AsDefault()),
+                      aRv);
+                  return nullptr;
+                },
+            [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+               const RefPtr<ReadableStream>& aReadable,
+               const RefPtr<TransformStream>& aStream)
+                MOZ_CAN_RUN_SCRIPT_BOUNDARY_LAMBDA
+            -> already_AddRefed<Promise> {
+                  // Step 5.2: If flushPromise was rejected with reason r, then:
+
+                  // Step 5.2.1: Perform ! TransformStreamError(stream, r).
+                  TransformStreamError(aCx, aStream, aValue, aRv);
+                  if (aRv.Failed()) {
+                    return nullptr;
+                  }
+
+                  // Step 5.2.2: Throw readable.[[storedError]].
+                  JS::Rooted<JS::Value> storedError(aCx,
+                                                    aReadable->StoredError());
+                  aRv.MightThrowJSException();
+                  aRv.ThrowJSException(aCx, storedError);
+                  return nullptr;
+                },
+            readable, mStream);
+
+    if (result.isErr()) {
+      aRv.Throw(result.unwrapErr());
+      return nullptr;
+    }
+    return result.unwrap().forget();
+  }
+
+ protected:
+  ~TransformStreamUnderlyingSinkAlgorithms() override = default;
+
+ private:
+  RefPtr<Promise> mStartPromise;
+  // MOZ_KNOWN_LIVE because it won't be reassigned
+  MOZ_KNOWN_LIVE RefPtr<TransformStream> mStream;
+};
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(TransformStreamUnderlyingSinkAlgorithms,
+                                   UnderlyingSinkAlgorithmsBase, mStartPromise,
+                                   mStream)
+NS_IMPL_ADDREF_INHERITED(TransformStreamUnderlyingSinkAlgorithms,
+                         UnderlyingSinkAlgorithmsBase)
+NS_IMPL_RELEASE_INHERITED(TransformStreamUnderlyingSinkAlgorithms,
+                          UnderlyingSinkAlgorithmsBase)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(TransformStreamUnderlyingSinkAlgorithms)
+NS_INTERFACE_MAP_END_INHERITING(UnderlyingSinkAlgorithmsBase)
+
+// https://streams.spec.whatwg.org/#initialize-transform-stream
+class TransformStreamUnderlyingSourceAlgorithms final
+    : public UnderlyingSourceAlgorithmsBase {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(
+      TransformStreamUnderlyingSourceAlgorithms, UnderlyingSourceAlgorithmsBase)
+
+  TransformStreamUnderlyingSourceAlgorithms(Promise* aStartPromise,
+                                            TransformStream* aStream)
+      : mStartPromise(aStartPromise), mStream(aStream) {}
+
+  void StartCallback(JSContext* aCx, ReadableStreamController& aController,
+                     JS::MutableHandle<JS::Value> aRetVal,
+                     ErrorResult& aRv) override {
+    // Step 1. Let startAlgorithm be an algorithm that returns startPromise.
+    // (Same as TransformStreamUnderlyingSinkAlgorithms::StartCallback)
+    aRetVal.setObject(*mStartPromise->PromiseObj());
+  }
+
+  already_AddRefed<Promise> PullCallback(JSContext* aCx,
+                                         ReadableStreamController& aController,
+                                         ErrorResult& aRv) override {
+    // Step 6. Let pullAlgorithm be the following steps:
+    // Step 6.1. Return ! TransformStreamDefaultSourcePullAlgorithm(stream).
+
+    // Inlining TransformStreamDefaultSourcePullAlgorithm here:
+    // https://streams.spec.whatwg.org/#transform-stream-default-source-pull-algorithm
+
+    // Step 1: Assert: stream.[[backpressure]] is true.
+    MOZ_ASSERT(mStream->Backpressure());
+
+    // Step 2: Assert: stream.[[backpressureChangePromise]] is not undefined.
+    MOZ_ASSERT(mStream->BackpressureChangePromise());
+
+    // Step 3: Perform ! TransformStreamSetBackpressure(stream, false).
+    mStream->SetBackpressure(false);
+
+    // Step 4: Return stream.[[backpressureChangePromise]].
+    return do_AddRef(mStream->BackpressureChangePromise());
+  }
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> CancelCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) override {
+    // Step 7. Let cancelAlgorithm be the following steps, taking a reason
+    // argument:
+    // Step 7.1. Perform ! TransformStreamErrorWritableAndUnblockWrite(stream,
+    // reason).
+    TransformStreamErrorWritableAndUnblockWrite(
+        aCx, mStream,
+        aReason.WasPassed() ? aReason.Value() : JS::UndefinedHandleValue, aRv);
+    if (aRv.Failed()) {
+      return nullptr;
+    }
+
+    // Step 7.2. Return a promise resolved with undefined.
+    return Promise::CreateResolvedWithUndefined(mStream->GetParentObject(),
+                                                aRv);
+  }
+
+ protected:
+  ~TransformStreamUnderlyingSourceAlgorithms() override = default;
+
+ private:
+  RefPtr<Promise> mStartPromise;
+  // MOZ_KNOWNLIVE because it will never be reassigned
+  MOZ_KNOWN_LIVE RefPtr<TransformStream> mStream;
+};
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED(TransformStreamUnderlyingSourceAlgorithms,
+                                   UnderlyingSourceAlgorithmsBase,
+                                   mStartPromise, mStream)
+NS_IMPL_ADDREF_INHERITED(TransformStreamUnderlyingSourceAlgorithms,
+                         UnderlyingSourceAlgorithmsBase)
+NS_IMPL_RELEASE_INHERITED(TransformStreamUnderlyingSourceAlgorithms,
+                          UnderlyingSourceAlgorithmsBase)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(
+    TransformStreamUnderlyingSourceAlgorithms)
+NS_INTERFACE_MAP_END_INHERITING(UnderlyingSourceAlgorithmsBase)
+
+// https://streams.spec.whatwg.org/#transform-stream-set-backpressure
+void TransformStream::SetBackpressure(bool aBackpressure) {
+  // Step 1. Assert: stream.[[backpressure]] is not backpressure.
+  MOZ_ASSERT(Backpressure() != aBackpressure);
+
+  // Step 2. If stream.[[backpressureChangePromise]] is not undefined, resolve
+  // stream.[[backpressureChangePromise]] with undefined.
+  if (Promise* promise = BackpressureChangePromise()) {
+    promise->MaybeResolveWithUndefined();
+  }
+
+  // Step 3. Set stream.[[backpressureChangePromise]] to a new promise.
+  RefPtr<Promise> promise = Promise::CreateInfallible(GetParentObject());
+  mBackpressureChangePromise = promise;
+
+  // Step 4. Set stream.[[backpressure]] to backpressure.
+  mBackpressure = aBackpressure;
+}
+
+// https://streams.spec.whatwg.org/#initialize-transform-stream
+void TransformStream::Initialize(JSContext* aCx, Promise* aStartPromise,
+                                 double aWritableHighWaterMark,
+                                 QueuingStrategySize* aWritableSizeAlgorithm,
+                                 double aReadableHighWaterMark,
+                                 QueuingStrategySize* aReadableSizeAlgorithm,
+                                 ErrorResult& aRv) {
+  // Step 1 - 4
+  auto sinkAlgorithms =
+      MakeRefPtr<TransformStreamUnderlyingSinkAlgorithms>(aStartPromise, this);
+
+  // Step 5. Set stream.[[writable]] to ! CreateWritableStream(startAlgorithm,
+  // writeAlgorithm, closeAlgorithm, abortAlgorithm, writableHighWaterMark,
+  // writableSizeAlgorithm).
+  mWritable = WritableStream::CreateAbstract(
+      aCx, MOZ_KnownLive(mGlobal), sinkAlgorithms, aWritableHighWaterMark,
+      aWritableSizeAlgorithm, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 6 - 7
+  auto sourceAlgorithms = MakeRefPtr<TransformStreamUnderlyingSourceAlgorithms>(
+      aStartPromise, this);
+
+  // Step 8. Set stream.[[readable]] to ! CreateReadableStream(startAlgorithm,
+  // pullAlgorithm, cancelAlgorithm, readableHighWaterMark,
+  // readableSizeAlgorithm).
+  mReadable = ReadableStream::CreateAbstract(
+      aCx, MOZ_KnownLive(mGlobal), sourceAlgorithms,
+      Some(aReadableHighWaterMark), aReadableSizeAlgorithm, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 9. Set stream.[[backpressure]] and
+  // stream.[[backpressureChangePromise]] to undefined.
+  // Note(krosylight): The spec allows setting [[backpressure]] as undefined,
+  // but I don't see why it should be. Since the spec also allows strict boolean
+  // type, and this is only to not trigger assertion inside the setter, we just
+  // set it as false.
+  mBackpressure = false;
+  mBackpressureChangePromise = nullptr;
+
+  // Step 10. Perform ! TransformStreamSetBackpressure(stream, true).
+  SetBackpressure(true);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 11. Set stream.[[controller]] to undefined.
+  mController = nullptr;
+}
+
+// https://streams.spec.whatwg.org/#ts-constructor
+already_AddRefed<TransformStream> TransformStream::Constructor(
+    const GlobalObject& aGlobal,
+    const Optional<JS::Handle<JSObject*>>& aTransformer,
+    const QueuingStrategy& aWritableStrategy,
+    const QueuingStrategy& aReadableStrategy, ErrorResult& aRv) {
+  // Step 1. If transformer is missing, set it to null.
+  JS::Rooted<JSObject*> transformerObj(
+      aGlobal.Context(),
+      aTransformer.WasPassed() ? aTransformer.Value() : nullptr);
+
+  // Step 2. Let transformerDict be transformer, converted to an IDL value of
+  // type Transformer.
+  RootedDictionary<Transformer> transformerDict(aGlobal.Context());
+  if (transformerObj) {
+    JS::Rooted<JS::Value> objValue(aGlobal.Context(),
+                                   JS::ObjectValue(*transformerObj));
+    dom::BindingCallContext callCx(aGlobal.Context(),
+                                   "TransformStream.constructor");
+    aRv.MightThrowJSException();
+    if (!transformerDict.Init(callCx, objValue)) {
+      aRv.StealExceptionFromJSContext(aGlobal.Context());
+      return nullptr;
+    }
+  }
+
+  // Step 3. If transformerDict["readableType"] exists, throw a RangeError
+  // exception.
+  if (!transformerDict.mReadableType.isUndefined()) {
+    aRv.ThrowRangeError(
+        "`readableType` is unsupported and preserved for future use");
+    return nullptr;
+  }
+
+  // Step 4. If transformerDict["writableType"] exists, throw a RangeError
+  // exception.
+  if (!transformerDict.mWritableType.isUndefined()) {
+    aRv.ThrowRangeError(
+        "`writableType` is unsupported and preserved for future use");
+    return nullptr;
+  }
+
+  // Step 5. Let readableHighWaterMark be ?
+  // ExtractHighWaterMark(readableStrategy, 0).
+  double readableHighWaterMark =
+      ExtractHighWaterMark(aReadableStrategy, 0, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 6. Let readableSizeAlgorithm be !
+  // ExtractSizeAlgorithm(readableStrategy).
+  // Note: Callers should recognize nullptr as a callback that returns 1. See
+  // also ReadableStream::Constructor for this design decision.
+  RefPtr<QueuingStrategySize> readableSizeAlgorithm =
+      aReadableStrategy.mSize.WasPassed() ? &aReadableStrategy.mSize.Value()
+                                          : nullptr;
+
+  // Step 7. Let writableHighWaterMark be ?
+  // ExtractHighWaterMark(writableStrategy, 1).
+  double writableHighWaterMark =
+      ExtractHighWaterMark(aWritableStrategy, 1, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 8. Let writableSizeAlgorithm be !
+  // ExtractSizeAlgorithm(writableStrategy).
+  // Note: Callers should recognize nullptr as a callback that returns 1. See
+  // also WritableStream::Constructor for this design decision.
+  RefPtr<QueuingStrategySize> writableSizeAlgorithm =
+      aWritableStrategy.mSize.WasPassed() ? &aWritableStrategy.mSize.Value()
+                                          : nullptr;
+
+  // Step 9. Let startPromise be a new promise.
+  nsCOMPtr<nsIGlobalObject> global = do_QueryInterface(aGlobal.GetAsSupports());
+  RefPtr<Promise> startPromise = Promise::CreateInfallible(global);
+
+  // Step 10. Perform ! InitializeTransformStream(this, startPromise,
+  // writableHighWaterMark, writableSizeAlgorithm, readableHighWaterMark,
+  // readableSizeAlgorithm).
+  RefPtr<TransformStream> transformStream = new TransformStream(global);
+  transformStream->Initialize(
+      aGlobal.Context(), startPromise, writableHighWaterMark,
+      writableSizeAlgorithm, readableHighWaterMark, readableSizeAlgorithm, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 11. Perform ?
+  // SetUpTransformStreamDefaultControllerFromTransformer(this, transformer,
+  // transformerDict).
+  SetUpTransformStreamDefaultControllerFromTransformer(
+      aGlobal.Context(), *transformStream, transformerObj, transformerDict);
+
+  // Step 12. If transformerDict["start"] exists, then resolve startPromise with
+  // the result of invoking transformerDict["start"] with argument list «
+  // this.[[controller]] » and callback this value transformer.
+  if (transformerDict.mStart.WasPassed()) {
+    RefPtr<TransformerStartCallback> callback = transformerDict.mStart.Value();
+    RefPtr<TransformStreamDefaultController> controller =
+        transformStream->Controller();
+    JS::Rooted<JS::Value> retVal(aGlobal.Context());
+    callback->Call(transformerObj, *controller, &retVal, aRv,
+                   "Transformer.start", CallbackFunction::eRethrowExceptions);
+    if (aRv.Failed()) {
+      return nullptr;
+    }
+
+    startPromise->MaybeResolve(retVal);
+  } else {
+    // Step 13. Otherwise, resolve startPromise with undefined.
+    startPromise->MaybeResolveWithUndefined();
+  }
+
+  return transformStream.forget();
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/TransformStream.h b/dom/streams/TransformStream.h
new file mode 100644
index 0000000000..ed9ec6f812
--- /dev/null
+++ b/dom/streams/TransformStream.h
@@ -0,0 +1,115 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 DOM_STREAMS_TRANSFORMSTREAM_H_
+#define DOM_STREAMS_TRANSFORMSTREAM_H_
+
+#include "TransformStreamDefaultController.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+
+#include "mozilla/dom/TransformerBinding.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsWrapperCache.h"
+
+namespace mozilla::dom {
+
+class WritableStream;
+class ReadableStream;
+class UniqueMessagePortId;
+class MessagePort;
+class TransformerAlgorithmsWrapper;
+
+class TransformStream final : public nsISupports, public nsWrapperCache {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(TransformStream)
+
+  // https://streams.spec.whatwg.org/#transformstream-set-up
+  // Intended to be used by interfaces using GenericTransformStream mixin.
+  MOZ_CAN_RUN_SCRIPT static already_AddRefed<TransformStream> CreateGeneric(
+      const GlobalObject& aGlobal, TransformerAlgorithmsWrapper& aAlgorithms,
+      ErrorResult& aRv);
+
+  // Internal slot accessors
+  bool Backpressure() const { return mBackpressure; }
+  Promise* BackpressureChangePromise() { return mBackpressureChangePromise; }
+  void SetBackpressure(bool aBackpressure);
+  MOZ_KNOWN_LIVE TransformStreamDefaultController* Controller() {
+    return mController;
+  }
+  void SetController(TransformStreamDefaultController& aController) {
+    MOZ_ASSERT(!mController);
+    mController = &aController;
+  }
+
+  // [Transferable]
+  // https://html.spec.whatwg.org/multipage/structured-data.html#transfer-steps
+  MOZ_CAN_RUN_SCRIPT bool Transfer(JSContext* aCx,
+                                   UniqueMessagePortId& aPortId1,
+                                   UniqueMessagePortId& aPortId2);
+  // https://html.spec.whatwg.org/multipage/structured-data.html#transfer-receiving-steps
+  static MOZ_CAN_RUN_SCRIPT bool ReceiveTransfer(
+      JSContext* aCx, nsIGlobalObject* aGlobal, MessagePort& aPort1,
+      MessagePort& aPort2, JS::MutableHandle<JSObject*> aReturnObject);
+
+ protected:
+  TransformStream(nsIGlobalObject* aGlobal, ReadableStream* aReadable,
+                  WritableStream* aWritable);
+  explicit TransformStream(nsIGlobalObject* aGlobal);
+
+  ~TransformStream();
+
+  MOZ_CAN_RUN_SCRIPT void Initialize(
+      JSContext* aCx, Promise* aStartPromise, double aWritableHighWaterMark,
+      QueuingStrategySize* aWritableSizeAlgorithm,
+      double aReadableHighWaterMark,
+      QueuingStrategySize* aReadableSizeAlgorithm, ErrorResult& aRv);
+
+ public:
+  nsIGlobalObject* GetParentObject() const { return mGlobal; }
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  // WebIDL methods
+  // TODO: Mark as MOZ_CAN_RUN_SCRIPT when IDL constructors can be (bug 1749042)
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY static already_AddRefed<TransformStream>
+  Constructor(const GlobalObject& aGlobal,
+              const Optional<JS::Handle<JSObject*>>& aTransformer,
+              const QueuingStrategy& aWritableStrategy,
+              const QueuingStrategy& aReadableStrategy, ErrorResult& aRv);
+
+  ReadableStream* Readable() const { return mReadable; }
+  WritableStream* Writable() const { return mWritable; }
+
+ private:
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+
+  // Internal slots
+  // MOZ_KNOWN_LIVE for slots that will never be reassigned
+  bool mBackpressure = false;
+  RefPtr<Promise> mBackpressureChangePromise;
+  RefPtr<TransformStreamDefaultController> mController;
+  RefPtr<ReadableStream> mReadable;
+  RefPtr<WritableStream> mWritable;
+};
+
+namespace streams_abstract {
+
+MOZ_CAN_RUN_SCRIPT void TransformStreamErrorWritableAndUnblockWrite(
+    JSContext* aCx, TransformStream* aStream, JS::Handle<JS::Value> aError,
+    ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void TransformStreamError(JSContext* aCx,
+                                             TransformStream* aStream,
+                                             JS::Handle<JS::Value> aError,
+                                             ErrorResult& aRv);
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
+
+#endif  // DOM_STREAMS_TRANSFORMSTREAM_H_
diff --git a/dom/streams/TransformStreamDefaultController.cpp b/dom/streams/TransformStreamDefaultController.cpp
new file mode 100644
index 0000000000..bbcb7f94f2
--- /dev/null
+++ b/dom/streams/TransformStreamDefaultController.cpp
@@ -0,0 +1,238 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/TransformStreamDefaultController.h"
+
+#include "TransformerCallbackHelpers.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/ReadableStreamDefaultController.h"
+#include "mozilla/dom/TransformStream.h"
+#include "mozilla/dom/TransformStreamDefaultControllerBinding.h"
+#include "nsWrapperCache.h"
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(TransformStreamDefaultController, mGlobal,
+                                      mStream, mTransformerAlgorithms)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(TransformStreamDefaultController)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(TransformStreamDefaultController)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(TransformStreamDefaultController)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+TransformStream* TransformStreamDefaultController::Stream() { return mStream; }
+
+void TransformStreamDefaultController::SetStream(TransformStream& aStream) {
+  MOZ_ASSERT(!mStream);
+  mStream = &aStream;
+}
+
+TransformerAlgorithmsBase* TransformStreamDefaultController::Algorithms() {
+  return mTransformerAlgorithms;
+}
+
+void TransformStreamDefaultController::SetAlgorithms(
+    TransformerAlgorithmsBase* aTransformerAlgorithms) {
+  mTransformerAlgorithms = aTransformerAlgorithms;
+}
+
+TransformStreamDefaultController::TransformStreamDefaultController(
+    nsIGlobalObject* aGlobal)
+    : mGlobal(aGlobal) {
+  mozilla::HoldJSObjects(this);
+}
+
+TransformStreamDefaultController::~TransformStreamDefaultController() {
+  mozilla::DropJSObjects(this);
+}
+
+JSObject* TransformStreamDefaultController::WrapObject(
+    JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
+  return TransformStreamDefaultController_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+// https://streams.spec.whatwg.org/#ts-default-controller-desired-size
+Nullable<double> TransformStreamDefaultController::GetDesiredSize() const {
+  // Step 1. Let readableController be
+  // this.[[stream]].[[readable]].[[controller]].
+  RefPtr<ReadableStreamDefaultController> readableController =
+      mStream->Readable()->Controller()->AsDefault();
+
+  // Step 2. Return !
+  // ReadableStreamDefaultControllerGetDesiredSize(readableController).
+  return ReadableStreamDefaultControllerGetDesiredSize(readableController);
+}
+
+// https://streams.spec.whatwg.org/#rs-default-controller-has-backpressure
+// Looks like a readable stream thing but the spec explicitly says this is for
+// TransformStream.
+static bool ReadableStreamDefaultControllerHasBackpressure(
+    ReadableStreamDefaultController* aController) {
+  // Step 1: If ! ReadableStreamDefaultControllerShouldCallPull(controller) is
+  // true, return false.
+  // Step 2: Otherwise, return true.
+  return !ReadableStreamDefaultControllerShouldCallPull(aController);
+}
+
+void TransformStreamDefaultController::Enqueue(JSContext* aCx,
+                                               JS::Handle<JS::Value> aChunk,
+                                               ErrorResult& aRv) {
+  // Step 1: Perform ? TransformStreamDefaultControllerEnqueue(this, chunk).
+
+  // Inlining TransformStreamDefaultControllerEnqueue here.
+  // https://streams.spec.whatwg.org/#transform-stream-default-controller-enqueue
+
+  // Step 1: Let stream be controller.[[stream]].
+  RefPtr<TransformStream> stream = mStream;
+
+  // Step 2: Let readableController be stream.[[readable]].[[controller]].
+  RefPtr<ReadableStreamDefaultController> readableController =
+      stream->Readable()->Controller()->AsDefault();
+
+  // Step 3: If !
+  // ReadableStreamDefaultControllerCanCloseOrEnqueue(readableController) is
+  // false, throw a TypeError exception.
+  if (!ReadableStreamDefaultControllerCanCloseOrEnqueueAndThrow(
+          readableController, CloseOrEnqueue::Enqueue, aRv)) {
+    return;
+  }
+
+  // Step 4: Let enqueueResult be
+  // ReadableStreamDefaultControllerEnqueue(readableController, chunk).
+  ErrorResult rv;
+  ReadableStreamDefaultControllerEnqueue(aCx, readableController, aChunk, rv);
+
+  // Step 5: If enqueueResult is an abrupt completion,
+  if (rv.MaybeSetPendingException(aCx)) {
+    JS::Rooted<JS::Value> error(aCx);
+    if (!JS_GetPendingException(aCx, &error)) {
+      // Uncatchable exception; we should mark aRv and return.
+      aRv.StealExceptionFromJSContext(aCx);
+      return;
+    }
+    JS_ClearPendingException(aCx);
+
+    // Step 5.1: Perform ! TransformStreamErrorWritableAndUnblockWrite(stream,
+    // enqueueResult.[[Value]]).
+    TransformStreamErrorWritableAndUnblockWrite(aCx, stream, error, aRv);
+
+    // Step 5.2: Throw stream.[[readable]].[[storedError]].
+    JS::Rooted<JS::Value> storedError(aCx, stream->Readable()->StoredError());
+    aRv.MightThrowJSException();
+    aRv.ThrowJSException(aCx, storedError);
+    return;
+  }
+
+  // Step 6: Let backpressure be !
+  // ReadableStreamDefaultControllerHasBackpressure(readableController).
+  bool backpressure =
+      ReadableStreamDefaultControllerHasBackpressure(readableController);
+
+  // Step 7: If backpressure is not stream.[[backpressure]],
+  if (backpressure != stream->Backpressure()) {
+    // Step 7.1: Assert: backpressure is true.
+    MOZ_ASSERT(backpressure);
+
+    // Step 7.2: Perform ! TransformStreamSetBackpressure(true).
+    stream->SetBackpressure(true);
+  }
+}
+
+// https://streams.spec.whatwg.org/#ts-default-controller-error
+void TransformStreamDefaultController::Error(JSContext* aCx,
+                                             JS::Handle<JS::Value> aError,
+                                             ErrorResult& aRv) {
+  // Step 1: Perform ? TransformStreamDefaultControllerError(this, e).
+
+  // Inlining TransformStreamDefaultControllerError here.
+  // https://streams.spec.whatwg.org/#transform-stream-default-controller-error
+
+  // Perform ! TransformStreamError(controller.[[stream]], e).
+  // mStream is set in initialization step and only modified in cycle
+  // collection.
+  // TODO: Move mStream initialization to a method/constructor and make it
+  // MOZ_KNOWN_LIVE again. (See bug 1769854)
+  TransformStreamError(aCx, MOZ_KnownLive(mStream), aError, aRv);
+}
+
+// https://streams.spec.whatwg.org/#ts-default-controller-terminate
+
+void TransformStreamDefaultController::Terminate(JSContext* aCx,
+                                                 ErrorResult& aRv) {
+  // Step 1: Perform ? TransformStreamDefaultControllerTerminate(this).
+
+  // Inlining TransformStreamDefaultControllerTerminate here.
+  // https://streams.spec.whatwg.org/#transform-stream-default-controller-terminate
+
+  // Step 1: Let stream be controller.[[stream]].
+  RefPtr<TransformStream> stream = mStream;
+
+  // Step 2: Let readableController be stream.[[readable]].[[controller]].
+  RefPtr<ReadableStreamDefaultController> readableController =
+      stream->Readable()->Controller()->AsDefault();
+
+  // Step 3: Perform ! ReadableStreamDefaultControllerClose(readableController).
+  ReadableStreamDefaultControllerClose(aCx, readableController, aRv);
+
+  // Step 4: Let error be a TypeError exception indicating that the stream has
+  // been terminated.
+  ErrorResult rv;
+  rv.ThrowTypeError("Terminating the stream");
+  JS::Rooted<JS::Value> error(aCx);
+  MOZ_ALWAYS_TRUE(ToJSValue(aCx, std::move(rv), &error));
+
+  // Step 5: Perform ! TransformStreamErrorWritableAndUnblockWrite(stream,
+  // error).
+  TransformStreamErrorWritableAndUnblockWrite(aCx, stream, error, aRv);
+}
+
+namespace streams_abstract {
+
+// https://streams.spec.whatwg.org/#set-up-transform-stream-default-controller
+void SetUpTransformStreamDefaultController(
+    JSContext* aCx, TransformStream& aStream,
+    TransformStreamDefaultController& aController,
+    TransformerAlgorithmsBase& aTransformerAlgorithms) {
+  // Step 1. Assert: stream implements TransformStream.
+  // Step 2. Assert: stream.[[controller]] is undefined.
+  MOZ_ASSERT(!aStream.Controller());
+
+  // Step 3. Set controller.[[stream]] to stream.
+  aController.SetStream(aStream);
+
+  // Step 4. Set stream.[[controller]] to controller.
+  aStream.SetController(aController);
+
+  // Step 5. Set controller.[[transformAlgorithm]] to transformAlgorithm.
+  // Step 6. Set controller.[[flushAlgorithm]] to flushAlgorithm.
+  aController.SetAlgorithms(&aTransformerAlgorithms);
+}
+
+// https://streams.spec.whatwg.org/#set-up-transform-stream-default-controller-from-transformer
+void SetUpTransformStreamDefaultControllerFromTransformer(
+    JSContext* aCx, TransformStream& aStream,
+    JS::Handle<JSObject*> aTransformer, Transformer& aTransformerDict) {
+  // Step 1. Let controller be a new TransformStreamDefaultController.
