Merge pull request 'Initial implementation of readable streams' (#2324) from dbsoft/UXP:readablestreams into master

Reviewed-on: https://repo.palemoon.org/MoonchildProductions/UXP/pulls/2324

29 files changed, 7436 insertions, 36 deletions

diff --git a/js/public/Stream.h b/js/public/Stream.h
new file mode 100644
index 0000000000..b91239ac9a
--- /dev/null
+++ b/js/public/Stream.h
@@ -0,0 +1,510 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * 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/. */
+
+/*
+ * JSAPI functions and callbacks related to WHATWG Stream objects.
+ *
+ * Much of the API here mirrors the JS API of ReadableStream and associated
+ * classes, e.g. ReadableStreamDefaultReader, ReadableStreamBYOBReader,
+ * ReadableStreamDefaultController, ReadableByteStreamController, and
+ * ReadableStreamBYOBRequest.
+ *
+ * There are some crucial differences, though: Functionality that's exposed
+ * as methods/accessors on controllers in JS is exposed as functions taking
+ * ReadableStream instances instead. This is because an analysis of how
+ * the API would be used showed that all functions that'd take controllers
+ * would do so by first getting the controller from the stream instance it's
+ * associated with and then call the function taking it. I.e., it would purely
+ * add boilerplate without any gains in ease of use of the API.
+ *
+ * Some functions exposed here deal with ReadableStream instances that have an
+ * embedding-provided underlying source. These instances are largely similar
+ * to byte streams as created using |new ReadableStream({type: "bytes"})|:
+ * They enable users to acquire ReadableStreamBYOBReaders and only vend chunks
+ * that're typed array instances.
+ *
+ * When creating an "external readable stream" using
+ * JS::NewReadableExternalSourceStreamObject, an underlying source and a set
+ * of flags can be passed to be stored on the stream. The underlying source is
+ * treated as an opaque void* pointer by the JS engine: it's purely meant as
+ * a reference to be used by the embedding to identify whatever actual source
+ * it uses to supply data for the stream. Similarly, the flags aren't
+ * interpreted by the JS engine, but are passed to some of the callbacks below
+ * and can be retrieved using JS::ReadableStreamGetEmbeddingFlags.
+ *
+ * External readable streams are optimized to allow the embedding to interact
+ * with them with a minimum of overhead: chunks aren't enqueued as individual
+ * typed array instances; instead, the embedding only updates the amount of
+ * data available using ReadableStreamUpdateDataAvailableFromSource.
+ * When content requests data by reading from a reader,
+ * WriteIntoReadRequestBufferCallback is invoked, asking the embedding to
+ * write data directly into the buffer we're about to hand to content.
+ *
+ * Additionally, ReadableStreamGetExternalUnderlyingSource can be used to
+ * get the void* pointer to the underlying source. This is equivalent to
+ * acquiring a reader for the stream in that it locks the stream until it
+ * is released again using JS::ReadableStreamReleaseExternalUnderlyingSource.
+ *
+ * Embeddings are expected to detect situations where an API exposed to JS
+ * takes a ReadableStream to read from that has an external underlying source.
+ * In those situations, it might be preferable to directly perform data
+ * transfers from the stream's underlying source to whatever sink the
+ * embedding uses, assuming that such direct transfers can be performed
+ * more efficiently.
+ */
+
+#ifndef js_Stream_h
+#define js_Stream_h
+
+#include "jstypes.h"
+
+#include "js/TypeDecls.h"
+
+namespace JS {
+
+/**
+ * Invoked whenever a reader desires more data from a ReadableStream's
+ * embedding-provided underlying source.
+ *
+ * The given |desiredSize| is the absolute size, not a delta from the previous
+ * desired size.
+ */
+typedef void
+(* RequestReadableStreamDataCallback)(JSContext* cx, HandleObject stream,
+                                      void* underlyingSource, uint8_t flags, size_t desiredSize);
+
+/**
+ * Invoked to cause the embedding to fill the given |buffer| with data from
+ * the given embedding-provided underlying source.
+ *
+ * This can only happen after the embedding has updated the amount of data
+ * available using JS::ReadableStreamUpdateDataAvailableFromSource. If at
+ * least one read request is pending when
+ * JS::ReadableStreamUpdateDataAvailableFromSource is called,
+ * the WriteIntoReadRequestBufferCallback is invoked immediately from under
+ * the call to JS::WriteIntoReadRequestBufferCallback. If not, it is invoked
+ * if and when a new read request is made.
+ *
+ * Note: This callback *must not cause GC*, because that could potentially
+ * invalidate the |buffer| pointer.
+ */
+typedef void
+(* WriteIntoReadRequestBufferCallback)(JSContext* cx, HandleObject stream,
+                                       void* underlyingSource, uint8_t flags, void* buffer,
+                                       size_t length, size_t* bytesWritten);
+
+/**
+ * Invoked in reaction to the ReadableStream being canceled to allow the
+ * embedding to free the underlying source.
+ *
+ * This is equivalent to calling |cancel| on non-external underlying sources
+ * provided to the ReadableStream constructor in JavaScript.
+ *
+ * The given |reason| is the JS::Value that was passed as an argument to
+ * ReadableStream#cancel().
+ *
+ * The returned JS::Value will be used to resolve the Promise returned by
+ * ReadableStream#cancel().
+ */
+typedef Value
+(* CancelReadableStreamCallback)(JSContext* cx, HandleObject stream,
+                                 void* underlyingSource, uint8_t flags, HandleValue reason);
+
+/**
+ * Invoked in reaction to a ReadableStream with an embedding-provided
+ * underlying source being closed.
+ */
+typedef void
+(* ReadableStreamClosedCallback)(JSContext* cx, HandleObject stream, void* underlyingSource,
+                                 uint8_t flags);
+
+/**
+ * Invoked in reaction to a ReadableStream with an embedding-provided
+ * underlying source being errored with the
+ * given reason.
+ */
+typedef void
+(* ReadableStreamErroredCallback)(JSContext* cx, HandleObject stream, void* underlyingSource,
+                                  uint8_t flags, HandleValue reason);
+
+/**
+ * Invoked in reaction to a ReadableStream with an embedding-provided
+ * underlying source being finalized. Only the underlying source is passed
+ * as an argument, while the ReadableStream itself is not to prevent the
+ * embedding from operating on a JSObject that might not be in a valid state
+ * anymore.
+ *
+ * Note: the ReadableStream might be finalized on a background thread. That
+ * means this callback might be invoked from an arbitrary thread, which the
+ * embedding must be able to handle.
+ */
+typedef void
+(* ReadableStreamFinalizeCallback)(void* underlyingSource, uint8_t flags);
+
+/**
+ * Sets runtime-wide callbacks to use for interacting with embedding-provided
+ * hooks for operating on ReadableStream instances.
+ *
+ * See the documentation for the individual callback types for details.
+ */
+extern JS_PUBLIC_API(void)
+SetReadableStreamCallbacks(JSContext* cx,
+                           RequestReadableStreamDataCallback dataRequestCallback,
+                           WriteIntoReadRequestBufferCallback writeIntoReadRequestCallback,
+                           CancelReadableStreamCallback cancelCallback,
+                           ReadableStreamClosedCallback closedCallback,
+                           ReadableStreamErroredCallback erroredCallback,
+                           ReadableStreamFinalizeCallback finalizeCallback);
+
+extern JS_PUBLIC_API(bool)
+HasReadableStreamCallbacks(JSContext* cx);
+
+/**
+ * Returns a new instance of the ReadableStream builtin class in the current
+ * compartment, configured as a default stream.
+ * If a |proto| is passed, that gets set as the instance's [[Prototype]]
+ * instead of the original value of |ReadableStream.prototype|.
+ */
+extern JS_PUBLIC_API(JSObject*)
+NewReadableDefaultStreamObject(JSContext* cx, HandleObject underlyingSource = nullptr,
+                               HandleFunction size = nullptr, double highWaterMark = 1,
+                               HandleObject proto = nullptr);
+
+/**
+ * Returns a new instance of the ReadableStream builtin class in the current
+ * compartment, configured as a byte stream.
+ * If a |proto| is passed, that gets set as the instance's [[Prototype]]
+ * instead of the original value of |ReadableStream.prototype|.
+ */
+extern JS_PUBLIC_API(JSObject*)
+NewReadableByteStreamObject(JSContext* cx, HandleObject underlyingSource = nullptr,
+                            double highWaterMark = 0, HandleObject proto = nullptr);
+
+/**
+ * Returns a new instance of the ReadableStream builtin class in the current
+ * compartment, with the right slot layout. If a |proto| is passed, that gets
+ * set as the instance's [[Prototype]] instead of the original value of
+ * |ReadableStream.prototype|.
+ *
+ * The instance is optimized for operating as a byte stream backed by an
+ * embedding-provided underlying source, using the callbacks set via
+ * |JS::SetReadableStreamCallbacks|.
+ *
+ * The given |flags| will be passed to all applicable callbacks and can be
+ * used to disambiguate between different types of stream sources the
+ * embedding might support.
+ *
+ * Note: the embedding is responsible for ensuring that the pointer to the
+ * underlying source stays valid as long as the stream can be read from.
+ * The underlying source can be freed if the tree is canceled or errored.
+ * It can also be freed if the stream is destroyed. The embedding is notified
+ * of that using ReadableStreamFinalizeCallback.
+ */
+extern JS_PUBLIC_API(JSObject*)
+NewReadableExternalSourceStreamObject(JSContext* cx, void* underlyingSource,
+                                      uint8_t flags = 0, HandleObject proto = nullptr);
+
+/**
+ * Returns the flags that were passed to NewReadableExternalSourceStreamObject
+ * when creating the given stream.
+ *
+ * Asserts that the given stream has an embedding-provided underlying source.
+ */
+extern JS_PUBLIC_API(uint8_t)
+ReadableStreamGetEmbeddingFlags(const JSObject* stream);
+
+/**
+ * Returns the embedding-provided underlying source of the given |stream|.
+ *
+ * Can be used to optimize operations if both the underlying source and the
+ * intended sink are embedding-provided. In that case it might be
+ * preferrable to pipe data directly from source to sink without interacting
+ * with the stream at all.
+ *
+ * Locks the stream until ReadableStreamReleaseExternalUnderlyingSource is
+ * called.
+ *
+ * Throws an exception if the stream is locked, i.e. if a reader has been
+ * acquired for the stream, or if ReadableStreamGetExternalUnderlyingSource
+ * has been used previously without releasing the external source again.
+ *
+ * Throws an exception if the stream isn't readable, i.e if it is errored or
+ * closed. This is different from ReadableStreamGetReader because we don't
+ * have a Promise to resolve/reject, which a reader provides.
+ *
+ * Asserts that the stream has an embedding-provided underlying source.
+ */
+extern JS_PUBLIC_API(bool)
+ReadableStreamGetExternalUnderlyingSource(JSContext* cx, HandleObject stream, void** source);
+
+/**
+ * Releases the embedding-provided underlying source of the given |stream|,
+ * returning the stream into an unlocked state.
+ *
+ * Asserts that the stream was locked through
+ * ReadableStreamGetExternalUnderlyingSource.
+ *
+ * Asserts that the stream has an embedding-provided underlying source.
+ */
+extern JS_PUBLIC_API(void)
+ReadableStreamReleaseExternalUnderlyingSource(JSObject* stream);
+
+/**
+ * Update the amount of data available at the underlying source of the given
+ * |stream|.
+ *
+ * Can only be used for streams with an embedding-provided underlying source.
+ * The JS engine will use the given value to satisfy read requests for the
+ * stream by invoking the JS::WriteIntoReadRequestBuffer callback.
+ */
+extern JS_PUBLIC_API(bool)
+ReadableStreamUpdateDataAvailableFromSource(JSContext* cx, HandleObject stream,
+                                            uint32_t availableData);
+
+/**
+ * Returns true if the given object is an unwrapped ReadableStream object,
+ * false otherwise.
+ */
+extern JS_PUBLIC_API(bool)
+IsReadableStream(const JSObject* obj);
+
+/**
+ * Returns true if the given object is an unwrapped
+ * ReadableStreamDefaultReader or ReadableStreamBYOBReader object,
+ * false otherwise.
+ */
+extern JS_PUBLIC_API(bool)
+IsReadableStreamReader(const JSObject* obj);
+
+/**
+ * Returns true if the given object is an unwrapped
+ * ReadableStreamDefaultReader object, false otherwise.
+ */
+extern JS_PUBLIC_API(bool)
+IsReadableStreamDefaultReader(const JSObject* obj);
+
+/**
+ * Returns true if the given object is an unwrapped
+ * ReadableStreamBYOBReader object, false otherwise.
+ */
+extern JS_PUBLIC_API(bool)
+IsReadableStreamBYOBReader(const JSObject* obj);
+
+enum class ReadableStreamMode {
+    Default,
+    Byte,
+    ExternalSource
+};
+
+/**
+ * Returns the stream's ReadableStreamMode. If the mode is |Byte| or
+ * |ExternalSource|, it's possible to acquire a BYOB reader for more optimized
+ * operations.
+ *
+ * Asserts that |stream| is an unwrapped ReadableStream instance.
+ */
+extern JS_PUBLIC_API(ReadableStreamMode)
+ReadableStreamGetMode(const JSObject* stream);
+
+enum class ReadableStreamReaderMode {
+    Default,
+    BYOB
+};
+
+/**
+ * Returns true if the given ReadableStream is readable, false if not.
+ *
+ * Asserts that |stream| is an unwrapped ReadableStream instance.
+ */
+extern JS_PUBLIC_API(bool)
+ReadableStreamIsReadable(const JSObject* stream);
+
+/**
+ * Returns true if the given ReadableStream is locked, false if not.
+ *
+ * Asserts that |stream| is an unwrapped ReadableStream instance.
+ */
+extern JS_PUBLIC_API(bool)
+ReadableStreamIsLocked(const JSObject* stream);
+
+/**
+ * Returns true if the given ReadableStream is disturbed, false if not.
+ *
+ * Asserts that |stream| is an ReadableStream instance.
+ */
+extern JS_PUBLIC_API(bool)
+ReadableStreamIsDisturbed(const JSObject* stream);
+
+/**
+ * Cancels the given ReadableStream with the given reason and returns a
+ * Promise resolved according to the result.
+ *
+ * Asserts that |stream| is an unwrapped ReadableStream instance.
+ */
+extern JS_PUBLIC_API(JSObject*)
+ReadableStreamCancel(JSContext* cx, HandleObject stream, HandleValue reason);
+
+/**
+ * Creates a reader of the type specified by the mode option and locks the
+ * stream to the new reader.
+ *
+ * Asserts that |stream| is an unwrapped ReadableStream instance.
+ */
+extern JS_PUBLIC_API(JSObject*)
+ReadableStreamGetReader(JSContext* cx, HandleObject stream, ReadableStreamReaderMode mode);
+
+/**
+ * Tees the given ReadableStream and stores the two resulting streams in
+ * outparams. Returns false if the operation fails, e.g. because the stream is
+ * locked.
+ *
+ * Asserts that |stream| is an unwrapped ReadableStream instance.
+ */
+extern JS_PUBLIC_API(bool)
+ReadableStreamTee(JSContext* cx, HandleObject stream,
+                  MutableHandleObject branch1Stream, MutableHandleObject branch2Stream);
+
+/**
+ * Retrieves the desired combined size of additional chunks to fill the given
+ * ReadableStream's queue. Stores the result in |value| and sets |hasValue| to
+ * true on success, returns false on failure.
+ *
+ * If the stream is errored, the call will succeed but no value will be stored
+ * in |value| and |hasValue| will be set to false.
+ *
+ * Note: This is semantically equivalent to the |desiredSize| getter on
+ * the stream controller's prototype in JS. We expose it with the stream
+ * itself as a target for simplicity.
+ *
+ * Asserts that |stream| is an unwrapped ReadableStream instance.
+ */
+extern JS_PUBLIC_API(void)
+ReadableStreamGetDesiredSize(JSObject* stream, bool* hasValue, double* value);
+
+/**
+ * Closes the given ReadableStream.
+ *
+ * Throws a TypeError and returns false if the closing operation fails.
+ *
+ * Note: This is semantically equivalent to the |close| method on
+ * the stream controller's prototype in JS. We expose it with the stream
+ * itself as a target for simplicity.
+ *
+ * Asserts that |stream| is an unwrapped ReadableStream instance.
+ */
+extern JS_PUBLIC_API(bool)
+ReadableStreamClose(JSContext* cx, HandleObject stream);
+
+/**
+ * Returns true if the given ReadableStream reader is locked, false otherwise.
+ *
+ * Asserts that |reader| is an unwrapped ReadableStreamDefaultReader or
+ * ReadableStreamBYOBReader instance.
+ */
+extern JS_PUBLIC_API(bool)
+ReadableStreamReaderIsClosed(const JSObject* reader);
+
+/**
+ * Enqueues the given chunk in the given ReadableStream.
+ *
+ * Throws a TypeError and returns false if the enqueing operation fails.
+ *
+ * Note: This is semantically equivalent to the |enqueue| method on
+ * the stream controller's prototype in JS. We expose it with the stream
+ * itself as a target for simplicity.
+ *
+ * If the ReadableStream has an underlying byte source, the given chunk must
+ * be a typed array or a DataView. Consider using
+ * ReadableByteStreamEnqueueBuffer.
+ *
+ * Asserts that |stream| is an unwrapped ReadableStream instance.
+ */
+extern JS_PUBLIC_API(bool)
+ReadableStreamEnqueue(JSContext* cx, HandleObject stream, HandleValue chunk);
+
+/**
+ * Enqueues the given buffer as a chunk in the given ReadableStream.
+ *
+ * Throws a TypeError and returns false if the enqueing operation fails.
+ *
+ * Note: This is semantically equivalent to the |enqueue| method on
+ * the stream controller's prototype in JS. We expose it with the stream
+ * itself as a target for simplicity. Additionally, the JS version only
+ * takes typed arrays and ArrayBufferView instances as arguments, whereas
+ * this takes an ArrayBuffer, obviating the need to wrap it into a typed
+ * array.