+  auto controller =
+      MakeRefPtr<TransformStreamDefaultController>(aStream.GetParentObject());
+
+  // Step 2 - 5:
+  auto algorithms = MakeRefPtr<TransformerAlgorithms>(
+      aStream.GetParentObject(), aTransformer, aTransformerDict);
+
+  // Step 6: Perform ! SetUpTransformStreamDefaultController(stream, controller,
+  // transformAlgorithm, flushAlgorithm).
+  SetUpTransformStreamDefaultController(aCx, aStream, *controller, *algorithms);
+}
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/TransformStreamDefaultController.h b/dom/streams/TransformStreamDefaultController.h
new file mode 100644
index 0000000000..71b92f69fe
--- /dev/null
+++ b/dom/streams/TransformStreamDefaultController.h
@@ -0,0 +1,74 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 DOM_STREAMS_TRANSFORMSTREAMDEFAULTCONTROLLER_H_
+#define DOM_STREAMS_TRANSFORMSTREAMDEFAULTCONTROLLER_H_
+
+#include "js/TypeDecls.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+
+#include "mozilla/dom/TransformerBinding.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsWrapperCache.h"
+
+namespace mozilla::dom {
+
+class TransformStream;
+class TransformerAlgorithmsBase;
+
+class TransformStreamDefaultController final : public nsISupports,
+                                               public nsWrapperCache {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(TransformStreamDefaultController)
+
+  MOZ_KNOWN_LIVE TransformStream* Stream();
+  void SetStream(TransformStream& aStream);
+  TransformerAlgorithmsBase* Algorithms();
+  void SetAlgorithms(TransformerAlgorithmsBase* aTransformerAlgorithms);
+
+  explicit TransformStreamDefaultController(nsIGlobalObject* aGlobal);
+
+ protected:
+  ~TransformStreamDefaultController();
+
+ public:
+  nsIGlobalObject* GetParentObject() const { return mGlobal; }
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  Nullable<double> GetDesiredSize() const;
+
+  MOZ_CAN_RUN_SCRIPT void Enqueue(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                                  ErrorResult& aRv);
+  MOZ_CAN_RUN_SCRIPT void Error(JSContext* aCx, JS::Handle<JS::Value> aError,
+                                ErrorResult& aRv);
+  MOZ_CAN_RUN_SCRIPT void Terminate(JSContext* aCx, ErrorResult& aRv);
+
+ private:
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+
+  // Internal slots
+  RefPtr<TransformStream> mStream;
+  RefPtr<TransformerAlgorithmsBase> mTransformerAlgorithms;
+};
+
+namespace streams_abstract {
+void SetUpTransformStreamDefaultController(
+    JSContext* aCx, TransformStream& aStream,
+    TransformStreamDefaultController& aController,
+    TransformerAlgorithmsBase& aTransformerAlgorithms);
+
+void SetUpTransformStreamDefaultControllerFromTransformer(
+    JSContext* aCx, TransformStream& aStream,
+    JS::Handle<JSObject*> aTransformer, Transformer& aTransformerDict);
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
+
+#endif  // DOM_STREAMS_TRANSFORMSTREAMDEFAULTCONTROLLER_H_
diff --git a/dom/streams/TransformerCallbackHelpers.cpp b/dom/streams/TransformerCallbackHelpers.cpp
new file mode 100644
index 0000000000..606c24adda
--- /dev/null
+++ b/dom/streams/TransformerCallbackHelpers.cpp
@@ -0,0 +1,111 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "TransformerCallbackHelpers.h"
+
+#include "StreamUtils.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/TransformStreamDefaultController.h"
+
+using namespace mozilla::dom;
+
+NS_IMPL_CYCLE_COLLECTION(TransformerAlgorithmsBase)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(TransformerAlgorithmsBase)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(TransformerAlgorithmsBase)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(TransformerAlgorithmsBase)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED_WITH_JS_MEMBERS(
+    TransformerAlgorithms, TransformerAlgorithmsBase,
+    (mGlobal, mTransformCallback, mFlushCallback), (mTransformer))
+NS_IMPL_ADDREF_INHERITED(TransformerAlgorithms, TransformerAlgorithmsBase)
+NS_IMPL_RELEASE_INHERITED(TransformerAlgorithms, TransformerAlgorithmsBase)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(TransformerAlgorithms)
+NS_INTERFACE_MAP_END_INHERITING(TransformerAlgorithmsBase)
+
+// https://streams.spec.whatwg.org/#set-up-transform-stream-default-controller-from-transformer
+already_AddRefed<Promise> TransformerAlgorithms::TransformCallback(
+    JSContext* aCx, JS::Handle<JS::Value> aChunk,
+    TransformStreamDefaultController& aController, ErrorResult& aRv) {
+  if (!mTransformCallback) {
+    // Step 2.1. Let result be
+    // TransformStreamDefaultControllerEnqueue(controller, chunk).
+    aController.Enqueue(aCx, aChunk, aRv);
+
+    // Step 2.2. If result is an abrupt completion, return a promise rejected
+    // with result.[[Value]].
+    if (aRv.MaybeSetPendingException(aCx)) {
+      JS::Rooted<JS::Value> error(aCx);
+      if (!JS_GetPendingException(aCx, &error)) {
+        // Uncatchable exception; we should mark aRv and return.
+        aRv.StealExceptionFromJSContext(aCx);
+        return nullptr;
+      }
+      JS_ClearPendingException(aCx);
+
+      return Promise::CreateRejected(aController.GetParentObject(), error, aRv);
+    }
+
+    // Step 2.3. Otherwise, return a promise resolved with undefined.
+    return Promise::CreateResolvedWithUndefined(aController.GetParentObject(),
+                                                aRv);
+  }
+  // Step 4. If transformerDict["transform"] exists, set transformAlgorithm to
+  // an algorithm which takes an argument chunk and returns the result of
+  // invoking transformerDict["transform"] with argument list « chunk,
+  // controller » and callback this value transformer.
+  JS::Rooted<JSObject*> thisObj(aCx, mTransformer);
+  return MOZ_KnownLive(mTransformCallback)
+      ->Call(thisObj, aChunk, aController, aRv,
+             "TransformStreamDefaultController.[[transformAlgorithm]]",
+             CallbackObject::eRethrowExceptions);
+}
+
+// https://streams.spec.whatwg.org/#set-up-transform-stream-default-controller-from-transformer
+already_AddRefed<Promise> TransformerAlgorithms::FlushCallback(
+    JSContext* aCx, TransformStreamDefaultController& aController,
+    ErrorResult& aRv) {
+  if (!mFlushCallback) {
+    // Step 3. Let flushAlgorithm be an algorithm which returns a promise
+    // resolved with undefined.
+    return Promise::CreateResolvedWithUndefined(aController.GetParentObject(),
+                                                aRv);
+  }
+  // Step 5. If transformerDict["flush"] exists, set flushAlgorithm to an
+  // algorithm which returns the result of invoking transformerDict["flush"]
+  // with argument list « controller » and callback this value transformer.
+  JS::Rooted<JSObject*> thisObj(aCx, mTransformer);
+  return MOZ_KnownLive(mFlushCallback)
+      ->Call(thisObj, aController, aRv,
+             "TransformStreamDefaultController.[[flushAlgorithm]]",
+             CallbackObject::eRethrowExceptions);
+}
+
+already_AddRefed<Promise> TransformerAlgorithmsWrapper::TransformCallback(
+    JSContext*, JS::Handle<JS::Value> aChunk,
+    TransformStreamDefaultController& aController, ErrorResult& aRv) {
+  nsCOMPtr<nsIGlobalObject> global = aController.GetParentObject();
+  return PromisifyAlgorithm(
+      global,
+      [this, &aChunk, &aController](ErrorResult& aRv)
+          MOZ_CAN_RUN_SCRIPT_FOR_DEFINITION {
+            return TransformCallbackImpl(aChunk, aController, aRv);
+          },
+      aRv);
+}
+
+already_AddRefed<Promise> TransformerAlgorithmsWrapper::FlushCallback(
+    JSContext*, TransformStreamDefaultController& aController,
+    ErrorResult& aRv) {
+  nsCOMPtr<nsIGlobalObject> global = aController.GetParentObject();
+  return PromisifyAlgorithm(
+      global,
+      [this, &aController](ErrorResult& aRv) MOZ_CAN_RUN_SCRIPT_FOR_DEFINITION {
+        return FlushCallbackImpl(aController, aRv);
+      },
+      aRv);
+}
diff --git a/dom/streams/TransformerCallbackHelpers.h b/dom/streams/TransformerCallbackHelpers.h
new file mode 100644
index 0000000000..8b12a31b0f
--- /dev/null
+++ b/dom/streams/TransformerCallbackHelpers.h
@@ -0,0 +1,102 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 DOM_STREAMS_TRANSFORMERCALLBACKHELPERS_H_
+#define DOM_STREAMS_TRANSFORMERCALLBACKHELPERS_H_
+
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/dom/TransformerBinding.h"
+#include "nsCycleCollectionParticipant.h"
+
+namespace mozilla::dom {
+
+class Promise;
+
+class TransformerAlgorithmsBase : public nsISupports {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(TransformerAlgorithmsBase)
+
+  MOZ_CAN_RUN_SCRIPT virtual already_AddRefed<Promise> TransformCallback(
+      JSContext* aCx, JS::Handle<JS::Value> aChunk,
+      TransformStreamDefaultController& aController, ErrorResult& aRv) = 0;
+
+  MOZ_CAN_RUN_SCRIPT virtual already_AddRefed<Promise> FlushCallback(
+      JSContext* aCx, TransformStreamDefaultController& aController,
+      ErrorResult& aRv) = 0;
+
+ protected:
+  virtual ~TransformerAlgorithmsBase() = default;
+};
+
+// https://streams.spec.whatwg.org/#set-up-transform-stream-default-controller-from-transformer
+class TransformerAlgorithms final : public TransformerAlgorithmsBase {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_INHERITED(
+      TransformerAlgorithms, TransformerAlgorithmsBase)
+
+  TransformerAlgorithms(nsIGlobalObject* aGlobal,
+                        JS::Handle<JSObject*> aTransformer,
+                        Transformer& aTransformerDict)
+      : mGlobal(aGlobal), mTransformer(aTransformer) {
+    // Step 4. (Step 2 is implicitly done through the initialization of
+    // mTransformCallback to null)
+    if (aTransformerDict.mTransform.WasPassed()) {
+      mTransformCallback = aTransformerDict.mTransform.Value();
+    }
+
+    // Step 5. (Step 3 is implicitly done through the initialization of
+    // mTransformCallback to null)
+    if (aTransformerDict.mFlush.WasPassed()) {
+      mFlushCallback = aTransformerDict.mFlush.Value();
+    }
+
+    mozilla::HoldJSObjects(this);
+  };
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> TransformCallback(
+      JSContext* aCx, JS::Handle<JS::Value> aChunk,
+      TransformStreamDefaultController& aController, ErrorResult& aRv) override;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> FlushCallback(
+      JSContext* aCx, TransformStreamDefaultController& aController,
+      ErrorResult& aRv) override;
+
+ protected:
+  ~TransformerAlgorithms() { mozilla::DropJSObjects(this); }
+
+ private:
+  // Virtually const, but are cycle collected
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+  JS::Heap<JSObject*> mTransformer;
+  MOZ_KNOWN_LIVE RefPtr<TransformerTransformCallback> mTransformCallback;
+  MOZ_KNOWN_LIVE RefPtr<TransformerFlushCallback> mFlushCallback;
+};
+
+// https://streams.spec.whatwg.org/#transformstream-set-up
+class TransformerAlgorithmsWrapper : public TransformerAlgorithmsBase {
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> TransformCallback(
+      JSContext*, JS::Handle<JS::Value> aChunk,
+      TransformStreamDefaultController& aController, ErrorResult& aRv) final;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> FlushCallback(
+      JSContext*, TransformStreamDefaultController& aController,
+      ErrorResult& aRv) final;
+
+  MOZ_CAN_RUN_SCRIPT virtual void TransformCallbackImpl(
+      JS::Handle<JS::Value> aChunk,
+      TransformStreamDefaultController& aController, ErrorResult& aRv) = 0;
+
+  MOZ_CAN_RUN_SCRIPT virtual void FlushCallbackImpl(
+      TransformStreamDefaultController& aController, ErrorResult& aRv) {
+    // flushAlgorithm is optional, do nothing by default
+  }
+};
+
+}  // namespace mozilla::dom
+
+#endif  // DOM_STREAMS_TRANSFORMERCALLBACKHELPERS_H_
diff --git a/dom/streams/UnderlyingSinkCallbackHelpers.cpp b/dom/streams/UnderlyingSinkCallbackHelpers.cpp
new file mode 100644
index 0000000000..c9767cbf3b
--- /dev/null
+++ b/dom/streams/UnderlyingSinkCallbackHelpers.cpp
@@ -0,0 +1,280 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/UnderlyingSinkCallbackHelpers.h"
+#include "StreamUtils.h"
+#include "mozilla/dom/UnionTypes.h"
+#include "mozilla/dom/WebTransportError.h"
+#include "nsHttp.h"
+
+using namespace mozilla::dom;
+
+NS_IMPL_CYCLE_COLLECTION(UnderlyingSinkAlgorithmsBase)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(UnderlyingSinkAlgorithmsBase)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(UnderlyingSinkAlgorithmsBase)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(UnderlyingSinkAlgorithmsBase)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED_WITH_JS_MEMBERS(
+    UnderlyingSinkAlgorithms, UnderlyingSinkAlgorithmsBase,
+    (mGlobal, mStartCallback, mWriteCallback, mCloseCallback, mAbortCallback),
+    (mUnderlyingSink))
+NS_IMPL_ADDREF_INHERITED(UnderlyingSinkAlgorithms, UnderlyingSinkAlgorithmsBase)
+NS_IMPL_RELEASE_INHERITED(UnderlyingSinkAlgorithms,
+                          UnderlyingSinkAlgorithmsBase)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(UnderlyingSinkAlgorithms)
+NS_INTERFACE_MAP_END_INHERITING(UnderlyingSinkAlgorithmsBase)
+
+// https://streams.spec.whatwg.org/#set-up-writable-stream-default-controller-from-underlying-sink
+void UnderlyingSinkAlgorithms::StartCallback(
+    JSContext* aCx, WritableStreamDefaultController& aController,
+    JS::MutableHandle<JS::Value> aRetVal, ErrorResult& aRv) {
+  if (!mStartCallback) {
+    // Step 2: Let startAlgorithm be an algorithm that returns undefined.
+    aRetVal.setUndefined();
+    return;
+  }
+
+  // Step 6: If underlyingSinkDict["start"] exists, then set startAlgorithm to
+  // an algorithm which returns the result of invoking
+  // underlyingSinkDict["start"] with argument list « controller » and callback
+  // this value underlyingSink.
+  JS::Rooted<JSObject*> thisObj(aCx, mUnderlyingSink);
+  return mStartCallback->Call(thisObj, aController, aRetVal, aRv,
+                              "UnderlyingSink.start",
+                              CallbackFunction::eRethrowExceptions);
+}
+
+// https://streams.spec.whatwg.org/#set-up-writable-stream-default-controller-from-underlying-sink
+already_AddRefed<Promise> UnderlyingSinkAlgorithms::WriteCallback(
+    JSContext* aCx, JS::Handle<JS::Value> aChunk,
+    WritableStreamDefaultController& aController, ErrorResult& aRv) {
+  if (!mWriteCallback) {
+    // Step 3: Let writeAlgorithm be an algorithm that returns a promise
+    // resolved with undefined.
+    return Promise::CreateResolvedWithUndefined(mGlobal, aRv);
+  }
+
+  // Step 7: If underlyingSinkDict["write"] exists, then set writeAlgorithm to
+  // an algorithm which takes an argument chunk and returns the result of
+  // invoking underlyingSinkDict["write"] with argument list « chunk, controller
+  // » and callback this value underlyingSink.
+  JS::Rooted<JSObject*> thisObj(aCx, mUnderlyingSink);
+  RefPtr<Promise> promise = mWriteCallback->Call(
+      thisObj, aChunk, aController, aRv, "UnderlyingSink.write",
+      CallbackFunction::eRethrowExceptions);
+  return promise.forget();
+}
+
+// https://streams.spec.whatwg.org/#set-up-writable-stream-default-controller-from-underlying-sink
+already_AddRefed<Promise> UnderlyingSinkAlgorithms::CloseCallback(
+    JSContext* aCx, ErrorResult& aRv) {
+  if (!mCloseCallback) {
+    // Step 4: Let closeAlgorithm be an algorithm that returns a promise
+    // resolved with undefined.
+    return Promise::CreateResolvedWithUndefined(mGlobal, aRv);
+  }
+
+  // Step 8: If underlyingSinkDict["close"] exists, then set closeAlgorithm to
+  // an algorithm which returns the result of invoking
+  // underlyingSinkDict["close"] with argument list «» and callback this value
+  // underlyingSink.
+  JS::Rooted<JSObject*> thisObj(aCx, mUnderlyingSink);
+  RefPtr<Promise> promise =
+      mCloseCallback->Call(thisObj, aRv, "UnderlyingSink.close",
+                           CallbackFunction::eRethrowExceptions);
+  return promise.forget();
+}
+
+// https://streams.spec.whatwg.org/#set-up-writable-stream-default-controller-from-underlying-sink
+already_AddRefed<Promise> UnderlyingSinkAlgorithms::AbortCallback(
+    JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+    ErrorResult& aRv) {
+  if (!mAbortCallback) {
+    // Step 5: Let abortAlgorithm be an algorithm that returns a promise
+    // resolved with undefined.
+    return Promise::CreateResolvedWithUndefined(mGlobal, aRv);
+  }
+
+  // Step 9: Let abortAlgorithm be an algorithm that returns a promise resolved
+  // with undefined.
+  JS::Rooted<JSObject*> thisObj(aCx, mUnderlyingSink);
+  RefPtr<Promise> promise =
+      mAbortCallback->Call(thisObj, aReason, aRv, "UnderlyingSink.abort",
+                           CallbackFunction::eRethrowExceptions);
+
+  return promise.forget();
+}
+
+// https://streams.spec.whatwg.org/#writable-set-up
+// Step 2.1: Let closeAlgorithmWrapper be an algorithm that runs these steps:
+already_AddRefed<Promise> UnderlyingSinkAlgorithmsWrapper::CloseCallback(
+    JSContext* aCx, ErrorResult& aRv) {
+  nsCOMPtr<nsIGlobalObject> global = xpc::CurrentNativeGlobal(aCx);
+  return PromisifyAlgorithm(
+      global, [&](ErrorResult& aRv) { return CloseCallbackImpl(aCx, aRv); },
+      aRv);
+}
+
+// https://streams.spec.whatwg.org/#writable-set-up
+// Step 3.1: Let abortAlgorithmWrapper be an algorithm that runs these steps:
+already_AddRefed<Promise> UnderlyingSinkAlgorithmsWrapper::AbortCallback(
+    JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+    ErrorResult& aRv) {
+  nsCOMPtr<nsIGlobalObject> global = xpc::CurrentNativeGlobal(aCx);
+  return PromisifyAlgorithm(
+      global,
+      [&](ErrorResult& aRv) { return AbortCallbackImpl(aCx, aReason, aRv); },
+      aRv);
+}
+
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED(WritableStreamToOutput,
+                                             UnderlyingSinkAlgorithmsBase,
+                                             nsIOutputStreamCallback)
+NS_IMPL_CYCLE_COLLECTION_INHERITED(WritableStreamToOutput,
+                                   UnderlyingSinkAlgorithmsBase, mParent,
+                                   mOutput, mPromise)
+
+NS_IMETHODIMP
+WritableStreamToOutput::OnOutputStreamReady(nsIAsyncOutputStream* aStream) {
+  if (!mData) {
+    return NS_OK;
+  }
+  MOZ_ASSERT(mPromise);
+  uint32_t written = 0;
+  nsresult rv = mOutput->Write(
+      reinterpret_cast<const char*>(mData->Elements() + mWritten),
+      mData->Length() - mWritten, &written);
+  if (NS_FAILED(rv) && rv != NS_BASE_STREAM_WOULD_BLOCK) {
+    mPromise->MaybeRejectWithAbortError("Error writing to stream"_ns);
+    ClearData();
+    // XXX should we add mErrored and fail future calls immediately?
+    // I presume new calls to Write() will fail though, too
+    return rv;
+  }
+  if (NS_SUCCEEDED(rv)) {
+    mWritten += written;
+    MOZ_ASSERT(mWritten <= mData->Length());
+    if (mWritten >= mData->Length()) {
+      mPromise->MaybeResolveWithUndefined();
+      ClearData();
+      return NS_OK;
+    }
+    // more to write
+  }
+  // wrote partial or nothing
+  // Wait for space
+  nsCOMPtr<nsIEventTarget> target = mozilla::GetCurrentSerialEventTarget();
+  rv = mOutput->AsyncWait(this, 0, 0, target);
+  if (NS_FAILED(rv)) {
+    mPromise->MaybeRejectWithUnknownError("error waiting to write data");
+    ClearData();
+    // XXX should we add mErrored and fail future calls immediately?
+    // New calls to Write() will fail, note
+    // See step 5.2 of
+    // https://streams.spec.whatwg.org/#writable-stream-default-controller-process-write.
+    return rv;
+  }
+  return NS_OK;
+}
+
+already_AddRefed<Promise> WritableStreamToOutput::WriteCallback(
+    JSContext* aCx, JS::Handle<JS::Value> aChunk,
+    WritableStreamDefaultController& aController, ErrorResult& aError) {
+  ArrayBufferViewOrArrayBuffer data;
+  if (!data.Init(aCx, aChunk)) {
+    aError.StealExceptionFromJSContext(aCx);
+    return nullptr;
+  }
+  // buffer/bufferView
+  MOZ_ASSERT(data.IsArrayBuffer() || data.IsArrayBufferView());
+
+  RefPtr<Promise> promise = Promise::Create(mParent, aError);
+  if (NS_WARN_IF(aError.Failed())) {
+    return nullptr;
+  }
+
+  // This is a duplicate of dom/encoding/TextDecoderStream.cpp#51-69
+  // PeterV will deal with that when he lands his patch for TypedArrays
+  auto dataSpan = [&data]() {
+    if (data.IsArrayBuffer()) {
+      const ArrayBuffer& buffer = data.GetAsArrayBuffer();
+      buffer.ComputeState();
+      return Span{buffer.Data(), buffer.Length()};
+    }
+    MOZ_ASSERT(data.IsArrayBufferView());
+    const ArrayBufferView& buffer = data.GetAsArrayBufferView();
+    buffer.ComputeState();
+    return Span{buffer.Data(), buffer.Length()};
+  }();
+
+  // Try to write first, and only enqueue data if we were already blocked
+  // or the write didn't write it all.  This avoids allocations and copies
+  // in common cases.
+  MOZ_ASSERT(!mPromise);
+  MOZ_ASSERT(mWritten == 0);
+  uint32_t written = 0;
+  nsresult rv = mOutput->Write(mozilla::AsChars(dataSpan).Elements(),
+                               dataSpan.Length(), &written);
+  if (NS_FAILED(rv) && rv != NS_BASE_STREAM_WOULD_BLOCK) {
+    promise->MaybeRejectWithAbortError("error writing data");
+    return promise.forget();
+  }
+  if (NS_SUCCEEDED(rv)) {
+    if (written == dataSpan.Length()) {
+      promise->MaybeResolveWithUndefined();
+      return promise.forget();
+    }
+    dataSpan = dataSpan.From(written);
+  }
+
+  auto buffer = Buffer<uint8_t>::CopyFrom(dataSpan);
+  if (buffer.isNothing()) {
+    promise->MaybeReject(NS_ERROR_OUT_OF_MEMORY);
+    return promise.forget();
+  }
+  mData = std::move(buffer);
+  mPromise = promise;
+
+  nsCOMPtr<nsIEventTarget> target = mozilla::GetCurrentSerialEventTarget();
+  rv = mOutput->AsyncWait(this, 0, 0, target);
+  if (NS_FAILED(rv)) {
+    ClearData();
+    promise->MaybeRejectWithUnknownError("error waiting to write data");
+  }
+  return promise.forget();
+}
+
+already_AddRefed<Promise> WritableStreamToOutput::AbortCallbackImpl(
+    JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+    ErrorResult& aRv) {
+  // https://streams.spec.whatwg.org/#writablestream-set-up
+  // Step 3. Let abortAlgorithmWrapper be an algorithm that runs these steps:
+
+  if (aReason.WasPassed() && aReason.Value().isObject()) {
+    JS::Rooted<JSObject*> obj(aCx, &aReason.Value().toObject());
+    RefPtr<WebTransportError> error;
+    UnwrapObject<prototypes::id::WebTransportError, WebTransportError>(
+        obj, error, nullptr);
+    if (error) {
+      mOutput->CloseWithStatus(net::GetNSResultFromWebTransportError(
+          error->GetStreamErrorCode().Value()));
+      return nullptr;
+    }
+  }
+
+  // XXX The close or rather a dedicated abort should be async. For now we have
+  // to always fall back to the Step 3.3 below.
+  // XXX how do we know this stream is used by webtransport?
+  mOutput->CloseWithStatus(NS_ERROR_WEBTRANSPORT_CODE_BASE);
+
+  // Step 3.3. Return a promise resolved with undefined.
+  // Wrapper handles this
+  return nullptr;
+}
+
+void WritableStreamToOutput::ReleaseObjects() { mOutput->Close(); }
diff --git a/dom/streams/UnderlyingSinkCallbackHelpers.h b/dom/streams/UnderlyingSinkCallbackHelpers.h
new file mode 100644
index 0000000000..c99c8709ce
--- /dev/null
+++ b/dom/streams/UnderlyingSinkCallbackHelpers.h
@@ -0,0 +1,202 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_UnderlyingSinkCallbackHelpers_h
+#define mozilla_dom_UnderlyingSinkCallbackHelpers_h
+
+#include "mozilla/Maybe.h"
+#include "mozilla/Buffer.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/UnderlyingSinkBinding.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsISupports.h"
+#include "nsISupportsImpl.h"
+#include "nsIAsyncOutputStream.h"
+
+/*
+ * See the comment in UnderlyingSourceCallbackHelpers.h!
+ *
+ * A native implementation of these callbacks is however currently not required.
+ */
+namespace mozilla::dom {
+
+class WritableStreamDefaultController;
+
+class UnderlyingSinkAlgorithmsBase : public nsISupports {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(UnderlyingSinkAlgorithmsBase)
+
+  MOZ_CAN_RUN_SCRIPT virtual void StartCallback(
+      JSContext* aCx, WritableStreamDefaultController& aController,
+      JS::MutableHandle<JS::Value> aRetVal, ErrorResult& aRv) = 0;
+
+  MOZ_CAN_RUN_SCRIPT virtual already_AddRefed<Promise> WriteCallback(
+      JSContext* aCx, JS::Handle<JS::Value> aChunk,
+      WritableStreamDefaultController& aController, ErrorResult& aRv) = 0;
+
+  MOZ_CAN_RUN_SCRIPT virtual already_AddRefed<Promise> CloseCallback(
+      JSContext* aCx, ErrorResult& aRv) = 0;
+
+  MOZ_CAN_RUN_SCRIPT virtual already_AddRefed<Promise> AbortCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) = 0;
+
+  // Implement this when you need to release underlying resources immediately
+  // from closed/errored(aborted) streams, without waiting for GC.