+ *
+ * Asserts that |stream| is an unwrapped ReadableStream instance and |buffer|
+ * an unwrapped ArrayBuffer instance.
+ */
+extern JS_PUBLIC_API(bool)
+ReadableByteStreamEnqueueBuffer(JSContext* cx, HandleObject stream, HandleObject buffer);
+
+/**
+ * Errors the given ReadableStream, causing all future interactions to fail
+ * with the given error value.
+ *
+ * Throws a TypeError and returns false if the erroring operation fails.
+ *
+ * Note: This is semantically equivalent to the |error| method on
+ * the stream controller's prototype in JS. We expose it with the stream
+ * itself as a target for simplicity.
+ *
+ * Asserts that |stream| is an unwrapped ReadableStream instance.
+ */
+extern JS_PUBLIC_API(bool)
+ReadableStreamError(JSContext* cx, HandleObject stream, HandleValue error);
+
+/**
+ * Cancels the given ReadableStream reader's associated stream.
+ *
+ * Throws a TypeError and returns false if the given reader isn't active.
+ *
+ * Asserts that |reader| is an unwrapped ReadableStreamDefaultReader or
+ * ReadableStreamBYOBReader instance.
+ */
+extern JS_PUBLIC_API(bool)
+ReadableStreamReaderCancel(JSContext* cx, HandleObject reader, HandleValue reason);
+
+/**
+ * Cancels the given ReadableStream reader's associated stream.
+ *
+ * Throws a TypeError and returns false if the given reader has pending
+ * read or readInto (for default or byob readers, respectively) requests.
+ *
+ * Asserts that |reader| is an unwrapped ReadableStreamDefaultReader or
+ * ReadableStreamBYOBReader instance.
+ */
+extern JS_PUBLIC_API(bool)
+ReadableStreamReaderReleaseLock(JSContext* cx, HandleObject reader);
+
+/**
+ * Requests a read from the reader's associated ReadableStream and returns the
+ * resulting PromiseObject.
+ *
+ * Returns a Promise that's resolved with the read result once available or
+ * rejected immediately if the stream is errored or the operation failed.
+ *
+ * Asserts that |reader| is an unwrapped ReadableStreamDefaultReader instance.
+ */
+extern JS_PUBLIC_API(JSObject*)
+ReadableStreamDefaultReaderRead(JSContext* cx, HandleObject reader);
+
+/**
+ * Requests a read from the reader's associated ReadableStream into the given
+ * ArrayBufferView and returns the resulting PromiseObject.
+ *
+ * Returns a Promise that's resolved with the read result once available or
+ * rejected immediately if the stream is errored or the operation failed.
+ *
+ * Asserts that |reader| is an unwrapped ReadableStreamDefaultReader and
+ * |view| an unwrapped typed array or DataView instance.
+ */
+extern JS_PUBLIC_API(JSObject*)
+ReadableStreamBYOBReaderRead(JSContext* cx, HandleObject reader, HandleObject view);
+
+} // namespace JS
+
+#endif // js_Realm_h
diff --git a/js/src/builtin/Stream.cpp b/js/src/builtin/Stream.cpp
new file mode 100644
index 0000000000..c8d8e3e324
--- /dev/null
+++ b/js/src/builtin/Stream.cpp
@@ -0,0 +1,5488 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "builtin/Stream.h"
+
+#include "js/Stream.h"
+
+#include "jscntxt.h"
+
+#include "gc/Heap.h"
+#include "vm/SelfHosting.h"
+
+#include "jsobjinlines.h"
+
+#include "vm/NativeObject-inl.h"
+
+using namespace js;
+
+enum StreamSlots {
+    StreamSlot_Controller,
+    StreamSlot_Reader,
+    StreamSlot_State,
+    StreamSlot_StoredError,
+    StreamSlotCount
+};
+
+enum ReaderSlots {
+    ReaderSlot_Stream,
+    ReaderSlot_Requests,
+    ReaderSlot_ClosedPromise,
+    ReaderSlotCount,
+};
+
+enum ReaderType {
+    ReaderType_Default,
+    ReaderType_BYOB
+};
+
+// ReadableStreamDefaultController and ReadableByteStreamController are both
+// queue containers and must have these slots at identical offsets.
+enum QueueContainerSlots {
+    QueueContainerSlot_Queue,
+    QueueContainerSlot_TotalSize,
+    QueueContainerSlotCount
+};
+
+// These slots are identical between the two types of ReadableStream
+// controllers.
+enum ControllerSlots {
+    ControllerSlot_Stream = QueueContainerSlotCount,
+    ControllerSlot_UnderlyingSource,
+    ControllerSlot_StrategyHWM,
+    ControllerSlot_Flags,
+    ControllerSlotCount
+};
+
+enum DefaultControllerSlots {
+    DefaultControllerSlot_StrategySize = ControllerSlotCount,
+    DefaultControllerSlotCount
+};
+
+enum ByteControllerSlots {
+    ByteControllerSlot_BYOBRequest = ControllerSlotCount,
+    ByteControllerSlot_PendingPullIntos,
+    ByteControllerSlot_AutoAllocateSize,
+    ByteControllerSlotCount
+};
+
+enum ControllerFlags {
+    ControllerFlag_Started        = 1 << 0,
+    ControllerFlag_Pulling        = 1 << 1,
+    ControllerFlag_PullAgain      = 1 << 2,
+    ControllerFlag_CloseRequested = 1 << 3,
+    ControllerFlag_TeeBranch      = 1 << 4,
+    ControllerFlag_TeeBranch1     = 1 << 5,
+    ControllerFlag_TeeBranch2     = 1 << 6,
+    ControllerFlag_ExternalSource = 1 << 7,
+    ControllerFlag_SourceLocked   = 1 << 8,
+};
+
+// Offset at which embedding flags are stored.
+constexpr uint8_t ControllerEmbeddingFlagsOffset = 24;
+
+enum BYOBRequestSlots {
+    BYOBRequestSlot_Controller,
+    BYOBRequestSlot_View,
+    BYOBRequestSlotCount
+};
+
+template<class T>
+MOZ_ALWAYS_INLINE bool
+Is(const HandleValue v)
+{
+    return v.isObject() && v.toObject().is<T>();
+}
+
+#ifdef DEBUG
+static bool
+IsReadableStreamController(const JSObject* controller)
+{
+    return controller->is<ReadableStreamDefaultController>() ||
+           controller->is<ReadableByteStreamController>();
+}
+#endif // DEBUG
+
+static inline uint32_t
+ControllerFlags(const NativeObject* controller)
+{
+    MOZ_ASSERT(IsReadableStreamController(controller));
+    return controller->getFixedSlot(ControllerSlot_Flags).toInt32();
+}
+
+static inline void
+AddControllerFlags(NativeObject* controller, uint32_t flags)
+{
+    MOZ_ASSERT(IsReadableStreamController(controller));
+    controller->setFixedSlot(ControllerSlot_Flags,
+                             Int32Value(ControllerFlags(controller) | flags));
+}
+
+static inline void
+RemoveControllerFlags(NativeObject* controller, uint32_t flags)
+{
+    MOZ_ASSERT(IsReadableStreamController(controller));
+    controller->setFixedSlot(ControllerSlot_Flags,
+                             Int32Value(ControllerFlags(controller) & ~flags));
+}
+
+static inline uint32_t
+StreamState(const ReadableStream* stream)
+{
+    return stream->getFixedSlot(StreamSlot_State).toInt32();
+}
+
+static inline void
+SetStreamState(ReadableStream* stream, uint32_t state)
+{
+    MOZ_ASSERT_IF(stream->disturbed(), state & ReadableStream::Disturbed);
+    MOZ_ASSERT_IF(stream->closed() || stream->errored(), !(state & ReadableStream::Readable));
+    stream->setFixedSlot(StreamSlot_State, Int32Value(state));
+}
+
+bool
+ReadableStream::readable() const
+{
+    return StreamState(this) & Readable;
+}
+
+bool
+ReadableStream::closed() const
+{
+    return StreamState(this) & Closed;
+}
+
+bool
+ReadableStream::errored() const
+{
+    return StreamState(this) & Errored;
+}
+
+bool
+ReadableStream::disturbed() const
+{
+    return StreamState(this) & Disturbed;
+}
+
+inline static bool
+ReaderHasStream(const NativeObject* reader)
+{
+    MOZ_ASSERT(JS::IsReadableStreamReader(reader));
+    return !reader->getFixedSlot(ReaderSlot_Stream).isUndefined();
+}
+
+bool
+js::ReadableStreamReaderIsClosed(const JSObject* reader)
+{
+    return !ReaderHasStream(&reader->as<NativeObject>());
+}
+
+inline static MOZ_MUST_USE ReadableStream*
+StreamFromController(const NativeObject* controller)
+{
+    MOZ_ASSERT(IsReadableStreamController(controller));
+    return &controller->getFixedSlot(ControllerSlot_Stream).toObject().as<ReadableStream>();
+}
+
+inline static MOZ_MUST_USE NativeObject*
+ControllerFromStream(const ReadableStream* stream)
+{
+    Value controllerVal = stream->getFixedSlot(StreamSlot_Controller);
+    MOZ_ASSERT(IsReadableStreamController(&controllerVal.toObject()));
+    return &controllerVal.toObject().as<NativeObject>();
+}
+
+inline static bool
+HasController(const ReadableStream* stream)
+{
+    return !stream->getFixedSlot(StreamSlot_Controller).isUndefined();
+}
+
+JS::ReadableStreamMode
+ReadableStream::mode() const
+{
+    NativeObject* controller = ControllerFromStream(this);
+    if (controller->is<ReadableStreamDefaultController>())
+        return JS::ReadableStreamMode::Default;
+    return controller->as<ReadableByteStreamController>().hasExternalSource()
+           ? JS::ReadableStreamMode::ExternalSource
+           : JS::ReadableStreamMode::Byte;
+}
+
+inline static MOZ_MUST_USE ReadableStream*
+StreamFromReader(const NativeObject* reader)
+{
+    MOZ_ASSERT(ReaderHasStream(reader));
+    return &reader->getFixedSlot(ReaderSlot_Stream).toObject().as<ReadableStream>();
+}
+
+inline static MOZ_MUST_USE NativeObject*
+ReaderFromStream(const NativeObject* stream)
+{
+    Value readerVal = stream->getFixedSlot(StreamSlot_Reader);
+    MOZ_ASSERT(JS::IsReadableStreamReader(&readerVal.toObject()));
+    return &readerVal.toObject().as<NativeObject>();
+}
+
+inline static bool
+HasReader(const ReadableStream* stream)
+{
+    return !stream->getFixedSlot(StreamSlot_Reader).isUndefined();
+}
+
+inline static MOZ_MUST_USE JSFunction*
+NewHandler(JSContext *cx, Native handler, HandleObject target)
+{
+    RootedAtom funName(cx, cx->names().empty);
+    RootedFunction handlerFun(cx, NewNativeFunction(cx, handler, 0, funName,
+                                                    gc::AllocKind::FUNCTION_EXTENDED,
+                                                    GenericObject));
+    if (!handlerFun)
+        return nullptr;
+    handlerFun->setExtendedSlot(0, ObjectValue(*target));
+    return handlerFun;
+}
+
+template<class T>
+inline static MOZ_MUST_USE T*
+TargetFromHandler(JSObject& handler)
+{
+    return &handler.as<JSFunction>().getExtendedSlot(0).toObject().as<T>();
+}
+
+inline static MOZ_MUST_USE bool
+ResetQueue(JSContext* cx, HandleNativeObject container);
+
+inline static MOZ_MUST_USE bool
+InvokeOrNoop(JSContext* cx, HandleValue O, HandlePropertyName P, HandleValue arg,
+             MutableHandleValue rval);
+
+static MOZ_MUST_USE JSObject*
+PromiseInvokeOrNoop(JSContext* cx, HandleValue O, HandlePropertyName P, HandleValue arg);
+
+static MOZ_MUST_USE JSObject*
+PromiseRejectedWithPendingError(JSContext* cx) {
+    // Not much we can do about uncatchable exceptions, just bail.
+    RootedValue exn(cx);
+    if (!GetAndClearException(cx, &exn))
+        return nullptr;
+    return PromiseObject::unforgeableReject(cx, exn);
+}
+
+static bool
+ReportArgTypeError(JSContext* cx, const char* funName, const char* expectedType,
+                   HandleValue arg)
+{
+    UniqueChars bytes = DecompileValueGenerator(cx, JSDVG_SEARCH_STACK, arg, nullptr);
+    if (!bytes)
+        return false;
+
+    return JS_ReportErrorFlagsAndNumberLatin1(cx, JSREPORT_ERROR, GetErrorMessage,
+                                              nullptr, JSMSG_NOT_EXPECTED_TYPE,
+                                              funName, expectedType, bytes.get());
+}
+
+static MOZ_MUST_USE bool
+ReturnPromiseRejectedWithPendingError(JSContext* cx, const CallArgs& args)
+{
+    JSObject* promise = PromiseRejectedWithPendingError(cx);
+    if (!promise)
+        return false;
+
+    args.rval().setObject(*promise);
+    return true;
+}
+
+static MOZ_MUST_USE bool
+RejectNonGenericMethod(JSContext* cx, const CallArgs& args,
+                       const char* className, const char* methodName)
+{
+    ReportValueError3(cx, JSMSG_INCOMPATIBLE_PROTO, JSDVG_SEARCH_STACK, args.thisv(),
+                      nullptr, className, methodName);
+
+    return ReturnPromiseRejectedWithPendingError(cx, args);
+}
+
+inline static MOZ_MUST_USE NativeObject*
+SetNewList(JSContext* cx, HandleNativeObject container, uint32_t slot)
+{
+    NativeObject* list = NewObjectWithNullTaggedProto<PlainObject>(cx);
+    if (!list)
+        return nullptr;
+    container->setFixedSlot(slot, ObjectValue(*list));
+    return list;
+}
+
+inline static MOZ_MUST_USE bool
+AppendToList(JSContext* cx, HandleNativeObject list, HandleValue value)
+{
+    uint32_t length = list->getDenseInitializedLength();
+
+    if (!list->ensureElements(cx, length + 1))
+        return false;
+
+    list->ensureDenseInitializedLength(cx, length, 1);
+    list->setDenseElement(length, value);
+
+    return true;
+}
+
+template<class T>
+inline static MOZ_MUST_USE T*
+PeekList(NativeObject* list)
+{
+    MOZ_ASSERT(list->getDenseInitializedLength() > 0);
+    return &list->getDenseElement(0).toObject().as<T>();
+}
+
+template<class T>
+inline static MOZ_MUST_USE T*
+ShiftFromList(JSContext* cx, HandleNativeObject list)
+{
+    uint32_t length = list->getDenseInitializedLength();
+    MOZ_ASSERT(length > 0);
+
+    Rooted<T*> entry(cx, &list->getDenseElement(0).toObject().as<T>());
+    list->moveDenseElements(0, 1, length - 1);
+    list->shrinkElements(cx, length - 1);
+
+    list->setDenseInitializedLength(length - 1);
+
+    return entry;
+}
+
+class ByteStreamChunk : public NativeObject
+{
+  private:
+    enum Slots {
+        Slot_Buffer = 0,
+        Slot_ByteOffset,
+        Slot_ByteLength,
+        SlotCount
+    };
+
+  public:
+    static const Class class_;
+
+    ArrayBufferObject* buffer() {
+        return &getFixedSlot(Slot_Buffer).toObject().as<ArrayBufferObject>();
+    }
+    uint32_t byteOffset() { return getFixedSlot(Slot_ByteOffset).toInt32(); }
+    void SetByteOffset(uint32_t offset) {
+        setFixedSlot(Slot_ByteOffset, Int32Value(offset));
+    }
+    uint32_t byteLength() { return getFixedSlot(Slot_ByteLength).toInt32(); }
+    void SetByteLength(uint32_t length) {
+        setFixedSlot(Slot_ByteLength, Int32Value(length));
+    }
+
+    static ByteStreamChunk* create(JSContext* cx, HandleObject buffer, uint32_t byteOffset,
+                                   uint32_t byteLength)
+   {
+        Rooted<ByteStreamChunk*> chunk(cx, NewObjectWithClassProto<ByteStreamChunk>(cx));
+        if (!chunk)
+            return nullptr;
+
+        chunk->setFixedSlot(Slot_Buffer, ObjectValue(*buffer));
+        chunk->setFixedSlot(Slot_ByteOffset, Int32Value(byteOffset));
+        chunk->setFixedSlot(Slot_ByteLength, Int32Value(byteLength));
+        return chunk;
+   }
+};
+
+const Class ByteStreamChunk::class_ = {
+    "ByteStreamChunk",
+    JSCLASS_HAS_RESERVED_SLOTS(SlotCount)
+};
+
+class PullIntoDescriptor : public NativeObject
+{
+  private:
+    enum Slots {
+        Slot_buffer,
+        Slot_ByteOffset,
+        Slot_ByteLength,
+        Slot_BytesFilled,
+        Slot_ElementSize,
+        Slot_Ctor,
+        Slot_ReaderType,
+        SlotCount
+    };
+  public:
+    static const Class class_;
+
+    ArrayBufferObject* buffer() {
+        return &getFixedSlot(Slot_buffer).toObject().as<ArrayBufferObject>();
+    }
+    void setBuffer(ArrayBufferObject* buffer) { setFixedSlot(Slot_buffer, ObjectValue(*buffer)); }
+    JSObject* ctor() { return getFixedSlot(Slot_Ctor).toObjectOrNull(); }
+    uint32_t byteOffset() const { return getFixedSlot(Slot_ByteOffset).toInt32(); }
+    uint32_t byteLength() const { return getFixedSlot(Slot_ByteLength).toInt32(); }
+    uint32_t bytesFilled() const { return getFixedSlot(Slot_BytesFilled).toInt32(); }
+    void setBytesFilled(int32_t bytes) { setFixedSlot(Slot_BytesFilled, Int32Value(bytes)); }
+    uint32_t elementSize() const { return getFixedSlot(Slot_ElementSize).toInt32(); }
+    uint32_t readerType() const { return getFixedSlot(Slot_ReaderType).toInt32(); }
+
+    static PullIntoDescriptor* create(JSContext* cx, HandleArrayBufferObject buffer,
+                                      uint32_t byteOffset, uint32_t byteLength,
+                                      uint32_t bytesFilled, uint32_t elementSize,
+                                      HandleObject ctor, uint32_t readerType)
+   {
+        Rooted<PullIntoDescriptor*> descriptor(cx, NewObjectWithClassProto<PullIntoDescriptor>(cx));
+        if (!descriptor)
+            return nullptr;
+
+        descriptor->setFixedSlot(Slot_buffer, ObjectValue(*buffer));
+        descriptor->setFixedSlot(Slot_Ctor, ObjectOrNullValue(ctor));
+        descriptor->setFixedSlot(Slot_ByteOffset, Int32Value(byteOffset));
+        descriptor->setFixedSlot(Slot_ByteLength, Int32Value(byteLength));
+        descriptor->setFixedSlot(Slot_BytesFilled, Int32Value(bytesFilled));
+        descriptor->setFixedSlot(Slot_ElementSize, Int32Value(elementSize));
+        descriptor->setFixedSlot(Slot_ReaderType, Int32Value(readerType));
+        return descriptor;
+   }
+};
+
+const Class PullIntoDescriptor::class_ = {
+    "PullIntoDescriptor",
+    JSCLASS_HAS_RESERVED_SLOTS(SlotCount)
+};
+
+class QueueEntry : public NativeObject
+{
+  private:
+    enum Slots {
+        Slot_Value = 0,
+        Slot_Size,
+        SlotCount
+    };
+
+  public:
+    static const Class class_;
+
+    Value value() { return getFixedSlot(Slot_Value); }
+    double size() { return getFixedSlot(Slot_Size).toNumber(); }
+
+    static QueueEntry* create(JSContext* cx, HandleValue value, double size)
+   {
+        Rooted<QueueEntry*> entry(cx, NewObjectWithClassProto<QueueEntry>(cx));
+        if (!entry)
+            return nullptr;
+
+        entry->setFixedSlot(Slot_Value, value);
+        entry->setFixedSlot(Slot_Size, NumberValue(size));
+
+        return entry;
+   }
+};
+
+const Class QueueEntry::class_ = {
+    "QueueEntry",
+    JSCLASS_HAS_RESERVED_SLOTS(SlotCount)
+};
+
+class TeeState : public NativeObject
+{
+  private:
+    enum Slots {
+        Slot_Flags = 0,
+        Slot_Reason1,
+        Slot_Reason2,
+        Slot_Promise,
+        Slot_Stream,
+        Slot_Branch1,
+        Slot_Branch2,
+        SlotCount
+    };
+
+    enum Flags
+    {
+        Flag_ClosedOrErrored = 1 << 0,
+        Flag_Canceled1 =       1 << 1,
+        Flag_Canceled2 =       1 << 2,
+        Flag_CloneForBranch2 = 1 << 3,
+    };
+    uint32_t flags() const { return getFixedSlot(Slot_Flags).toInt32(); }
+    void setFlags(uint32_t flags) { setFixedSlot(Slot_Flags, Int32Value(flags)); }
+
+  public:
+    static const Class class_;
+
+    bool cloneForBranch2() const { return flags() & Flag_CloneForBranch2; }
+
+    bool closedOrErrored() const { return flags() & Flag_ClosedOrErrored; }
+    void setClosedOrErrored() {
+        MOZ_ASSERT(!(flags() & Flag_ClosedOrErrored));
+        setFlags(flags() | Flag_ClosedOrErrored);
+    }
+
+    bool canceled1() const { return flags() & Flag_Canceled1; }
+    void setCanceled1(HandleValue reason) {
+        MOZ_ASSERT(!(flags() & Flag_Canceled1));
+        setFlags(flags() | Flag_Canceled1);
+        setFixedSlot(Slot_Reason1, reason);
+    }
+
+    bool canceled2() const { return flags() & Flag_Canceled2; }
+    void setCanceled2(HandleValue reason) {
+        MOZ_ASSERT(!(flags() & Flag_Canceled2));
+        setFlags(flags() | Flag_Canceled2);
+        setFixedSlot(Slot_Reason2, reason);
+    }
+
+    Value reason1() const {
+        MOZ_ASSERT(canceled1());
+        return getFixedSlot(Slot_Reason1);
+    }
+
+    Value reason2() const {
+        MOZ_ASSERT(canceled2());
+        return getFixedSlot(Slot_Reason2);
+    }
+
+    PromiseObject* promise() {
+        return &getFixedSlot(Slot_Promise).toObject().as<PromiseObject>();
+    }
+    ReadableStream* stream() {
+        return &getFixedSlot(Slot_Stream).toObject().as<ReadableStream>();
+    }
+    ReadableStreamDefaultReader* reader() {
+        return &ReaderFromStream(stream())->as<ReadableStreamDefaultReader>();
+    }
+
+    ReadableStreamDefaultController* branch1() {
+        ReadableStreamDefaultController* controller = &getFixedSlot(Slot_Branch1).toObject()
+                                                       .as<ReadableStreamDefaultController>();
+        MOZ_ASSERT(ControllerFlags(controller) & ControllerFlag_TeeBranch);
+        MOZ_ASSERT(ControllerFlags(controller) & ControllerFlag_TeeBranch1);
+        return controller;
+    }
+    void setBranch1(ReadableStreamDefaultController* controller) {
+        MOZ_ASSERT(ControllerFlags(controller) & ControllerFlag_TeeBranch);
+        MOZ_ASSERT(ControllerFlags(controller) & ControllerFlag_TeeBranch1);
+        setFixedSlot(Slot_Branch1, ObjectValue(*controller));
+    }
+
+    ReadableStreamDefaultController* branch2() {
+        ReadableStreamDefaultController* controller = &getFixedSlot(Slot_Branch2).toObject()
+                                                       .as<ReadableStreamDefaultController>();
+        MOZ_ASSERT(ControllerFlags(controller) & ControllerFlag_TeeBranch);
+        MOZ_ASSERT(ControllerFlags(controller) & ControllerFlag_TeeBranch2);
+        return controller;
+    }
+    void setBranch2(ReadableStreamDefaultController* controller) {
+        MOZ_ASSERT(ControllerFlags(controller) & ControllerFlag_TeeBranch);
+        MOZ_ASSERT(ControllerFlags(controller) & ControllerFlag_TeeBranch2);
+        setFixedSlot(Slot_Branch2, ObjectValue(*controller));
+    }
+
+    static TeeState* create(JSContext* cx, Handle<ReadableStream*> stream) {
+        Rooted<TeeState*> state(cx, NewObjectWithClassProto<TeeState>(cx));
+        if (!state)
+            return nullptr;
+
+        Rooted<PromiseObject*> promise(cx, PromiseObject::createSkippingExecutor(cx));
+        if (!promise)
+            return nullptr;
+
+        state->setFixedSlot(Slot_Flags, Int32Value(0));
+        state->setFixedSlot(Slot_Promise, ObjectValue(*promise));
+        state->setFixedSlot(Slot_Stream, ObjectValue(*stream));
+
+        return state;
+   }
+};
+
+const Class TeeState::class_ = {
+    "TeeState",
+    JSCLASS_HAS_RESERVED_SLOTS(SlotCount)
+};
+
+#define CLASS_SPEC(cls, nCtorArgs, nSlots, specFlags, classFlags, classOps) \
+const ClassSpec cls::classSpec_ = { \
+    GenericCreateConstructor<cls::constructor, nCtorArgs, gc::AllocKind::FUNCTION>, \
+    GenericCreatePrototype, \
+    nullptr, \
+    nullptr, \
+    cls##_methods, \
+    cls##_properties, \
+    nullptr, \
+    specFlags \
+}; \
+\
+const Class cls::class_ = { \
+    #cls, \
+    JSCLASS_HAS_RESERVED_SLOTS(nSlots) | \
+    JSCLASS_HAS_CACHED_PROTO(JSProto_##cls) | \
+    classFlags, \
+    classOps, \
+    &cls::classSpec_ \
+}; \
+\
+const Class cls::protoClass_ = { \
+    "object", \
+    JSCLASS_HAS_CACHED_PROTO(JSProto_##cls), \
+    JS_NULL_CLASS_OPS, \
+    &cls::classSpec_ \
+};
+
+// Streams spec, 3.2.3., steps 1-4.
+ReadableStream*
+ReadableStream::createStream(JSContext* cx, HandleObject proto /* = nullptr */)
+{
+    Rooted<ReadableStream*> stream(cx, NewObjectWithClassProto<ReadableStream>(cx, proto));
+    if (!stream)
+        return nullptr;
+
+    // Step 1: Set this.[[state]] to "readable".
+    // Step 2: Set this.[[reader]] and this.[[storedError]] to undefined (implicit).
+    // Step 3: Set this.[[disturbed]] to false (implicit).
+    // Step 4: Set this.[[readableStreamController]] to undefined (implicit).
+    stream->setFixedSlot(StreamSlot_State, Int32Value(Readable));
+
+    return stream;
+}
+
+static MOZ_MUST_USE ReadableStreamDefaultController*
+CreateReadableStreamDefaultController(JSContext* cx, Handle<ReadableStream*> stream,
+                                      HandleValue underlyingSource, HandleValue size,
+                                      HandleValue highWaterMarkVal);
+
+// Streams spec, 3.2.3., steps 1-4, 8.
+ReadableStream*
+ReadableStream::createDefaultStream(JSContext* cx, HandleValue underlyingSource,
+                                    HandleValue size, HandleValue highWaterMark,
+                                    HandleObject proto /* = nullptr */)
+{
+    // Steps 1-4.
+    Rooted<ReadableStream*> stream(cx, createStream(cx));
+    if (!stream)
+        return nullptr;
+
+    // Step 8.b: Set this.[[readableStreamController]] to
+    //           ? Construct(ReadableStreamDefaultController,
+    //                       « this, underlyingSource, size,
+    //                         highWaterMark »).
+    RootedObject controller(cx, CreateReadableStreamDefaultController(cx, stream,
+                                                                      underlyingSource,
+                                                                      size,
+                                                                      highWaterMark));
+    if (!controller)
+        return nullptr;
+
+    stream->setFixedSlot(StreamSlot_Controller, ObjectValue(*controller));
+
+    return stream;
+}
+
+static MOZ_MUST_USE ReadableByteStreamController*
+CreateReadableByteStreamController(JSContext* cx, Handle<ReadableStream*> stream,
+                                   HandleValue underlyingByteSource,
+                                   HandleValue highWaterMarkVal);
+
+// Streams spec, 3.2.3., steps 1-4, 7.
+ReadableStream*
+ReadableStream::createByteStream(JSContext* cx, HandleValue underlyingSource,
+                                 HandleValue highWaterMark, HandleObject proto /* = nullptr */)
+{
+    // Steps 1-4.
+    Rooted<ReadableStream*> stream(cx, createStream(cx, proto));
+    if (!stream)
+        return nullptr;
+
+    // Step 7.b: Set this.[[readableStreamController]] to
+    //           ? Construct(ReadableByteStreamController,
+    //                       « this, underlyingSource, highWaterMark »).
+    RootedObject controller(cx, CreateReadableByteStreamController(cx, stream,
+                                                                   underlyingSource,
+                                                                   highWaterMark));
+    if (!controller)
+        return nullptr;
+
+    stream->setFixedSlot(StreamSlot_Controller, ObjectValue(*controller));
+
+    return stream;
+}
+
+static MOZ_MUST_USE ReadableByteStreamController*
+CreateReadableByteStreamController(JSContext* cx, Handle<ReadableStream*> stream,
+                                   void* underlyingSource);
+
+ReadableStream*
+ReadableStream::createExternalSourceStream(JSContext* cx, void* underlyingSource,
+                                           uint8_t flags, HandleObject proto /* = nullptr */)
+{
+    Rooted<ReadableStream*> stream(cx, createStream(cx, proto));
+    if (!stream)
+        return nullptr;
+
+    RootedNativeObject controller(cx, CreateReadableByteStreamController(cx, stream,
+                                                                         underlyingSource));
+    if (!controller)
+        return nullptr;
+
+    stream->setFixedSlot(StreamSlot_Controller, ObjectValue(*controller));
+    AddControllerFlags(controller, flags << ControllerEmbeddingFlagsOffset);
+
+    return stream;
+}
+
+// Streams spec, 3.2.3.
+bool
+ReadableStream::constructor(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    RootedValue val(cx, args.get(0));
+    RootedValue underlyingSource(cx, args.get(0));
+    RootedValue options(cx, args.get(1));
+
+    // Do argument handling first to keep the right order of error reporting.
+    if (underlyingSource.isUndefined()) {
+        RootedObject sourceObj(cx, NewBuiltinClassInstance<PlainObject>(cx));
+        if (!sourceObj)
+            return false;
+        underlyingSource = ObjectValue(*sourceObj);
+    }
+    RootedValue size(cx);
+    RootedValue highWaterMark(cx);
+
+    if (!options.isUndefined()) {
+        if (!GetProperty(cx, options, cx->names().size, &size))
+            return false;
+
+        if (!GetProperty(cx, options, cx->names().highWaterMark, &highWaterMark))
+            return false;
+    }
+
+    if (!ThrowIfNotConstructing(cx, args, "ReadableStream"))
+        return false;
+
+    // Step 5: Let type be ? GetV(underlyingSource, "type").
+    RootedValue typeVal(cx);
+    if (!GetProperty(cx, underlyingSource, cx->names().type, &typeVal))
+        return false;
+
+    // Step 6: Let typeString be ? ToString(type).
+    RootedString type(cx, ToString<CanGC>(cx, typeVal));
+    if (!type)
+        return false;
+
+    int32_t notByteStream;
+    if (!CompareStrings(cx, type, cx->names().bytes, &notByteStream))
+        return false;
+
+    // Step 7.a & 8.a (reordered): If highWaterMark is undefined, let
+    //                             highWaterMark be 1 (or 0 for byte streams).
+    if (highWaterMark.isUndefined())
+        highWaterMark = Int32Value(notByteStream ? 1 : 0);
+
+    Rooted<ReadableStream*> stream(cx);
+
+    // Step 7: If typeString is "bytes",
+    if (!notByteStream) {
+        // Step 7.b: Set this.[[readableStreamController]] to
+        //           ? Construct(ReadableByteStreamController,
+        //                       « this, underlyingSource, highWaterMark »).
+        stream = createByteStream(cx, underlyingSource, highWaterMark);
+    } else if (typeVal.isUndefined()) {
+        // Step 8: Otherwise, if type is undefined,
+        // Step 8.b: Set this.[[readableStreamController]] to
+        //           ? Construct(ReadableStreamDefaultController,
+        //                       « this, underlyingSource, size, highWaterMark »).
+        stream = createDefaultStream(cx, underlyingSource, size, highWaterMark);
+    } else {
+        // Step 9: Otherwise, throw a RangeError exception.
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAM_UNDERLYINGSOURCE_TYPE_WRONG);
+        return false;
+    }
+    if (!stream)
+        return false;
+
+    args.rval().setObject(*stream);
+    return true;
+}
+
+// Streams spec, 3.2.4.1. get locked
+static MOZ_MUST_USE bool
+ReadableStream_locked_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableStream*> stream(cx, &args.thisv().toObject().as<ReadableStream>());
+
+    // Step 2: Return ! IsReadableStreamLocked(this).
+    args.rval().setBoolean(stream->locked());
+    return true;
+}
+
+static bool
+ReadableStream_locked(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableStream(this) is false, throw a TypeError exception.
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableStream>, ReadableStream_locked_impl>(cx, args);
+}
+
+// Streams spec, 3.2.4.2. cancel ( reason )
+static MOZ_MUST_USE bool
+ReadableStream_cancel(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    // Step 1: If ! IsReadableStream(this) is false, return a promise rejected
+    //         with a TypeError exception.
+    if (!Is<ReadableStream>(args.thisv())) {
+        ReportValueError3(cx, JSMSG_INCOMPATIBLE_PROTO, JSDVG_SEARCH_STACK, args.thisv(),
+                          nullptr, "cancel", "");
+        return ReturnPromiseRejectedWithPendingError(cx, args);
+    }
+
+    Rooted<ReadableStream*> stream(cx, &args.thisv().toObject().as<ReadableStream>());
+
+    // Step 2: If ! IsReadableStreamLocked(this) is true, return a promise
+    //         rejected with a TypeError exception.
+    if (stream->locked()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAM_NOT_LOCKED, "cancel");
+        return ReturnPromiseRejectedWithPendingError(cx, args);
+    }
+
+    // Step 3: Return ! ReadableStreamCancel(this, reason).
+    RootedObject cancelPromise(cx, ReadableStream::cancel(cx, stream, args.get(0)));
+    if (!cancelPromise)
+        return false;
+    args.rval().setObject(*cancelPromise);
+    return true;
+}
+
+static MOZ_MUST_USE ReadableStreamDefaultReader*
+CreateReadableStreamDefaultReader(JSContext* cx, Handle<ReadableStream*> stream);
+
+static MOZ_MUST_USE ReadableStreamBYOBReader*
+CreateReadableStreamBYOBReader(JSContext* cx, Handle<ReadableStream*> stream);
+
+// Streams spec, 3.2.4.3. getReader()
+static MOZ_MUST_USE bool
+ReadableStream_getReader_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableStream*> stream(cx, &args.thisv().toObject().as<ReadableStream>());
+    RootedObject reader(cx);
+
+    // Step 2: If mode is undefined, return
+    //         ? AcquireReadableStreamDefaultReader(this).
+    RootedValue modeVal(cx);
+    HandleValue optionsVal = args.get(0);
+    if (!optionsVal.isUndefined()) {
+        if (!GetProperty(cx, optionsVal, cx->names().mode, &modeVal))
+            return false;
+    }
+
+    if (modeVal.isUndefined()) {
+        reader = CreateReadableStreamDefaultReader(cx, stream);
+    } else {
+        // Step 3: Set mode to ? ToString(mode) (implicit).