+  virtual void ReleaseObjects() {}
+
+ protected:
+  virtual ~UnderlyingSinkAlgorithmsBase() = default;
+};
+
+// https://streams.spec.whatwg.org/#set-up-writable-stream-default-controller-from-underlying-sink
+class UnderlyingSinkAlgorithms final : public UnderlyingSinkAlgorithmsBase {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_INHERITED(
+      UnderlyingSinkAlgorithms, UnderlyingSinkAlgorithmsBase)
+
+  UnderlyingSinkAlgorithms(nsIGlobalObject* aGlobal,
+                           JS::Handle<JSObject*> aUnderlyingSink,
+                           UnderlyingSink& aUnderlyingSinkDict)
+      : mGlobal(aGlobal), mUnderlyingSink(aUnderlyingSink) {
+    // Step 6. (implicit Step 2.)
+    if (aUnderlyingSinkDict.mStart.WasPassed()) {
+      mStartCallback = aUnderlyingSinkDict.mStart.Value();
+    }
+
+    // Step 7. (implicit Step 3.)
+    if (aUnderlyingSinkDict.mWrite.WasPassed()) {
+      mWriteCallback = aUnderlyingSinkDict.mWrite.Value();
+    }
+
+    // Step 8. (implicit Step 4.)
+    if (aUnderlyingSinkDict.mClose.WasPassed()) {
+      mCloseCallback = aUnderlyingSinkDict.mClose.Value();
+    }
+
+    // Step 9. (implicit Step 5.)
+    if (aUnderlyingSinkDict.mAbort.WasPassed()) {
+      mAbortCallback = aUnderlyingSinkDict.mAbort.Value();
+    }
+
+    mozilla::HoldJSObjects(this);
+  };
+
+  MOZ_CAN_RUN_SCRIPT void StartCallback(
+      JSContext* aCx, WritableStreamDefaultController& aController,
+      JS::MutableHandle<JS::Value> aRetVal, ErrorResult& aRv) override;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> WriteCallback(
+      JSContext* aCx, JS::Handle<JS::Value> aChunk,
+      WritableStreamDefaultController& aController, ErrorResult& aRv) override;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> CloseCallback(
+      JSContext* aCx, ErrorResult& aRv) override;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> AbortCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) override;
+
+ protected:
+  ~UnderlyingSinkAlgorithms() override { mozilla::DropJSObjects(this); }
+
+ private:
+  // Virtually const, but are cycle collected
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+  JS::Heap<JSObject*> mUnderlyingSink;
+  MOZ_KNOWN_LIVE RefPtr<UnderlyingSinkStartCallback> mStartCallback;
+  MOZ_KNOWN_LIVE RefPtr<UnderlyingSinkWriteCallback> mWriteCallback;
+  MOZ_KNOWN_LIVE RefPtr<UnderlyingSinkCloseCallback> mCloseCallback;
+  MOZ_KNOWN_LIVE RefPtr<UnderlyingSinkAbortCallback> mAbortCallback;
+};
+
+// https://streams.spec.whatwg.org/#writablestream-set-up
+// Wrappers defined by the "Set up" methods in the spec.
+// (closeAlgorithmWrapper, abortAlgorithmWrapper)
+// This helps you just return nullptr when 1) the algorithm is synchronous, or
+// 2) an error occurred, as this wrapper will return a resolved or rejected
+// promise respectively.
+// Note that StartCallback is only for JS consumers to access the
+// controller, and thus is no-op here since native consumers can call
+// `ErrorNative()` etc. without direct controller access.
+class UnderlyingSinkAlgorithmsWrapper : public UnderlyingSinkAlgorithmsBase {
+ public:
+  void StartCallback(JSContext* aCx,
+                     WritableStreamDefaultController& aController,
+                     JS::MutableHandle<JS::Value> aRetVal,
+                     ErrorResult& aRv) final {
+    // Step 1: Let startAlgorithm be an algorithm that returns undefined.
+    aRetVal.setUndefined();
+  }
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> CloseCallback(
+      JSContext* aCx, ErrorResult& aRv) final;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> AbortCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) final;
+
+  virtual already_AddRefed<Promise> CloseCallbackImpl(JSContext* aCx,
+                                                      ErrorResult& aRv) {
+    // (closeAlgorithm is optional, give null by default)
+    return nullptr;
+  }
+
+  virtual already_AddRefed<Promise> AbortCallbackImpl(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) {
+    // (abortAlgorithm is optional, give null by default)
+    return nullptr;
+  }
+};
+
+class WritableStreamToOutput final : public UnderlyingSinkAlgorithmsWrapper,
+                                     public nsIOutputStreamCallback {
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIOUTPUTSTREAMCALLBACK
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(WritableStreamToOutput,
+                                           UnderlyingSinkAlgorithmsBase)
+
+  WritableStreamToOutput(nsIGlobalObject* aParent,
+                         nsIAsyncOutputStream* aOutput)
+      : mWritten(0), mParent(aParent), mOutput(aOutput) {}
+
+  // Streams algorithms
+
+  already_AddRefed<Promise> WriteCallback(
+      JSContext* aCx, JS::Handle<JS::Value> aChunk,
+      WritableStreamDefaultController& aController, ErrorResult& aRv) override;
+
+  // No CloseCallbackImpl() since ReleaseObjects() will call Close()
+
+  already_AddRefed<Promise> AbortCallbackImpl(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) override;
+
+  void ReleaseObjects() override;
+
+ private:
+  ~WritableStreamToOutput() override = default;
+
+  void ClearData() {
+    mData = Nothing();
+    mPromise = nullptr;
+    mWritten = 0;
+  }
+
+  uint32_t mWritten;
+  nsCOMPtr<nsIGlobalObject> mParent;
+  nsCOMPtr<nsIAsyncOutputStream> mOutput;
+  RefPtr<Promise> mPromise;  // Resolved when entirely written
+  Maybe<Buffer<uint8_t>> mData;
+};
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/streams/UnderlyingSourceCallbackHelpers.cpp b/dom/streams/UnderlyingSourceCallbackHelpers.cpp
new file mode 100644
index 0000000000..373e46390f
--- /dev/null
+++ b/dom/streams/UnderlyingSourceCallbackHelpers.cpp
@@ -0,0 +1,516 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "StreamUtils.h"
+#include "mozilla/dom/ReadableStream.h"
+#include "mozilla/dom/ReadableStreamDefaultController.h"
+#include "mozilla/dom/UnderlyingSourceCallbackHelpers.h"
+#include "mozilla/dom/UnderlyingSourceBinding.h"
+#include "mozilla/dom/WorkerCommon.h"
+#include "mozilla/dom/WorkerPrivate.h"
+#include "mozilla/dom/WorkerRunnable.h"
+#include "js/experimental/TypedData.h"
+#include "nsStreamUtils.h"
+
+namespace mozilla::dom {
+
+// UnderlyingSourceAlgorithmsBase
+NS_IMPL_CYCLE_COLLECTION(UnderlyingSourceAlgorithmsBase)
+NS_IMPL_CYCLE_COLLECTING_ADDREF(UnderlyingSourceAlgorithmsBase)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(UnderlyingSourceAlgorithmsBase)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(UnderlyingSourceAlgorithmsBase)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION_INHERITED_WITH_JS_MEMBERS(
+    UnderlyingSourceAlgorithms, UnderlyingSourceAlgorithmsBase,
+    (mGlobal, mStartCallback, mPullCallback, mCancelCallback),
+    (mUnderlyingSource))
+NS_IMPL_ADDREF_INHERITED(UnderlyingSourceAlgorithms,
+                         UnderlyingSourceAlgorithmsBase)
+NS_IMPL_RELEASE_INHERITED(UnderlyingSourceAlgorithms,
+                          UnderlyingSourceAlgorithmsBase)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(UnderlyingSourceAlgorithms)
+NS_INTERFACE_MAP_END_INHERITING(UnderlyingSourceAlgorithmsBase)
+
+// https://streams.spec.whatwg.org/#set-up-readable-stream-default-controller-from-underlying-source
+void UnderlyingSourceAlgorithms::StartCallback(
+    JSContext* aCx, ReadableStreamController& aController,
+    JS::MutableHandle<JS::Value> aRetVal, ErrorResult& aRv) {
+  if (!mStartCallback) {
+    // Step 2: Let startAlgorithm be an algorithm that returns undefined.
+    aRetVal.setUndefined();
+    return;
+  }
+
+  // Step 5: If underlyingSourceDict["start"] exists, then set startAlgorithm to
+  // an algorithm which returns the result of invoking
+  // underlyingSourceDict["start"] with argument list « controller » and
+  // callback this value underlyingSource.
+  JS::Rooted<JSObject*> thisObj(aCx, mUnderlyingSource);
+  ReadableStreamDefaultControllerOrReadableByteStreamController controller;
+  if (aController.IsDefault()) {
+    controller.SetAsReadableStreamDefaultController() = aController.AsDefault();
+  } else {
+    controller.SetAsReadableByteStreamController() = aController.AsByte();
+  }
+
+  return mStartCallback->Call(thisObj, controller, aRetVal, aRv,
+                              "UnderlyingSource.start",
+                              CallbackFunction::eRethrowExceptions);
+}
+
+// https://streams.spec.whatwg.org/#set-up-readable-stream-default-controller-from-underlying-source
+already_AddRefed<Promise> UnderlyingSourceAlgorithms::PullCallback(
+    JSContext* aCx, ReadableStreamController& aController, ErrorResult& aRv) {
+  JS::Rooted<JSObject*> thisObj(aCx, mUnderlyingSource);
+  if (!mPullCallback) {
+    // Step 3: Let pullAlgorithm be an algorithm that returns a promise resolved
+    // with undefined.
+    return Promise::CreateResolvedWithUndefined(mGlobal, aRv);
+  }
+
+  // Step 6: If underlyingSourceDict["pull"] exists, then set pullAlgorithm to
+  // an algorithm which returns the result of invoking
+  // underlyingSourceDict["pull"] with argument list « controller » and callback
+  // this value underlyingSource.
+  ReadableStreamDefaultControllerOrReadableByteStreamController controller;
+  if (aController.IsDefault()) {
+    controller.SetAsReadableStreamDefaultController() = aController.AsDefault();
+  } else {
+    controller.SetAsReadableByteStreamController() = aController.AsByte();
+  }
+
+  RefPtr<Promise> promise =
+      mPullCallback->Call(thisObj, controller, aRv, "UnderlyingSource.pull",
+                          CallbackFunction::eRethrowExceptions);
+
+  return promise.forget();
+}
+
+// https://streams.spec.whatwg.org/#set-up-readable-stream-default-controller-from-underlying-source
+already_AddRefed<Promise> UnderlyingSourceAlgorithms::CancelCallback(
+    JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+    ErrorResult& aRv) {
+  if (!mCancelCallback) {
+    // Step 4: Let cancelAlgorithm be an algorithm that returns a promise
+    // resolved with undefined.
+    return Promise::CreateResolvedWithUndefined(mGlobal, aRv);
+  }
+
+  // Step 7: If underlyingSourceDict["cancel"] exists, then set cancelAlgorithm
+  // to an algorithm which takes an argument reason and returns the result of
+  // invoking underlyingSourceDict["cancel"] with argument list « reason » and
+  // callback this value underlyingSource.
+  JS::Rooted<JSObject*> thisObj(aCx, mUnderlyingSource);
+  RefPtr<Promise> promise =
+      mCancelCallback->Call(thisObj, aReason, aRv, "UnderlyingSource.cancel",
+                            CallbackFunction::eRethrowExceptions);
+
+  return promise.forget();
+}
+
+// Shared between:
+// https://streams.spec.whatwg.org/#readablestream-set-up
+// https://streams.spec.whatwg.org/#readablestream-set-up-with-byte-reading-support
+// Step 1: Let startAlgorithm be an algorithm that returns undefined.
+void UnderlyingSourceAlgorithmsWrapper::StartCallback(
+    JSContext*, ReadableStreamController&, JS::MutableHandle<JS::Value> aRetVal,
+    ErrorResult&) {
+  aRetVal.setUndefined();
+}
+
+// Shared between:
+// https://streams.spec.whatwg.org/#readablestream-set-up
+// https://streams.spec.whatwg.org/#readablestream-set-up-with-byte-reading-support
+// Step 2: Let pullAlgorithmWrapper be an algorithm that runs these steps:
+already_AddRefed<Promise> UnderlyingSourceAlgorithmsWrapper::PullCallback(
+    JSContext* aCx, ReadableStreamController& aController, ErrorResult& aRv) {
+  nsCOMPtr<nsIGlobalObject> global = aController.GetParentObject();
+  return PromisifyAlgorithm(
+      global,
+      [&](ErrorResult& aRv) MOZ_CAN_RUN_SCRIPT_FOR_DEFINITION {
+        return PullCallbackImpl(aCx, aController, aRv);
+      },
+      aRv);
+}
+
+// Shared between:
+// https://streams.spec.whatwg.org/#readablestream-set-up
+// https://streams.spec.whatwg.org/#readablestream-set-up-with-byte-reading-support
+// Step 3: Let cancelAlgorithmWrapper be an algorithm that runs these steps:
+already_AddRefed<Promise> UnderlyingSourceAlgorithmsWrapper::CancelCallback(
+    JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+    ErrorResult& aRv) {
+  nsCOMPtr<nsIGlobalObject> global = xpc::CurrentNativeGlobal(aCx);
+  return PromisifyAlgorithm(
+      global,
+      [&](ErrorResult& aRv) { return CancelCallbackImpl(aCx, aReason, aRv); },
+      aRv);
+}
+
+NS_IMPL_ISUPPORTS(InputStreamHolder, nsIInputStreamCallback)
+
+InputStreamHolder::InputStreamHolder(nsIGlobalObject* aGlobal,
+                                     InputToReadableStreamAlgorithms* aCallback,
+                                     nsIAsyncInputStream* aInput)
+    : GlobalTeardownObserver(aGlobal), mCallback(aCallback), mInput(aInput) {}
+
+void InputStreamHolder::Init(JSContext* aCx) {
+  if (!NS_IsMainThread()) {
+    // We're in a worker
+    WorkerPrivate* workerPrivate = GetWorkerPrivateFromContext(aCx);
+    MOZ_ASSERT(workerPrivate);
+
+    workerPrivate->AssertIsOnWorkerThread();
+
+    // Note, this will create a ref-cycle between the holder and the stream.
+    // The cycle is broken when the stream is closed or the worker begins
+    // shutting down.
+    mWorkerRef = StrongWorkerRef::Create(workerPrivate, "InputStreamHolder",
+                                         [self = RefPtr{this}]() {});
+    if (NS_WARN_IF(!mWorkerRef)) {
+      return;
+    }
+  }
+}
+
+InputStreamHolder::~InputStreamHolder() = default;
+
+void InputStreamHolder::DisconnectFromOwner() {
+  Shutdown();
+  GlobalTeardownObserver::DisconnectFromOwner();
+}
+
+void InputStreamHolder::Shutdown() {
+  if (mInput) {
+    mInput->Close();
+  }
+  // NOTE(krosylight): Dropping mAsyncWaitAlgorithms here means letting cycle
+  // collection happen on the underlying source, which can cause a dangling
+  // read promise that never resolves. Doing so shouldn't be a problem at
+  // shutdown phase.
+  // Note that this is currently primarily for Fetch which does not explicitly
+  // close its streams at shutdown. (i.e. to prevent memory leak for cases e.g
+  // WPT /fetch/api/basic/stream-response.any.html)
+  mAsyncWaitAlgorithms = nullptr;
+  // If we have an AsyncWait running, we'll get a callback and clear
+  // the mAsyncWaitWorkerRef
+  mWorkerRef = nullptr;
+}
+
+nsresult InputStreamHolder::AsyncWait(uint32_t aFlags, uint32_t aRequestedCount,
+                                      nsIEventTarget* aEventTarget) {
+  nsresult rv = mInput->AsyncWait(this, aFlags, aRequestedCount, aEventTarget);
+  if (NS_SUCCEEDED(rv)) {
+    mAsyncWaitWorkerRef = mWorkerRef;
+    mAsyncWaitAlgorithms = mCallback;
+  }
+  return rv;
+}
+
+NS_IMETHODIMP InputStreamHolder::OnInputStreamReady(
+    nsIAsyncInputStream* aStream) {
+  mAsyncWaitWorkerRef = nullptr;
+  mAsyncWaitAlgorithms = nullptr;
+  // We may get called back after ::Shutdown()
+  if (mCallback) {
+    return mCallback->OnInputStreamReady(aStream);
+  }
+  return NS_ERROR_FAILURE;
+}
+
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED(InputToReadableStreamAlgorithms,
+                                             UnderlyingSourceAlgorithmsWrapper,
+                                             nsIInputStreamCallback)
+NS_IMPL_CYCLE_COLLECTION_WEAK_PTR_INHERITED(InputToReadableStreamAlgorithms,
+                                            UnderlyingSourceAlgorithmsWrapper,
+                                            mPullPromise, mStream)
+
+InputToReadableStreamAlgorithms::InputToReadableStreamAlgorithms(
+    JSContext* aCx, nsIAsyncInputStream* aInput, ReadableStream* aStream)
+    : mOwningEventTarget(GetCurrentSerialEventTarget()),
+      mInput(new InputStreamHolder(aStream->GetParentObject(), this, aInput)),
+      mStream(aStream) {
+  mInput->Init(aCx);
+}
+
+already_AddRefed<Promise> InputToReadableStreamAlgorithms::PullCallbackImpl(
+    JSContext* aCx, ReadableStreamController& aController, ErrorResult& aRv) {
+  MOZ_ASSERT(aController.IsByte());
+  ReadableStream* stream = aController.Stream();
+  MOZ_ASSERT(stream);
+
+  MOZ_DIAGNOSTIC_ASSERT(stream->Disturbed());
+
+  MOZ_DIAGNOSTIC_ASSERT(!IsClosed());
+  MOZ_ASSERT(!mPullPromise);
+  mPullPromise = Promise::CreateInfallible(aController.GetParentObject());
+
+  MOZ_DIAGNOSTIC_ASSERT(mInput);
+
+  nsresult rv = mInput->AsyncWait(0, 0, mOwningEventTarget);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    ErrorPropagation(aCx, stream, rv);
+    return nullptr;
+  }
+
+  // All good.
+  return do_AddRef(mPullPromise);
+}
+
+NS_IMETHODIMP
+InputToReadableStreamAlgorithms::OnInputStreamReady(
+    nsIAsyncInputStream* aStream) {
+  MOZ_DIAGNOSTIC_ASSERT(aStream);
+
+  // Already closed. We have nothing else to do here.
+  if (IsClosed()) {
+    return NS_OK;
+  }
+
+  AutoEntryScript aes(mStream->GetParentObject(),
+                      "InputToReadableStream data available");
+
+  MOZ_DIAGNOSTIC_ASSERT(mInput);
+
+  JSContext* cx = aes.cx();
+
+  uint64_t size = 0;
+  nsresult rv = mInput->Available(&size);
+  MOZ_ASSERT_IF(NS_SUCCEEDED(rv), size > 0);
+
+  // No warning for stream closed.
+  if (rv == NS_BASE_STREAM_CLOSED || NS_WARN_IF(NS_FAILED(rv))) {
+    ErrorPropagation(cx, mStream, rv);
+    return NS_OK;
+  }
+
+  // Not having a promise means we are pinged by stream closure
+  // (WAIT_CLOSURE_ONLY below), but here we still have more data to read. Let's
+  // wait for the next read request in that case.
+  if (!mPullPromise) {
+    return NS_OK;
+  }
+
+  MOZ_DIAGNOSTIC_ASSERT(mPullPromise->State() ==
+                        Promise::PromiseState::Pending);
+
+  ErrorResult errorResult;
+  EnqueueChunkWithSizeIntoStream(cx, mStream, size, errorResult);
+  errorResult.WouldReportJSException();
+  if (errorResult.Failed()) {
+    ErrorPropagation(cx, mStream, errorResult.StealNSResult());
+    return NS_OK;
+  }
+
+  // Enqueuing triggers read request chunk steps which may execute JS, but:
+  // 1. The nsIAsyncInputStream should hold the reference of `this` so it should
+  // be safe from cycle collection
+  // 2. AsyncWait is called after enqueuing and thus OnInputStreamReady can't be
+  // synchronously called again
+  //
+  // That said, it's generally good to be cautious as there's no guarantee that
+  // the interface is implemented in a safest way.
+  MOZ_DIAGNOSTIC_ASSERT(mPullPromise);
+  if (mPullPromise) {
+    mPullPromise->MaybeResolveWithUndefined();
+    mPullPromise = nullptr;
+  }
+
+  return NS_OK;
+}
+
+void InputToReadableStreamAlgorithms::WriteIntoReadRequestBuffer(
+    JSContext* aCx, ReadableStream* aStream, JS::Handle<JSObject*> aBuffer,
+    uint32_t aLength, uint32_t* aByteWritten) {
+  MOZ_DIAGNOSTIC_ASSERT(aBuffer);
+  MOZ_DIAGNOSTIC_ASSERT(aByteWritten);
+  MOZ_DIAGNOSTIC_ASSERT(mInput);
+  MOZ_DIAGNOSTIC_ASSERT(!IsClosed());
+  MOZ_DIAGNOSTIC_ASSERT(mPullPromise->State() ==
+                        Promise::PromiseState::Pending);
+
+  uint32_t written;
+  nsresult rv;
+  void* buffer;
+  {
+    // Bug 1754513: Hazard suppression.
+    //
+    // Because mInput->Read is detected as possibly GCing by the
+    // current state of our static hazard analysis, we need to do the
+    // suppression here. This can be removed with future improvements
+    // to the static analysis.
+    JS::AutoSuppressGCAnalysis suppress;
+    JS::AutoCheckCannotGC noGC;
+    bool isSharedMemory;
+
+    buffer = JS_GetArrayBufferViewData(aBuffer, &isSharedMemory, noGC);
+    MOZ_ASSERT(!isSharedMemory);
+
+    rv = mInput->Read(static_cast<char*>(buffer), aLength, &written);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      ErrorPropagation(aCx, aStream, rv);
+      return;
+    }
+  }
+
+  *aByteWritten = written;
+
+  if (written == 0) {
+    CloseAndReleaseObjects(aCx, aStream);
+    return;
+  }
+
+  // All good.
+}
+
+// Whenever one or more bytes are available and stream is not
+// errored, enqueue a Uint8Array wrapping an ArrayBuffer containing the
+// available bytes into stream.
+void InputToReadableStreamAlgorithms::EnqueueChunkWithSizeIntoStream(
+    JSContext* aCx, ReadableStream* aStream, uint64_t aAvailableData,
+    ErrorResult& aRv) {
+  // To avoid OOMing up on huge amounts of available data on a 32 bit system,
+  // as well as potentially overflowing nsIInputStream's Read method's
+  // parameter, let's limit our maximum chunk size to 256MB.
+  uint32_t ableToRead =
+      std::min(static_cast<uint64_t>(256 * 1024 * 1024), aAvailableData);
+
+  // Create Chunk
+  aRv.MightThrowJSException();
+  JS::Rooted<JSObject*> chunk(aCx, JS_NewUint8Array(aCx, ableToRead));
+  if (!chunk) {
+    aRv.StealExceptionFromJSContext(aCx);
+    return;
+  }
+
+  {
+    uint32_t bytesWritten = 0;
+
+    WriteIntoReadRequestBuffer(aCx, aStream, chunk, ableToRead, &bytesWritten);
+
+    // If bytesWritten is zero, then the stream has been closed; return
+    // rather than enqueueing a chunk filled with zeros.
+    if (bytesWritten == 0) {
+      return;
+    }
+
+    // If we don't read every byte we've allocated in the Uint8Array
+    // we risk enqueuing a chunk that is padded with trailing zeros,
+    // corrupting future processing of the chunks:
+    MOZ_DIAGNOSTIC_ASSERT((ableToRead - bytesWritten) == 0);
+  }
+
+  MOZ_ASSERT(aStream->Controller()->IsByte());
+  JS::Rooted<JS::Value> chunkValue(aCx);
+  chunkValue.setObject(*chunk);
+  aStream->EnqueueNative(aCx, chunkValue, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Subscribe WAIT_CLOSURE_ONLY so that OnInputStreamReady can be called when
+  // mInput is closed.
+  nsresult rv = mInput->AsyncWait(nsIAsyncInputStream::WAIT_CLOSURE_ONLY, 0,
+                                  mOwningEventTarget);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    aRv.Throw(rv);
+    return;
+  }
+}
+
+void InputToReadableStreamAlgorithms::CloseAndReleaseObjects(
+    JSContext* aCx, ReadableStream* aStream) {
+  MOZ_DIAGNOSTIC_ASSERT(!IsClosed());
+
+  ReleaseObjects();
+
+  if (aStream->State() == ReadableStream::ReaderState::Readable) {
+    IgnoredErrorResult rv;
+    aStream->CloseNative(aCx, rv);
+    NS_WARNING_ASSERTION(!rv.Failed(), "Failed to Close Stream");
+  }
+}
+
+void InputToReadableStreamAlgorithms::ReleaseObjects() {
+  if (mInput) {
+    mInput->CloseWithStatus(NS_BASE_STREAM_CLOSED);
+    mInput->Shutdown();
+    mInput = nullptr;
+  }
+
+  // It's okay to leave a potentially unsettled promise as-is as this is only
+  // used to prevent reentrant to PullCallback. CloseNative() or ErrorNative()
+  // will settle the read requests for us.
+  mPullPromise = nullptr;
+}
+
+void InputToReadableStreamAlgorithms::ErrorPropagation(JSContext* aCx,
+                                                       ReadableStream* aStream,
+                                                       nsresult aError) {
+  // Nothing to do.
+  if (IsClosed()) {
+    return;
+  }
+
+  // Let's close the stream.
+  if (aError == NS_BASE_STREAM_CLOSED) {
+    CloseAndReleaseObjects(aCx, aStream);
+    return;
+  }
+
+  // Let's use a generic error.
+  ErrorResult rv;
+  // XXXbz can we come up with a better error message here to tell the
+  // consumer what went wrong?
+  rv.ThrowTypeError("Error in input stream");
+
+  JS::Rooted<JS::Value> errorValue(aCx);
+  bool ok = ToJSValue(aCx, std::move(rv), &errorValue);
+  MOZ_RELEASE_ASSERT(ok, "ToJSValue never fails for ErrorResult");
+
+  {
+    // This will be ignored if it's already errored.
+    IgnoredErrorResult rv;
+    aStream->ErrorNative(aCx, errorValue, rv);
+    NS_WARNING_ASSERTION(!rv.Failed(), "Failed to error InputToReadableStream");
+  }
+
+  MOZ_ASSERT(IsClosed());
+}
+
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(
+    NonAsyncInputToReadableStreamAlgorithms, UnderlyingSourceAlgorithmsWrapper)
+NS_IMPL_CYCLE_COLLECTION_INHERITED(NonAsyncInputToReadableStreamAlgorithms,
+                                   UnderlyingSourceAlgorithmsWrapper,
+                                   mAsyncAlgorithms)
+
+already_AddRefed<Promise>
+NonAsyncInputToReadableStreamAlgorithms::PullCallbackImpl(
+    JSContext* aCx, ReadableStreamController& aController, ErrorResult& aRv) {
+  if (!mAsyncAlgorithms) {
+    nsCOMPtr<nsIAsyncInputStream> asyncStream;
+
+    // NS_MakeAsyncNonBlockingInputStream may immediately start a stream read
+    // via nsInputStreamTransport::OpenInputStream, which is why this should be
+    // called on a pull callback instead of in the constructor.