+        RootedString mode(cx, ToString<CanGC>(cx, modeVal));
+        if (!mode)
+            return false;
+
+        // Step 4: If mode is "byob", return ? AcquireReadableStreamBYOBReader(this).
+        int32_t notByob;
+        if (!CompareStrings(cx, mode, cx->names().byob, &notByob))
+            return false;
+        if (notByob) {
+            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                      JSMSG_READABLESTREAM_INVALID_READER_MODE);
+            // Step 5: Throw a RangeError exception.
+            return false;
+
+        }
+        reader = CreateReadableStreamBYOBReader(cx, stream);
+    }
+
+    // Reordered second part of steps 2 and 4.
+    if (!reader)
+        return false;
+    args.rval().setObject(*reader);
+    return true;
+}
+
+static bool
+ReadableStream_getReader(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableStream(this) is false, throw a TypeError exception.
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableStream>, ReadableStream_getReader_impl>(cx, args);
+}
+
+// Streams spec, 3.2.4.4. pipeThrough({ writable, readable }, options)
+static MOZ_MUST_USE bool
+ReadableStream_pipeThrough(JSContext* cx, unsigned argc, Value* vp)
+{
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_READABLESTREAM_METHOD_NOT_IMPLEMENTED, "pipeThrough");
+    return false;
+    // // Step 1: Perform ? Invoke(this, "pipeTo", « writable, options »).
+
+    // // Step 2: Return readable.
+    // return readable;
+}
+
+// Streams spec, 3.2.4.5. pipeTo(dest, { preventClose, preventAbort, preventCancel } = {})
+// TODO: Unimplemented since spec is not complete yet.
+static MOZ_MUST_USE bool
+ReadableStream_pipeTo(JSContext* cx, unsigned argc, Value* vp)
+{
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_READABLESTREAM_METHOD_NOT_IMPLEMENTED, "pipeTo");
+    return false;
+}
+
+static MOZ_MUST_USE bool
+ReadableStreamTee(JSContext* cx, Handle<ReadableStream*> stream, bool cloneForBranch2,
+                  MutableHandle<ReadableStream*> branch1, MutableHandle<ReadableStream*> branch2);
+
+// Streams spec, 3.2.4.6. tee()
+static MOZ_MUST_USE bool
+ReadableStream_tee_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableStream*> stream(cx, &args.thisv().toObject().as<ReadableStream>());
+
+    // Step 2: Let branches be ? ReadableStreamTee(this, false).
+    Rooted<ReadableStream*> branch1(cx);
+    Rooted<ReadableStream*> branch2(cx);
+    if (!ReadableStreamTee(cx, stream, false, &branch1, &branch2))
+        return false;
+
+    // Step 3: Return ! CreateArrayFromList(branches).
+    RootedNativeObject branches(cx, NewDenseFullyAllocatedArray(cx, 2));
+    if (!branches)
+        return false;
+    branches->setDenseInitializedLength(2);
+    branches->initDenseElement(0, ObjectValue(*branch1));
+    branches->initDenseElement(1, ObjectValue(*branch2));
+
+    args.rval().setObject(*branches);
+    return true;
+}
+
+static bool
+ReadableStream_tee(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableStream(this) is false, throw a TypeError exception
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableStream>, ReadableStream_tee_impl>(cx, args);
+}
+
+static const JSFunctionSpec ReadableStream_methods[] = {
+    JS_FN("cancel",         ReadableStream_cancel,      1, 0),
+    JS_FN("getReader",      ReadableStream_getReader,   0, 0),
+    JS_FN("pipeThrough",    ReadableStream_pipeThrough, 2, 0),
+    JS_FN("pipeTo",         ReadableStream_pipeTo,      1, 0),
+    JS_FN("tee",            ReadableStream_tee,         0, 0),
+    JS_FS_END
+};
+
+static const JSPropertySpec ReadableStream_properties[] = {
+    JS_PSG("locked", ReadableStream_locked, 0),
+    JS_PS_END
+};
+
+CLASS_SPEC(ReadableStream, 0, StreamSlotCount, 0, 0, JS_NULL_CLASS_OPS);
+
+// Streams spec, 3.3.1. AcquireReadableStreamBYOBReader ( stream )
+// Always inlined.
+
+// Streams spec, 3.3.2. AcquireReadableStreamDefaultReader ( stream )
+// Always inlined.
+
+// Streams spec, 3.3.3. IsReadableStream ( x )
+// Using is<T> instead.
+
+// Streams spec, 3.3.4. IsReadableStreamDisturbed ( stream )
+// Using stream->disturbed() instead.
+
+// Streams spec, 3.3.5. IsReadableStreamLocked ( stream )
+bool
+ReadableStream::locked() const
+{
+    // Step 1: Assert: ! IsReadableStream(stream) is true (implicit).
+    // Step 2: If stream.[[reader]] is undefined, return false.
+    // Step 3: Return true.
+    // Special-casing for streams with external sources. Those can be locked
+    // explicitly via JSAPI, which is indicated by a controller flag.
+    // IsReadableStreamLocked is called from the controller's constructor, at
+    // which point we can't yet call ControllerFromStream(stream), but the
+    // source also can't be locked yet.
+    if (HasController(this) &&
+        (ControllerFlags(ControllerFromStream(this)) & ControllerFlag_SourceLocked))
+    {
+        return true;
+    }
+    return HasReader(this);
+}
+
+static MOZ_MUST_USE bool
+ReadableStreamDefaultControllerClose(JSContext* cx,
+                                     Handle<ReadableStreamDefaultController*> controller);
+static MOZ_MUST_USE bool
+ReadableStreamDefaultControllerEnqueue(JSContext* cx,
+                                       Handle<ReadableStreamDefaultController*> controller,
+                                       HandleValue chunk);
+
+static bool
+TeeReaderReadHandler(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    Rooted<TeeState*> teeState(cx, TargetFromHandler<TeeState>(args.callee()));
+    HandleValue resultVal = args.get(0);
+
+    // Step a: Assert: Type(result) is Object.
+    RootedObject result(cx, &resultVal.toObject());
+
+    // Step b: Let value be ? Get(result, "value").
+    RootedValue value(cx);
+    if (!GetPropertyPure(cx, result, NameToId(cx->names().value), value.address()))
+        return false;
+
+    // Step c: Let done be ? Get(result, "done").
+    RootedValue doneVal(cx);
+    if (!GetPropertyPure(cx, result, NameToId(cx->names().done), doneVal.address()))
+        return false;
+
+    // Step d: Assert: Type(done) is Boolean.
+    bool done = doneVal.toBoolean();
+
+    // Step e: If done is true and teeState.[[closedOrErrored]] is false,
+    if (done && !teeState->closedOrErrored()) {
+        // Step i: If teeState.[[canceled1]] is false,
+        if (!teeState->canceled1()) {
+            // Step 1: Perform ! ReadableStreamDefaultControllerClose(branch1).
+            Rooted<ReadableStreamDefaultController*> branch1(cx, teeState->branch1());
+            if (!ReadableStreamDefaultControllerClose(cx, branch1))
+                return false;
+        }
+
+        // Step ii: If teeState.[[canceled2]] is false,
+        if (!teeState->canceled2()) {
+            // Step 1: Perform ! ReadableStreamDefaultControllerClose(branch1).
+            Rooted<ReadableStreamDefaultController*> branch2(cx, teeState->branch2());
+            if (!ReadableStreamDefaultControllerClose(cx, branch2))
+                return false;
+        }
+
+        // Step iii: Set teeState.[[closedOrErrored]] to true.
+        teeState->setClosedOrErrored();
+    }
+
+    // Step f: If teeState.[[closedOrErrored]] is true, return.
+    if (teeState->closedOrErrored())
+        return true;
+
+    // Step g: Let value1 and value2 be value.
+    RootedValue value1(cx, value);
+    RootedValue value2(cx, value);
+
+    // Step h: If teeState.[[canceled2]] is false and cloneForBranch2 is
+    //         true, set value2 to
+    //         ? StructuredDeserialize(StructuredSerialize(value2),
+    //                                 the current Realm Record).
+    // TODO: add StructuredClone() intrinsic.
+    MOZ_ASSERT(!teeState->cloneForBranch2(), "tee(cloneForBranch2=true) should not be exposed");
+
+    // Step i: If teeState.[[canceled1]] is false, perform
+    //         ? ReadableStreamDefaultControllerEnqueue(branch1, value1).
+    Rooted<ReadableStreamDefaultController*> controller(cx);
+    if (!teeState->canceled1()) {
+        controller = teeState->branch1();
+        if (!ReadableStreamDefaultControllerEnqueue(cx, controller, value1))
+            return false;
+    }
+
+    // Step j: If teeState.[[canceled2]] is false,
+    //         perform ? ReadableStreamDefaultControllerEnqueue(branch2, value2).
+    if (!teeState->canceled2()) {
+        controller = teeState->branch2();
+        if (!ReadableStreamDefaultControllerEnqueue(cx, controller, value2))
+            return false;
+    }
+
+    args.rval().setUndefined();
+    return true;
+}
+
+static MOZ_MUST_USE JSObject*
+ReadableStreamTee_Pull(JSContext* cx, Handle<TeeState*> teeState,
+                       Handle<ReadableStream*> branchStream)
+{
+    // Step 1: Let reader be F.[[reader]], branch1 be F.[[branch1]],
+    //         branch2 be F.[[branch2]], teeState be F.[[teeState]], and
+    //         cloneForBranch2 be F.[[cloneForBranch2]].
+
+    // Step 2: Return the result of transforming
+    //         ! ReadableStreamDefaultReaderRead(reader) by a fulfillment
+    //         handler which takes the argument result and performs the
+    //         following steps:
+    Rooted<ReadableStreamDefaultReader*> reader(cx, teeState->reader());
+    RootedObject readPromise(cx, ReadableStreamDefaultReader::read(cx, reader));
+    if (!readPromise)
+        return nullptr;
+
+    RootedObject onFulfilled(cx, NewHandler(cx, TeeReaderReadHandler, teeState));
+    if (!onFulfilled)
+        return nullptr;
+
+    return JS::CallOriginalPromiseThen(cx, readPromise, onFulfilled, nullptr);
+}
+
+static MOZ_MUST_USE JSObject*
+ReadableStreamTee_Cancel(JSContext* cx, Handle<TeeState*> teeState,
+                         Handle<ReadableStreamDefaultController*> branch, HandleValue reason)
+{
+    // Step 1: Let stream be F.[[stream]] and teeState be F.[[teeState]].
+    Rooted<ReadableStream*> stream(cx, teeState->stream());
+
+    bool bothBranchesCanceled = false;
+
+    // Step 2: Set teeState.[[canceled1]] to true.
+    // Step 3: Set teeState.[[reason1]] to reason.
+    if (ControllerFlags(branch) & ControllerFlag_TeeBranch1) {
+        teeState->setCanceled1(reason);
+        bothBranchesCanceled = teeState->canceled2();
+    } else {
+        MOZ_ASSERT(ControllerFlags(branch) & ControllerFlag_TeeBranch2);
+        teeState->setCanceled2(reason);
+        bothBranchesCanceled = teeState->canceled1();
+    }
+
+    // Step 4: If teeState.[[canceled1]] is true,
+    // Step 4: If teeState.[[canceled2]] is true,
+    if (bothBranchesCanceled) {
+        // Step a: Let compositeReason be
+        //         ! CreateArrayFromList(« teeState.[[reason1]], teeState.[[reason2]] »).
+        RootedNativeObject compositeReason(cx, NewDenseFullyAllocatedArray(cx, 2));
+        if (!compositeReason)
+            return nullptr;
+
+        compositeReason->setDenseInitializedLength(2);
+        compositeReason->initDenseElement(0, teeState->reason1());
+        compositeReason->initDenseElement(1, teeState->reason2());
+        RootedValue compositeReasonVal(cx, ObjectValue(*compositeReason));
+
+        Rooted<PromiseObject*> promise(cx, teeState->promise());
+
+        // Step b: Let cancelResult be ! ReadableStreamCancel(stream, compositeReason).
+        RootedObject cancelResult(cx, ReadableStream::cancel(cx, stream, compositeReasonVal));
+        if (!cancelResult) {
+            if (!RejectPromiseWithPendingError(cx, promise))
+                return nullptr;
+        } else {
+            // Step c: Resolve teeState.[[promise]] with cancelResult.
+            RootedValue resultVal(cx, ObjectValue(*cancelResult));
+            if (!PromiseObject::resolve(cx, promise, resultVal))
+                return nullptr;
+        }
+    }
+
+    // Step 5: Return teeState.[[promise]].
+    return teeState->promise();
+}
+
+static MOZ_MUST_USE bool
+ReadableStreamControllerError(JSContext* cx, HandleNativeObject controller, HandleValue e);
+
+// Streams spec, 3.3.6. step 21:
+// Upon rejection of reader.[[closedPromise]] with reason r,
+static bool
+TeeReaderClosedHandler(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    Rooted<TeeState*> teeState(cx, TargetFromHandler<TeeState>(args.callee()));
+    HandleValue reason = args.get(0);
+
+    // Step a: If teeState.[[closedOrErrored]] is false, then:
+    if (!teeState->closedOrErrored()) {
+        // Step a.i: Perform ! ReadableStreamDefaultControllerError(pull.[[branch1]], r).
+        Rooted<ReadableStreamDefaultController*> branch1(cx, teeState->branch1());
+        if (!ReadableStreamControllerError(cx, branch1, reason))
+            return false;
+
+        // Step a.ii: Perform ! ReadableStreamDefaultControllerError(pull.[[branch2]], r).
+        Rooted<ReadableStreamDefaultController*> branch2(cx, teeState->branch2());
+        if (!ReadableStreamControllerError(cx, branch2, reason))
+            return false;
+
+        // Step a.iii: Set teeState.[[closedOrErrored]] to true.
+        teeState->setClosedOrErrored();
+    }
+
+    return true;
+}
+
+// Streams spec, 3.3.6. ReadableStreamTee ( stream, cloneForBranch2 )
+static MOZ_MUST_USE bool
+ReadableStreamTee(JSContext* cx, Handle<ReadableStream*> stream, bool cloneForBranch2,
+                  MutableHandle<ReadableStream*> branch1Stream,
+                  MutableHandle<ReadableStream*> branch2Stream)
+{
+    // Step 1: Assert: ! IsReadableStream(stream) is true (implicit).
+    // Step 2: Assert: Type(cloneForBranch2) is Boolean (implicit).
+
+    // Step 3: Let reader be ? AcquireReadableStreamDefaultReader(stream).
+    Rooted<ReadableStreamDefaultReader*> reader(cx, CreateReadableStreamDefaultReader(cx, stream));
+    if (!reader)
+        return false;
+
+    // Step 4: Let teeState be Record {[[closedOrErrored]]: false,
+    //                                 [[canceled1]]: false,
+    //                                 [[canceled2]]: false,
+    //                                 [[reason1]]: undefined,
+    //                                 [[reason2]]: undefined,
+    //                                 [[promise]]: a new promise}.
+    Rooted<TeeState*> teeState(cx, TeeState::create(cx, stream));
+    if (!teeState)
+        return false;
+
+    // Steps 5-10 omitted because our implementation works differently.
+
+    // Step 5: Let pull be a new ReadableStreamTee pull function.
+    // Step 6: Set pull.[[reader]] to reader, pull.[[teeState]] to teeState, and
+    //         pull.[[cloneForBranch2]] to cloneForBranch2.
+    // Step 7: Let cancel1 be a new ReadableStreamTee branch 1 cancel function.
+    // Step 8: Set cancel1.[[stream]] to stream and cancel1.[[teeState]] to
+    //         teeState.
+
+    // Step 9: Let cancel2 be a new ReadableStreamTee branch 2 cancel function.
+    // Step 10: Set cancel2.[[stream]] to stream and cancel2.[[teeState]] to
+    //          teeState.
+
+    // Step 11: Let underlyingSource1 be ! ObjectCreate(%ObjectPrototype%).
+    // Step 12: Perform ! CreateDataProperty(underlyingSource1, "pull", pull).
+    // Step 13: Perform ! CreateDataProperty(underlyingSource1, "cancel", cancel1).
+
+    // Step 14: Let branch1Stream be ! Construct(ReadableStream, underlyingSource1).
+    RootedValue hwmValue(cx, NumberValue(1));
+    RootedValue underlyingSource(cx, ObjectValue(*teeState));
+    branch1Stream.set(ReadableStream::createDefaultStream(cx, underlyingSource,
+                                                          UndefinedHandleValue,
+                                                          hwmValue));
+    if (!branch1Stream)
+        return false;
+
+    Rooted<ReadableStreamDefaultController*> branch1(cx);
+    branch1 = &ControllerFromStream(branch1Stream)->as<ReadableStreamDefaultController>();
+    AddControllerFlags(branch1, ControllerFlag_TeeBranch | ControllerFlag_TeeBranch1);
+    teeState->setBranch1(branch1);
+
+    // Step 15: Let underlyingSource2 be ! ObjectCreate(%ObjectPrototype%).
+    // Step 16: Perform ! CreateDataProperty(underlyingSource2, "pull", pull).
+    // Step 17: Perform ! CreateDataProperty(underlyingSource2, "cancel", cancel2).
+
+    // Step 18: Let branch2Stream be ! Construct(ReadableStream, underlyingSource2).
+    branch2Stream.set(ReadableStream::createDefaultStream(cx, underlyingSource,
+                                                          UndefinedHandleValue,
+                                                          hwmValue));
+    if (!branch2Stream)
+        return false;
+
+    Rooted<ReadableStreamDefaultController*> branch2(cx);
+    branch2 = &ControllerFromStream(branch2Stream)->as<ReadableStreamDefaultController>();
+    AddControllerFlags(branch2, ControllerFlag_TeeBranch | ControllerFlag_TeeBranch2);
+    teeState->setBranch2(branch2);
+
+    // Step 19: Set pull.[[branch1]] to branch1Stream.[[readableStreamController]].
+    // Step 20: Set pull.[[branch2]] to branch2Stream.[[readableStreamController]].
+
+    // Step 21: Upon rejection of reader.[[closedPromise]] with reason r,
+    RootedObject closedPromise(cx, &reader->getFixedSlot(ReaderSlot_ClosedPromise).toObject());
+
+    RootedObject onRejected(cx, NewHandler(cx, TeeReaderClosedHandler, teeState));
+    if (!onRejected)
+        return false;
+
+    if (!JS::AddPromiseReactions(cx, closedPromise, nullptr, onRejected))
+        return false;
+
+    // Step 22: Return « branch1, branch2 ».
+    return true;
+}
+
+// Streams spec, 3.4.1. ReadableStreamAddReadIntoRequest ( stream )
+static MOZ_MUST_USE PromiseObject*
+ReadableStreamAddReadIntoRequest(JSContext* cx, Handle<ReadableStream*> stream)
+{
+    // Step 1: MOZ_ASSERT: ! IsReadableStreamBYOBReader(stream.[[reader]]) is true.
+    RootedValue val(cx, stream->getFixedSlot(StreamSlot_Reader));
+    RootedNativeObject reader(cx, &val.toObject().as<ReadableStreamBYOBReader>());
+
+    // Step 2: MOZ_ASSERT: stream.[[state]] is "readable" or "closed".
+    MOZ_ASSERT(stream->readable() || stream->closed());
+
+    // Step 3: Let promise be a new promise.
+    Rooted<PromiseObject*> promise(cx, PromiseObject::createSkippingExecutor(cx));
+    if (!promise)
+        return nullptr;
+
+    // Step 4: Let readIntoRequest be Record {[[promise]]: promise}.
+    // Step 5: Append readIntoRequest as the last element of stream.[[reader]].[[readIntoRequests]].
+    val = reader->getFixedSlot(ReaderSlot_Requests);
+    RootedNativeObject readIntoRequests(cx, &val.toObject().as<NativeObject>());
+    // Since [[promise]] is the Record's only field, we store it directly.
+    val = ObjectValue(*promise);
+    if (!AppendToList(cx, readIntoRequests, val))
+        return nullptr;
+
+    // Step 6: Return promise.
+    return promise;
+}
+
+// Streams spec, 3.4.2. ReadableStreamAddReadRequest ( stream )
+static MOZ_MUST_USE PromiseObject*
+ReadableStreamAddReadRequest(JSContext* cx, Handle<ReadableStream*> stream)
+{
+  MOZ_ASSERT(stream->is<ReadableStream>());
+
+  // Step 1: Assert: ! IsReadableStreamDefaultReader(stream.[[reader]]) is true.
+  RootedNativeObject reader(cx, ReaderFromStream(stream));
+
+  // Step 2: Assert: stream.[[state]] is "readable".
+  MOZ_ASSERT(stream->readable());
+
+  // Step 3: Let promise be a new promise.
+  Rooted<PromiseObject*> promise(cx, PromiseObject::createSkippingExecutor(cx));
+  if (!promise)
+      return nullptr;
+
+  // Step 4: Let readRequest be Record {[[promise]]: promise}.
+  // Step 5: Append readRequest as the last element of stream.[[reader]].[[readRequests]].
+  RootedValue val(cx, reader->getFixedSlot(ReaderSlot_Requests));
+  RootedNativeObject readRequests(cx, &val.toObject().as<NativeObject>());
+
+  // Since [[promise]] is the Record's only field, we store it directly.
+  val = ObjectValue(*promise);
+  if (!AppendToList(cx, readRequests, val))
+      return nullptr;
+
+  // Step 6: Return promise.
+  return promise;
+}
+
+static MOZ_MUST_USE JSObject*
+ReadableStreamControllerCancelSteps(JSContext* cx,
+                                    HandleNativeObject controller, HandleValue reason);
+
+// Used for transforming the result of promise fulfillment/rejection.
+static bool
+ReturnUndefined(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setUndefined();
+    return true;
+}
+
+MOZ_MUST_USE bool
+ReadableStreamCloseInternal(JSContext* cx, Handle<ReadableStream*> stream);
+
+// Streams spec, 3.4.3. ReadableStreamCancel ( stream, reason )
+/* static */ MOZ_MUST_USE JSObject*
+ReadableStream::cancel(JSContext* cx, Handle<ReadableStream*> stream, HandleValue reason)
+{
+    // Step 1: Set stream.[[disturbed]] to true.
+    uint32_t state = StreamState(stream) | ReadableStream::Disturbed;
+    SetStreamState(stream, state);
+
+    // Step 2: If stream.[[state]] is "closed", return a new promise resolved
+    //         with undefined.
+    if (stream->closed())
+        return PromiseObject::unforgeableResolve(cx, UndefinedHandleValue);
+
+    // Step 3: If stream.[[state]] is "errored", return a new promise rejected
+    //         with stream.[[storedError]].
+    if (stream->errored()) {
+        RootedValue storedError(cx, stream->getFixedSlot(StreamSlot_StoredError));
+        return PromiseObject::unforgeableReject(cx, storedError);
+    }
+
+    // Step 4: Perform ! ReadableStreamClose(stream).
+    if (!ReadableStreamCloseInternal(cx, stream))
+        return nullptr;
+
+    // Step 5: Let sourceCancelPromise be
+    //         ! stream.[[readableStreamController]].[[CancelSteps]](reason).
+    RootedNativeObject controller(cx, ControllerFromStream(stream));
+    RootedObject sourceCancelPromise(cx);
+    sourceCancelPromise = ReadableStreamControllerCancelSteps(cx, controller, reason);
+    if (!sourceCancelPromise)
+        return nullptr;
+
+    // Step 6: Return the result of transforming sourceCancelPromise by a
+    //         fulfillment handler that returns undefined.
+    RootedAtom funName(cx, cx->names().empty);
+    RootedFunction returnUndefined(cx, NewNativeFunction(cx, ReturnUndefined, 0, funName));
+    if (!returnUndefined)
+        return nullptr;
+    return JS::CallOriginalPromiseThen(cx, sourceCancelPromise, returnUndefined, nullptr);
+}
+
+// Streams spec, 3.4.4. ReadableStreamClose ( stream )
+MOZ_MUST_USE bool
+ReadableStreamCloseInternal(JSContext* cx, Handle<ReadableStream*> stream)
+{
+  // Step 1: Assert: stream.[[state]] is "readable".
+  MOZ_ASSERT(stream->readable());
+
+  uint32_t state = StreamState(stream);
+  // Step 2: Set stream.[[state]] to "closed".
+  SetStreamState(stream, (state & ReadableStream::Disturbed) | ReadableStream::Closed);
+
+  // Step 3: Let reader be stream.[[reader]].
+  RootedValue val(cx, stream->getFixedSlot(StreamSlot_Reader));
+
+  // Step 4: If reader is undefined, return.
+  if (val.isUndefined())
+      return true;
+
+  // Step 5: If ! IsReadableStreamDefaultReader(reader) is true,
+  RootedNativeObject reader(cx, &val.toObject().as<NativeObject>());
+  if (reader->is<ReadableStreamDefaultReader>()) {
+      // Step a: Repeat for each readRequest that is an element of
+      //         reader.[[readRequests]],
+      val = reader->getFixedSlot(ReaderSlot_Requests);
+      if (!val.isUndefined()) {
+          RootedNativeObject readRequests(cx, &val.toObject().as<NativeObject>());
+          uint32_t len = readRequests->getDenseInitializedLength();
+          RootedObject readRequest(cx);
+          RootedObject resultObj(cx);
+          RootedValue resultVal(cx);
+          for (uint32_t i = 0; i < len; i++) {
+              // Step i: Resolve readRequest.[[promise]] with
+              //         ! CreateIterResultObject(undefined, true).
+              readRequest = &readRequests->getDenseElement(i).toObject();
+              resultObj = CreateIterResultObject(cx, UndefinedHandleValue, true);
+              if (!resultObj)
+                  return false;
+              resultVal = ObjectValue(*resultObj);
+              if (!ResolvePromise(cx, readRequest, resultVal))
+                  return false;
+          }
+
+          // Step b: Set reader.[[readRequests]] to an empty List.
+          reader->setFixedSlot(ReaderSlot_Requests, UndefinedValue());
+      }
+  }
+
+  // Step 6: Resolve reader.[[closedPromise]] with undefined.
+  // Step 7: Return (implicit).
+  RootedObject closedPromise(cx, &reader->getFixedSlot(ReaderSlot_ClosedPromise).toObject());
+  if (!ResolvePromise(cx, closedPromise, UndefinedHandleValue))
+      return false;
+
+  if (stream->mode() == JS::ReadableStreamMode::ExternalSource &&
+      cx->runtime()->readableStreamClosedCallback)
+  {
+      NativeObject* controller = ControllerFromStream(stream);
+      void* source = controller->getFixedSlot(ControllerSlot_UnderlyingSource).toPrivate();
+      cx->runtime()->readableStreamClosedCallback(cx, stream, source, stream->embeddingFlags());
+  }
+
+  return true;
+}
+
+// Streams spec, 3.4.5. ReadableStreamError ( stream, e )
+MOZ_MUST_USE bool
+ReadableStreamErrorInternal(JSContext* cx, Handle<ReadableStream*> stream, HandleValue e)
+{
+    // Step 1: Assert: ! IsReadableStream(stream) is true (implicit).
+
+    // Step 2: Assert: stream.[[state]] is "readable".
+    MOZ_ASSERT(stream->readable());
+
+    // Step 3: Set stream.[[state]] to "errored".
+    uint32_t state = StreamState(stream);
+    SetStreamState(stream, (state & ReadableStream::Disturbed) | ReadableStream::Errored);
+
+    // Step 4: Set stream.[[storedError]] to e.
+    stream->setFixedSlot(StreamSlot_StoredError, e);
+
+    // Step 5: Let reader be stream.[[reader]].
+    RootedValue val(cx, stream->getFixedSlot(StreamSlot_Reader));
+
+    // Step 6: If reader is undefined, return.
+    if (val.isUndefined())
+        return true;
+    RootedNativeObject reader(cx, &val.toObject().as<NativeObject>());
+
+    // Steps 7,8: (Identical in our implementation.)
+    // Step a: Repeat for each readRequest that is an element of
+    //         reader.[[readRequests]],
+    val = reader->getFixedSlot(ReaderSlot_Requests);
+    RootedNativeObject readRequests(cx, &val.toObject().as<NativeObject>());
+    Rooted<PromiseObject*> readRequest(cx);
+    uint32_t len = readRequests->getDenseInitializedLength();
+    for (uint32_t i = 0; i < len; i++) {
+        // Step i: Reject readRequest.[[promise]] with e.
+        val = readRequests->getDenseElement(i);
+        readRequest = &val.toObject().as<PromiseObject>();
+        if (!PromiseObject::reject(cx, readRequest, e))
+            return false;
+    }
+
+    // Step b: Set reader.[[readRequests]] to a new empty List.
+    if (!SetNewList(cx, reader, ReaderSlot_Requests))
+        return false;
+
+    // Step 9: Reject reader.[[closedPromise]] with e.
+    val = reader->getFixedSlot(ReaderSlot_ClosedPromise);
+    Rooted<PromiseObject*> closedPromise(cx, &val.toObject().as<PromiseObject>());
+    if (!PromiseObject::reject(cx, closedPromise, e))
+        return false;
+
+    if (stream->mode() == JS::ReadableStreamMode::ExternalSource &&
+        cx->runtime()->readableStreamErroredCallback)
+    {
+        NativeObject* controller = ControllerFromStream(stream);
+        void* source = controller->getFixedSlot(ControllerSlot_UnderlyingSource).toPrivate();
+        cx->runtime()->readableStreamErroredCallback(cx, stream, source,
+                                                     stream->embeddingFlags(), e);
+    }
+
+    return true;
+}
+
+// Streams spec, 3.4.6. ReadableStreamFulfillReadIntoRequest( stream, chunk, done )
+// Streams spec, 3.4.7. ReadableStreamFulfillReadRequest ( stream, chunk, done )
+// These two spec functions are identical in our implementation.
+static MOZ_MUST_USE bool
+ReadableStreamFulfillReadOrReadIntoRequest(JSContext* cx, Handle<ReadableStream*> stream,
+                                           HandleValue chunk, bool done)
+{
+    // Step 1: Let reader be stream.[[reader]].
+    RootedValue val(cx, stream->getFixedSlot(StreamSlot_Reader));
+    RootedNativeObject reader(cx, &val.toObject().as<NativeObject>());
+
+    // Step 2: Let readIntoRequest be the first element of
+    //         reader.[[readIntoRequests]].
+    // Step 3: Remove readIntoRequest from reader.[[readIntoRequests]], shifting
+    //         all other elements downward (so that the second becomes the first,
+    //         and so on).
+    val = reader->getFixedSlot(ReaderSlot_Requests);
+    RootedNativeObject readIntoRequests(cx, &val.toObject().as<NativeObject>());
+    Rooted<PromiseObject*> readIntoRequest(cx);
+    readIntoRequest = ShiftFromList<PromiseObject>(cx, readIntoRequests);
+    MOZ_ASSERT(readIntoRequest);
+
+    // Step 4: Resolve readIntoRequest.[[promise]] with
+    //         ! CreateIterResultObject(chunk, done).
+    RootedObject iterResult(cx, CreateIterResultObject(cx, chunk, done));
+    if (!iterResult)
+        return false;
+    val = ObjectValue(*iterResult);
+    return PromiseObject::resolve(cx, readIntoRequest, val);
+}
+
+// Streams spec, 3.4.8. ReadableStreamGetNumReadIntoRequests ( stream )
+// Streams spec, 3.4.9. ReadableStreamGetNumReadRequests ( stream )
+// (Identical implementation.)
+static uint32_t
+ReadableStreamGetNumReadRequests(ReadableStream* stream)
+{
+    // Step 1: Return the number of elements in
+    //         stream.[[reader]].[[readRequests]].
+    if (!HasReader(stream))
+        return 0;
+    NativeObject* reader = ReaderFromStream(stream);
+    Value readRequests = reader->getFixedSlot(ReaderSlot_Requests);
+    return readRequests.toObject().as<NativeObject>().getDenseInitializedLength();
+}
+
+// Stream spec 3.4.10. ReadableStreamHasBYOBReader ( stream )
+static MOZ_MUST_USE bool
+ReadableStreamHasBYOBReader(ReadableStream* stream)
+{
+    // Step 1: Let reader be stream.[[reader]].
+    // Step 2: If reader is undefined, return false.
+    // Step 3: If ! IsReadableStreamBYOBReader(reader) is false, return false.
+    // Step 4: Return true.
+    Value reader = stream->getFixedSlot(StreamSlot_Reader);
+    return reader.isObject() && reader.toObject().is<ReadableStreamBYOBReader>();
+}
+
+// Streap spec 3.4.11. ReadableStreamHasDefaultReader ( stream )
+static MOZ_MUST_USE bool
+ReadableStreamHasDefaultReader(ReadableStream* stream)
+{
+    // Step 1: Let reader be stream.[[reader]].
+    // Step 2: If reader is undefined, return false.
+    // Step 3: If ! ReadableStreamDefaultReader(reader) is false, return false.
+    // Step 4: Return true.
+    Value reader = stream->getFixedSlot(StreamSlot_Reader);
+    return reader.isObject() && reader.toObject().is<ReadableStreamDefaultReader>();
+}
+
+static MOZ_MUST_USE bool
+ReadableStreamReaderGenericInitialize(JSContext* cx,
+                                      HandleNativeObject reader,
+                                      Handle<ReadableStream*> stream);
+
+// Stream spec, 3.5.3. new ReadableStreamDefaultReader ( stream )
+// Steps 2-4.
+static MOZ_MUST_USE ReadableStreamDefaultReader*
+CreateReadableStreamDefaultReader(JSContext* cx, Handle<ReadableStream*> stream)
+{
+    Rooted<ReadableStreamDefaultReader*> reader(cx);
+    reader = NewBuiltinClassInstance<ReadableStreamDefaultReader>(cx);
+    if (!reader)
+        return nullptr;
+
+    // Step 2: If ! IsReadableStreamLocked(stream) is true, throw a TypeError
+    //         exception.
+    if (stream->locked()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAM_LOCKED);
+        return nullptr;
+    }
+
+    // Step 3: Perform ! ReadableStreamReaderGenericInitialize(this, stream).
+    if (!ReadableStreamReaderGenericInitialize(cx, reader, stream))
+        return nullptr;
+
+    // Step 4: Set this.[[readRequests]] to a new empty List.
+    if (!SetNewList(cx, reader, ReaderSlot_Requests))
+        return nullptr;
+
+    return reader;
+}
+
+// Stream spec, 3.5.3. new ReadableStreamDefaultReader ( stream )
+bool
+ReadableStreamDefaultReader::constructor(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    if (!ThrowIfNotConstructing(cx, args, "ReadableStreamDefaultReader"))
+        return false;
+
+    // Step 1: If ! IsReadableStream(stream) is false, throw a TypeError exception.
+    if (!Is<ReadableStream>(args.get(0))) {
+        ReportArgTypeError(cx, "ReadableStreamDefaultReader", "ReadableStream",
+                           args.get(0));
+        return false;
+    }
+
+    Rooted<ReadableStream*> stream(cx, &args.get(0).toObject().as<ReadableStream>());
+
+    RootedObject reader(cx, CreateReadableStreamDefaultReader(cx, stream));
+    if (!reader)
+        return false;
+
+    args.rval().setObject(*reader);
+    return true;
+}
+
+// Streams spec, 3.5.4.1 get closed
+static MOZ_MUST_USE bool
+ReadableStreamDefaultReader_closed(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    // Step 1: If ! IsReadableStreamDefaultReader(this) is false, return a promise
+    //         rejected with a TypeError exception.
+    if (!Is<ReadableStreamDefaultReader>(args.thisv()))
+        return RejectNonGenericMethod(cx, args, "ReadableStreamDefaultReader", "get closed");
+
+    // Step 2: Return this.[[closedPromise]].