+    nsresult rv = NS_MakeAsyncNonBlockingInputStream(
+        mInput.forget(), getter_AddRefs(asyncStream));
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      aRv.Throw(rv);
+      return nullptr;
+    }
+
+    mAsyncAlgorithms = MakeRefPtr<InputToReadableStreamAlgorithms>(
+        aCx, asyncStream, aController.Stream());
+  }
+
+  MOZ_ASSERT(!mInput);
+  return mAsyncAlgorithms->PullCallbackImpl(aCx, aController, aRv);
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/UnderlyingSourceCallbackHelpers.h b/dom/streams/UnderlyingSourceCallbackHelpers.h
new file mode 100644
index 0000000000..1859204f32
--- /dev/null
+++ b/dom/streams/UnderlyingSourceCallbackHelpers.h
@@ -0,0 +1,318 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_UnderlyingSourceCallbackHelpers_h
+#define mozilla_dom_UnderlyingSourceCallbackHelpers_h
+
+#include "mozilla/DOMEventTargetHelper.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/UnderlyingSourceBinding.h"
+#include "mozilla/WeakPtr.h"
+#include "nsIAsyncInputStream.h"
+#include "nsISupports.h"
+#include "nsISupportsImpl.h"
+
+/* Since the streams specification has native descriptions of some callbacks
+ * (i.e. described in prose, rather than provided by user code), we need to be
+ * able to pass around native callbacks. To handle this, we define polymorphic
+ * classes That cover the difference between native callback and user-provided.
+ *
+ * The Streams specification wants us to invoke these callbacks, run through
+ * WebIDL as if they were methods. So we have to preserve the underlying object
+ * to use as the This value on invocation.
+ */
+enum class nsresult : uint32_t;
+
+namespace mozilla::dom {
+
+class StrongWorkerRef;
+class BodyStreamHolder;
+class ReadableStreamController;
+class ReadableStream;
+
+class UnderlyingSourceAlgorithmsBase : public nsISupports {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(UnderlyingSourceAlgorithmsBase)
+
+  MOZ_CAN_RUN_SCRIPT virtual void StartCallback(
+      JSContext* aCx, ReadableStreamController& aController,
+      JS::MutableHandle<JS::Value> aRetVal, ErrorResult& aRv) = 0;
+
+  // A promise-returning algorithm that pulls data from the underlying byte
+  // source
+  MOZ_CAN_RUN_SCRIPT virtual already_AddRefed<Promise> PullCallback(
+      JSContext* aCx, ReadableStreamController& aController,
+      ErrorResult& aRv) = 0;
+
+  // A promise-returning algorithm, taking one argument (the cancel reason),
+  // which communicates a requested cancelation to the underlying byte source
+  MOZ_CAN_RUN_SCRIPT virtual already_AddRefed<Promise> CancelCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) = 0;
+
+  // Implement this when you need to release underlying resources immediately
+  // from closed(canceled)/errored streams, without waiting for GC.
+  virtual void ReleaseObjects() {}
+
+  // Fetch wants to special-case nsIInputStream-based streams
+  virtual nsIInputStream* MaybeGetInputStreamIfUnread() { return nullptr; }
+
+  // https://streams.spec.whatwg.org/#other-specs-rs-create
+  // By "native" we mean "instances initialized via the above set up or set up
+  // with byte reading support algorithms (not, e.g., on web-developer-created
+  // instances)"
+  virtual bool IsNative() { return true; }
+
+ protected:
+  virtual ~UnderlyingSourceAlgorithmsBase() = default;
+};
+
+class UnderlyingSourceAlgorithms final : public UnderlyingSourceAlgorithmsBase {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_INHERITED(
+      UnderlyingSourceAlgorithms, UnderlyingSourceAlgorithmsBase)
+
+  UnderlyingSourceAlgorithms(nsIGlobalObject* aGlobal,
+                             JS::Handle<JSObject*> aUnderlyingSource,
+                             UnderlyingSource& aUnderlyingSourceDict)
+      : mGlobal(aGlobal), mUnderlyingSource(aUnderlyingSource) {
+    // Step 6. (implicit Step 2.)
+    if (aUnderlyingSourceDict.mStart.WasPassed()) {
+      mStartCallback = aUnderlyingSourceDict.mStart.Value();
+    }
+
+    // Step 7. (implicit Step 3.)
+    if (aUnderlyingSourceDict.mPull.WasPassed()) {
+      mPullCallback = aUnderlyingSourceDict.mPull.Value();
+    }
+
+    // Step 8. (implicit Step 4.)
+    if (aUnderlyingSourceDict.mCancel.WasPassed()) {
+      mCancelCallback = aUnderlyingSourceDict.mCancel.Value();
+    }
+
+    mozilla::HoldJSObjects(this);
+  };
+
+  MOZ_CAN_RUN_SCRIPT void StartCallback(JSContext* aCx,
+                                        ReadableStreamController& aController,
+                                        JS::MutableHandle<JS::Value> aRetVal,
+                                        ErrorResult& aRv) override;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> PullCallback(
+      JSContext* aCx, ReadableStreamController& aController,
+      ErrorResult& aRv) override;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> CancelCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) override;
+
+  bool IsNative() override { return false; }
+
+ protected:
+  ~UnderlyingSourceAlgorithms() override { mozilla::DropJSObjects(this); };
+
+ private:
+  // Virtually const, but are cycle collected
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+  JS::Heap<JSObject*> mUnderlyingSource;
+  MOZ_KNOWN_LIVE RefPtr<UnderlyingSourceStartCallback> mStartCallback;
+  MOZ_KNOWN_LIVE RefPtr<UnderlyingSourcePullCallback> mPullCallback;
+  MOZ_KNOWN_LIVE RefPtr<UnderlyingSourceCancelCallback> mCancelCallback;
+};
+
+// https://streams.spec.whatwg.org/#readablestream-set-up
+// https://streams.spec.whatwg.org/#readablestream-set-up-with-byte-reading-support
+// Wrappers defined by the "Set up" methods in the spec. This helps you just
+// return nullptr when an error occurred as this wrapper converts it to a
+// rejected promise.
+// Note that StartCallback is only for JS consumers to access
+// the controller, and thus is no-op here since native consumers can call
+// `EnqueueNative()` etc. without direct controller access.
+class UnderlyingSourceAlgorithmsWrapper
+    : public UnderlyingSourceAlgorithmsBase {
+  void StartCallback(JSContext*, ReadableStreamController&,
+                     JS::MutableHandle<JS::Value> aRetVal, ErrorResult&) final;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> PullCallback(
+      JSContext* aCx, ReadableStreamController& aController,
+      ErrorResult& aRv) final;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> CancelCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) final;
+
+  MOZ_CAN_RUN_SCRIPT virtual already_AddRefed<Promise> PullCallbackImpl(
+      JSContext* aCx, ReadableStreamController& aController, ErrorResult& aRv) {
+    // pullAlgorithm is optional, return null by default
+    return nullptr;
+  }
+
+  virtual already_AddRefed<Promise> CancelCallbackImpl(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) {
+    // cancelAlgorithm is optional, return null by default
+    return nullptr;
+  }
+};
+
+class InputToReadableStreamAlgorithms;
+
+// This class exists to isolate InputToReadableStreamAlgorithms from the
+// nsIAsyncInputStream.  If we call AsyncWait(this,...), it holds a
+// reference to 'this' which can't be cc'd, and we can leak the stream,
+// causing a Worker to assert with globalScopeAlive.  By isolating
+// ourselves from the inputstream, we can safely be CC'd if needed and
+// will inform the inputstream to shut down.
+class InputStreamHolder final : public nsIInputStreamCallback,
+                                public GlobalTeardownObserver {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIINPUTSTREAMCALLBACK
+
+  InputStreamHolder(nsIGlobalObject* aGlobal,
+                    InputToReadableStreamAlgorithms* aCallback,
+                    nsIAsyncInputStream* aInput);
+
+  void Init(JSContext* aCx);
+
+  void DisconnectFromOwner() override;
+
+  // Used by global teardown
+  void Shutdown();
+
+  // These just proxy the calls to the nsIAsyncInputStream
+  nsresult AsyncWait(uint32_t aFlags, uint32_t aRequestedCount,
+                     nsIEventTarget* aEventTarget);
+  nsresult Available(uint64_t* aSize) { return mInput->Available(aSize); }
+  nsresult Read(char* aBuffer, uint32_t aLength, uint32_t* aWritten) {
+    return mInput->Read(aBuffer, aLength, aWritten);
+  }
+  nsresult CloseWithStatus(nsresult aStatus) {
+    return mInput->CloseWithStatus(aStatus);
+  }
+
+ private:
+  ~InputStreamHolder();
+
+  // WeakPtr to avoid cycles
+  WeakPtr<InputToReadableStreamAlgorithms> mCallback;
+  // To ensure the worker sticks around
+  RefPtr<StrongWorkerRef> mAsyncWaitWorkerRef;
+  RefPtr<StrongWorkerRef> mWorkerRef;
+  nsCOMPtr<nsIAsyncInputStream> mInput;
+
+  // To ensure the underlying source sticks around during an ongoing read
+  // operation. mAlgorithms is not cycle collected on purpose, and this holder
+  // is responsible to keep the underlying source algorithms until
+  // nsIAsyncInputStream responds.
+  //
+  // This is done because otherwise the whole stream objects may be cycle
+  // collected, including the promises created from read(), as our JS engine may
+  // throw unsettled promises away for optimization. See bug 1849860.
+  RefPtr<InputToReadableStreamAlgorithms> mAsyncWaitAlgorithms;
+};
+
+// Using this class means you are also passing the lifetime control of your
+// nsIAsyncInputStream, as it will be closed when this class tears down.
+class InputToReadableStreamAlgorithms final
+    : public UnderlyingSourceAlgorithmsWrapper,
+      public nsIInputStreamCallback,
+      public SupportsWeakPtr {
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIINPUTSTREAMCALLBACK
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(InputToReadableStreamAlgorithms,
+                                           UnderlyingSourceAlgorithmsWrapper)
+
+  InputToReadableStreamAlgorithms(JSContext* aCx, nsIAsyncInputStream* aInput,
+                                  ReadableStream* aStream);
+
+  // Streams algorithms
+
+  already_AddRefed<Promise> PullCallbackImpl(
+      JSContext* aCx, ReadableStreamController& aController,
+      ErrorResult& aRv) override;
+
+  void ReleaseObjects() override;
+
+ private:
+  ~InputToReadableStreamAlgorithms() {
+    if (mInput) {
+      mInput->Shutdown();
+    }
+  }
+
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY void CloseAndReleaseObjects(
+      JSContext* aCx, ReadableStream* aStream);
+
+  void WriteIntoReadRequestBuffer(JSContext* aCx, ReadableStream* aStream,
+                                  JS::Handle<JSObject*> aBuffer,
+                                  uint32_t aLength, uint32_t* aByteWritten);
+
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY void EnqueueChunkWithSizeIntoStream(
+      JSContext* aCx, ReadableStream* aStream, uint64_t aAvailableData,
+      ErrorResult& aRv);
+  void ErrorPropagation(JSContext* aCx, ReadableStream* aStream,
+                        nsresult aError);
+
+  // Common methods
+
+  bool IsClosed() { return !mInput; }
+
+  nsCOMPtr<nsIEventTarget> mOwningEventTarget;
+
+  // This promise is created by PullCallback and resolved when
+  // OnInputStreamReady succeeds. No need to try hard to settle it though, see
+  // also ReleaseObjects() for the reason.
+  RefPtr<Promise> mPullPromise;
+
+  RefPtr<InputStreamHolder> mInput;
+  RefPtr<ReadableStream> mStream;
+};
+
+class NonAsyncInputToReadableStreamAlgorithms
+    : public UnderlyingSourceAlgorithmsWrapper {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(
+      NonAsyncInputToReadableStreamAlgorithms,
+      UnderlyingSourceAlgorithmsWrapper)
+
+  explicit NonAsyncInputToReadableStreamAlgorithms(nsIInputStream& aInput)
+      : mInput(&aInput) {}
+
+  already_AddRefed<Promise> PullCallbackImpl(
+      JSContext* aCx, ReadableStreamController& aController,
+      ErrorResult& aRv) override;
+
+  void ReleaseObjects() override {
+    if (RefPtr<InputToReadableStreamAlgorithms> algorithms =
+            mAsyncAlgorithms.forget()) {
+      algorithms->ReleaseObjects();
+    }
+    if (nsCOMPtr<nsIInputStream> input = mInput.forget()) {
+      input->Close();
+    }
+  }
+
+  nsIInputStream* MaybeGetInputStreamIfUnread() override {
+    MOZ_ASSERT(mInput, "Should be only called on non-disturbed streams");
+    return mInput;
+  }
+
+ private:
+  ~NonAsyncInputToReadableStreamAlgorithms() = default;
+
+  nsCOMPtr<nsIInputStream> mInput;
+  RefPtr<InputToReadableStreamAlgorithms> mAsyncAlgorithms;
+};
+
+}  // namespace mozilla::dom
+
+#endif
diff --git a/dom/streams/WritableStream.cpp b/dom/streams/WritableStream.cpp
new file mode 100644
index 0000000000..8dab9a564e
--- /dev/null
+++ b/dom/streams/WritableStream.cpp
@@ -0,0 +1,811 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/WritableStream.h"
+
+#include "StreamUtils.h"
+#include "js/Array.h"
+#include "js/PropertyAndElement.h"
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/CycleCollectedJSContext.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/dom/AbortSignal.h"
+#include "mozilla/dom/BindingCallContext.h"
+#include "mozilla/dom/QueueWithSizes.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+#include "mozilla/dom/ReadRequest.h"
+#include "mozilla/dom/RootedDictionary.h"
+#include "mozilla/dom/UnderlyingSinkBinding.h"
+#include "mozilla/dom/WritableStreamBinding.h"
+#include "mozilla/dom/WritableStreamDefaultController.h"
+#include "mozilla/dom/WritableStreamDefaultWriter.h"
+#include "nsCOMPtr.h"
+
+#include "mozilla/dom/Promise-inl.h"
+#include "nsIGlobalObject.h"
+#include "nsISupports.h"
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE_WITH_JS_MEMBERS(
+    WritableStream,
+    (mGlobal, mCloseRequest, mController, mInFlightWriteRequest,
+     mInFlightCloseRequest, mPendingAbortRequestPromise, mWriter,
+     mWriteRequests),
+    (mPendingAbortRequestReason, mStoredError))
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(WritableStream)
+NS_IMPL_CYCLE_COLLECTING_RELEASE_WITH_LAST_RELEASE(WritableStream,
+                                                   LastRelease())
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(WritableStream)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+WritableStream::WritableStream(nsIGlobalObject* aGlobal,
+                               HoldDropJSObjectsCaller aHoldDropCaller)
+    : mGlobal(aGlobal), mHoldDropCaller(aHoldDropCaller) {
+  if (mHoldDropCaller == HoldDropJSObjectsCaller::Implicit) {
+    mozilla::HoldJSObjects(this);
+  }
+}
+
+WritableStream::WritableStream(const GlobalObject& aGlobal,
+                               HoldDropJSObjectsCaller aHoldDropCaller)
+    : mGlobal(do_QueryInterface(aGlobal.GetAsSupports())),
+      mHoldDropCaller(aHoldDropCaller) {
+  if (mHoldDropCaller == HoldDropJSObjectsCaller::Implicit) {
+    mozilla::HoldJSObjects(this);
+  }
+}
+
+WritableStream::~WritableStream() {
+  if (mHoldDropCaller == HoldDropJSObjectsCaller::Implicit) {
+    mozilla::DropJSObjects(this);
+  }
+}
+
+JSObject* WritableStream::WrapObject(JSContext* aCx,
+                                     JS::Handle<JSObject*> aGivenProto) {
+  return WritableStream_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-deal-with-rejection
+void WritableStream::DealWithRejection(JSContext* aCx,
+                                       JS::Handle<JS::Value> aError,
+                                       ErrorResult& aRv) {
+  // Step 1. Let state be stream.[[state]].
+  // Step 2. If state is "writable",
+  if (mState == WriterState::Writable) {
+    // Step 2.1. Perform ! WritableStreamStartErroring(stream, error).
+    StartErroring(aCx, aError, aRv);
+
+    // Step 2.2. Return.
+    return;
+  }
+
+  // Step 3. Assert: state is "erroring".
+  MOZ_ASSERT(mState == WriterState::Erroring);
+
+  // Step 4. Perform ! WritableStreamFinishErroring(stream).
+  FinishErroring(aCx, aRv);
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-finish-erroring
+void WritableStream::FinishErroring(JSContext* aCx, ErrorResult& aRv) {
+  // Step 1. Assert: stream.[[state]] is "erroring".
+  MOZ_ASSERT(mState == WriterState::Erroring);
+
+  // Step 2. Assert: ! WritableStreamHasOperationMarkedInFlight(stream) is
+  // false.
+  MOZ_ASSERT(!HasOperationMarkedInFlight());
+
+  // Step 3. Set stream.[[state]] to "errored".
+  mState = WriterState::Errored;
+
+  // Step 4. Perform ! stream.[[controller]].[[ErrorSteps]]().
+  Controller()->ErrorSteps();
+
+  // Step 5. Let storedError be stream.[[storedError]].
+  JS::Rooted<JS::Value> storedError(aCx, mStoredError);
+
+  // Step 6. For each writeRequest of stream.[[writeRequests]]:
+  for (const RefPtr<Promise>& writeRequest : mWriteRequests) {
+    // Step 6.1. Reject writeRequest with storedError.
+    writeRequest->MaybeReject(storedError);
+  }
+
+  // Step 7. Set stream.[[writeRequests]] to an empty list.
+  mWriteRequests.Clear();
+
+  // Step 8. If stream.[[pendingAbortRequest]] is undefined,
+  if (!mPendingAbortRequestPromise) {
+    // Step 8.1. Perform !
+    // WritableStreamRejectCloseAndClosedPromiseIfNeeded(stream).
+    RejectCloseAndClosedPromiseIfNeeded();
+
+    // Step 8.2. Return.
+    return;
+  }
+
+  // Step 9. Let abortRequest be stream.[[pendingAbortRequest]].
+  RefPtr<Promise> abortPromise = mPendingAbortRequestPromise;
+  JS::Rooted<JS::Value> abortReason(aCx, mPendingAbortRequestReason);
+  bool abortWasAlreadyErroring = mPendingAbortRequestWasAlreadyErroring;
+
+  // Step 10. Set stream.[[pendingAbortRequest]] to undefined.
+  SetPendingAbortRequest(nullptr, JS::UndefinedHandleValue, false);
+
+  // Step 11. If abortRequest’s was already erroring is true,
+  if (abortWasAlreadyErroring) {
+    // Step 11.1. Reject abortRequest’s promise with storedError.
+    abortPromise->MaybeReject(storedError);
+
+    // Step 11.2. Perform !
+    // WritableStreamRejectCloseAndClosedPromiseIfNeeded(stream).
+    RejectCloseAndClosedPromiseIfNeeded();
+
+    // Step 11.3. Return.
+    return;
+  }
+
+  // Step 12. Let promise be !
+  // stream.[[controller]].[[AbortSteps]](abortRequest’s reason).
+  RefPtr<WritableStreamDefaultController> controller = mController;
+  RefPtr<Promise> promise = controller->AbortSteps(aCx, abortReason, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 13 + 14.
+  promise->AddCallbacksWithCycleCollectedArgs(
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         Promise* aAbortRequestPromise, WritableStream* aStream) {
+        // Step 13. Upon fulfillment of promise,
+        // Step 13.1. Resolve abortRequest’s promise with undefined.
+        aAbortRequestPromise->MaybeResolveWithUndefined();
+
+        // Step 13.2. Perform !
+        // WritableStreamRejectCloseAndClosedPromiseIfNeeded(stream).
+        aStream->RejectCloseAndClosedPromiseIfNeeded();
+      },
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         Promise* aAbortRequestPromise, WritableStream* aStream) {
+        // Step 14. Upon rejection of promise with reason reason,
+        // Step 14.1. Reject abortRequest’s promise with reason.
+        aAbortRequestPromise->MaybeReject(aValue);
+
+        // Step 14.2. Perform !
+        // WritableStreamRejectCloseAndClosedPromiseIfNeeded(stream).
+        aStream->RejectCloseAndClosedPromiseIfNeeded();
+      },
+      RefPtr(abortPromise), RefPtr(this));
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-finish-in-flight-close
+void WritableStream::FinishInFlightClose() {
+  // Step 1. Assert: stream.[[inFlightCloseRequest]] is not undefined.
+  MOZ_ASSERT(mInFlightCloseRequest);
+
+  // Step 2. Resolve stream.[[inFlightCloseRequest]] with undefined.
+  mInFlightCloseRequest->MaybeResolveWithUndefined();
+
+  // Step 3. Set stream.[[inFlightCloseRequest]] to undefined.
+  mInFlightCloseRequest = nullptr;
+
+  // Step 4. Let state be stream.[[state]].
+  // Step 5. Assert: stream.[[state]] is "writable" or "erroring".
+  MOZ_ASSERT(mState == WriterState::Writable ||
+             mState == WriterState::Erroring);
+
+  // Step 6. If state is "erroring",
+  if (mState == WriterState::Erroring) {
+    // Step 6.1. Set stream.[[storedError]] to undefined.
+    mStoredError.setUndefined();
+
+    // Step 6.2. If stream.[[pendingAbortRequest]] is not undefined,
+    if (mPendingAbortRequestPromise) {
+      // Step 6.2.1. Resolve stream.[[pendingAbortRequest]]'s promise with
+      // undefined.
+      mPendingAbortRequestPromise->MaybeResolveWithUndefined();
+
+      // Step 6.2.2. Set stream.[[pendingAbortRequest]] to undefined.
+      SetPendingAbortRequest(nullptr, JS::UndefinedHandleValue, false);
+    }
+  }
+
+  // Step 7. Set stream.[[state]] to "closed".
+  mState = WriterState::Closed;
+
+  // Step 8. Let writer be stream.[[writer]].
+  // Step 9. If writer is not undefined, resolve writer.[[closedPromise]] with
+  // undefined.
+  if (mWriter) {
+    mWriter->ClosedPromise()->MaybeResolveWithUndefined();
+  }
+
+  // Step 10. Assert: stream.[[pendingAbortRequest]] is undefined.
+  MOZ_ASSERT(!mPendingAbortRequestPromise);
+  // Assert: stream.[[storedError]] is undefined.
+  MOZ_ASSERT(mStoredError.isUndefined());
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-finish-in-flight-close-with-error
+void WritableStream::FinishInFlightCloseWithError(JSContext* aCx,
+                                                  JS::Handle<JS::Value> aError,
+                                                  ErrorResult& aRv) {
+  // Step 1. Assert: stream.[[inFlightCloseRequest]] is not undefined.
+  MOZ_ASSERT(mInFlightCloseRequest);
+
+  // Step 2. Reject stream.[[inFlightCloseRequest]] with error.
+  mInFlightCloseRequest->MaybeReject(aError);
+
+  // Step 3. Set stream.[[inFlightCloseRequest]] to undefined.
+  mInFlightCloseRequest = nullptr;
+
+  // Step 4. Assert: stream.[[state]] is "writable" or "erroring".
+  MOZ_ASSERT(mState == WriterState::Writable ||
+             mState == WriterState::Erroring);
+
+  // Step 5. If stream.[[pendingAbortRequest]] is not undefined,
+  if (mPendingAbortRequestPromise) {
+    // Step 5.1. Reject stream.[[pendingAbortRequest]]'s promise with error.
+    mPendingAbortRequestPromise->MaybeReject(aError);
+
+    // Step 5.2. Set stream.[[pendingAbortRequest]] to undefined.
+    SetPendingAbortRequest(nullptr, JS::UndefinedHandleValue, false);
+  }
+
+  // Step 6. Perform ! WritableStreamDealWithRejection(stream, error).
+  DealWithRejection(aCx, aError, aRv);
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-finish-in-flight-write
+void WritableStream::FinishInFlightWrite() {
+  // Step 1. Assert: stream.[[inFlightWriteRequest]] is not undefined.
+  MOZ_ASSERT(mInFlightWriteRequest);
+
+  // Step 2. Resolve stream.[[inFlightWriteRequest]] with undefined.
+  mInFlightWriteRequest->MaybeResolveWithUndefined();
+
+  // Step 3. Set stream.[[inFlightWriteRequest]] to undefined.
+  mInFlightWriteRequest = nullptr;
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-finish-in-flight-write-with-error
+void WritableStream::FinishInFlightWriteWithError(JSContext* aCx,
+                                                  JS::Handle<JS::Value> aError,
+                                                  ErrorResult& aRv) {
+  // Step 1. Assert: stream.[[inFlightWriteRequest]] is not undefined.
+  MOZ_ASSERT(mInFlightWriteRequest);
+
+  // Step 2. Reject stream.[[inFlightWriteRequest]] with error.
+  mInFlightWriteRequest->MaybeReject(aError);
+
+  // Step 3. Set stream.[[inFlightWriteRequest]] to undefined.
+  mInFlightWriteRequest = nullptr;
+
+  // Step 4. Assert: stream.[[state]] is "writable" or "erroring".
+  MOZ_ASSERT(mState == WriterState::Writable ||
+             mState == WriterState::Erroring);
+
+  // Step 5. Perform ! WritableStreamDealWithRejection(stream, error).
+  DealWithRejection(aCx, aError, aRv);
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-mark-close-request-in-flight
+void WritableStream::MarkCloseRequestInFlight() {
+  // Step 1. Assert: stream.[[inFlightCloseRequest]] is undefined.
+  MOZ_ASSERT(!mInFlightCloseRequest);
+
+  // Step 2. Assert: stream.[[closeRequest]] is not undefined.
+  MOZ_ASSERT(mCloseRequest);
+
+  // Step 3. Set stream.[[inFlightCloseRequest]] to stream.[[closeRequest]].
+  mInFlightCloseRequest = mCloseRequest;
+
+  // Step 4. Set stream.[[closeRequest]] to undefined.
+  mCloseRequest = nullptr;
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-mark-first-write-request-in-flight
+void WritableStream::MarkFirstWriteRequestInFlight() {
+  // Step 1. Assert: stream.[[inFlightWriteRequest]] is undefined.
+  MOZ_ASSERT(!mInFlightWriteRequest);
+
+  // Step 2. Assert: stream.[[writeRequests]] is not empty.
+  MOZ_ASSERT(!mWriteRequests.IsEmpty());
+
+  // Step 3. Let writeRequest be stream.[[writeRequests]][0].
+  RefPtr<Promise> writeRequest = mWriteRequests.ElementAt(0);
+
+  // Step 4. Remove writeRequest from stream.[[writeRequests]].
+  mWriteRequests.RemoveElementAt(0);
+
+  // Step 5. Set stream.[[inFlightWriteRequest]] to writeRequest.
+  mInFlightWriteRequest = writeRequest;
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-reject-close-and-closed-promise-if-needed
+void WritableStream::RejectCloseAndClosedPromiseIfNeeded() {
+  // Step 1. Assert: stream.[[state]] is "errored".
+  MOZ_ASSERT(mState == WriterState::Errored);
+
+  JS::Rooted<JS::Value> storedError(RootingCx(), mStoredError);
+  // Step 2. If stream.[[closeRequest]] is not undefined,
+  if (mCloseRequest) {
+    // Step 2.1. Assert: stream.[[inFlightCloseRequest]] is undefined.
+    MOZ_ASSERT(!mInFlightCloseRequest);
+
+    // Step 2.2. Reject stream.[[closeRequest]] with stream.[[storedError]].
+    mCloseRequest->MaybeReject(storedError);
+
+    // Step 2.3. Set stream.[[closeRequest]] to undefined.
+    mCloseRequest = nullptr;
+  }
+
+  // Step 3. Let writer be stream.[[writer]].
+  RefPtr<WritableStreamDefaultWriter> writer = mWriter;
+
+  // Step 4. If writer is not undefined,
+  if (writer) {
+    // Step 4.1. Reject writer.[[closedPromise]] with stream.[[storedError]].