+    NativeObject* reader = &args.thisv().toObject().as<NativeObject>();
+    args.rval().set(reader->getFixedSlot(ReaderSlot_ClosedPromise));
+    return true;
+}
+
+static MOZ_MUST_USE JSObject*
+ReadableStreamReaderGenericCancel(JSContext* cx, HandleNativeObject reader, HandleValue reason);
+
+// Streams spec, 3.5.4.2. cancel ( reason )
+static MOZ_MUST_USE bool
+ReadableStreamDefaultReader_cancel(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    // Step 1: If ! IsReadableStreamDefaultReader(this) is false, return a promise
+    //         rejected with a TypeError exception.
+    if (!Is<ReadableStreamDefaultReader>(args.thisv()))
+        return RejectNonGenericMethod(cx, args, "ReadableStreamDefaultReader", "cancel");
+
+    // Step 2: If this.[[ownerReadableStream]] is undefined, return a promise
+    //         rejected with a TypeError exception.
+    RootedNativeObject reader(cx, &args.thisv().toObject().as<NativeObject>());
+    if (!ReaderHasStream(reader)) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMREADER_NOT_OWNED, "cancel");
+        return ReturnPromiseRejectedWithPendingError(cx, args);
+    }
+
+    // Step 3: Return ! ReadableStreamReaderGenericCancel(this, reason).
+    JSObject* cancelPromise = ReadableStreamReaderGenericCancel(cx, reader, args.get(0));
+    if (!cancelPromise)
+        return false;
+    args.rval().setObject(*cancelPromise);
+    return true;
+}
+
+// Streams spec, 3.5.4.3 read ( )
+static MOZ_MUST_USE bool
+ReadableStreamDefaultReader_read(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    // Step 1: If ! IsReadableStreamDefaultReader(this) is false, return a promise
+    //         rejected with a TypeError exception.
+    if (!Is<ReadableStreamDefaultReader>(args.thisv()))
+        return RejectNonGenericMethod(cx, args, "ReadableStreamDefaultReader", "read");
+
+    // Step 2: If this.[[ownerReadableStream]] is undefined, return a promise
+    //         rejected with a TypeError exception.
+    Rooted<ReadableStreamDefaultReader*> reader(cx);
+    reader = &args.thisv().toObject().as<ReadableStreamDefaultReader>();
+    if (!ReaderHasStream(reader)) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMREADER_NOT_OWNED, "read");
+        return ReturnPromiseRejectedWithPendingError(cx, args);
+    }
+
+    // Step 3: Return ! ReadableStreamDefaultReaderRead(this).
+    JSObject* readPromise = ReadableStreamDefaultReader::read(cx, reader);
+    if (!readPromise)
+        return false;
+    args.rval().setObject(*readPromise);
+    return true;
+}
+
+static MOZ_MUST_USE bool
+ReadableStreamReaderGenericRelease(JSContext* cx, HandleNativeObject reader);
+
+// Streams spec, 3.5.4.4. releaseLock ( )
+static MOZ_MUST_USE bool
+ReadableStreamDefaultReader_releaseLock_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableStreamDefaultReader*> reader(cx);
+    reader = &args.thisv().toObject().as<ReadableStreamDefaultReader>();
+
+    // Step 2: If this.[[ownerReadableStream]] is undefined, return.
+    if (!ReaderHasStream(reader)) {
+        args.rval().setUndefined();
+        return true;
+    }
+
+    // Step 3: If this.[[readRequests]] is not empty, throw a TypeError exception.
+    Value val = reader->getFixedSlot(ReaderSlot_Requests);
+    if (!val.isUndefined()) {
+        NativeObject* readRequests = &val.toObject().as<NativeObject>();
+        uint32_t len = readRequests->getDenseInitializedLength();
+        if (len != 0) {
+            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                      JSMSG_READABLESTREAMREADER_NOT_EMPTY,
+                                      "releaseLock");
+            return false;
+        }
+    }
+
+    // Step 4: Perform ! ReadableStreamReaderGenericRelease(this).
+    return ReadableStreamReaderGenericRelease(cx, reader);
+}
+
+static bool
+ReadableStreamDefaultReader_releaseLock(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableStreamDefaultReader(this) is false,
+    //         throw a TypeError exception.
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableStreamDefaultReader>,
+                                ReadableStreamDefaultReader_releaseLock_impl>(cx, args);
+}
+
+static const JSFunctionSpec ReadableStreamDefaultReader_methods[] = {
+    JS_FN("cancel",         ReadableStreamDefaultReader_cancel,         1, 0),
+    JS_FN("read",           ReadableStreamDefaultReader_read,           0, 0),
+    JS_FN("releaseLock",    ReadableStreamDefaultReader_releaseLock,    0, 0),
+    JS_FS_END
+};
+
+static const JSPropertySpec ReadableStreamDefaultReader_properties[] = {
+    JS_PSG("closed", ReadableStreamDefaultReader_closed, 0),
+    JS_PS_END
+};
+
+CLASS_SPEC(ReadableStreamDefaultReader, 1, ReaderSlotCount, ClassSpec::DontDefineConstructor, 0,
+           JS_NULL_CLASS_OPS);
+
+
+// Streams spec, 3.6.3 new ReadableStreamBYOBReader ( stream )
+// Steps 2-5.
+static MOZ_MUST_USE ReadableStreamBYOBReader*
+CreateReadableStreamBYOBReader(JSContext* cx, Handle<ReadableStream*> stream)
+{
+    // Step 2: If ! IsReadableByteStreamController(stream.[[readableStreamController]])
+    //         is false, throw a TypeError exception.
+    if (!ControllerFromStream(stream)->is<ReadableByteStreamController>()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAM_NOT_BYTE_STREAM_CONTROLLER,
+                                  "ReadableStream.getReader('byob')");
+        return nullptr;
+    }
+
+    // Step 3: If ! IsReadableStreamLocked(stream) is true, throw a TypeError
+    //         exception.
+    if (stream->locked()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_READABLESTREAM_LOCKED);
+        return nullptr;
+    }
+
+    Rooted<ReadableStreamBYOBReader*> reader(cx);
+    reader = NewBuiltinClassInstance<ReadableStreamBYOBReader>(cx);
+    if (!reader)
+        return nullptr;
+
+    // Step 4: Perform ! ReadableStreamReaderGenericInitialize(this, stream).
+    if (!ReadableStreamReaderGenericInitialize(cx, reader, stream))
+        return nullptr;
+
+    // Step 5: Set this.[[readIntoRequests]] to a new empty List.
+    if (!SetNewList(cx, reader, ReaderSlot_Requests))
+        return nullptr;
+
+    return reader;
+}
+
+// Streams spec, 3.6.3 new ReadableStreamBYOBReader ( stream )
+bool
+ReadableStreamBYOBReader::constructor(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    if (!ThrowIfNotConstructing(cx, args, "ReadableStreamBYOBReader"))
+        return false;
+
+    // Step 1: If ! IsReadableStream(stream) is false, throw a TypeError exception.
+    if (!Is<ReadableStream>(args.get(0))) {
+        ReportArgTypeError(cx, "ReadableStreamBYOBReader", "ReadableStream", args.get(0));
+        return false;
+    }
+
+    Rooted<ReadableStream*> stream(cx, &args.get(0).toObject().as<ReadableStream>());
+    RootedObject reader(cx, CreateReadableStreamBYOBReader(cx, stream));
+    if (!reader)
+        return false;
+
+    args.rval().setObject(*reader);
+    return true;
+}
+
+// Streams spec, 3.6.4.1 get closed
+static MOZ_MUST_USE bool
+ReadableStreamBYOBReader_closed(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    // Step 1: If ! IsReadableStreamBYOBReader(this) is false, return a promise
+    //         rejected with a TypeError exception.
+    if (!Is<ReadableStreamBYOBReader>(args.thisv()))
+        return RejectNonGenericMethod(cx, args, "ReadableStreamBYOBReader", "get closed");
+
+    // Step 2: Return this.[[closedPromise]].
+    NativeObject* reader = &args.thisv().toObject().as<NativeObject>();
+    args.rval().set(reader->getFixedSlot(ReaderSlot_ClosedPromise));
+    return true;
+}
+
+// Streams spec, 3.6.4.2. cancel ( reason )
+static MOZ_MUST_USE bool
+ReadableStreamBYOBReader_cancel(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    // Step 1: If ! IsReadableStreamBYOBReader(this) is false, return a promise
+    //         rejected with a TypeError exception.
+    if (!Is<ReadableStreamBYOBReader>(args.thisv()))
+        return RejectNonGenericMethod(cx, args, "ReadableStreamBYOBReader", "cancel");
+
+    // Step 2: If this.[[ownerReadableStream]] is undefined, return a promise
+    //         rejected with a TypeError exception.
+    RootedNativeObject reader(cx, &args.thisv().toObject().as<NativeObject>());
+    if (!ReaderHasStream(reader)) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMREADER_NOT_OWNED, "cancel");
+        return ReturnPromiseRejectedWithPendingError(cx, args);
+    }
+
+    // Step 3: Return ! ReadableStreamReaderGenericCancel(this, reason).
+    JSObject* cancelPromise = ReadableStreamReaderGenericCancel(cx, reader, args.get(0));
+    if (!cancelPromise)
+        return false;
+    args.rval().setObject(*cancelPromise);
+    return true;
+}
+
+// Streams spec, 3.6.4.3 read ( )
+static MOZ_MUST_USE bool
+ReadableStreamBYOBReader_read(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    HandleValue viewVal = args.get(0);
+
+    // Step 1: If ! IsReadableStreamBYOBReader(this) is false, return a promise
+    //         rejected with a TypeError exception.
+    if (!Is<ReadableStreamBYOBReader>(args.thisv()))
+        return RejectNonGenericMethod(cx, args, "ReadableStreamBYOBReader", "read");
+
+    // Step 2: If this.[[ownerReadableStream]] is undefined, return a promise
+    //         rejected with a TypeError exception.
+    Rooted<ReadableStreamBYOBReader*> reader(cx);
+    reader = &args.thisv().toObject().as<ReadableStreamBYOBReader>();
+    if (!ReaderHasStream(reader)) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMREADER_NOT_OWNED, "read");
+        return ReturnPromiseRejectedWithPendingError(cx, args);
+    }
+
+    // Step 3: If Type(view) is not Object, return a promise rejected with a
+    //         TypeError exception.
+    // Step 4: If view does not have a [[ViewedArrayBuffer]] internal slot,
+    //         return a promise rejected with a TypeError exception.
+    if (!Is<ArrayBufferViewObject>(viewVal)) {
+        ReportArgTypeError(cx, "ReadableStreamBYOBReader.read", "Typed Array", viewVal);
+        return ReturnPromiseRejectedWithPendingError(cx, args);
+    }
+
+    Rooted<ArrayBufferViewObject*> view(cx, &viewVal.toObject().as<ArrayBufferViewObject>());
+
+    // Step 5: If view.[[ByteLength]] is 0, return a promise rejected with a
+    //         TypeError exception.
+    // Note: It's ok to use the length in number of elements here because all we
+    // want to know is whether it's < 0.
+    if (JS_GetArrayBufferViewByteLength(view) == 0) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMBYOBREADER_READ_EMPTY_VIEW);
+        return ReturnPromiseRejectedWithPendingError(cx, args);
+    }
+
+    // Step 6: Return ! ReadableStreamBYOBReaderRead(this, view).
+    JSObject* readPromise = ReadableStreamBYOBReader::read(cx, reader, view);
+    if (!readPromise)
+        return false;
+    args.rval().setObject(*readPromise);
+    return true;
+}
+
+static MOZ_MUST_USE bool
+ReadableStreamReaderGenericRelease(JSContext* cx, HandleNativeObject reader);
+
+// Streams spec, 3.6.4.4. releaseLock ( )
+static MOZ_MUST_USE bool
+ReadableStreamBYOBReader_releaseLock_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableStreamBYOBReader*> reader(cx);
+    reader = &args.thisv().toObject().as<ReadableStreamBYOBReader>();
+
+    // Step 2: If this.[[ownerReadableStream]] is undefined, return.
+    if (!ReaderHasStream(reader)) {
+        args.rval().setUndefined();
+        return true;
+    }
+
+    // Step 3: If this.[[readRequests]] is not empty, throw a TypeError exception.
+    Value val = reader->getFixedSlot(ReaderSlot_Requests);
+    if (!val.isUndefined()) {
+        NativeObject* readRequests = &val.toObject().as<NativeObject>();
+        uint32_t len = readRequests->getDenseInitializedLength();
+        if (len != 0) {
+            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                      JSMSG_READABLESTREAMREADER_NOT_EMPTY, "releaseLock");
+            return false;
+        }
+    }
+
+    // Step 4: Perform ! ReadableStreamReaderGenericRelease(this).
+    return ReadableStreamReaderGenericRelease(cx, reader);
+}
+
+static bool
+ReadableStreamBYOBReader_releaseLock(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableStreamBYOBReader(this) is false,
+    //         throw a TypeError exception.
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableStreamBYOBReader>,
+                                ReadableStreamBYOBReader_releaseLock_impl>(cx, args);
+}
+
+static const JSPropertySpec ReadableStreamBYOBReader_properties[] = {
+    JS_PSG("closed", ReadableStreamBYOBReader_closed, 0),
+    JS_PS_END
+};
+
+static const JSFunctionSpec ReadableStreamBYOBReader_methods[] = {
+    JS_FN("cancel",         ReadableStreamBYOBReader_cancel,        1, 0),
+    JS_FN("read",           ReadableStreamBYOBReader_read,          1, 0),
+    JS_FN("releaseLock",    ReadableStreamBYOBReader_releaseLock,   0, 0),
+    JS_FS_END
+};
+
+CLASS_SPEC(ReadableStreamBYOBReader, 1, 3, ClassSpec::DontDefineConstructor, 0, JS_NULL_CLASS_OPS);
+
+inline static MOZ_MUST_USE bool
+ReadableStreamControllerCallPullIfNeeded(JSContext* cx, HandleNativeObject controller);
+
+// Streams spec, 3.7.1. IsReadableStreamDefaultReader ( x )
+// Implemented via intrinsic_isInstanceOfBuiltin<ReadableStreamDefaultReader>()
+
+// Streams spec, 3.7.2. IsReadableStreamBYOBReader ( x )
+// Implemented via intrinsic_isInstanceOfBuiltin<ReadableStreamBYOBReader>()
+
+// Streams spec, 3.7.3. ReadableStreamReaderGenericCancel ( reader, reason )
+static MOZ_MUST_USE JSObject*
+ReadableStreamReaderGenericCancel(JSContext* cx, HandleNativeObject reader, HandleValue reason)
+{
+    // Step 1: Let stream be reader.[[ownerReadableStream]].
+    Rooted<ReadableStream*> stream(cx, StreamFromReader(reader));
+
+    // Step 2: Assert: stream is not undefined (implicit).
+
+    // Step 3: Return ! ReadableStreamCancel(stream, reason).
+    return &ReadableStreamCancel(cx, stream, reason)->as<PromiseObject>();
+}
+
+// Streams spec, 3.7.4. ReadableStreamReaderGenericInitialize ( reader, stream )
+static MOZ_MUST_USE bool
+ReadableStreamReaderGenericInitialize(JSContext* cx, HandleNativeObject reader,
+                                      Handle<ReadableStream*> stream)
+{
+    // Step 1: Set reader.[[ownerReadableStream]] to stream.
+    reader->setFixedSlot(ReaderSlot_Stream, ObjectValue(*stream));
+
+    // Step 2: Set stream.[[reader]] to reader.
+    stream->setFixedSlot(StreamSlot_Reader, ObjectValue(*reader));
+
+    // Step 3: If stream.[[state]] is "readable",
+    RootedObject promise(cx);
+    if (stream->readable()) {
+        // Step a: Set reader.[[closedPromise]] to a new promise.
+        promise = PromiseObject::createSkippingExecutor(cx);
+    } else if (stream->closed()) {
+        // Step 4: Otherwise
+        // Step a: If stream.[[state]] is "closed",
+        // Step i: Set reader.[[closedPromise]] to a new promise resolved with
+        //         undefined.
+        promise = PromiseObject::unforgeableResolve(cx, UndefinedHandleValue);
+    } else {
+        // Step b: Otherwise,
+        // Step i: Assert: stream.[[state]] is "errored".
+        MOZ_ASSERT(stream->errored());
+
+        // Step ii: Set reader.[[closedPromise]] to a new promise rejected with
+        //          stream.[[storedError]].
+        RootedValue storedError(cx, stream->getFixedSlot(StreamSlot_StoredError));
+        promise = PromiseObject::unforgeableReject(cx, storedError);
+    }
+
+    if (!promise)
+        return false;
+
+    reader->setFixedSlot(ReaderSlot_ClosedPromise, ObjectValue(*promise));
+    return true;
+}
+
+// Streams spec, 3.7.5. ReadableStreamReaderGenericRelease ( reader )
+static MOZ_MUST_USE bool
+ReadableStreamReaderGenericRelease(JSContext* cx, HandleNativeObject reader)
+{
+    // Step 1: Assert: reader.[[ownerReadableStream]] is not undefined.
+    Rooted<ReadableStream*> stream(cx, StreamFromReader(reader));
+
+    // Step 2: Assert: reader.[[ownerReadableStream]].[[reader]] is reader.
+    MOZ_ASSERT(&stream->getFixedSlot(StreamSlot_Reader).toObject() == reader);
+
+    // Create an exception to reject promises with below. We don't have a
+    // clean way to do this, unfortunately.
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_READABLESTREAMREADER_RELEASED);
+    RootedValue exn(cx);
+    // Not much we can do about uncatchable exceptions, just bail.
+    if (!GetAndClearException(cx, &exn))
+        return false;
+
+    // Step 3: If reader.[[ownerReadableStream]].[[state]] is "readable", reject
+    //         reader.[[closedPromise]] with a TypeError exception.
+    if (stream->readable()) {
+            Value val = reader->getFixedSlot(ReaderSlot_ClosedPromise);
+            Rooted<PromiseObject*> closedPromise(cx, &val.toObject().as<PromiseObject>());
+            if (!PromiseObject::reject(cx, closedPromise, exn))
+                return false;
+    } else {
+        // Step 4: Otherwise, set reader.[[closedPromise]] to a new promise rejected
+        //         with a TypeError exception.
+        RootedObject closedPromise(cx, PromiseObject::unforgeableReject(cx, exn));
+        if (!closedPromise)
+            return false;
+        reader->setFixedSlot(ReaderSlot_ClosedPromise, ObjectValue(*closedPromise));
+    }
+
+    // Step 5: Set reader.[[ownerReadableStream]].[[reader]] to undefined.
+    stream->setFixedSlot(StreamSlot_Reader, UndefinedValue());
+
+    // Step 6: Set reader.[[ownerReadableStream]] to undefined.
+    reader->setFixedSlot(ReaderSlot_Stream, UndefinedValue());
+
+    return true;
+}
+
+static MOZ_MUST_USE JSObject*
+ReadableByteStreamControllerPullInto(JSContext* cx,
+                                     Handle<ReadableByteStreamController*> controller,
+                                     Handle<ArrayBufferViewObject*> view);
+
+// Streams spec, 3.7.6. ReadableStreamBYOBReaderRead ( reader, view )
+/* static */ MOZ_MUST_USE JSObject*
+ReadableStreamBYOBReader::read(JSContext* cx, Handle<ReadableStreamBYOBReader*> reader,
+                               Handle<ArrayBufferViewObject*> view)
+{
+    MOZ_ASSERT(reader->is<ReadableStreamBYOBReader>());
+
+    // Step 1: Let stream be reader.[[ownerReadableStream]].
+    // Step 2: Assert: stream is not undefined.
+    Rooted<ReadableStream*> stream(cx, StreamFromReader(reader));
+
+    // Step 3: Set stream.[[disturbed]] to true.
+    SetStreamState(stream, StreamState(stream) | ReadableStream::Disturbed);
+
+    // Step 4: If stream.[[state]] is "errored", return a promise rejected with
+    //         stream.[[storedError]].
+    if (stream->errored()) {
+        RootedValue storedError(cx, stream->getFixedSlot(StreamSlot_StoredError));
+        return PromiseObject::unforgeableReject(cx, storedError);
+    }
+
+    // Step 5: Return ! ReadableByteStreamControllerPullInto(stream.[[readableStreamController]], view).
+    Rooted<ReadableByteStreamController*> controller(cx);
+    controller = &ControllerFromStream(stream)->as<ReadableByteStreamController>();
+    return ReadableByteStreamControllerPullInto(cx, controller, view);
+}
+
+static MOZ_MUST_USE JSObject*
+ReadableStreamControllerPullSteps(JSContext* cx, HandleNativeObject controller);
+
+// Streams spec, 3.7.7. ReadableStreamDefaultReaderRead ( reader )
+MOZ_MUST_USE JSObject*
+ReadableStreamDefaultReader::read(JSContext* cx, Handle<ReadableStreamDefaultReader*> reader)
+{
+    // Step 1: Let stream be reader.[[ownerReadableStream]].
+    // Step 2: Assert: stream is not undefined.
+    Rooted<ReadableStream*> stream(cx, StreamFromReader(reader));
+
+    // Step 3: Set stream.[[disturbed]] to true.
+    SetStreamState(stream, StreamState(stream) | ReadableStream::Disturbed);
+
+    // Step 4: If stream.[[state]] is "closed", return a new promise resolved with
+    //         ! CreateIterResultObject(undefined, true).
+    if (stream->closed()) {
+        RootedObject iterResult(cx, CreateIterResultObject(cx, UndefinedHandleValue, true));
+        if (!iterResult)
+            return nullptr;
+        RootedValue iterResultVal(cx, ObjectValue(*iterResult));
+        return PromiseObject::unforgeableResolve(cx, iterResultVal);
+    }
+
+    // Step 5: If stream.[[state]] is "errored", return a new promise rejected with
+    //         stream.[[storedError]].
+    if (stream->errored()) {
+        RootedValue storedError(cx, stream->getFixedSlot(StreamSlot_StoredError));
+        return PromiseObject::unforgeableReject(cx, storedError);
+    }
+
+    // Step 6: Assert: stream.[[state]] is "readable".
+    MOZ_ASSERT(stream->readable());
+
+    // Step 7: Return ! stream.[[readableStreamController]].[[PullSteps]]().
+    RootedNativeObject controller(cx, ControllerFromStream(stream));
+    return ReadableStreamControllerPullSteps(cx, controller);
+}
+
+// Streams spec, 3.8.3, step 11.a.
+// and
+// Streams spec, 3.10.3, step 16.a.
+static bool
+ControllerStartHandler(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    RootedNativeObject controller(cx, TargetFromHandler<NativeObject>(args.callee()));
+
+    // Step i: Set controller.[[started]] to true.
+    AddControllerFlags(controller, ControllerFlag_Started);
+
+    // Step ii: Assert: controller.[[pulling]] is false.
+    // Step iii: Assert: controller.[[pullAgain]] is false.
+    MOZ_ASSERT(!(ControllerFlags(controller) &
+                 (ControllerFlag_Pulling | ControllerFlag_PullAgain)));
+
+    // Step iv: Perform ! ReadableStreamDefaultControllerCallPullIfNeeded(controller).
+    // or
+    // Step iv: Perform ! ReadableByteStreamControllerCallPullIfNeeded((controller).
+    if (!ReadableStreamControllerCallPullIfNeeded(cx, controller))
+        return false;
+    args.rval().setUndefined();
+    return true;
+}
+
+static MOZ_MUST_USE bool
+ReadableStreamDefaultControllerErrorIfNeeded(JSContext* cx,
+                                             Handle<ReadableStreamDefaultController*> controller,
+                                             HandleValue e);
+
+static MOZ_MUST_USE bool
+ReadableStreamControllerError(JSContext* cx, HandleNativeObject controller, HandleValue e);
+
+// Streams spec, 3.8.3, step 11.b.
+// and
+// Streams spec, 3.10.3, step 16.b.
+static bool
+ControllerStartFailedHandler(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    RootedNativeObject controllerObj(cx, TargetFromHandler<NativeObject>(args.callee()));
+
+    // 3.8.3, Step 11.b.i:
+    // Perform ! ReadableStreamDefaultControllerErrorIfNeeded(controller, r).
+    if (controllerObj->is<ReadableStreamDefaultController>()) {
+        Rooted<ReadableStreamDefaultController*> controller(cx);
+        controller = &controllerObj->as<ReadableStreamDefaultController>();
+        return ReadableStreamDefaultControllerErrorIfNeeded(cx, controller, args.get(0));
+    }
+
+    // 3.10.3, Step 16.b.i: If stream.[[state]] is "readable", perform
+    //                      ! ReadableByteStreamControllerError(controller, r).
+    if (StreamFromController(controllerObj)->readable())
+        return ReadableStreamControllerError(cx, controllerObj, args.get(0));
+
+    args.rval().setUndefined();
+    return true;
+}
+
+static MOZ_MUST_USE bool
+ValidateAndNormalizeHighWaterMark(JSContext* cx,
+                                  HandleValue highWaterMarkVal,
+                                  double* highWaterMark);
+
+static MOZ_MUST_USE bool
+ValidateAndNormalizeQueuingStrategy(JSContext* cx,
+                                    HandleValue size,
+                                    HandleValue highWaterMarkVal,
+                                    double* highWaterMark);
+
+// Streams spec, 3.8.3 new ReadableStreamDefaultController ( stream, underlyingSource,
+//                                                           size, highWaterMark )
+// Steps 3 - 11.
+static MOZ_MUST_USE ReadableStreamDefaultController*
+CreateReadableStreamDefaultController(JSContext* cx, Handle<ReadableStream*> stream,
+                                      HandleValue underlyingSource, HandleValue size,
+                                      HandleValue highWaterMarkVal)
+{
+    Rooted<ReadableStreamDefaultController*> controller(cx);
+    controller = NewBuiltinClassInstance<ReadableStreamDefaultController>(cx);
+    if (!controller)
+        return nullptr;
+
+    // Step 3: Set this.[[controlledReadableStream]] to stream.
+    controller->setFixedSlot(ControllerSlot_Stream, ObjectValue(*stream));
+
+    // Step 4: Set this.[[underlyingSource]] to underlyingSource.
+    controller->setFixedSlot(ControllerSlot_UnderlyingSource, underlyingSource);
+
+    // Step 5: Perform ! ResetQueue(this).
+    if (!ResetQueue(cx, controller))
+        return nullptr;
+
+    // Step 6: Set this.[[started]], this.[[closeRequested]], this.[[pullAgain]],
+    //         and this.[[pulling]] to false.
+    controller->setFixedSlot(ControllerSlot_Flags, Int32Value(0));
+
+    // Step 7: Let normalizedStrategy be
+    //         ? ValidateAndNormalizeQueuingStrategy(size, highWaterMark).
+    double highWaterMark;
+    if (!ValidateAndNormalizeQueuingStrategy(cx, size, highWaterMarkVal, &highWaterMark))
+        return nullptr;
+
+    // Step 8: Set this.[[strategySize]] to normalizedStrategy.[[size]] and
+    //         this.[[strategyHWM]] to normalizedStrategy.[[highWaterMark]].
+    controller->setFixedSlot(DefaultControllerSlot_StrategySize, size);
+    controller->setFixedSlot(ControllerSlot_StrategyHWM, NumberValue(highWaterMark));
+
+    // Step 9: Let controller be this (implicit).
+
+    // Step 10: Let startResult be
+    //          ? InvokeOrNoop(underlyingSource, "start", « this »).
+    RootedValue startResult(cx);
+    RootedValue controllerVal(cx, ObjectValue(*controller));
+    if (!InvokeOrNoop(cx, underlyingSource, cx->names().start, controllerVal, &startResult))
+        return nullptr;
+
+    // Step 11: Let startPromise be a promise resolved with startResult:
+    RootedObject startPromise(cx, PromiseObject::unforgeableResolve(cx, startResult));
+    if (!startPromise)
+        return nullptr;
+
+    RootedObject onStartFulfilled(cx, NewHandler(cx, ControllerStartHandler, controller));
+    if (!onStartFulfilled)
+        return nullptr;
+
+    RootedObject onStartRejected(cx, NewHandler(cx, ControllerStartFailedHandler, controller));
+    if (!onStartRejected)
+        return nullptr;
+
+    if (!JS::AddPromiseReactions(cx, startPromise, onStartFulfilled, onStartRejected))
+        return nullptr;
+
+    return controller;
+}
+
+// Streams spec, 3.8.3.
+// new ReadableStreamDefaultController( stream, underlyingSource, size,
+//                                      highWaterMark )
+bool
+ReadableStreamDefaultController::constructor(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    if (!ThrowIfNotConstructing(cx, args, "ReadableStreamDefaultController"))
+        return false;
+
+    // Step 1: If ! IsReadableStream(stream) is false, throw a TypeError exception.
+    HandleValue streamVal = args.get(0);
+    if (!Is<ReadableStream>(streamVal)) {
+        ReportArgTypeError(cx, "ReadableStreamDefaultController", "ReadableStream",
+                           args.get(0));
+        return false;
+    }
+
+    Rooted<ReadableStream*> stream(cx, &streamVal.toObject().as<ReadableStream>());
+
+    // Step 2: If stream.[[readableStreamController]] is not undefined, throw a
+    //         TypeError exception.
+    if (HasController(stream)) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAM_CONTROLLER_SET);
+        return false;
+    }
+
+    // Steps 3-11.
+    RootedObject controller(cx, CreateReadableStreamDefaultController(cx, stream, args.get(1),
+                                                                      args.get(2), args.get(3)));
+    if (!controller)
+        return false;
+
+    args.rval().setObject(*controller);
+    return true;
+}
+
+static MOZ_MUST_USE double
+ReadableStreamControllerGetDesiredSizeUnchecked(NativeObject* controller);
+
+// Streams spec, 3.8.4.1. get desiredSize
+// and
+// Streams spec, 3.10.4.2. get desiredSize
+static MOZ_MUST_USE bool
+ReadableStreamController_desiredSize_impl(JSContext* cx, const CallArgs& args)
+{
+    RootedNativeObject controller(cx);
+    controller = &args.thisv().toObject().as<NativeObject>();
+
+    // Streams spec, 3.9.8. steps 1-4.
+    // 3.9.8. Step 1: Let stream be controller.[[controlledReadableStream]].
+    ReadableStream* stream = StreamFromController(controller);
+
+    // 3.9.8. Step 2: Let state be stream.[[state]].
+    // 3.9.8. Step 3: If state is "errored", return null.
+    if (stream->errored()) {
+        args.rval().setNull();
+        return true;
+    }
+
+    // 3.9.8. Step 4: If state is "closed", return 0.
+    if (stream->closed()) {
+        args.rval().setInt32(0);
+        return true;
+    }
+
+    // Step 2: Return ! ReadableStreamDefaultControllerGetDesiredSize(this).
+    args.rval().setNumber(ReadableStreamControllerGetDesiredSizeUnchecked(controller));
+    return true;
+}
+
+static bool
+ReadableStreamDefaultController_desiredSize(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableStreamDefaultController(this) is false, throw a
+    //         TypeError exception.
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableStreamDefaultController>,
+                                ReadableStreamController_desiredSize_impl>(cx, args);
+}
+
+static MOZ_MUST_USE bool
+ReadableStreamDefaultControllerClose(JSContext* cx,
+                                     Handle<ReadableStreamDefaultController*> controller);
+
+// Unified implementation of steps 2-3 of 3.8.4.2 and 3.10.4.3.
+static MOZ_MUST_USE bool
+VerifyControllerStateForClosing(JSContext* cx, HandleNativeObject controller)
+{
+    // Step 2: If this.[[closeRequested]] is true, throw a TypeError exception.
+    if (ControllerFlags(controller) & ControllerFlag_CloseRequested) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMCONTROLLER_CLOSED, "close");
+        return false;
+    }
+
+    // Step 3: If this.[[controlledReadableStream]].[[state]] is not "readable",
+    //         throw a TypeError exception.
+    ReadableStream* stream = StreamFromController(controller);
+    if (!stream->readable()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMCONTROLLER_NOT_READABLE, "close");
+        return false;
+    }
+
+    return true;
+}
+
+// Streams spec, 3.8.4.2 close()
+static MOZ_MUST_USE bool
+ReadableStreamDefaultController_close_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableStreamDefaultController*> controller(cx);
+    controller = &args.thisv().toObject().as<ReadableStreamDefaultController>();
+
+    // Steps 2-3.
+    if (!VerifyControllerStateForClosing(cx, controller))
+        return false;
+
+    // Step 4: Perform ! ReadableStreamDefaultControllerClose(this).
+    if (!ReadableStreamDefaultControllerClose(cx, controller))
+        return false;
+    args.rval().setUndefined();
+    return true;
+}
+
+static bool
+ReadableStreamDefaultController_close(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableStreamDefaultController(this) is false, throw a
+    //         TypeError exception.
+
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableStreamDefaultController>,
+                                ReadableStreamDefaultController_close_impl>(cx, args);
+}
+
+static MOZ_MUST_USE bool
+ReadableStreamDefaultControllerEnqueue(JSContext* cx,
+                                       Handle<ReadableStreamDefaultController*> controller,
+                                       HandleValue chunk);
+
+// Streams spec, 3.8.4.3. enqueue ( chunk )
+static MOZ_MUST_USE bool
+ReadableStreamDefaultController_enqueue_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableStreamDefaultController*> controller(cx);
+    controller = &args.thisv().toObject().as<ReadableStreamDefaultController>();
+
+    // Step 2: If this.[[closeRequested]] is true, throw a TypeError exception.
+    if (ControllerFlags(controller) & ControllerFlag_CloseRequested) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMCONTROLLER_CLOSED, "close");
+        return false;
+    }
+
+    // Step 3: If this.[[controlledReadableStream]].[[state]] is not "readable",
+    //         throw a TypeError exception.
+    ReadableStream* stream = StreamFromController(controller);
+    if (!stream->readable()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMCONTROLLER_NOT_READABLE, "close");
+        return false;
+    }
+
+    // Step 4: Return ! ReadableStreamDefaultControllerEnqueue(this, chunk).
+    if (!ReadableStreamDefaultControllerEnqueue(cx, controller, args.get(0)))
+        return false;
+    args.rval().setUndefined();
+    return true;
+}
+
+static bool
+ReadableStreamDefaultController_enqueue(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableStreamDefaultController(this) is false, throw a
+    //         TypeError exception.
+
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableStreamDefaultController>,
+                                ReadableStreamDefaultController_enqueue_impl>(cx, args);
+}
+
+// Streams spec, 3.8.4.4. error ( e )
+static MOZ_MUST_USE bool
+ReadableStreamDefaultController_error_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableStreamDefaultController*> controller(cx);
+    controller = &args.thisv().toObject().as<ReadableStreamDefaultController>();
+
+    // Step 2: Let stream be this.[[controlledReadableStream]].
+    // Step 3: If stream.[[state]] is not "readable", throw a TypeError exception.
+    if (!StreamFromController(controller)->readable()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMCONTROLLER_NOT_READABLE, "error");
+        return false;
+    }
+
+    // Step 4: Perform ! ReadableStreamDefaultControllerError(this, e).
+    if (!ReadableStreamControllerError(cx, controller, args.get(0)))
+        return false;
+    args.rval().setUndefined();
+    return true;
+}
+
+static bool
+ReadableStreamDefaultController_error(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableStreamDefaultController(this) is false, throw a
+    //         TypeError exception.
+
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableStreamDefaultController>,
+                                ReadableStreamDefaultController_error_impl>(cx, args);
+}
+
+static const JSPropertySpec ReadableStreamDefaultController_properties[] = {
+    JS_PSG("desiredSize", ReadableStreamDefaultController_desiredSize, 0),
+    JS_PS_END
+};
+
+static const JSFunctionSpec ReadableStreamDefaultController_methods[] = {
+    JS_FN("close",      ReadableStreamDefaultController_close,      0, 0),
+    JS_FN("enqueue",    ReadableStreamDefaultController_enqueue,    1, 0),
+    JS_FN("error",      ReadableStreamDefaultController_error,      1, 0),
+    JS_FS_END
+};
+
+CLASS_SPEC(ReadableStreamDefaultController, 4, 7, ClassSpec::DontDefineConstructor, 0,
+           JS_NULL_CLASS_OPS);
+
+/**
+ * Unified implementation of ReadableStream controllers' [[CancelSteps]] internal
+ * methods.