+    RefPtr<Promise> closedPromise = writer->ClosedPromise();
+    closedPromise->MaybeReject(storedError);
+
+    // Step 4.2. Set writer.[[closedPromise]].[[PromiseIsHandled]] to true.
+    closedPromise->SetSettledPromiseIsHandled();
+  }
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-start-erroring
+void WritableStream::StartErroring(JSContext* aCx,
+                                   JS::Handle<JS::Value> aReason,
+                                   ErrorResult& aRv) {
+  // Step 1. Assert: stream.[[storedError]] is undefined.
+  MOZ_ASSERT(mStoredError.isUndefined());
+
+  // Step 2. Assert: stream.[[state]] is "writable".
+  MOZ_ASSERT(mState == WriterState::Writable);
+
+  // Step 3. Let controller be stream.[[controller]].
+  RefPtr<WritableStreamDefaultController> controller = mController;
+  // Step 4. Assert: controller is not undefined.
+  MOZ_ASSERT(controller);
+
+  // Step 5. Set stream.[[state]] to "erroring".
+  mState = WriterState::Erroring;
+
+  // Step 6. Set stream.[[storedError]] to reason.
+  mStoredError = aReason;
+
+  // Step 7. Let writer be stream.[[writer]].
+  RefPtr<WritableStreamDefaultWriter> writer = mWriter;
+  // Step 8. If writer is not undefined, perform !
+  // WritableStreamDefaultWriterEnsureReadyPromiseRejected(writer, reason).
+  if (writer) {
+    WritableStreamDefaultWriterEnsureReadyPromiseRejected(writer, aReason);
+  }
+
+  // Step 9. If ! WritableStreamHasOperationMarkedInFlight(stream) is false
+  // and controller.[[started]] is true,
+  // perform !WritableStreamFinishErroring(stream).
+  if (!HasOperationMarkedInFlight() && controller->Started()) {
+    FinishErroring(aCx, aRv);
+  }
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-update-backpressure
+void WritableStream::UpdateBackpressure(bool aBackpressure) {
+  // Step 1. Assert: stream.[[state]] is "writable".
+  MOZ_ASSERT(mState == WriterState::Writable);
+  // Step 2. Assert: ! WritableStreamCloseQueuedOrInFlight(stream) is false.
+  MOZ_ASSERT(!CloseQueuedOrInFlight());
+
+  // Step 3. Let writer be stream.[[writer]].
+  RefPtr<WritableStreamDefaultWriter> writer = mWriter;
+
+  // Step 4. If writer is not undefined and backpressure is not
+  // stream.[[backpressure]],
+  if (writer && aBackpressure != mBackpressure) {
+    // Step 4.1. If backpressure is true, set writer.[[readyPromise]] to a new
+    // promise.
+    if (aBackpressure) {
+      RefPtr<Promise> promise =
+          Promise::CreateInfallible(writer->GetParentObject());
+      writer->SetReadyPromise(promise);
+    } else {
+      // Step 4.2. Otherwise,
+      // Step 4.2.1. Assert: backpressure is false.
+      // Step 4.2.2. Resolve writer.[[readyPromise]] with undefined.
+      writer->ReadyPromise()->MaybeResolveWithUndefined();
+    }
+  }
+
+  // Step 5. Set stream.[[backpressure]] to backpressure.
+  mBackpressure = aBackpressure;
+}
+
+// https://streams.spec.whatwg.org/#ws-constructor
+already_AddRefed<WritableStream> WritableStream::Constructor(
+    const GlobalObject& aGlobal,
+    const Optional<JS::Handle<JSObject*>>& aUnderlyingSink,
+    const QueuingStrategy& aStrategy, ErrorResult& aRv) {
+  // Step 1. If underlyingSink is missing, set it to null.
+  JS::Rooted<JSObject*> underlyingSinkObj(
+      aGlobal.Context(),
+      aUnderlyingSink.WasPassed() ? aUnderlyingSink.Value() : nullptr);
+
+  // Step 2. Let underlyingSinkDict be underlyingSink, converted to
+  //         an IDL value of type UnderlyingSink.
+  RootedDictionary<UnderlyingSink> underlyingSinkDict(aGlobal.Context());
+  if (underlyingSinkObj) {
+    JS::Rooted<JS::Value> objValue(aGlobal.Context(),
+                                   JS::ObjectValue(*underlyingSinkObj));
+    dom::BindingCallContext callCx(aGlobal.Context(),
+                                   "WritableStream.constructor");
+    aRv.MightThrowJSException();
+    if (!underlyingSinkDict.Init(callCx, objValue)) {
+      aRv.StealExceptionFromJSContext(aGlobal.Context());
+      return nullptr;
+    }
+  }
+
+  // Step 3. If underlyingSinkDict["type"] exists, throw a RangeError exception.
+  if (!underlyingSinkDict.mType.isUndefined()) {
+    aRv.ThrowRangeError("Implementation preserved member 'type'");
+    return nullptr;
+  }
+
+  // Step 4. Perform ! InitializeWritableStream(this).
+  RefPtr<WritableStream> writableStream =
+      new WritableStream(aGlobal, HoldDropJSObjectsCaller::Implicit);
+
+  // Step 5. Let sizeAlgorithm be ! ExtractSizeAlgorithm(strategy).
+  //
+  // Implementation Note: The specification demands that if the size doesn't
+  // exist, we instead would provide an algorithm that returns 1. Instead, we
+  // will teach callers that a missing callback should simply return 1, rather
+  // than gin up a fake callback here.
+  //
+  // This decision may need to be revisited if the default action ever diverges
+  // within the specification.
+  RefPtr<QueuingStrategySize> sizeAlgorithm =
+      aStrategy.mSize.WasPassed() ? &aStrategy.mSize.Value() : nullptr;
+
+  // Step 6. Let highWaterMark be ? ExtractHighWaterMark(strategy, 1).
+  double highWaterMark = ExtractHighWaterMark(aStrategy, 1, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 7. Perform ? SetUpWritableStreamDefaultControllerFromUnderlyingSink(
+  // this, underlyingSink, underlyingSinkDict, highWaterMark, sizeAlgorithm).
+  SetUpWritableStreamDefaultControllerFromUnderlyingSink(
+      aGlobal.Context(), writableStream, underlyingSinkObj, underlyingSinkDict,
+      highWaterMark, sizeAlgorithm, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  return writableStream.forget();
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#writable-stream-abort
+already_AddRefed<Promise> WritableStreamAbort(JSContext* aCx,
+                                              WritableStream* aStream,
+                                              JS::Handle<JS::Value> aReason,
+                                              ErrorResult& aRv) {
+  // Step 1. If stream.[[state]] is "closed" or "errored", return a promise
+  // resolved with undefined.
+  if (aStream->State() == WritableStream::WriterState::Closed ||
+      aStream->State() == WritableStream::WriterState::Errored) {
+    RefPtr<Promise> promise =
+        Promise::CreateInfallible(aStream->GetParentObject());
+    promise->MaybeResolveWithUndefined();
+    return promise.forget();
+  }
+
+  // Step 2. Signal abort on stream.[[controller]].[[signal]] with reason.
+  RefPtr<WritableStreamDefaultController> controller = aStream->Controller();
+  controller->Signal()->SignalAbort(aReason);
+
+  // Step 3. Let state be stream.[[state]].
+  WritableStream::WriterState state = aStream->State();
+
+  // Step 4. If state is "closed" or "errored", return a promise resolved with
+  // undefined. Note: We re-check the state because signaling abort runs author
+  // code and that might have changed the state.
+  if (aStream->State() == WritableStream::WriterState::Closed ||
+      aStream->State() == WritableStream::WriterState::Errored) {
+    RefPtr<Promise> promise =
+        Promise::CreateInfallible(aStream->GetParentObject());
+    promise->MaybeResolveWithUndefined();
+    return promise.forget();
+  }
+
+  // Step 5. If stream.[[pendingAbortRequest]] is not undefined, return
+  // stream.[[pendingAbortRequest]]'s promise.
+  if (aStream->GetPendingAbortRequestPromise()) {
+    RefPtr<Promise> promise = aStream->GetPendingAbortRequestPromise();
+    return promise.forget();
+  }
+
+  // Step 6. Assert: state is "writable" or "erroring".
+  MOZ_ASSERT(state == WritableStream::WriterState::Writable ||
+             state == WritableStream::WriterState::Erroring);
+
+  // Step 7. Let wasAlreadyErroring be false.
+  bool wasAlreadyErroring = false;
+
+  // Step 8. If state is "erroring",
+  JS::Rooted<JS::Value> reason(aCx, aReason);
+  if (state == WritableStream::WriterState::Erroring) {
+    // Step 8.1. Set wasAlreadyErroring to true.
+    wasAlreadyErroring = true;
+    // Step 8.2. Set reason to undefined.
+    reason.setUndefined();
+  }
+
+  // Step 9. Let promise be a new promise.
+  RefPtr<Promise> promise =
+      Promise::CreateInfallible(aStream->GetParentObject());
+
+  // Step 10. Set stream.[[pendingAbortRequest]] to a new pending abort request
+  // whose promise is promise, reason is reason, and was already erroring is
+  // wasAlreadyErroring.
+  aStream->SetPendingAbortRequest(promise, reason, wasAlreadyErroring);
+
+  // Step 11. If wasAlreadyErroring is false, perform !
+  // WritableStreamStartErroring(stream, reason).
+  if (!wasAlreadyErroring) {
+    aStream->StartErroring(aCx, reason, aRv);
+    if (aRv.Failed()) {
+      return nullptr;
+    }
+  }
+
+  // Step 12. Return promise.
+  return promise.forget();
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#ws-abort
+already_AddRefed<Promise> WritableStream::Abort(JSContext* aCx,
+                                                JS::Handle<JS::Value> aReason,
+                                                ErrorResult& aRv) {
+  // Step 1. If ! IsWritableStreamLocked(this) is true, return a promise
+  // rejected with a TypeError exception.
+  if (Locked()) {
+    return Promise::CreateRejectedWithTypeError(
+        GetParentObject(), "Canceled Locked Stream"_ns, aRv);
+  }
+
+  // Step 2. Return ! WritableStreamAbort(this, reason).
+  RefPtr<WritableStream> thisRefPtr = this;
+  return WritableStreamAbort(aCx, thisRefPtr, aReason, aRv);
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#writable-stream-close
+already_AddRefed<Promise> WritableStreamClose(JSContext* aCx,
+                                              WritableStream* aStream,
+                                              ErrorResult& aRv) {
+  // Step 1. Let state be stream.[[state]].
+  WritableStream::WriterState state = aStream->State();
+
+  // Step 2. If state is "closed" or "errored", return a promise rejected with a
+  // TypeError exception.
+  if (state == WritableStream::WriterState::Closed ||
+      state == WritableStream::WriterState::Errored) {
+    return Promise::CreateRejectedWithTypeError(
+        aStream->GetParentObject(),
+        "Can not close stream after closing or error"_ns, aRv);
+  }
+
+  // Step 3. Assert: state is "writable" or "erroring".
+  MOZ_ASSERT(state == WritableStream::WriterState::Writable ||
+             state == WritableStream::WriterState::Erroring);
+
+  // Step 4. Assert: ! WritableStreamCloseQueuedOrInFlight(stream) is false.
+  MOZ_ASSERT(!aStream->CloseQueuedOrInFlight());
+
+  // Step 5. Let promise be a new promise.
+  RefPtr<Promise> promise =
+      Promise::CreateInfallible(aStream->GetParentObject());
+
+  // Step 6. Set stream.[[closeRequest]] to promise.
+  aStream->SetCloseRequest(promise);
+
+  // Step 7. Let writer be stream.[[writer]].
+  RefPtr<WritableStreamDefaultWriter> writer = aStream->GetWriter();
+
+  // Step 8. If writer is not undefined, and stream.[[backpressure]] is true,
+  // and state is "writable", resolve writer.[[readyPromise]] with undefined.
+  if (writer && aStream->Backpressure() &&
+      state == WritableStream::WriterState::Writable) {
+    writer->ReadyPromise()->MaybeResolveWithUndefined();
+  }
+
+  // Step 9.
+  // Perform ! WritableStreamDefaultControllerClose(stream.[[controller]]).
+  RefPtr<WritableStreamDefaultController> controller = aStream->Controller();
+  WritableStreamDefaultControllerClose(aCx, controller, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 10. Return promise.
+  return promise.forget();
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#ws-close
+already_AddRefed<Promise> WritableStream::Close(JSContext* aCx,
+                                                ErrorResult& aRv) {
+  // Step 1. If ! IsWritableStreamLocked(this) is true, return a promise
+  // rejected with a TypeError exception.
+  if (Locked()) {
+    return Promise::CreateRejectedWithTypeError(
+        GetParentObject(), "Can not close locked stream"_ns, aRv);
+  }
+
+  // Step 2. If ! WritableStreamCloseQueuedOrInFlight(this) is true, return a
+  // promise rejected with a TypeError exception.
+  if (CloseQueuedOrInFlight()) {
+    return Promise::CreateRejectedWithTypeError(
+        GetParentObject(), "Stream is already closing"_ns, aRv);
+  }
+
+  // Step 3. Return ! WritableStreamClose(this).
+  RefPtr<WritableStream> thisRefPtr = this;
+  return WritableStreamClose(aCx, thisRefPtr, aRv);
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#acquire-writable-stream-default-writer
+already_AddRefed<WritableStreamDefaultWriter>
+AcquireWritableStreamDefaultWriter(WritableStream* aStream, ErrorResult& aRv) {
+  // Step 1. Let writer be a new WritableStreamDefaultWriter.
+  RefPtr<WritableStreamDefaultWriter> writer =
+      new WritableStreamDefaultWriter(aStream->GetParentObject());
+
+  // Step 2. Perform ? SetUpWritableStreamDefaultWriter(writer, stream).
+  SetUpWritableStreamDefaultWriter(writer, aStream, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 3. Return writer.
+  return writer.forget();
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#create-writable-stream
+already_AddRefed<WritableStream> WritableStream::CreateAbstract(
+    JSContext* aCx, nsIGlobalObject* aGlobal,
+    UnderlyingSinkAlgorithmsBase* aAlgorithms, double aHighWaterMark,
+    QueuingStrategySize* aSizeAlgorithm, ErrorResult& aRv) {
+  // Step 1: Assert: ! IsNonNegativeNumber(highWaterMark) is true.
+  MOZ_ASSERT(IsNonNegativeNumber(aHighWaterMark));
+
+  // Step 2: Let stream be a new WritableStream.
+  // Step 3: Perform ! InitializeWritableStream(stream).
+  RefPtr<WritableStream> stream = new WritableStream(
+      aGlobal, WritableStream::HoldDropJSObjectsCaller::Implicit);
+
+  // Step 4: Let controller be a new WritableStreamDefaultController.
+  auto controller =
+      MakeRefPtr<WritableStreamDefaultController>(aGlobal, *stream);
+
+  // Step 5: Perform ? SetUpWritableStreamDefaultController(stream, controller,
+  // startAlgorithm, writeAlgorithm, closeAlgorithm, abortAlgorithm,
+  // highWaterMark, sizeAlgorithm).
+  SetUpWritableStreamDefaultController(aCx, stream, controller, aAlgorithms,
+                                       aHighWaterMark, aSizeAlgorithm, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 6: Return stream.
+  return stream.forget();
+}
+
+already_AddRefed<WritableStreamDefaultWriter> WritableStream::GetWriter(
+    ErrorResult& aRv) {
+  return AcquireWritableStreamDefaultWriter(this, aRv);
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#writable-stream-add-write-request
+already_AddRefed<Promise> WritableStreamAddWriteRequest(
+    WritableStream* aStream) {
+  // Step 1. Assert: ! IsWritableStreamLocked(stream) is true.
+  MOZ_ASSERT(IsWritableStreamLocked(aStream));
+
+  // Step 2. Assert: stream.[[state]] is "writable".
+  MOZ_ASSERT(aStream->State() == WritableStream::WriterState::Writable);
+
+  // Step 3. Let promise be a new promise.
+  RefPtr<Promise> promise =
+      Promise::CreateInfallible(aStream->GetParentObject());
+
+  // Step 4. Append promise to stream.[[writeRequests]].
+  aStream->AppendWriteRequest(promise);
+
+  // Step 5. Return promise.
+  return promise.forget();
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#writablestream-set-up
+// _BOUNDARY because `aAlgorithms->StartCallback` (called by
+// SetUpWritableStreamDefaultController below) should not be able to run script
+// in this case.
+MOZ_CAN_RUN_SCRIPT_BOUNDARY void WritableStream::SetUpNative(
+    JSContext* aCx, UnderlyingSinkAlgorithmsWrapper& aAlgorithms,
+    Maybe<double> aHighWaterMark, QueuingStrategySize* aSizeAlgorithm,
+    ErrorResult& aRv) {
+  // an optional number highWaterMark (default 1)
+  double highWaterMark = aHighWaterMark.valueOr(1);
+  // and if given, highWaterMark must be a non-negative, non-NaN number.
+  MOZ_ASSERT(IsNonNegativeNumber(highWaterMark));
+
+  // Step 1: Let startAlgorithm be an algorithm that returns undefined.
+  // Step 2: Let closeAlgorithmWrapper be an algorithm that runs these steps:
+  // Step 3: Let abortAlgorithmWrapper be an algorithm that runs these steps:
+  // (Covered by UnderlyingSinkAlgorithmsWrapper)
+
+  // Step 4: If sizeAlgorithm was not given, then set it to an algorithm that
+  // returns 1. (Callers will treat nullptr as such, see
+  // WritableStream::Constructor for details)
+
+  // Step 5: Perform ! InitializeWritableStream(stream).
+  // (Covered by the constructor)
+
+  // Step 6: Let controller be a new WritableStreamDefaultController.
+  auto controller =
+      MakeRefPtr<WritableStreamDefaultController>(GetParentObject(), *this);
+
+  // Step 7: Perform ! SetUpWritableStreamDefaultController(stream, controller,
+  // startAlgorithm, writeAlgorithm, closeAlgorithmWrapper,
+  // abortAlgorithmWrapper, highWaterMark, sizeAlgorithm).
+  SetUpWritableStreamDefaultController(aCx, this, controller, &aAlgorithms,
+                                       highWaterMark, aSizeAlgorithm, aRv);
+}
+
+already_AddRefed<WritableStream> WritableStream::CreateNative(
+    JSContext* aCx, nsIGlobalObject& aGlobal,
+    UnderlyingSinkAlgorithmsWrapper& aAlgorithms, Maybe<double> aHighWaterMark,
+    QueuingStrategySize* aSizeAlgorithm, ErrorResult& aRv) {
+  RefPtr<WritableStream> stream = new WritableStream(
+      &aGlobal, WritableStream::HoldDropJSObjectsCaller::Implicit);
+  stream->SetUpNative(aCx, aAlgorithms, aHighWaterMark, aSizeAlgorithm, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+  return stream.forget();
+}
+
+// https://streams.spec.whatwg.org/#writablestream-error
+// To error a WritableStream stream given a JavaScript value e, perform !
+// WritableStreamDefaultControllerErrorIfNeeded(stream.[[controller]], e).
+void WritableStream::ErrorNative(JSContext* aCx, JS::Handle<JS::Value> aError,
+                                 ErrorResult& aRv) {
+  // MOZ_KnownLive here instead of MOZ_KNOWN_LIVE at the field, because
+  // mController is set outside of the constructor
+  WritableStreamDefaultControllerErrorIfNeeded(aCx, MOZ_KnownLive(mController),
+                                               aError, aRv);
+}
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/WritableStream.h b/dom/streams/WritableStream.h
new file mode 100644
index 0000000000..8b380020fe
--- /dev/null
+++ b/dom/streams/WritableStream.h
@@ -0,0 +1,269 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_WritableStream_h
+#define mozilla_dom_WritableStream_h
+
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+#include "mozilla/dom/WritableStreamDefaultController.h"
+#include "mozilla/dom/WritableStreamDefaultWriter.h"
+
+#include "nsCycleCollectionParticipant.h"
+#include "nsWrapperCache.h"
+
+namespace mozilla::dom {
+
+class Promise;
+class WritableStreamDefaultController;
+class WritableStreamDefaultWriter;
+class UnderlyingSinkAlgorithmsBase;
+class UniqueMessagePortId;
+class MessagePort;
+
+class WritableStream : public nsISupports, public nsWrapperCache {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(WritableStream)
+
+  friend class ReadableStream;
+
+ protected:
+  virtual ~WritableStream();
+
+  virtual void LastRelease() {}
+
+  // If one extends WritableStream with another cycle collectable class,
+  // calling HoldJSObjects and DropJSObjects should happen using 'this' of
+  // that extending class. And in that case Explicit should be passed to the
+  // constructor of WriteableStream so that it doesn't make those calls.
+  // See also https://bugzilla.mozilla.org/show_bug.cgi?id=1801214.
+  enum class HoldDropJSObjectsCaller { Implicit, Explicit };
+
+  explicit WritableStream(const GlobalObject& aGlobal,
+                          HoldDropJSObjectsCaller aHoldDropCaller);
+  explicit WritableStream(nsIGlobalObject* aGlobal,
+                          HoldDropJSObjectsCaller aHoldDropCaller);
+
+ public:
+  // Slot Getter/Setters:
+  bool Backpressure() const { return mBackpressure; }
+  void SetBackpressure(bool aBackpressure) { mBackpressure = aBackpressure; }
+
+  Promise* GetCloseRequest() { return mCloseRequest; }
+  void SetCloseRequest(Promise* aRequest) { mCloseRequest = aRequest; }
+
+  MOZ_KNOWN_LIVE WritableStreamDefaultController* Controller() {
+    return mController;
+  }
+  void SetController(WritableStreamDefaultController& aController) {
+    MOZ_ASSERT(!mController);
+    mController = &aController;
+  }
+
+  Promise* GetInFlightWriteRequest() const { return mInFlightWriteRequest; }
+
+  Promise* GetPendingAbortRequestPromise() const {
+    return mPendingAbortRequestPromise;
+  }
+
+  void SetPendingAbortRequest(Promise* aPromise, JS::Handle<JS::Value> aReason,
+                              bool aWasAlreadyErroring) {
+    mPendingAbortRequestPromise = aPromise;
+    mPendingAbortRequestReason = aReason;
+    mPendingAbortRequestWasAlreadyErroring = aWasAlreadyErroring;
+  }
+
+  WritableStreamDefaultWriter* GetWriter() const { return mWriter; }
+  void SetWriter(WritableStreamDefaultWriter* aWriter) { mWriter = aWriter; }
+
+  enum class WriterState { Writable, Closed, Erroring, Errored };
+
+  WriterState State() const { return mState; }
+  void SetState(const WriterState& aState) { mState = aState; }
+
+  JS::Value StoredError() const { return mStoredError; }
+  void SetStoredError(JS::Handle<JS::Value> aStoredError) {
+    mStoredError = aStoredError;
+  }
+
+  void AppendWriteRequest(RefPtr<Promise>& aRequest) {
+    mWriteRequests.AppendElement(aRequest);
+  }
+
+  // CreateWritableStream
+  MOZ_CAN_RUN_SCRIPT static already_AddRefed<WritableStream> CreateAbstract(
+      JSContext* aCx, nsIGlobalObject* aGlobal,
+      UnderlyingSinkAlgorithmsBase* aAlgorithms, double aHighWaterMark,
+      QueuingStrategySize* aSizeAlgorithm, ErrorResult& aRv);
+
+  // WritableStreamCloseQueuedOrInFlight
+  bool CloseQueuedOrInFlight() const {
+    return mCloseRequest || mInFlightCloseRequest;
+  }
+
+  // WritableStreamDealWithRejection
+  MOZ_CAN_RUN_SCRIPT void DealWithRejection(JSContext* aCx,
+                                            JS::Handle<JS::Value> aError,
+                                            ErrorResult& aRv);
+
+  // WritableStreamFinishErroring
+  MOZ_CAN_RUN_SCRIPT void FinishErroring(JSContext* aCx, ErrorResult& aRv);
+
+  // WritableStreamFinishInFlightClose
+  void FinishInFlightClose();
+
+  // WritableStreamFinishInFlightCloseWithError
+  MOZ_CAN_RUN_SCRIPT void FinishInFlightCloseWithError(
+      JSContext* aCx, JS::Handle<JS::Value> aError, ErrorResult& aRv);
+
+  // WritableStreamFinishInFlightWrite
+  void FinishInFlightWrite();
+
+  // WritableStreamFinishInFlightWriteWithError
+  MOZ_CAN_RUN_SCRIPT void FinishInFlightWriteWithError(
+      JSContext* aCX, JS::Handle<JS::Value> aError, ErrorResult& aR);
+
+  // WritableStreamHasOperationMarkedInFlight
+  bool HasOperationMarkedInFlight() const {
+    return mInFlightWriteRequest || mInFlightCloseRequest;
+  }
+
+  // WritableStreamMarkCloseRequestInFlight
+  void MarkCloseRequestInFlight();
+
+  // WritableStreamMarkFirstWriteRequestInFlight
+  void MarkFirstWriteRequestInFlight();
+
+  // WritableStreamRejectCloseAndClosedPromiseIfNeeded
+  void RejectCloseAndClosedPromiseIfNeeded();
+
+  // WritableStreamStartErroring
+  MOZ_CAN_RUN_SCRIPT void StartErroring(JSContext* aCx,
+                                        JS::Handle<JS::Value> aReason,
+                                        ErrorResult& aRv);
+
+  // WritableStreamUpdateBackpressure
+  void UpdateBackpressure(bool aBackpressure);
+
+  // [Transferable]
+  // https://html.spec.whatwg.org/multipage/structured-data.html#transfer-steps
+  MOZ_CAN_RUN_SCRIPT bool Transfer(JSContext* aCx,
+                                   UniqueMessagePortId& aPortId);
+  // https://html.spec.whatwg.org/multipage/structured-data.html#transfer-receiving-steps
+  MOZ_CAN_RUN_SCRIPT static already_AddRefed<WritableStream>
+  ReceiveTransferImpl(JSContext* aCx, nsIGlobalObject* aGlobal,
+                      MessagePort& aPort);
+  MOZ_CAN_RUN_SCRIPT static bool ReceiveTransfer(
+      JSContext* aCx, nsIGlobalObject* aGlobal, MessagePort& aPort,
+      JS::MutableHandle<JSObject*> aReturnObject);
+
+  // Public functions to implement other specs
+  // https://streams.spec.whatwg.org/#other-specs-ws
+
+  // https://streams.spec.whatwg.org/#writablestream-set-up
+ protected:
+  // Sets up the WritableStream. Intended for subclasses.
+  void SetUpNative(JSContext* aCx, UnderlyingSinkAlgorithmsWrapper& aAlgorithms,
+                   Maybe<double> aHighWaterMark,
+                   QueuingStrategySize* aSizeAlgorithm, ErrorResult& aRv);
+
+ public:
+  // Creates and sets up a WritableStream. Use SetUpNative for this purpose in
+  // subclasses.
+  static already_AddRefed<WritableStream> CreateNative(
+      JSContext* aCx, nsIGlobalObject& aGlobal,
+      UnderlyingSinkAlgorithmsWrapper& aAlgorithms,
+      Maybe<double> aHighWaterMark, QueuingStrategySize* aSizeAlgorithm,
+      ErrorResult& aRv);
+
+  // The following definitions must only be used on WritableStream instances
+  // initialized via the above set up algorithm:
+
+  // https://streams.spec.whatwg.org/#writablestream-error
+  MOZ_CAN_RUN_SCRIPT void ErrorNative(JSContext* aCx,
+                                      JS::Handle<JS::Value> aError,
+                                      ErrorResult& aRv);
+
+  // IDL layer functions
+
+  nsIGlobalObject* GetParentObject() const { return mGlobal; }
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  // IDL methods
+
+  // TODO: Use MOZ_CAN_RUN_SCRIPT when IDL constructors can use it (bug 1749042)
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY static already_AddRefed<WritableStream>
+  Constructor(const GlobalObject& aGlobal,
+              const Optional<JS::Handle<JSObject*>>& aUnderlyingSink,
+              const QueuingStrategy& aStrategy, ErrorResult& aRv);
+
+  bool Locked() const { return !!mWriter; }
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> Abort(
+      JSContext* cx, JS::Handle<JS::Value> aReason, ErrorResult& aRv);
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> Close(JSContext* aCx,
+                                                     ErrorResult& aRv);
+
+  already_AddRefed<WritableStreamDefaultWriter> GetWriter(ErrorResult& aRv);
+
+ protected:
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+
+  // Internal Slots:
+ private:
+  bool mBackpressure = false;
+  RefPtr<Promise> mCloseRequest;
+  RefPtr<WritableStreamDefaultController> mController;
+  RefPtr<Promise> mInFlightWriteRequest;
+  RefPtr<Promise> mInFlightCloseRequest;
+
+  // We inline all members of [[pendingAbortRequest]] in this class.
+  // The absence (i.e. undefined) of the [[pendingAbortRequest]]
+  // is indicated by mPendingAbortRequestPromise = nullptr.
+  RefPtr<Promise> mPendingAbortRequestPromise;
+  JS::Heap<JS::Value> mPendingAbortRequestReason;
+  bool mPendingAbortRequestWasAlreadyErroring = false;
+
+  WriterState mState = WriterState::Writable;
+  JS::Heap<JS::Value> mStoredError;
+  RefPtr<WritableStreamDefaultWriter> mWriter;
+  nsTArray<RefPtr<Promise>> mWriteRequests;
+
+  HoldDropJSObjectsCaller mHoldDropCaller;
+};
+
+namespace streams_abstract {
+
+inline bool IsWritableStreamLocked(WritableStream* aStream) {
+  return aStream->Locked();
+}
+
+MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> WritableStreamAbort(
+    JSContext* aCx, WritableStream* aStream, JS::Handle<JS::Value> aReason,
+    ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> WritableStreamClose(
+    JSContext* aCx, WritableStream* aStream, ErrorResult& aRv);
+
+already_AddRefed<Promise> WritableStreamAddWriteRequest(
+    WritableStream* aStream);
+
+already_AddRefed<WritableStreamDefaultWriter>
+AcquireWritableStreamDefaultWriter(WritableStream* aStream, ErrorResult& aRv);
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_WritableStream_h
diff --git a/dom/streams/WritableStreamDefaultController.cpp b/dom/streams/WritableStreamDefaultController.cpp
new file mode 100644
index 0000000000..c1ee4bd8ea
--- /dev/null
+++ b/dom/streams/WritableStreamDefaultController.cpp
@@ -0,0 +1,564 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "js/Exception.h"
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/dom/AbortSignal.h"
+#include "mozilla/dom/Promise.h"
+#include "mozilla/dom/Promise-inl.h"
+#include "mozilla/dom/WritableStream.h"
+#include "mozilla/dom/WritableStreamDefaultController.h"
+#include "mozilla/dom/WritableStreamDefaultControllerBinding.h"
+#include "mozilla/dom/UnderlyingSinkBinding.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsDebug.h"
+#include "nsISupports.h"
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+// Note: Using the individual macros vs NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE
+// because I need to specificy a manual implementation of
+// NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN.
+NS_IMPL_CYCLE_COLLECTION_CLASS(WritableStreamDefaultController)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(WritableStreamDefaultController)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mGlobal, mSignal, mStrategySizeAlgorithm,
+                                  mAlgorithms, mStream)
+  tmp->mQueue.clear();
+  NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(WritableStreamDefaultController)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mGlobal, mSignal, mStrategySizeAlgorithm,
+                                    mAlgorithms, mStream)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(WritableStreamDefaultController)
+  NS_IMPL_CYCLE_COLLECTION_TRACE_PRESERVED_WRAPPER
+  // Trace the associated queue.
+  for (const auto& queueEntry : tmp->mQueue) {
+    aCallbacks.Trace(&queueEntry->mValue, "mQueue.mValue", aClosure);
+  }
+NS_IMPL_CYCLE_COLLECTION_TRACE_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(WritableStreamDefaultController)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(WritableStreamDefaultController)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(WritableStreamDefaultController)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+WritableStreamDefaultController::WritableStreamDefaultController(
+    nsISupports* aGlobal, WritableStream& aStream)
+    : mGlobal(do_QueryInterface(aGlobal)), mStream(&aStream) {
+  mozilla::HoldJSObjects(this);
+}
+
+WritableStreamDefaultController::~WritableStreamDefaultController() {
+  // MG:XXX: LinkedLists are required to be empty at destruction, but it seems
+  //         it is possible to have a controller be destructed while still
+  //         having entries in its queue.
+  //
+  //         This needs to be verified as not indicating some other issue.
+  mQueue.clear();
+  mozilla::DropJSObjects(this);
+}
+
+JSObject* WritableStreamDefaultController::WrapObject(
+    JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
+  return WritableStreamDefaultController_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+// https://streams.spec.whatwg.org/#ws-default-controller-error
+void WritableStreamDefaultController::Error(JSContext* aCx,
+                                            JS::Handle<JS::Value> aError,
+                                            ErrorResult& aRv) {
+  // Step 1. Let state be this.[[stream]].[[state]].
+  // Step 2. If state is not "writable", return.
+  if (mStream->State() != WritableStream::WriterState::Writable) {
+    return;
+  }
+  // Step 3. Perform ! WritableStreamDefaultControllerError(this, e).
+  RefPtr<WritableStreamDefaultController> thisRefPtr = this;
+  WritableStreamDefaultControllerError(aCx, thisRefPtr, aError, aRv);
+}
+
+// https://streams.spec.whatwg.org/#ws-default-controller-private-abort
+already_AddRefed<Promise> WritableStreamDefaultController::AbortSteps(
+    JSContext* aCx, JS::Handle<JS::Value> aReason, ErrorResult& aRv) {
+  // Step 1. Let result be the result of performing this.[[abortAlgorithm]],
+  // passing reason.
+  RefPtr<UnderlyingSinkAlgorithmsBase> algorithms = mAlgorithms;
+  Optional<JS::Handle<JS::Value>> optionalReason(aCx, aReason);
+  RefPtr<Promise> abortPromise =
+      algorithms->AbortCallback(aCx, optionalReason, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 2. Perform ! WritableStreamDefaultControllerClearAlgorithms(this).
+  ClearAlgorithms();
+
+  // Step 3. Return result.
+  return abortPromise.forget();
+}
+
+// https://streams.spec.whatwg.org/#ws-default-controller-private-error
+void WritableStreamDefaultController::ErrorSteps() {
+  // Step 1. Perform ! ResetQueue(this).
+  ResetQueue(this);
+}
+
+void WritableStreamDefaultController::SetSignal(AbortSignal* aSignal) {
+  MOZ_ASSERT(aSignal);
+  mSignal = aSignal;
+}
+
+namespace streams_abstract {
+
+MOZ_CAN_RUN_SCRIPT static void
+WritableStreamDefaultControllerAdvanceQueueIfNeeded(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    ErrorResult& aRv);
+
+// https://streams.spec.whatwg.org/#set-up-writable-stream-default-controller
+void SetUpWritableStreamDefaultController(
+    JSContext* aCx, WritableStream* aStream,
+    WritableStreamDefaultController* aController,
+    UnderlyingSinkAlgorithmsBase* aAlgorithms, double aHighWaterMark,
+    QueuingStrategySize* aSizeAlgorithm, ErrorResult& aRv) {
+  // Step 1. Assert: stream implements WritableStream.
+  // Step 2. Assert: stream.[[controller]] is undefined.
+  MOZ_ASSERT(!aStream->Controller());
+
+  // Step 3. Set controller.[[stream]] to stream.
+  // Note: Already set in
+  // SetUpWritableStreamDefaultControllerFromUnderlyingSink.
+  MOZ_ASSERT(aController->Stream() == aStream);
+
+  // Step 4. Set stream.[[controller]] to controller.
+  aStream->SetController(*aController);
+
+  // Step 5. Perform ! ResetQueue(controller).
+  ResetQueue(aController);
+
+  // Step 6. Set controller.[[signal]] to a new AbortSignal.
+  RefPtr<AbortSignal> signal = new AbortSignal(aController->GetParentObject(),
+                                               false, JS::UndefinedHandleValue);
+  aController->SetSignal(signal);
+
+  // Step 7. Set controller.[[started]] to false.
+  aController->SetStarted(false);
+
+  // Step 8. Set controller.[[strategySizeAlgorithm]] to sizeAlgorithm.
+  aController->SetStrategySizeAlgorithm(aSizeAlgorithm);
+
+  // Step 9. Set controller.[[strategyHWM]] to highWaterMark.
+  aController->SetStrategyHWM(aHighWaterMark);
+
+  // Step 10. Set controller.[[writeAlgorithm]] to writeAlgorithm.
+  // Step 11. Set controller.[[closeAlgorithm]] to closeAlgorithm.
+  // Step 12. Set controller.[[abortAlgorithm]] to abortAlgorithm.
+  aController->SetAlgorithms(*aAlgorithms);
+
+  // Step 13. Let backpressure be !
+  // WritableStreamDefaultControllerGetBackpressure(controller).
+  bool backpressure = aController->GetBackpressure();
+
+  // Step 14. Perform ! WritableStreamUpdateBackpressure(stream, backpressure).
+  aStream->UpdateBackpressure(backpressure);
+
+  // Step 15. Let startResult be the result of performing startAlgorithm. (This
+  // may throw an exception.)
+  JS::Rooted<JS::Value> startResult(aCx, JS::UndefinedValue());
+  RefPtr<WritableStreamDefaultController> controller(aController);
+  aAlgorithms->StartCallback(aCx, *controller, &startResult, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 16. Let startPromise be a promise resolved with startResult.
+  RefPtr<Promise> startPromise =
+      Promise::CreateInfallible(aStream->GetParentObject());
+  startPromise->MaybeResolve(startResult);
+
+  // Step 17/18.
+  startPromise->AddCallbacksWithCycleCollectedArgs(
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         WritableStreamDefaultController* aController)
+          MOZ_CAN_RUN_SCRIPT_BOUNDARY {
+            // Step 17. Upon fulfillment of startPromise,
+            // Step 17.1. Assert: stream.[[state]] is "writable" or "erroring".
+            MOZ_ASSERT(aController->Stream()->State() ==
+                           WritableStream::WriterState::Writable ||
+                       aController->Stream()->State() ==
+                           WritableStream::WriterState::Erroring);
+            // Step 17.2. Set controller.[[started]] to true.
+            aController->SetStarted(true);
+            // Step 17.3 Perform
+            // !WritableStreamDefaultControllerAdvanceQueueIfNeeded(controller).
+            WritableStreamDefaultControllerAdvanceQueueIfNeeded(
+                aCx, MOZ_KnownLive(aController), aRv);
+          },
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         WritableStreamDefaultController* aController)
+          MOZ_CAN_RUN_SCRIPT_BOUNDARY {
+            RefPtr<WritableStream> stream = aController->Stream();
+            // Step 18. Upon rejection of startPromise with reason r,
+            // Step 18.1. Assert: stream.[[state]] is "writable" or "erroring".
+            MOZ_ASSERT(
+                stream->State() == WritableStream::WriterState::Writable ||
+                stream->State() == WritableStream::WriterState::Erroring);
+            // Step 18.2. Set controller.[[started]] to true.
+            aController->SetStarted(true);
+            // Step 18.3. Perform ! WritableStreamDealWithRejection(stream, r).
+            stream->DealWithRejection(aCx, aValue, aRv);
+          },
+      RefPtr(aController));
+}
+
+// https://streams.spec.whatwg.org/#set-up-writable-stream-default-controller-from-underlying-sink
+void SetUpWritableStreamDefaultControllerFromUnderlyingSink(
+    JSContext* aCx, WritableStream* aStream,
+    JS::Handle<JSObject*> aUnderlyingSink, UnderlyingSink& aUnderlyingSinkDict,
+    double aHighWaterMark, QueuingStrategySize* aSizeAlgorithm,
+    ErrorResult& aRv) {
+  // Step 1.
+  RefPtr<WritableStreamDefaultController> controller =
+      new WritableStreamDefaultController(aStream->GetParentObject(), *aStream);
+
+  // Step 2 - 9.
+  auto algorithms = MakeRefPtr<UnderlyingSinkAlgorithms>(
+      aStream->GetParentObject(), aUnderlyingSink, aUnderlyingSinkDict);
+
+  // Step 10.
+  SetUpWritableStreamDefaultController(aCx, aStream, controller, algorithms,
+                                       aHighWaterMark, aSizeAlgorithm, aRv);
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-default-controller-process-close
+MOZ_CAN_RUN_SCRIPT static void WritableStreamDefaultControllerProcessClose(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    ErrorResult& aRv) {
+  // Step 1. Let stream be controller.[[stream]].
+  RefPtr<WritableStream> stream = aController->Stream();
+
+  // Step 2. Perform ! WritableStreamMarkCloseRequestInFlight(stream).
+  stream->MarkCloseRequestInFlight();
+
+  // Step 3. Perform ! DequeueValue(controller).
+  JS::Rooted<JS::Value> value(aCx);
+  DequeueValue(aController, &value);
+
+  // Step 4. Assert: controller.[[queue]] is empty.
+  MOZ_ASSERT(aController->Queue().isEmpty());
+
+  // Step 5. Let sinkClosePromise be the result of performing
+  // controller.[[closeAlgorithm]].
+  RefPtr<UnderlyingSinkAlgorithmsBase> algorithms =
+      aController->GetAlgorithms();
+  RefPtr<Promise> sinkClosePromise = algorithms->CloseCallback(aCx, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 6. Perform !
+  // WritableStreamDefaultControllerClearAlgorithms(controller).
+  aController->ClearAlgorithms();
+
+  // Step 7 + 8.
+  sinkClosePromise->AddCallbacksWithCycleCollectedArgs(
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         WritableStreamDefaultController* aController) {
+        RefPtr<WritableStream> stream = aController->Stream();
+        // Step 7. Upon fulfillment of sinkClosePromise,
+        // Step 7.1. Perform ! WritableStreamFinishInFlightClose(stream).
+        stream->FinishInFlightClose();
+      },
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         WritableStreamDefaultController* aController)
+          MOZ_CAN_RUN_SCRIPT_BOUNDARY {
+            RefPtr<WritableStream> stream = aController->Stream();
+            // Step 8. Upon rejection of sinkClosePromise with reason reason,
+            // Step 8.1. Perform
+            // ! WritableStreamFinishInFlightCloseWithError(stream, reason).
+            stream->FinishInFlightCloseWithError(aCx, aValue, aRv);
+          },
+      RefPtr(aController));
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-default-controller-process-write
+MOZ_CAN_RUN_SCRIPT static void WritableStreamDefaultControllerProcessWrite(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    JS::Handle<JS::Value> aChunk, ErrorResult& aRv) {
+  // Step 1. Let stream be controller.[[stream]].
+  RefPtr<WritableStream> stream = aController->Stream();
+
+  // Step 2. Perform ! WritableStreamMarkFirstWriteRequestInFlight(stream).
+  stream->MarkFirstWriteRequestInFlight();
+
+  // Step 3. Let sinkWritePromise be the result of performing
+  // controller.[[writeAlgorithm]], passing in chunk.
+  RefPtr<UnderlyingSinkAlgorithmsBase> algorithms =
+      aController->GetAlgorithms();
+  RefPtr<Promise> sinkWritePromise =
+      algorithms->WriteCallback(aCx, aChunk, *aController, aRv);
+  if (aRv.Failed()) {
+    return;
+  }
+
+  // Step 4 + 5:
+  sinkWritePromise->AddCallbacksWithCycleCollectedArgs(
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         WritableStreamDefaultController* aController)
+          MOZ_CAN_RUN_SCRIPT_BOUNDARY {
+            RefPtr<WritableStream> stream = aController->Stream();
+
+            // Step 4.1. Perform ! WritableStreamFinishInFlightWrite(stream).
+            stream->FinishInFlightWrite();
+
+            // Step 4.2. Let state be stream.[[state]].
+            WritableStream::WriterState state = stream->State();
+
+            // Step 4.3. Assert: state is "writable" or "erroring".
+            MOZ_ASSERT(state == WritableStream::WriterState::Writable ||
+                       state == WritableStream::WriterState::Erroring);
+
+            // Step 4.4. Perform ! DequeueValue(controller).
+            JS::Rooted<JS::Value> value(aCx);
+            DequeueValue(aController, &value);
+
+            // Step 4.5. If ! WritableStreamCloseQueuedOrInFlight(stream) is
+            // false and state is "writable",
+            if (!stream->CloseQueuedOrInFlight() &&
+                state == WritableStream::WriterState::Writable) {
+              // Step 4.5.1. Let backpressure be !
+              // WritableStreamDefaultControllerGetBackpressure(controller).
+              bool backpressure = aController->GetBackpressure();
+              // Step 4.5.2. Perform ! WritableStreamUpdateBackpressure(stream,
+              // backpressure).
+              stream->UpdateBackpressure(backpressure);
+            }
+
+            // Step 4.6. Perform !
+            // WritableStreamDefaultControllerAdvanceQueueIfNeeded(controller).
+            WritableStreamDefaultControllerAdvanceQueueIfNeeded(
+                aCx, MOZ_KnownLive(aController), aRv);
+          },
+      [](JSContext* aCx, JS::Handle<JS::Value> aValue, ErrorResult& aRv,
+         WritableStreamDefaultController* aController)
+          MOZ_CAN_RUN_SCRIPT_BOUNDARY {
+            RefPtr<WritableStream> stream = aController->Stream();
+
+            // Step 5.1. If stream.[[state]] is "writable", perform !
+            // WritableStreamDefaultControllerClearAlgorithms(controller).
+            if (stream->State() == WritableStream::WriterState::Writable) {
+              aController->ClearAlgorithms();
+            }
+
+            // Step 5.2. Perform !
+            // WritableStreamFinishInFlightWriteWithError(stream, reason)
+            stream->FinishInFlightWriteWithError(aCx, aValue, aRv);
+          },
+      RefPtr(aController));
+}
+
+// We use a JS::MagicValue to represent the close sentinel required by the spec.
+// Normal JavaScript code can not generate magic values, so we can use this
+// as a special value. However care has to be taken to not leak the magic value
+// to other code.
+constexpr JSWhyMagic CLOSE_SENTINEL = JS_GENERIC_MAGIC;
+
+// https://streams.spec.whatwg.org/#writable-stream-default-controller-advance-queue-if-needed
+static void WritableStreamDefaultControllerAdvanceQueueIfNeeded(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    ErrorResult& aRv) {
+  // Step 1. Let stream be controller.[[stream]].
+  RefPtr<WritableStream> stream = aController->Stream();
+
+  // Step 2. If controller.[[started]] is false, return.
+  if (!aController->Started()) {
+    return;
+  }
+
+  // Step 3. If stream.[[inFlightWriteRequest]] is not undefined, return.
+  if (stream->GetInFlightWriteRequest()) {
+    return;
+  }
+
+  // Step 4. Let state be stream.[[state]].
+  WritableStream::WriterState state = stream->State();
+
+  // Step 5. Assert: state is not "closed" or "errored".
+  MOZ_ASSERT(state != WritableStream::WriterState::Closed &&
+             state != WritableStream::WriterState::Errored);
+
+  // Step 6. If state is "erroring",
+  if (state == WritableStream::WriterState::Erroring) {
+    // Step 6.1. Perform ! WritableStreamFinishErroring(stream).
+    stream->FinishErroring(aCx, aRv);
+
+    // Step 6.2. Return.
+    return;
+  }
+
+  // Step 7. If controller.[[queue]] is empty, return.
+  if (aController->Queue().isEmpty()) {
+    return;
+  }
+
+  // Step 8. Let value be ! PeekQueueValue(controller).
+  JS::Rooted<JS::Value> value(aCx);
+  PeekQueueValue(aController, &value);
+
+  // Step 9. If value is the close sentinel, perform !
+  // WritableStreamDefaultControllerProcessClose(controller).
+  if (value.isMagic(CLOSE_SENTINEL)) {
+    WritableStreamDefaultControllerProcessClose(aCx, aController, aRv);
+    return;
+  }
+
+  // Step 10. Otherwise, perform !
+  // WritableStreamDefaultControllerProcessWrite(controller, value).
+  WritableStreamDefaultControllerProcessWrite(aCx, aController, value, aRv);
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-default-controller-close
+void WritableStreamDefaultControllerClose(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    ErrorResult& aRv) {
+  // Step 1. Perform ! EnqueueValueWithSize(controller, close sentinel, 0).
+  JS::Rooted<JS::Value> aCloseSentinel(aCx, JS::MagicValue(CLOSE_SENTINEL));
+  EnqueueValueWithSize(aController, aCloseSentinel, 0, aRv);
+  MOZ_ASSERT(!aRv.Failed());
+
+  // Step 2. Perform !
+  // WritableStreamDefaultControllerAdvanceQueueIfNeeded(controller).
+  WritableStreamDefaultControllerAdvanceQueueIfNeeded(aCx, aController, aRv);
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-default-controller-write
+void WritableStreamDefaultControllerWrite(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    JS::Handle<JS::Value> aChunk, double chunkSize, ErrorResult& aRv) {
+  // Step 1. Let enqueueResult be EnqueueValueWithSize(controller, chunk,
+  // chunkSize).
+  IgnoredErrorResult rv;
+  EnqueueValueWithSize(aController, aChunk, chunkSize, rv);
+
+  // Step 2. If enqueueResult is an abrupt completion,
+  if (rv.MaybeSetPendingException(aCx,
+                                  "WritableStreamDefaultController.write")) {
+    JS::Rooted<JS::Value> error(aCx);
+    JS_GetPendingException(aCx, &error);
+    JS_ClearPendingException(aCx);
+
+    // Step 2.1. Perform !
+    // WritableStreamDefaultControllerErrorIfNeeded(controller,
+    // enqueueResult.[[Value]]).
+    WritableStreamDefaultControllerErrorIfNeeded(aCx, aController, error, aRv);
+
+    // Step 2.2. Return.
+    return;
+  }
+
+  // Step 3. Let stream be controller.[[stream]].
+  RefPtr<WritableStream> stream = aController->Stream();
+
+  // Step 4. If ! WritableStreamCloseQueuedOrInFlight(stream) is false and
+  // stream.[[state]] is "writable",
+  if (!stream->CloseQueuedOrInFlight() &&
+      stream->State() == WritableStream::WriterState::Writable) {
+    // Step 4.1. Let backpressure be
+    // !WritableStreamDefaultControllerGetBackpressure(controller).
+    bool backpressure = aController->GetBackpressure();
+
+    // Step 4.2. Perform ! WritableStreamUpdateBackpressure(stream,
+    // backpressure).
+    stream->UpdateBackpressure(backpressure);
+  }
+
+  // Step 5. Perform
+  // ! WritableStreamDefaultControllerAdvanceQueueIfNeeded(controller).
+  WritableStreamDefaultControllerAdvanceQueueIfNeeded(aCx, aController, aRv);
+}
+
+void WritableStreamDefaultControllerError(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    JS::Handle<JS::Value> aError, ErrorResult& aRv) {
+  // Step 1. Let stream be controller.[[stream]].
+  RefPtr<WritableStream> stream = aController->Stream();
+
+  // Step 2. Assert: stream.[[state]] is "writable".
+  MOZ_ASSERT(stream->State() == WritableStream::WriterState::Writable);
+
+  // Step 3. Perform
+  // ! WritableStreamDefaultControllerClearAlgorithms(controller).
+  aController->ClearAlgorithms();
+
+  // Step 4.Perform ! WritableStreamStartErroring(stream, error).
+  stream->StartErroring(aCx, aError, aRv);
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-default-controller-error-if-needed
+void WritableStreamDefaultControllerErrorIfNeeded(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    JS::Handle<JS::Value> aError, ErrorResult& aRv) {
+  // Step 1. If controller.[[stream]].[[state]] is "writable", perform
+  // !WritableStreamDefaultControllerError(controller, error).
+  if (aController->Stream()->State() == WritableStream::WriterState::Writable) {
+    WritableStreamDefaultControllerError(aCx, aController, aError, aRv);
+  }
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-default-controller-get-chunk-size
+double WritableStreamDefaultControllerGetChunkSize(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    JS::Handle<JS::Value> aChunk, ErrorResult& aRv) {
+  // Step 1. Let returnValue be the result of performing
+  // controller.[[strategySizeAlgorithm]], passing in chunk, and interpreting
+  // the result as a completion record.
+  RefPtr<QueuingStrategySize> sizeAlgorithm(
+      aController->StrategySizeAlgorithm());
+
+  // If !sizeAlgorithm, we return 1, which is inlined from
+  // https://streams.spec.whatwg.org/#make-size-algorithm-from-size-function
+  Optional<JS::Handle<JS::Value>> optionalChunk(aCx, aChunk);
+
+  double chunkSize =
+      sizeAlgorithm
+          ? sizeAlgorithm->Call(
+                optionalChunk, aRv,
+                "WritableStreamDefaultController.[[strategySizeAlgorithm]]",
+                CallbackObject::eRethrowExceptions)
+          : 1.0;
+
+  // Step 2. If returnValue is an abrupt completion,
+  if (aRv.MaybeSetPendingException(
+          aCx, "WritableStreamDefaultController.[[strategySizeAlgorithm]]")) {
+    JS::Rooted<JS::Value> error(aCx);
+    JS_GetPendingException(aCx, &error);
+    JS_ClearPendingException(aCx);
+
+    // Step 2.1. Perform !
+    // WritableStreamDefaultControllerErrorIfNeeded(controller,
+    // returnValue.[[Value]]).
+    WritableStreamDefaultControllerErrorIfNeeded(aCx, aController, error, aRv);
+
+    // Step 2.2. Return 1.
+    return 1.0;
+  }
+
+  // Step 3. Return returnValue.[[Value]].
+  return chunkSize;
+}
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/WritableStreamDefaultController.h b/dom/streams/WritableStreamDefaultController.h
new file mode 100644
index 0000000000..d41014594c
--- /dev/null
+++ b/dom/streams/WritableStreamDefaultController.h
@@ -0,0 +1,178 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_WritableStreamDefaultController_h
+#define mozilla_dom_WritableStreamDefaultController_h
+
+#include "js/TypeDecls.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+#include "mozilla/dom/QueueWithSizes.h"
+#include "mozilla/dom/ReadRequest.h"
+#include "mozilla/dom/UnderlyingSinkCallbackHelpers.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsWrapperCache.h"
+#include "mozilla/dom/Nullable.h"
+#include "nsTArray.h"
+#include "nsISupports.h"
+
+namespace mozilla::dom {
+
+class AbortSignal;
+class WritableStream;
+struct UnderlyingSink;
+
+class WritableStreamDefaultController final : public nsISupports,
+                                              public nsWrapperCache {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(WritableStreamDefaultController)
+
+  explicit WritableStreamDefaultController(nsISupports* aGlobal,
+                                           WritableStream& aStream);
+
+ protected:
+  ~WritableStreamDefaultController();
+
+ public:
+  nsIGlobalObject* GetParentObject() const { return mGlobal; }
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  // WebIDL methods/properties
+
+  AbortSignal* Signal() { return mSignal; }
+
+  MOZ_CAN_RUN_SCRIPT void Error(JSContext* aCx, JS::Handle<JS::Value> aError,
+                                ErrorResult& aRv);
+
+  // [[AbortSteps]]
+  MOZ_CAN_RUN_SCRIPT virtual already_AddRefed<Promise> AbortSteps(
+      JSContext* aCx, JS::Handle<JS::Value> aReason, ErrorResult& aRv);
+
+  // [[ErrorSteps]]
+  virtual void ErrorSteps();
+
+  // Internal Slot Accessors
+
+  QueueWithSizes& Queue() { return mQueue; }
+
+  double QueueTotalSize() const { return mQueueTotalSize; }
+  void SetQueueTotalSize(double aQueueTotalSize) {
+    mQueueTotalSize = aQueueTotalSize;
+  }
+
+  void SetSignal(AbortSignal* aSignal);
+
+  bool Started() const { return mStarted; }
+  void SetStarted(bool aStarted) { mStarted = aStarted; }
+
+  double StrategyHWM() const { return mStrategyHWM; }
+  void SetStrategyHWM(double aStrategyHWM) { mStrategyHWM = aStrategyHWM; }
+
+  QueuingStrategySize* StrategySizeAlgorithm() const {
+    return mStrategySizeAlgorithm;
+  }
+  void SetStrategySizeAlgorithm(QueuingStrategySize* aStrategySizeAlgorithm) {
+    mStrategySizeAlgorithm = aStrategySizeAlgorithm;
+  }
+
+  UnderlyingSinkAlgorithmsBase* GetAlgorithms() { return mAlgorithms; }
+  void SetAlgorithms(UnderlyingSinkAlgorithmsBase& aAlgorithms) {
+    mAlgorithms = &aAlgorithms;
+  }
+
+  WritableStream* Stream() { return mStream; }
+
+  // WritableStreamDefaultControllerGetBackpressure
+  // https://streams.spec.whatwg.org/#writable-stream-default-controller-get-backpressure
+  bool GetBackpressure() const {
+    // Step 1. Let desiredSize be !
+    // WritableStreamDefaultControllerGetDesiredSize(controller).
+    double desiredSize = GetDesiredSize();
+    // Step 2. Return true if desiredSize ≤ 0, or false otherwise.
+    return desiredSize <= 0;
+  }
+
+  // WritableStreamDefaultControllerGetDesiredSize
+  // https://streams.spec.whatwg.org/#writable-stream-default-controller-get-desired-size
+  double GetDesiredSize() const { return mStrategyHWM - mQueueTotalSize; }
+
+  // WritableStreamDefaultControllerClearAlgorithms
+  // https://streams.spec.whatwg.org/#writable-stream-default-controller-clear-algorithms
+  void ClearAlgorithms() {
+    // Step 1. Set controller.[[writeAlgorithm]] to undefined.
+    // Step 2. Set controller.[[closeAlgorithm]] to undefined.
+    // Step 3. Set controller.[[abortAlgorithm]] to undefined.
+    // (As written in the spec, this can happen multiple time. Try running
+    // wpt/streams/transform-streams/errors.any.js for example.)
+    if (RefPtr<UnderlyingSinkAlgorithmsBase> algorithms =
+            mAlgorithms.forget()) {
+      algorithms->ReleaseObjects();
+    }
+
+    // Step 4. Set controller.[[strategySizeAlgorithm]] to undefined.
+    mStrategySizeAlgorithm = nullptr;
+  }
+
+ private:
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+
+  // Internal Slots
+  QueueWithSizes mQueue = {};
+  double mQueueTotalSize = 0.0;
+  RefPtr<AbortSignal> mSignal;
+  bool mStarted = false;
+  double mStrategyHWM = 0.0;
+
+  RefPtr<QueuingStrategySize> mStrategySizeAlgorithm;
+  RefPtr<UnderlyingSinkAlgorithmsBase> mAlgorithms;
+  RefPtr<WritableStream> mStream;
+};
+
+namespace streams_abstract {
+
+MOZ_CAN_RUN_SCRIPT void SetUpWritableStreamDefaultController(
+    JSContext* aCx, WritableStream* aStream,
+    WritableStreamDefaultController* aController,
+    UnderlyingSinkAlgorithmsBase* aAlgorithms, double aHighWaterMark,
+    QueuingStrategySize* aSizeAlgorithm, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void SetUpWritableStreamDefaultControllerFromUnderlyingSink(
+    JSContext* aCx, WritableStream* aStream,
+    JS::Handle<JSObject*> aUnderlyingSink, UnderlyingSink& aUnderlyingSinkDict,
+    double aHighWaterMark, QueuingStrategySize* aSizeAlgorithm,
+    ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void WritableStreamDefaultControllerClose(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void WritableStreamDefaultControllerWrite(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    JS::Handle<JS::Value> aChunk, double chunkSize, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void WritableStreamDefaultControllerError(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    JS::Handle<JS::Value> aError, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT void WritableStreamDefaultControllerErrorIfNeeded(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    JS::Handle<JS::Value> aError, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT double WritableStreamDefaultControllerGetChunkSize(
+    JSContext* aCx, WritableStreamDefaultController* aController,
+    JS::Handle<JS::Value> aChunk, ErrorResult& aRv);
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_WritableStreamDefaultController_h
diff --git a/dom/streams/WritableStreamDefaultWriter.cpp b/dom/streams/WritableStreamDefaultWriter.cpp
new file mode 100644
index 0000000000..589eada7e2
--- /dev/null
+++ b/dom/streams/WritableStreamDefaultWriter.cpp
@@ -0,0 +1,544 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 "mozilla/dom/WritableStreamDefaultWriter.h"
+#include "js/Array.h"
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/CycleCollectedJSContext.h"
+#include "mozilla/HoldDropJSObjects.h"
+#include "mozilla/dom/WritableStream.h"
+#include "mozilla/dom/WritableStreamDefaultWriterBinding.h"
+#include "nsCOMPtr.h"
+
+#include "mozilla/dom/Promise-inl.h"
+#include "nsIGlobalObject.h"
+#include "nsISupports.h"
+
+namespace mozilla::dom {
+
+using namespace streams_abstract;
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(WritableStreamDefaultWriter)
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(WritableStreamDefaultWriter)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mGlobal, mStream, mReadyPromise,
+                                  mClosedPromise)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(WritableStreamDefaultWriter)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mGlobal, mStream, mReadyPromise,
+                                    mClosedPromise)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(WritableStreamDefaultWriter)
+  NS_IMPL_CYCLE_COLLECTION_TRACE_PRESERVED_WRAPPER
+NS_IMPL_CYCLE_COLLECTION_TRACE_END
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(WritableStreamDefaultWriter)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(WritableStreamDefaultWriter)
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(WritableStreamDefaultWriter)
+  NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+NS_INTERFACE_MAP_END
+
+WritableStreamDefaultWriter::WritableStreamDefaultWriter(
+    nsIGlobalObject* aGlobal)
+    : mGlobal(aGlobal) {
+  mozilla::HoldJSObjects(this);
+}
+
+WritableStreamDefaultWriter::~WritableStreamDefaultWriter() {
+  mozilla::DropJSObjects(this);
+}
+
+void WritableStreamDefaultWriter::SetReadyPromise(Promise* aPromise) {
+  MOZ_ASSERT(aPromise);
+  mReadyPromise = aPromise;
+}
+
+void WritableStreamDefaultWriter::SetClosedPromise(Promise* aPromise) {
+  MOZ_ASSERT(aPromise);
+  mClosedPromise = aPromise;
+}
+
+JSObject* WritableStreamDefaultWriter::WrapObject(
+    JSContext* aCx, JS::Handle<JSObject*> aGivenProto) {
+  return WritableStreamDefaultWriter_Binding::Wrap(aCx, this, aGivenProto);
+}
+
+/* static */
+already_AddRefed<WritableStreamDefaultWriter>
+WritableStreamDefaultWriter::Constructor(const GlobalObject& aGlobal,
+                                         WritableStream& aStream,
+                                         ErrorResult& aRv) {
+  nsCOMPtr<nsIGlobalObject> global = do_QueryInterface(aGlobal.GetAsSupports());
+  RefPtr<WritableStreamDefaultWriter> writer =
+      new WritableStreamDefaultWriter(global);
+  SetUpWritableStreamDefaultWriter(writer, &aStream, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+  return writer.forget();
+}
+
+already_AddRefed<Promise> WritableStreamDefaultWriter::Closed() {
+  RefPtr<Promise> closedPromise = mClosedPromise;
+  return closedPromise.forget();
+}
+
+already_AddRefed<Promise> WritableStreamDefaultWriter::Ready() {
+  RefPtr<Promise> readyPromise = mReadyPromise;
+  return readyPromise.forget();
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#writable-stream-default-writer-get-desired-size
+Nullable<double> WritableStreamDefaultWriterGetDesiredSize(
+    WritableStreamDefaultWriter* aWriter) {
+  // Step 1. Let stream be writer.[[stream]].
+  RefPtr<WritableStream> stream = aWriter->GetStream();
+
+  // Step 2. Let state be stream.[[state]].
+  WritableStream::WriterState state = stream->State();
+
+  // Step 3. If state is "errored" or "erroring", return null.
+  if (state == WritableStream::WriterState::Errored ||
+      state == WritableStream::WriterState::Erroring) {
+    return nullptr;
+  }
+
+  // Step 4. If state is "closed", return 0.
+  if (state == WritableStream::WriterState::Closed) {
+    return 0.0;
+  }
+
+  // Step 5. Return
+  // ! WritableStreamDefaultControllerGetDesiredSize(stream.[[controller]]).
+  return stream->Controller()->GetDesiredSize();
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#default-writer-desired-size
+Nullable<double> WritableStreamDefaultWriter::GetDesiredSize(ErrorResult& aRv) {
+  // Step 1. If this.[[stream]] is undefined, throw a TypeError exception.
+  if (!mStream) {
+    aRv.ThrowTypeError("Missing stream");
+    return nullptr;
+  }
+
+  // Step 2. Return ! WritableStreamDefaultWriterGetDesiredSize(this).
+  RefPtr<WritableStreamDefaultWriter> thisRefPtr = this;
+  return WritableStreamDefaultWriterGetDesiredSize(thisRefPtr);
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-default-writer-abort
+MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> WritableStreamDefaultWriterAbort(
+    JSContext* aCx, WritableStreamDefaultWriter* aWriter,
+    JS::Handle<JS::Value> aReason, ErrorResult& aRv) {
+  // Step 1. Let stream be writer.[[stream]].
+  RefPtr<WritableStream> stream = aWriter->GetStream();
+
+  // Step 2. Assert: stream is not undefined.
+  MOZ_ASSERT(stream);
+
+  // Step 3. Return ! WritableStreamAbort(stream, reason).
+  return WritableStreamAbort(aCx, stream, aReason, aRv);
+}
+
+// https://streams.spec.whatwg.org/#default-writer-abort
+already_AddRefed<Promise> WritableStreamDefaultWriter::Abort(
+    JSContext* aCx, JS::Handle<JS::Value> aReason, ErrorResult& aRv) {
+  // Step 1. If this.[[stream]] is undefined, return a promise rejected with a
+  // TypeError exception.
+  if (!mStream) {
+    aRv.ThrowTypeError("Missing stream");
+    return nullptr;
+  }
+
+  // Step 2. Return ! WritableStreamDefaultWriterAbort(this, reason).
+  RefPtr<WritableStreamDefaultWriter> thisRefPtr = this;
+  return WritableStreamDefaultWriterAbort(aCx, thisRefPtr, aReason, aRv);
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-default-writer-close
+MOZ_CAN_RUN_SCRIPT static already_AddRefed<Promise>
+WritableStreamDefaultWriterClose(JSContext* aCx,
+                                 WritableStreamDefaultWriter* aWriter,
+                                 ErrorResult& aRv) {
+  // Step 1. Let stream be writer.[[stream]].
+  RefPtr<WritableStream> stream = aWriter->GetStream();
+
+  // Step 2. Assert: stream is not undefined.
+  MOZ_ASSERT(stream);
+
+  // Step 3. Return ! WritableStreamClose(stream).
+  return WritableStreamClose(aCx, stream, aRv);
+}
+
+// https://streams.spec.whatwg.org/#default-writer-close
+already_AddRefed<Promise> WritableStreamDefaultWriter::Close(JSContext* aCx,
+                                                             ErrorResult& aRv) {
+  // Step 1. Let stream be this.[[stream]].
+  RefPtr<WritableStream> stream = mStream;
+
+  // Step 2. If stream is undefined, return a promise rejected with a TypeError
+  // exception.
+  if (!stream) {
+    aRv.ThrowTypeError("Missing stream");
+    return nullptr;
+  }
+
+  // Step 3. If ! WritableStreamCloseQueuedOrInFlight(stream) is true,
+  // return a promise rejected with a TypeError exception.
+  if (stream->CloseQueuedOrInFlight()) {
+    aRv.ThrowTypeError("Stream is closing");
+    return nullptr;
+  }
+
+  // Step 3. Return ! WritableStreamDefaultWriterClose(this).
+  RefPtr<WritableStreamDefaultWriter> thisRefPtr = this;
+  return WritableStreamDefaultWriterClose(aCx, thisRefPtr, aRv);
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#writable-stream-default-writer-release
+void WritableStreamDefaultWriterRelease(JSContext* aCx,
+                                        WritableStreamDefaultWriter* aWriter) {
+  // Step 1. Let stream be writer.[[stream]].
+  RefPtr<WritableStream> stream = aWriter->GetStream();
+
+  // Step 2. Assert: stream is not undefined.
+  MOZ_ASSERT(stream);
+
+  // Step 3. Assert: stream.[[writer]] is writer.
+  MOZ_ASSERT(stream->GetWriter() == aWriter);
+
+  // Step 4. Let releasedError be a new TypeError.
+  JS::Rooted<JS::Value> releasedError(RootingCx(), JS::UndefinedValue());
+  {
+    ErrorResult rv;
+    rv.ThrowTypeError("Releasing lock");
+    bool ok = ToJSValue(aCx, std::move(rv), &releasedError);
+    MOZ_RELEASE_ASSERT(ok, "must be ok");
+  }
+
+  // Step 5. Perform !
+  // WritableStreamDefaultWriterEnsureReadyPromiseRejected(writer,
+  // releasedError).
+  WritableStreamDefaultWriterEnsureReadyPromiseRejected(aWriter, releasedError);
+
+  // Step 6. Perform !
+  // WritableStreamDefaultWriterEnsureClosedPromiseRejected(writer,
+  // releasedError).
+  WritableStreamDefaultWriterEnsureClosedPromiseRejected(aWriter,
+                                                         releasedError);
+
+  // Step 7. Set stream.[[writer]] to undefined.
+  stream->SetWriter(nullptr);
+
+  // Step 8. Set writer.[[stream]] to undefined.
+  aWriter->SetStream(nullptr);
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#default-writer-release-lock
+void WritableStreamDefaultWriter::ReleaseLock(JSContext* aCx) {
+  // Step 1. Let stream be this.[[stream]].
+  RefPtr<WritableStream> stream = mStream;
+
+  // Step 2. If stream is undefined, return.
+  if (!stream) {
+    return;
+  }
+
+  // Step 3. Assert: stream.[[writer]] is not undefined.
+  MOZ_ASSERT(stream->GetWriter());
+
+  // Step 4. Perform ! WritableStreamDefaultWriterRelease(this).
+  RefPtr<WritableStreamDefaultWriter> thisRefPtr = this;
+  return WritableStreamDefaultWriterRelease(aCx, thisRefPtr);
+}
+
+namespace streams_abstract {
+// https://streams.spec.whatwg.org/#writable-stream-default-writer-write
+already_AddRefed<Promise> WritableStreamDefaultWriterWrite(
+    JSContext* aCx, WritableStreamDefaultWriter* aWriter,
+    JS::Handle<JS::Value> aChunk, ErrorResult& aRv) {
+  // Step 1. Let stream be writer.[[stream]].
+  RefPtr<WritableStream> stream = aWriter->GetStream();
+
+  // Step 2. Assert: stream is not undefined.
+  MOZ_ASSERT(stream);
+
+  // Step 3. Let controller be stream.[[controller]].
+  RefPtr<WritableStreamDefaultController> controller = stream->Controller();
+
+  // Step 4. Let chunkSize be !
+  // WritableStreamDefaultControllerGetChunkSize(controller, chunk).
+  double chunkSize =
+      WritableStreamDefaultControllerGetChunkSize(aCx, controller, aChunk, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 5. If stream is not equal to writer.[[stream]], return a promise
+  // rejected with a TypeError exception.
+  if (stream != aWriter->GetStream()) {
+    aRv.ThrowTypeError(
+        "Can not write on WritableStream owned by another writer.");
+    return nullptr;
+  }
+
+  // Step 6. Let state be stream.[[state]].
+  WritableStream::WriterState state = stream->State();
+
+  // Step 7. If state is "errored", return a promise rejected with
+  // stream.[[storedError]].
+  if (state == WritableStream::WriterState::Errored) {
+    JS::Rooted<JS::Value> error(aCx, stream->StoredError());
+    return Promise::CreateRejected(aWriter->GetParentObject(), error, aRv);
+  }
+
+  // Step 8. If ! WritableStreamCloseQueuedOrInFlight(stream) is true or state
+  // is "closed", return a promise rejected with a TypeError exception
+  // indicating that the stream is closing or closed.
+  if (stream->CloseQueuedOrInFlight() ||
+      state == WritableStream::WriterState::Closed) {
+    return Promise::CreateRejectedWithTypeError(
+        aWriter->GetParentObject(), "Stream is closed or closing"_ns, aRv);
+  }
+
+  // Step 9. If state is "erroring", return a promise rejected with
+  // stream.[[storedError]].
+  if (state == WritableStream::WriterState::Erroring) {
+    JS::Rooted<JS::Value> error(aCx, stream->StoredError());
+    return Promise::CreateRejected(aWriter->GetParentObject(), error, aRv);
+  }
+
+  // Step 10. Assert: state is "writable".
+  MOZ_ASSERT(state == WritableStream::WriterState::Writable);
+
+  // Step 11. Let promise be ! WritableStreamAddWriteRequest(stream).
+  RefPtr<Promise> promise = WritableStreamAddWriteRequest(stream);
+
+  // Step 12. Perform ! WritableStreamDefaultControllerWrite(controller, chunk,
+  // chunkSize).
+  WritableStreamDefaultControllerWrite(aCx, controller, aChunk, chunkSize, aRv);
+  if (aRv.Failed()) {
+    return nullptr;
+  }
+
+  // Step 13. Return promise.
+  return promise.forget();
+}
+}  // namespace streams_abstract
+
+// https://streams.spec.whatwg.org/#default-writer-write
+already_AddRefed<Promise> WritableStreamDefaultWriter::Write(
+    JSContext* aCx, JS::Handle<JS::Value> aChunk, ErrorResult& aRv) {
+  // Step 1. If this.[[stream]] is undefined, return a promise rejected with a
+  // TypeError exception.
+  if (!mStream) {
+    aRv.ThrowTypeError("Missing stream");
+    return nullptr;
+  }
+
+  // Step 2. Return ! WritableStreamDefaultWriterWrite(this, chunk).
+  return WritableStreamDefaultWriterWrite(aCx, this, aChunk, aRv);
+}
+
+namespace streams_abstract {
+
+// https://streams.spec.whatwg.org/#set-up-writable-stream-default-writer
+void SetUpWritableStreamDefaultWriter(WritableStreamDefaultWriter* aWriter,
+                                      WritableStream* aStream,
+                                      ErrorResult& aRv) {
+  // Step 1. If ! IsWritableStreamLocked(stream) is true, throw a TypeError
+  // exception.
+  if (IsWritableStreamLocked(aStream)) {
+    aRv.ThrowTypeError("WritableStream is already locked!");
+    return;
+  }
+
+  // Step 2. Set writer.[[stream]] to stream.
+  aWriter->SetStream(aStream);
+
+  // Step 3. Set stream.[[writer]] to writer.
+  aStream->SetWriter(aWriter);
+
+  // Step 4. Let state be stream.[[state]].
+  WritableStream::WriterState state = aStream->State();
+
+  // Step 5. If state is "writable",
+  if (state == WritableStream::WriterState::Writable) {
+    RefPtr<Promise> readyPromise =
+        Promise::CreateInfallible(aWriter->GetParentObject());
+
+    // Step 5.1 If ! WritableStreamCloseQueuedOrInFlight(stream) is false and
+    // stream.[[backpressure]] is true, set writer.[[readyPromise]] to a new
+    // promise.
+    if (!aStream->CloseQueuedOrInFlight() && aStream->Backpressure()) {
+      aWriter->SetReadyPromise(readyPromise);
+    } else {
+      // Step 5.2. Otherwise, set writer.[[readyPromise]] to a promise resolved
+      // with undefined.
+      readyPromise->MaybeResolveWithUndefined();
+      aWriter->SetReadyPromise(readyPromise);
+    }
+
+    // Step 5.3. Set writer.[[closedPromise]] to a new promise.
+    RefPtr<Promise> closedPromise =
+        Promise::CreateInfallible(aWriter->GetParentObject());
+    aWriter->SetClosedPromise(closedPromise);
+  } else if (state == WritableStream::WriterState::Erroring) {
+    // Step 6. Otherwise, if state is "erroring",
+
+    // Step 6.1. Set writer.[[readyPromise]] to a promise rejected with
+    // stream.[[storedError]].
+    JS::Rooted<JS::Value> storedError(RootingCx(), aStream->StoredError());
+    RefPtr<Promise> readyPromise =
+        Promise::CreateInfallible(aWriter->GetParentObject());
+    readyPromise->MaybeReject(storedError);
+    aWriter->SetReadyPromise(readyPromise);
+
+    // Step 6.2. Set writer.[[readyPromise]].[[PromiseIsHandled]] to true.
+    readyPromise->SetSettledPromiseIsHandled();
+
+    // Step 6.3. Set writer.[[closedPromise]] to a new promise.
+    RefPtr<Promise> closedPromise =
+        Promise::CreateInfallible(aWriter->GetParentObject());
+    aWriter->SetClosedPromise(closedPromise);
+  } else if (state == WritableStream::WriterState::Closed) {
+    // Step 7. Otherwise, if state is "closed",
+    // Step 7.1. Set writer.[[readyPromise]] to a promise resolved with
+    // undefined.
+    RefPtr<Promise> readyPromise =
+        Promise::CreateResolvedWithUndefined(aWriter->GetParentObject(), aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+    aWriter->SetReadyPromise(readyPromise);
+
+    // Step 7.2. Set writer.[[closedPromise]] to a promise resolved with
+    // undefined.
+    RefPtr<Promise> closedPromise =
+        Promise::CreateResolvedWithUndefined(aWriter->GetParentObject(), aRv);
+    if (aRv.Failed()) {
+      return;
+    }
+    aWriter->SetClosedPromise(closedPromise);
+  } else {
+    // Step 8. Otherwise,
+    // Step 8.1 Assert: state is "errored".
+    MOZ_ASSERT(state == WritableStream::WriterState::Errored);
+
+    // Step 8.2. Step Let storedError be stream.[[storedError]].
+    JS::Rooted<JS::Value> storedError(RootingCx(), aStream->StoredError());
+
+    // Step 8.3. Set writer.[[readyPromise]] to a promise rejected with
+    // storedError.
+    RefPtr<Promise> readyPromise =
+        Promise::CreateInfallible(aWriter->GetParentObject());
+    readyPromise->MaybeReject(storedError);
+    aWriter->SetReadyPromise(readyPromise);
+
+    // Step 8.4. Set writer.[[readyPromise]].[[PromiseIsHandled]] to true.
+    readyPromise->SetSettledPromiseIsHandled();
+
+    // Step 8.5. Set writer.[[closedPromise]] to a promise rejected with
+    // storedError.
+    RefPtr<Promise> closedPromise =
+        Promise::CreateInfallible(aWriter->GetParentObject());
+    closedPromise->MaybeReject(storedError);
+    aWriter->SetClosedPromise(closedPromise);
+
+    // Step 8.6 Set writer.[[closedPromise]].[[PromiseIsHandled]] to true.
+    closedPromise->SetSettledPromiseIsHandled();
+  }
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-default-writer-ensure-closed-promise-rejected
+void WritableStreamDefaultWriterEnsureClosedPromiseRejected(
+    WritableStreamDefaultWriter* aWriter, JS::Handle<JS::Value> aError) {
+  RefPtr<Promise> closedPromise = aWriter->ClosedPromise();
+  // Step 1. If writer.[[closedPromise]].[[PromiseState]] is "pending", reject
+  // writer.[[closedPromise]] with error.
+  if (closedPromise->State() == Promise::PromiseState::Pending) {
+    closedPromise->MaybeReject(aError);
+  } else {
+    // Step 2. Otherwise, set writer.[[closedPromise]] to a promise rejected
+    // with error.
+    closedPromise = Promise::CreateInfallible(aWriter->GetParentObject());
+    closedPromise->MaybeReject(aError);
+    aWriter->SetClosedPromise(closedPromise);
+  }
+
+  // Step 3. Set writer.[[closedPromise]].[[PromiseIsHandled]] to true.
+  closedPromise->SetSettledPromiseIsHandled();
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-default-writer-ensure-ready-promise-rejected
+void WritableStreamDefaultWriterEnsureReadyPromiseRejected(
+    WritableStreamDefaultWriter* aWriter, JS::Handle<JS::Value> aError) {
+  RefPtr<Promise> readyPromise = aWriter->ReadyPromise();
+  // Step 1. If writer.[[readyPromise]].[[PromiseState]] is "pending", reject
+  // writer.[[readyPromise]] with error.
+  if (readyPromise->State() == Promise::PromiseState::Pending) {
+    readyPromise->MaybeReject(aError);
+  } else {
+    // Step 2. Otherwise, set writer.[[readyPromise]] to a promise rejected with
+    // error.
+    readyPromise = Promise::CreateInfallible(aWriter->GetParentObject());
+    readyPromise->MaybeReject(aError);
+    aWriter->SetReadyPromise(readyPromise);
+  }
+
+  // Step 3. Set writer.[[readyPromise]].[[PromiseIsHandled]] to true.
+  readyPromise->SetSettledPromiseIsHandled();
+}
+
+// https://streams.spec.whatwg.org/#writable-stream-default-writer-close-with-error-propagation
+already_AddRefed<Promise> WritableStreamDefaultWriterCloseWithErrorPropagation(
+    JSContext* aCx, WritableStreamDefaultWriter* aWriter, ErrorResult& aRv) {
+  // Step 1. Let stream be writer.[[stream]].
+  RefPtr<WritableStream> stream = aWriter->GetStream();
+
+  // Step 2. Assert: stream is not undefined.
+  MOZ_ASSERT(stream);
+
+  // Step 3. Let state be stream.[[state]].
+  WritableStream::WriterState state = stream->State();
+
+  // Step 4. If ! WritableStreamCloseQueuedOrInFlight(stream) is true
+  // or state is "closed", return a promise resolved with undefined.
+  if (stream->CloseQueuedOrInFlight() ||
+      state == WritableStream::WriterState::Closed) {
+    return Promise::CreateResolvedWithUndefined(aWriter->GetParentObject(),
+                                                aRv);
+  }
+
+  // Step 5. If state is "errored",
+  // return a promise rejected with stream.[[storedError]].
+  if (state == WritableStream::WriterState::Errored) {
+    JS::Rooted<JS::Value> error(aCx, stream->StoredError());
+    return Promise::CreateRejected(aWriter->GetParentObject(), error, aRv);
+  }
+
+  // Step 6. Assert: state is "writable" or "erroring".
+  MOZ_ASSERT(state == WritableStream::WriterState::Writable ||
+             state == WritableStream::WriterState::Erroring);
+
+  // Step 7. Return ! WritableStreamDefaultWriterClose(writer).
+  return WritableStreamDefaultWriterClose(aCx, aWriter, aRv);
+}
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
diff --git a/dom/streams/WritableStreamDefaultWriter.h b/dom/streams/WritableStreamDefaultWriter.h
new file mode 100644
index 0000000000..487a504cd1
--- /dev/null
+++ b/dom/streams/WritableStreamDefaultWriter.h
@@ -0,0 +1,112 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 mozilla_dom_WritableStreamDefaultWriter_h
+#define mozilla_dom_WritableStreamDefaultWriter_h
+
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/QueuingStrategyBinding.h"
+
+#include "nsCycleCollectionParticipant.h"
+#include "nsWrapperCache.h"
+
+namespace mozilla::dom {
+
+class Promise;
+class WritableStream;
+
+class WritableStreamDefaultWriter final : public nsISupports,
+                                          public nsWrapperCache {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(WritableStreamDefaultWriter)
+
+ protected:
+  ~WritableStreamDefaultWriter();
+
+ public:
+  explicit WritableStreamDefaultWriter(nsIGlobalObject* aGlobal);
+
+  // Slot Getter/Setters:
+ public:
+  WritableStream* GetStream() const { return mStream; }
+  void SetStream(WritableStream* aStream) { mStream = aStream; }
+
+  Promise* ReadyPromise() const { return mReadyPromise; }
+  void SetReadyPromise(Promise* aPromise);
+
+  Promise* ClosedPromise() const { return mClosedPromise; }
+  void SetClosedPromise(Promise* aPromise);
+
+ public:
+  nsIGlobalObject* GetParentObject() const { return mGlobal; }
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  // IDL Methods
+  static already_AddRefed<WritableStreamDefaultWriter> Constructor(
+      const GlobalObject& aGlobal, WritableStream& aStream, ErrorResult& aRv);
+
+  already_AddRefed<Promise> Closed();
+  already_AddRefed<Promise> Ready();
+
+  Nullable<double> GetDesiredSize(ErrorResult& aRv);
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> Abort(
+      JSContext* aCx, JS::Handle<JS::Value> aReason, ErrorResult& aRv);
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> Close(JSContext* aCx,
+                                                     ErrorResult& aRv);
+
+  void ReleaseLock(JSContext* aCx);
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> Write(
+      JSContext* aCx, JS::Handle<JS::Value> aChunk, ErrorResult& aRv);
+
+  // Internal Slots:
+ private:
+  nsCOMPtr<nsIGlobalObject> mGlobal;
+  RefPtr<WritableStream> mStream;
+  RefPtr<Promise> mReadyPromise;
+  RefPtr<Promise> mClosedPromise;
+};
+
+namespace streams_abstract {
+
+void SetUpWritableStreamDefaultWriter(WritableStreamDefaultWriter* aWriter,
+                                      WritableStream* aStream,
+                                      ErrorResult& aRv);
+
+void WritableStreamDefaultWriterEnsureClosedPromiseRejected(
+    WritableStreamDefaultWriter* aWriter, JS::Handle<JS::Value> aError);
+
+void WritableStreamDefaultWriterEnsureReadyPromiseRejected(
+    WritableStreamDefaultWriter* aWriter, JS::Handle<JS::Value> aError);
+
+Nullable<double> WritableStreamDefaultWriterGetDesiredSize(
+    WritableStreamDefaultWriter* aWriter);
+
+void WritableStreamDefaultWriterRelease(JSContext* aCx,
+                                        WritableStreamDefaultWriter* aWriter);
+
+MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> WritableStreamDefaultWriterWrite(
+    JSContext* aCx, WritableStreamDefaultWriter* aWriter,
+    JS::Handle<JS::Value> aChunk, ErrorResult& aRv);
+
+MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise>
+WritableStreamDefaultWriterCloseWithErrorPropagation(
+    JSContext* aCx, WritableStreamDefaultWriter* aWriter, ErrorResult& aRv);
+
+}  // namespace streams_abstract
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_WritableStreamDefaultWriter_h
diff --git a/dom/streams/crashtests/1764222.html b/dom/streams/crashtests/1764222.html
new file mode 100644
index 0000000000..a48a1168a1
--- /dev/null
+++ b/dom/streams/crashtests/1764222.html
@@ -0,0 +1,10 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<script>
+  // This can't be in WPT because of bug 1766711.
+  let a = new AbortController()
+  a.abort()
+  let b = new File(['ó ¬—a'], 'a', {})
+  let c = new WritableStream()
+  b.stream().tee()[0].pipeTo(c, { 'signal': a.signal })
+</script>
diff --git a/dom/streams/crashtests/crashtests.list b/dom/streams/crashtests/crashtests.list
new file mode 100644
index 0000000000..c19e7107dc
--- /dev/null
+++ b/dom/streams/crashtests/crashtests.list
@@ -0,0 +1 @@
+load 1764222.html
diff --git a/dom/streams/moz.build b/dom/streams/moz.build
new file mode 100644
index 0000000000..85f267150a
--- /dev/null
+++ b/dom/streams/moz.build
@@ -0,0 +1,72 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# 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/.
+
+with Files("**"):
+    BUG_COMPONENT = ("Core", "DOM: Streams")
+
+EXPORTS.mozilla.dom += [
+    "BaseQueuingStrategy.h",
+    "ByteLengthQueuingStrategy.h",
+    "ByteStreamHelpers.h",
+    "CountQueuingStrategy.h",
+    "QueueWithSizes.h",
+    "ReadableByteStreamController.h",
+    "ReadableStream.h",
+    "ReadableStreamBYOBReader.h",
+    "ReadableStreamBYOBRequest.h",
+    "ReadableStreamController.h",
+    "ReadableStreamDefaultController.h",
+    "ReadableStreamDefaultReader.h",
+    "ReadableStreamGenericReader.h",
+    "ReadRequest.h",
+    "TransformerCallbackHelpers.h",
+    "TransformStream.h",
+    "TransformStreamDefaultController.h",
+    "UnderlyingSinkCallbackHelpers.h",
+    "UnderlyingSourceCallbackHelpers.h",
+    "WritableStream.h",
+    "WritableStreamDefaultController.h",
+    "WritableStreamDefaultWriter.h",
+]
+
+UNIFIED_SOURCES += [
+    "ByteLengthQueuingStrategy.cpp",
+    "ByteStreamHelpers.cpp",
+    "CountQueuingStrategy.cpp",
+    "ReadableByteStreamController.cpp",
+    "ReadableStream.cpp",
+    "ReadableStreamBYOBReader.cpp",
+    "ReadableStreamBYOBRequest.cpp",
+    "ReadableStreamDefaultController.cpp",
+    "ReadableStreamDefaultReader.cpp",
+    "ReadableStreamPipeTo.cpp",
+    "ReadableStreamTee.cpp",
+    "StreamUtils.cpp",
+    "TeeState.cpp",
+    "Transferable.cpp",
+    "TransformerCallbackHelpers.cpp",
+    "TransformStream.cpp",
+    "TransformStreamDefaultController.cpp",
+    "UnderlyingSinkCallbackHelpers.cpp",
+    "UnderlyingSourceCallbackHelpers.cpp",
+    "WritableStream.cpp",
+    "WritableStreamDefaultController.cpp",
+    "WritableStreamDefaultWriter.cpp",
+]
+
+include("/ipc/chromium/chromium-config.mozbuild")  # to import MessagePort.h
+
+FINAL_LIBRARY = "xul"
+
+LOCAL_INCLUDES += [
+    "/dom/base",
+    "/dom/ipc",
+]
+
+# MOCHITEST_MANIFESTS += ["tests/mochitest.ini"]
+# ROWSER_CHROME_MANIFESTS += ["tests/browser/browser.ini"]
+
+XPCSHELL_TESTS_MANIFESTS += ["test/xpcshell/xpcshell.ini"]
diff --git a/dom/streams/test/xpcshell/bug-1387503-1.js b/dom/streams/test/xpcshell/bug-1387503-1.js
new file mode 100644
index 0000000000..777f614c44
--- /dev/null
+++ b/dom/streams/test/xpcshell/bug-1387503-1.js
@@ -0,0 +1,44 @@
+// Test uncatchable error when a stream source's pull() method is called.
+let readerCreated = false;
+let fnFinished = false;
+let g;
+
+add_task(async function test() {
+  // Make `debugger;` raise an uncatchable error.
+  g = newGlobal({ newCompartment: true });
+  g.parent = this;
+  g.hit = false;
+  g.eval(
+    ` new Debugger(parent).onDebuggerStatement = _frame => (hit = true, null);`
+  );
+
+  // Create a stream whose pull() method raises an uncatchable error,
+  // and try reading from it.
+
+  async function fn() {
+    try {
+      let stream = new ReadableStream({
+        start(controller) {},
+        pull(controller) {
+          // eslint-disable-next-line no-debugger
+          debugger;
+        },
+      });
+
+      let reader = stream.getReader();
+      let p = reader.read();
+      readerCreated = true;
+      await p;
+    } finally {
+      fnFinished = true;
+    }
+  }
+
+  fn();
+});
+
+add_task(() => {
+  equal(readerCreated, true);
+  equal(g.hit, true);
+  equal(fnFinished, false);
+});
diff --git a/dom/streams/test/xpcshell/bug-1387503-2.js b/dom/streams/test/xpcshell/bug-1387503-2.js
new file mode 100644
index 0000000000..1678b46649
--- /dev/null
+++ b/dom/streams/test/xpcshell/bug-1387503-2.js
@@ -0,0 +1,52 @@
+// Test uncatchable error when a stream's queuing strategy's size() method is called.
+/* global newGlobal */
+
+let fnFinished = false;
+let g;
+add_task(async function test() {
+  // Make `debugger;` raise an uncatchable exception.
+  g = newGlobal();
+  g.parent = this;
+  g.hit = false;
+  g.info = info;
+  g.eval(`
+    var dbg = new Debugger(parent);
+    dbg.onDebuggerStatement = (_frame, exc) => {hit = true; info("hit"); return null};
+`);
+
+  async function fn() {
+    // Await once to postpone the uncatchable error until we're running inside
+    // a reaction job. We don't want the rest of the test to be terminated.
+    // (`drainJobQueue` catches uncatchable errors!)
+    await 1;
+
+    try {
+      // Create a stream with a strategy whose .size() method raises an
+      // uncatchable exception, and have it call that method.
+      new ReadableStream(
+        {
+          start(controller) {
+            controller.enqueue("FIRST POST"); // this calls .size()
+          },
+        },
+        {
+          size() {
+            // eslint-disable-next-line no-debugger
+            debugger;
+          },
+        }
+      );
+    } finally {
+      fnFinished = true;
+    }
+  }
+
+  fn()
+    .then(() => info("Resolved"))
+    .catch(() => info("Rejected"));
+});
+
+add_task(() => {
+  equal(g.hit, true, "We hit G");
+  equal(fnFinished, false, "We didn't hit the finally block");
+});
diff --git a/dom/streams/test/xpcshell/bug-1503406.js b/dom/streams/test/xpcshell/bug-1503406.js
new file mode 100644
index 0000000000..8e45291faf
--- /dev/null
+++ b/dom/streams/test/xpcshell/bug-1503406.js
@@ -0,0 +1,20 @@
+let read;
+let reader;
+
+add_task(async function test() {
+  let g = newGlobal({ wantGlobalProperties: ["ReadableStream"] });
+  reader = g.eval(`
+    let stream = new ReadableStream({
+        start(controller) {
+            controller.enqueue([]);
+        },
+    });
+    let [b1, b2] = stream.tee();
+    b1.getReader();
+`);
+  read = new ReadableStream({}).getReader().read;
+});
+
+add_task(async function test2() {
+  read.call(reader);
+});
diff --git a/dom/streams/test/xpcshell/bug-1773237.js b/dom/streams/test/xpcshell/bug-1773237.js
new file mode 100644
index 0000000000..0d0107fe85
--- /dev/null
+++ b/dom/streams/test/xpcshell/bug-1773237.js
@@ -0,0 +1,11 @@
+// This test fails if there is an unhandled promise rejection
+var stream = new ReadableStream({
+  pull() {
+    return Promise.reject("foobar");
+  },
+});
+var response = new Response(stream);
+var text = response.text().then(
+  () => {},
+  e => {}
+);
diff --git a/dom/streams/test/xpcshell/dom_stream_prototype_test.js b/dom/streams/test/xpcshell/dom_stream_prototype_test.js
new file mode 100644
index 0000000000..b127368318
--- /dev/null
+++ b/dom/streams/test/xpcshell/dom_stream_prototype_test.js
@@ -0,0 +1,29 @@
+"use strict";
+
+var log = [];
+const stream = new ReadableStream({
+  start(controller) {
+    log.push("started");
+  },
+  pull(controller) {
+    log.push("pulled");
+    controller.enqueue("hi from pull");
+  },
+  cancel() {
+    log.push("cancelled");
+  },
+});
+
+print(log); // Currently prints "started"!
+
+add_task(async function helper() {
+  var reader = stream.getReader();
+  var readPromise = reader.read();
+  readPromise.then(x => print(`Printing promise result ${x}, log ${log}`));
+  print(log);
+
+  var x = await readPromise;
+  print(`Promise result ${x} ${x.value}`);
+  Assert.equal(x.value, "hi from pull");
+  Assert.ok(true);
+});
diff --git a/dom/streams/test/xpcshell/fetch.js b/dom/streams/test/xpcshell/fetch.js
new file mode 100644
index 0000000000..03c8471f80
--- /dev/null
+++ b/dom/streams/test/xpcshell/fetch.js
@@ -0,0 +1,36 @@
+"use strict";
+
+const { AddonTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/AddonTestUtils.sys.mjs"
+);
+
+AddonTestUtils.init(this);
+AddonTestUtils.createAppInfo(
+  "xpcshell@tests.mozilla.org",
+  "XPCShell",
+  "42",
+  "42"
+);
+
+Cu.importGlobalProperties(["fetch"]);
+add_task(async function helper() {
+  do_get_profile();
+
+  // The SearchService is also needed in order to construct the initial state,
+  // which means that the AddonManager needs to be available.
+  await AddonTestUtils.promiseStartupManager();
+
+  // The example.com domain will be used to host the dynamic layout JSON and
+  // the top stories JSON.
+  let server = AddonTestUtils.createHttpServer({ hosts: ["example.com"] });
+  server.registerDirectory("/", do_get_cwd());
+
+  Assert.equal(true, fetch instanceof Function);
+  var k = await fetch("http://example.com/");
+  console.log(k);
+  console.log(k.body);
+  var r = k.body.getReader();
+  console.log(r);
+  var v = await r.read();
+  console.log(v);
+});
diff --git a/dom/streams/test/xpcshell/head.js b/dom/streams/test/xpcshell/head.js
new file mode 100644
index 0000000000..510cabe757
--- /dev/null
+++ b/dom/streams/test/xpcshell/head.js
@@ -0,0 +1,45 @@
+"use strict";
+
+const { addDebuggerToGlobal } = ChromeUtils.importESModule(
+  "resource://gre/modules/jsdebugger.sys.mjs"
+);
+
+const SYSTEM_PRINCIPAL = Cc["@mozilla.org/systemprincipal;1"].createInstance(
+  Ci.nsIPrincipal
+);
+
+function addTestingFunctionsToGlobal(global) {
+  global.eval(
+    `
+      const testingFunctions = Cu.getJSTestingFunctions();
+      for (let k in testingFunctions) {
+
+        this[k] = testingFunctions[k];
+      }
+      `
+  );
+  if (!global.print) {
+    global.print = info;
+  }
+  if (!global.newGlobal) {
+    global.newGlobal = newGlobal;
+  }
+  if (!global.Debugger) {
+    addDebuggerToGlobal(global);
+  }
+}
+
+addTestingFunctionsToGlobal(this);
+
+/* Create a new global, with all the JS shell testing functions. Similar to the
+ * newGlobal function exposed to JS shells, and useful for porting JS shell
+ * tests to xpcshell tests.
+ */
+function newGlobal(args) {
+  const global = new Cu.Sandbox(SYSTEM_PRINCIPAL, {
+    freshCompartment: true,
+    ...args,
+  });
+  addTestingFunctionsToGlobal(global);
+  return global;
+}
diff --git a/dom/streams/test/xpcshell/large-pipeto.js b/dom/streams/test/xpcshell/large-pipeto.js
new file mode 100644
index 0000000000..420adfea20
--- /dev/null
+++ b/dom/streams/test/xpcshell/large-pipeto.js
@@ -0,0 +1,101 @@
+const { AppConstants } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppConstants.sys.mjs"
+);
+
+// Stamp an array buffer with a pattern; verified by verify_chunk below.
+function init(array) {
+  for (var i = 0; i < array.length; i++) {
+    array[i] = i % 256;
+  }
+}
+
+// The construction of the file below ends up with 12 instances
+// of the array buffer -- we want this to be larger than 2**32 bytes
+// to exercise potential truncation of nsIInputStream::Read's count
+// parameter.
+const ABLENGTH = (2 ** 32 + 8) / 12;
+
+// Get a large file (bigger than 2GB!)
+function get_file() {
+  const array = new ArrayBuffer(ABLENGTH);
+  const buff = new Uint8Array(array);
+
+  // Stamp with pattern.
+  init(buff);
+
+  const blob = new Blob([buff, buff], {});
+  return new File([blob, blob, blob, blob, blob, blob], {});
+}
+
+// Verify that the chunks the stream recieve correspond to the initialization
+function verify_chunk(chunk, verification_state) {
+  for (var j = 0; j < chunk.length; j++) {
+    // If we don't match the fill pattern.
+    if (chunk[j] != verification_state.expected) {
+      // we ran out of array buffer; so we should be looking at the first byte of the array again.
+      if (
+        verification_state.total_index % ABLENGTH != 0 ||
+        chunk[j] != 0 /* ASSUME THAT THE INITIALIZATION OF THE BUFFER IS ZERO */
+      ) {
+        throw new Error(
+          `Mismatch: chunk[${j}] (${chunk[j]}) != ${verification_state.expected} (total_index ${verification_state.total_index})`
+        );
+      }
+      // Reset the fill expectation to 1 for next round.
+      verification_state.expected = 1;
+    } else {
+      // We are inside regular fill section
+      verification_state.expected = (verification_state.expected + 1) % 256;
+    }
+    verification_state.total_index++;
+  }
+}
+
+// Pipe To Testing: Win32 can't handle the file size created in this test and OOMs.
+add_task(
+  {
+    skip_if: () => AppConstants.platform == "win" && !Services.appinfo.is64Bit,
+  },
+  async () => {
+    var chunk_verification_state = {
+      expected: 0,
+      total_index: 0,
+    };
+
+    const file = get_file();
+
+    await file.stream().pipeTo(
+      new WritableStream({
+        write(chunk) {
+          verify_chunk(chunk, chunk_verification_state);
+        },
+      })
+    );
+  }
+);
+
+// Do the same test as above, but this time don't use pipeTo.
+add_task(
+  {
+    skip_if: () => AppConstants.platform == "win" && !Services.appinfo.is64Bit,
+  },
+  async () => {
+    var file = get_file();
+
+    var chunk_verification_state = {
+      expected: 0,
+      total_index: 0,
+    };
+
+    var streamReader = file.stream().getReader();
+
+    while (true) {
+      var res = await streamReader.read();
+      if (res.done) {
+        break;
+      }
+      var chunk = res.value;
+      verify_chunk(chunk, chunk_verification_state);
+    }
+  }
+);
diff --git a/dom/streams/test/xpcshell/proper-realm-cancel.js b/dom/streams/test/xpcshell/proper-realm-cancel.js
new file mode 100644
index 0000000000..1de6db3172
--- /dev/null
+++ b/dom/streams/test/xpcshell/proper-realm-cancel.js
@@ -0,0 +1,5 @@
+// This test passes if we don't have a CCW assertion.
+var g = newGlobal();
+var ccwCancelMethod = new g.Function("return 17;");
+
+new ReadableStream({ cancel: ccwCancelMethod }).cancel("bye");
diff --git a/dom/streams/test/xpcshell/proper-realm-pull.js b/dom/streams/test/xpcshell/proper-realm-pull.js
new file mode 100644
index 0000000000..9d42d9a65d
--- /dev/null
+++ b/dom/streams/test/xpcshell/proper-realm-pull.js
@@ -0,0 +1,6 @@
+// This test passes if we don't have a CCW assertion.
+
+var g = newGlobal({ newCompartment: true });
+var ccwPullMethod = new g.Function("return 17;");
+
+new ReadableStream({ pull: ccwPullMethod });
diff --git a/dom/streams/test/xpcshell/response.js b/dom/streams/test/xpcshell/response.js
new file mode 100644
index 0000000000..ead1be527f
--- /dev/null
+++ b/dom/streams/test/xpcshell/response.js
@@ -0,0 +1,30 @@
+"use strict";
+
+add_task(async function (test) {
+  return new Response(new Blob([], { type: "text/plain" })).body.cancel();
+});
+
+add_task(function (test) {
+  var response = new Response(
+    new Blob(["This is data"], { type: "text/plain" })
+  );
+  var reader = response.body.getReader();
+  reader.read();
+  return reader.cancel();
+});
+
+add_task(function (test) {
+  var response = new Response(new Blob(["T"], { type: "text/plain" }));
+  var reader = response.body.getReader();
+
+  var closedPromise = reader.closed.then(function () {
+    return reader.cancel();
+  });
+  reader.read().then(function readMore({ done, value }) {
+    if (!done) {
+      return reader.read().then(readMore);
+    }
+    return undefined;
+  });
+  return closedPromise;
+});
diff --git a/dom/streams/test/xpcshell/subclassing.js b/dom/streams/test/xpcshell/subclassing.js
new file mode 100644
index 0000000000..b2b86cf353
--- /dev/null
+++ b/dom/streams/test/xpcshell/subclassing.js
@@ -0,0 +1,123 @@
+// Adapted from js/src/tests/non262/ReadableStream/subclassing.js to suit requirements of xpcshell-testing.
+
+function assertEq(a, b) {
+  Assert.equal(a, b);
+}
+function assertThrowsInstanceOf(fun, err) {
+  var regexp = new RegExp(err.name);
+  print(regexp);
+  Assert.throws(fun, regexp);
+}
+
+// Spot-check subclassing of stream constructors.
+
+// ReadableStream can be subclassed.
+class PartyStreamer extends ReadableStream {}
+
+let started = false;
+add_task(function subclass_helper() {
+  // The base class constructor is called.
+  let stream = new PartyStreamer({
+    // (The ReadableStream constructor calls this start method.)
+    start(c) {
+      started = true;
+    },
+  });
+
+  assertEq(started, true);
+
+  // The instance's prototype chain is correct.
+  assertEq(stream.__proto__, PartyStreamer.prototype);
+  assertEq(stream.__proto__.__proto__, ReadableStream.prototype);
+  assertEq(stream.__proto__.__proto__.__proto__, Object.prototype);
+  assertEq(stream.__proto__.__proto__.__proto__.__proto__, null);
+  assertEq(stream instanceof ReadableStream, true);
+
+  // Non-generic methods can be called on the resulting stream.
+  stream.getReader();
+  assertEq(stream.locked, true);
+});
+
+add_task(function strategy_helper() {
+  // CountQueuingStrategy can be subclassed.
+  class PixelStrategy extends CountQueuingStrategy {}
+  assertEq(
+    new PixelStrategy({ highWaterMark: 4 }).__proto__,
+    PixelStrategy.prototype
+  );
+
+  // The base class constructor is called.
+  assertThrowsInstanceOf(() => new PixelStrategy(), TypeError);
+  assertEq(new PixelStrategy({ highWaterMark: -1 }).highWaterMark, -1);
+
+  // // VerySmartStrategy can be subclassed.
+  // class VerySmartStrategy extends ByteLengthQueuingStrategy {
+  //     size(chunk) {
+  //         return super.size(chunk) * 8;
+  //     }
+  // }
+  // let vss = new VerySmartStrategy({ highWaterMark: 12 });
+  // assertEq(vss.size(new ArrayBuffer(8)), 64);
+  // assertEq(vss.__proto__, VerySmartStrategy.prototype);
+});
+
+// Even ReadableStreamDefaultReader can be subclassed.
+add_task(async function readerTest() {
+  const ReadableStreamDefaultReader = new ReadableStream().getReader()
+    .constructor;
+  class MindReader extends ReadableStreamDefaultReader {
+    async read() {
+      let foretold = { value: "death", done: false };
+      let actual = await super.read();
+      actual = foretold; // ZOMG I WAS RIGHT, EXACTLY AS FORETOLD they should call me a righter
+      return actual;
+    }
+  }
+
+  let stream = new ReadableStream({
+    start(c) {
+      c.enqueue("one");
+      c.enqueue("two");
+    },
+    pull(c) {
+      c.close();
+    },
+  });
+  let reader = new MindReader(stream);
+  let result = await reader.read();
+  assertEq(result.value, "death");
+  reader.releaseLock();
+
+  reader = stream.getReader();
+  result = await reader.read();
+  assertEq(result.done, false);
+  assertEq(result.value, "two");
+  result = await reader.read();
+  assertEq(result.done, true);
+  assertEq(result.value, undefined);
+});
+
+add_task(function default_controller() {
+  // Even ReadableStreamDefaultController, which can't be constructed,
+  // can be subclassed.
+  let ReadableStreamDefaultController;
+  new ReadableStream({
+    start(c) {
+      ReadableStreamDefaultController = c.constructor;
+    },
+  });
+  class MasterController extends ReadableStreamDefaultController {
+    constructor() {
+      // don't call super, it'll just throw
+      return Object.create(MasterController.prototype);
+    }
+  }
+  let c = new MasterController();
+
+  // The prototype chain is per spec.
+  assertEq(c instanceof ReadableStreamDefaultController, true);
+
+  // But the instance does not have the internal slots of a
+  // ReadableStreamDefaultController, so the non-generic methods can't be used.
+  assertThrowsInstanceOf(() => c.enqueue("horse"), TypeError);
+});
diff --git a/dom/streams/test/xpcshell/too-big-array-buffer.js b/dom/streams/test/xpcshell/too-big-array-buffer.js
new file mode 100644
index 0000000000..b80c36e813
--- /dev/null
+++ b/dom/streams/test/xpcshell/too-big-array-buffer.js
@@ -0,0 +1,15 @@
+add_task(async function helper() {
+  // Note: this test assumes the largest possible ArrayBuffer is
+  // smaller than 10GB -- if that changes, this test will fail.
+  let rs = new ReadableStream({
+    type: "bytes",
+    autoAllocateChunkSize: 10 * 1024 * 1024 * 1024,
+  });
+  let reader = rs.getReader();
+  try {
+    await reader.read();
+    Assert.equal(true, false, "Shouldn't succeed at reading");
+  } catch (e) {
+    Assert.equal(e instanceof RangeError, true, "Should throw RangeError");
+  }
+});
diff --git a/dom/streams/test/xpcshell/xpcshell.ini b/dom/streams/test/xpcshell/xpcshell.ini
new file mode 100644
index 0000000000..7c509fe7f9
--- /dev/null
+++ b/dom/streams/test/xpcshell/xpcshell.ini
@@ -0,0 +1,22 @@
+[DEFAULT]
+head = head.js
+skip-if = toolkit == 'android'
+support-files =
+
+[response.js]
+[fetch.js]
+[dom_stream_prototype_test.js]
+[subclassing.js]
+[bug-1387503-1.js]
+prefs=
+  security.allow_parent_unrestricted_js_loads=true
+[bug-1503406.js]
+[proper-realm-cancel.js]
+[proper-realm-pull.js]
+[large-pipeto.js]
+skip-if =
+  os == "win"
+  tsan # Causes claim expired errors; see Bug 1770170.
+run-sequentially = very high failure rate in parallel
+[too-big-array-buffer.js]
+[bug-1773237.js]