+ * Streams spec, 3.8.5.1. [[CancelSteps]] ( reason )
+ * and
+ * Streams spec, 3.10.5.1. [[CancelSteps]] ( reason )
+ */
+static MOZ_MUST_USE JSObject*
+ReadableStreamControllerCancelSteps(JSContext* cx, HandleNativeObject controller,
+                                    HandleValue reason)
+{
+    MOZ_ASSERT(IsReadableStreamController(controller));
+
+    // Step 1 of 3.10.5.1: If this.[[pendingPullIntos]] is not empty,
+    if (!controller->is<ReadableStreamDefaultController>()) {
+        Value val = controller->getFixedSlot(ByteControllerSlot_PendingPullIntos);
+        RootedNativeObject pendingPullIntos(cx, &val.toObject().as<NativeObject>());
+
+        if (pendingPullIntos->getDenseInitializedLength() != 0) {
+            // Step a: Let firstDescriptor be the first element of
+            //         this.[[pendingPullIntos]].
+            // Step b: Set firstDescriptor.[[bytesFilled]] to 0.
+            Rooted<PullIntoDescriptor*> firstDescriptor(cx);
+            firstDescriptor = PeekList<PullIntoDescriptor>(pendingPullIntos);
+            firstDescriptor->setBytesFilled(0);
+        }
+    }
+
+    // Step 1 of 3.8.5.1, step 2 of 3.10.5.1: Perform ! ResetQueue(this).
+    if (!ResetQueue(cx, controller))
+        return nullptr;
+
+    // Step 2 of 3.8.5.1, step 3 of 3.10.5.1:
+    // Return ! PromiseInvokeOrNoop(this.[[underlying(Byte)Source]],
+    //                              "cancel", « reason »)
+    RootedValue underlyingSource(cx);
+    underlyingSource = controller->getFixedSlot(ControllerSlot_UnderlyingSource);
+
+    if (Is<TeeState>(underlyingSource)) {
+        Rooted<TeeState*> teeState(cx, &underlyingSource.toObject().as<TeeState>());
+        Rooted<ReadableStreamDefaultController*> defaultController(cx);
+        defaultController = &controller->as<ReadableStreamDefaultController>();
+        return ReadableStreamTee_Cancel(cx, teeState, defaultController, reason);
+    }
+
+    if (ControllerFlags(controller) & ControllerFlag_ExternalSource) {
+        void* source = underlyingSource.toPrivate();
+        Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+        RootedValue rval(cx);
+        rval = cx->runtime()->readableStreamCancelCallback(cx, stream, source,
+                                                           stream->embeddingFlags(), reason);
+        return PromiseObject::unforgeableResolve(cx, rval);
+    }
+
+    return PromiseInvokeOrNoop(cx, underlyingSource, cx->names().cancel, reason);
+}
+
+inline static MOZ_MUST_USE bool
+DequeueValue(JSContext* cx, HandleNativeObject container, MutableHandleValue chunk);
+
+// Streams spec, 3.8.5.2. ReadableStreamDefaultController [[PullSteps]]()
+static JSObject*
+ReadableStreamDefaultControllerPullSteps(JSContext* cx, HandleNativeObject controller)
+{
+    MOZ_ASSERT(controller->is<ReadableStreamDefaultController>());
+
+    // Step 1: Let stream be this.[[controlledReadableStream]].
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+
+    // Step 2: If this.[[queue]] is not empty,
+    RootedNativeObject queue(cx);
+    RootedValue val(cx, controller->getFixedSlot(QueueContainerSlot_Queue));
+    if (val.isObject())
+        queue = &val.toObject().as<NativeObject>();
+
+    if (queue && queue->getDenseInitializedLength() != 0) {
+        // Step a: Let chunk be ! DequeueValue(this.[[queue]]).
+        RootedValue chunk(cx);
+        if (!DequeueValue(cx, controller, &chunk))
+            return nullptr;
+
+        // Step b: If this.[[closeRequested]] is true and this.[[queue]] is empty,
+        //         perform ! ReadableStreamClose(stream).
+        bool closeRequested = ControllerFlags(controller) & ControllerFlag_CloseRequested;
+        if (closeRequested && queue->getDenseInitializedLength() == 0) {
+            if (!ReadableStreamCloseInternal(cx, stream))
+                return nullptr;
+        }
+
+        // Step c: Otherwise, perform ! ReadableStreamDefaultControllerCallPullIfNeeded(this).
+        else {
+        if (!ReadableStreamControllerCallPullIfNeeded(cx, controller))
+            return nullptr;
+        }
+
+        // Step d: Return a promise resolved with ! CreateIterResultObject(chunk, false).
+        RootedObject iterResultObj(cx, CreateIterResultObject(cx, chunk, false));
+        if (!iterResultObj)
+          return nullptr;
+        RootedValue iterResult(cx, ObjectValue(*iterResultObj));
+        return PromiseObject::unforgeableResolve(cx, iterResult);
+    }
+
+    // Step 3: Let pendingPromise be ! ReadableStreamAddReadRequest(stream).
+    Rooted<PromiseObject*> pendingPromise(cx, ReadableStreamAddReadRequest(cx, stream));
+    if (!pendingPromise)
+        return nullptr;
+
+    // Step 4: Perform ! ReadableStreamDefaultControllerCallPullIfNeeded(this).
+    if (!ReadableStreamControllerCallPullIfNeeded(cx, controller))
+        return nullptr;
+
+    // Step 5: Return pendingPromise.
+    return pendingPromise;
+}
+
+// Streams spec, 3.9.2 and 3.12.3. step 7:
+// Upon fulfillment of pullPromise,
+static bool
+ControllerPullHandler(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    RootedNativeObject controller(cx, TargetFromHandler<NativeObject>(args.callee()));
+    uint32_t flags = ControllerFlags(controller);
+
+    // Step a: Set controller.[[pulling]] to false.
+    // Step b.i: Set controller.[[pullAgain]] to false.
+    RemoveControllerFlags(controller, ControllerFlag_Pulling | ControllerFlag_PullAgain);
+
+    // Step b: If controller.[[pullAgain]] is true,
+    if (flags & ControllerFlag_PullAgain) {
+        // Step ii: Perform ! ReadableByteStreamControllerCallPullIfNeeded(controller).
+        if (!ReadableStreamControllerCallPullIfNeeded(cx, controller))
+            return false;
+    }
+
+    args.rval().setUndefined();
+    return true;
+}
+
+// Streams spec, 3.9.2 and 3.12.3. step 8:
+// Upon rejection of pullPromise with reason e,
+static bool
+ControllerPullFailedHandler(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    RootedNativeObject controller(cx, TargetFromHandler<NativeObject>(args.callee()));
+    HandleValue e = args.get(0);
+
+    // Step a: If controller.[[controlledReadableStream]].[[state]] is "readable",
+    //         perform ! ReadableByteStreamControllerError(controller, e).
+    if (StreamFromController(controller)->readable()) {
+        if (!ReadableStreamControllerError(cx, controller, e))
+            return false;
+    }
+
+    args.rval().setUndefined();
+    return true;
+}
+
+static bool
+ReadableStreamControllerShouldCallPull(NativeObject* controller);
+
+static MOZ_MUST_USE double
+ReadableStreamControllerGetDesiredSizeUnchecked(NativeObject* controller);
+
+// Streams spec, 3.9.2 ReadableStreamDefaultControllerCallPullIfNeeded ( controller )
+// and
+// Streams spec, 3.12.3. ReadableByteStreamControllerCallPullIfNeeded ( controller )
+inline static MOZ_MUST_USE bool
+ReadableStreamControllerCallPullIfNeeded(JSContext* cx, HandleNativeObject controller)
+{
+    // Step 1: Let shouldPull be
+    //         ! ReadableByteStreamControllerShouldCallPull(controller).
+    bool shouldPull = ReadableStreamControllerShouldCallPull(controller);
+
+    // Step 2: If shouldPull is false, return.
+    if (!shouldPull)
+        return true;
+
+    // Step 3: If controller.[[pulling]] is true,
+    if (ControllerFlags(controller) & ControllerFlag_Pulling) {
+        // Step a: Set controller.[[pullAgain]] to true.
+        AddControllerFlags(controller, ControllerFlag_PullAgain);
+
+        // Step b: Return.
+        return true;
+    }
+
+    // Step 4: Assert: controller.[[pullAgain]] is false.
+    MOZ_ASSERT(!(ControllerFlags(controller) & ControllerFlag_PullAgain));
+
+    // Step 5: Set controller.[[pulling]] to true.
+    AddControllerFlags(controller, ControllerFlag_Pulling);
+
+    // Step 6: Let pullPromise be
+    //         ! PromiseInvokeOrNoop(controller.[[underlyingByteSource]], "pull", controller).
+    RootedObject pullPromise(cx);
+    RootedValue underlyingSource(cx);
+    underlyingSource = controller->getFixedSlot(ControllerSlot_UnderlyingSource);
+    RootedValue controllerVal(cx, ObjectValue(*controller));
+
+    if (Is<TeeState>(underlyingSource)) {
+        Rooted<TeeState*> teeState(cx, &underlyingSource.toObject().as<TeeState>());
+        Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+        pullPromise = ReadableStreamTee_Pull(cx, teeState, stream);
+    } else if (ControllerFlags(controller) & ControllerFlag_ExternalSource) {
+        void* source = underlyingSource.toPrivate();
+        Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+        double desiredSize = ReadableStreamControllerGetDesiredSizeUnchecked(controller);
+        cx->runtime()->readableStreamDataRequestCallback(cx, stream, source,
+                                                         stream->embeddingFlags(), desiredSize);
+        pullPromise = PromiseObject::unforgeableResolve(cx, UndefinedHandleValue);
+    } else {
+        pullPromise = PromiseInvokeOrNoop(cx, underlyingSource, cx->names().pull, controllerVal);
+    }
+    if (!pullPromise)
+        return false;
+
+    RootedObject onPullFulfilled(cx, NewHandler(cx, ControllerPullHandler, controller));
+    if (!onPullFulfilled)
+        return false;
+
+    RootedObject onPullRejected(cx, NewHandler(cx, ControllerPullFailedHandler, controller));
+    if (!onPullRejected)
+        return false;
+
+    return JS::AddPromiseReactions(cx, pullPromise, onPullFulfilled, onPullRejected);
+
+    // Steps 7-8 implemented in functions above.
+}
+
+// Streams spec, 3.9.3. ReadableStreamDefaultControllerShouldCallPull ( controller )
+// and
+// Streams spec, 3.12.24. ReadableByteStreamControllerShouldCallPull ( controller )
+static bool
+ReadableStreamControllerShouldCallPull(NativeObject* controller)
+{
+    // Step 1: Let stream be controller.[[controlledReadableStream]].
+    ReadableStream* stream = StreamFromController(controller);
+
+    // Step 2: If stream.[[state]] is "closed" or stream.[[state]] is "errored",
+    //         return false.
+    // or, equivalently
+    // Step 2: If stream.[[state]] is not "readable", return false.
+    if (!stream->readable())
+        return false;
+
+    // Step 3: If controller.[[closeRequested]] is true, return false.
+    uint32_t flags = ControllerFlags(controller);
+    if (flags & ControllerFlag_CloseRequested)
+        return false;
+
+    // Step 4: If controller.[[started]] is false, return false.
+    if (!(flags & ControllerFlag_Started))
+        return false;
+
+    // Step 5: If ! IsReadableStreamLocked(stream) is true and
+    //         ! ReadableStreamGetNumReadRequests(stream) > 0, return true.
+    // Steps 5-6 of 3.12.24 are equivalent in our implementation.
+    if (stream->locked() && ReadableStreamGetNumReadRequests(stream) > 0)
+        return true;
+
+    // Step 6: Let desiredSize be ReadableStreamDefaultControllerGetDesiredSize(controller).
+    double desiredSize = ReadableStreamControllerGetDesiredSizeUnchecked(controller);
+
+    // Step 7: If desiredSize > 0, return true.
+    // Step 8: Return false.
+    // Steps 7-8 of 3.12.24 are equivalent in our implementation.
+    return desiredSize > 0;
+}
+
+// Streams spec, 3.9.4. ReadableStreamDefaultControllerClose ( controller )
+static MOZ_MUST_USE bool
+ReadableStreamDefaultControllerClose(JSContext* cx,
+                                     Handle<ReadableStreamDefaultController*> controller)
+{
+    // Step 1: Let stream be controller.[[controlledReadableStream]].
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+
+    // Step 2: Assert: controller.[[closeRequested]] is false.
+    MOZ_ASSERT(!(ControllerFlags(controller) & ControllerFlag_CloseRequested));
+
+    // Step 3: Assert: stream.[[state]] is "readable".
+    MOZ_ASSERT(stream->readable());
+
+    // Step 4: Set controller.[[closeRequested]] to true.
+    AddControllerFlags(controller, ControllerFlag_CloseRequested);
+
+    // Step 5: If controller.[[queue]] is empty, perform ! ReadableStreamClose(stream).
+    RootedNativeObject queue(cx);
+    queue = &controller->getFixedSlot(QueueContainerSlot_Queue).toObject().as<NativeObject>();
+    if (queue->getDenseInitializedLength() == 0)
+        return ReadableStreamCloseInternal(cx, stream);
+
+    return true;
+}
+
+static MOZ_MUST_USE bool
+EnqueueValueWithSize(JSContext* cx, HandleNativeObject container, HandleValue value,
+                     HandleValue sizeVal);
+
+// Streams spec, 3.9.5. ReadableStreamDefaultControllerEnqueue ( controller, chunk )
+static MOZ_MUST_USE bool
+ReadableStreamDefaultControllerEnqueue(JSContext* cx,
+                                       Handle<ReadableStreamDefaultController*> controller,
+                                       HandleValue chunk)
+{
+    // Step 1: Let stream be controller.[[controlledReadableStream]].
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+
+    // Step 2: Assert: controller.[[closeRequested]] is false.
+    MOZ_ASSERT(!(ControllerFlags(controller) & ControllerFlag_CloseRequested));
+
+    // Step 3: Assert: stream.[[state]] is "readable".
+    MOZ_ASSERT(stream->readable());
+
+    // Step 4: If ! IsReadableStreamLocked(stream) is true and
+    //         ! ReadableStreamGetNumReadRequests(stream) > 0, perform
+    //         ! ReadableStreamFulfillReadRequest(stream, chunk, false).
+    if (stream->locked() && ReadableStreamGetNumReadRequests(stream) > 0) {
+        if (!ReadableStreamFulfillReadOrReadIntoRequest(cx, stream, chunk, false))
+            return false;
+    } else {
+        // Step 5: Otherwise,
+        // Step a: Let chunkSize be 1.
+        RootedValue chunkSize(cx, NumberValue(1));
+        bool success = true;
+
+        // Step b: If controller.[[strategySize]] is not undefined,
+        RootedValue strategySize(cx);
+        strategySize = controller->getFixedSlot(DefaultControllerSlot_StrategySize);
+        if (!strategySize.isUndefined()) {
+            // Step i: Set chunkSize to Call(stream.[[strategySize]], undefined, chunk).
+            success = Call(cx, strategySize, UndefinedHandleValue, chunk, &chunkSize);
+        }
+
+        // Step c: Let enqueueResult be
+        //         EnqueueValueWithSize(controller, chunk, chunkSize).
+        if (success)
+            success = EnqueueValueWithSize(cx, controller, chunk, chunkSize);
+
+        if (!success) {
+            // Step b.ii: If chunkSize is an abrupt completion,
+            // and
+            // Step d: If enqueueResult is an abrupt completion,
+            RootedValue exn(cx);
+            if (!cx->getPendingException(&exn))
+                return false;
+
+            // Step b.ii.1: Perform
+            //         ! ReadableStreamDefaultControllerErrorIfNeeded(controller,
+            //                                                        chunkSize.[[Value]]).
+            if (!ReadableStreamDefaultControllerErrorIfNeeded(cx, controller, exn))
+                return false;
+
+            // Step b.ii.2: Return chunkSize.
+            return false;
+        }
+    }
+
+    // Step 6: Perform ! ReadableStreamDefaultControllerCallPullIfNeeded(controller).
+    if (!ReadableStreamControllerCallPullIfNeeded(cx, controller))
+        return false;
+
+    // Step 7: Return.
+    return true;
+}
+
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerClearPendingPullIntos(JSContext* cx, HandleNativeObject controller);
+
+// Streams spec, 3.9.6. ReadableStreamDefaultControllerError ( controller, e )
+// and
+// Streams spec, 3.12.10. ReadableByteStreamControllerError ( controller, e )
+static MOZ_MUST_USE bool
+ReadableStreamControllerError(JSContext* cx, HandleNativeObject controller, HandleValue e)
+{
+    MOZ_ASSERT(IsReadableStreamController(controller));
+
+    // Step 1: Let stream be controller.[[controlledReadableStream]].
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+
+    // Step 2: Assert: stream.[[state]] is "readable".
+    MOZ_ASSERT(stream->readable());
+
+    // Step 3 of 3.12.10:
+    // Perform ! ReadableByteStreamControllerClearPendingPullIntos(controller).
+    if (controller->is<ReadableByteStreamController>()) {
+        Rooted<ReadableByteStreamController*> byteStreamController(cx);
+        byteStreamController = &controller->as<ReadableByteStreamController>();
+        if (!ReadableByteStreamControllerClearPendingPullIntos(cx, byteStreamController))
+            return false;
+    }
+
+    // Step 3 (or 4): Perform ! ResetQueue(controller).
+    if (!ResetQueue(cx, controller))
+        return false;
+
+    // Step 4 (or 5): Perform ! ReadableStreamError(stream, e).
+    return ReadableStreamErrorInternal(cx, stream, e);
+}
+
+// Streams spec, 3.9.7. ReadableStreamDefaultControllerErrorIfNeeded ( controller, e ) nothrow
+static MOZ_MUST_USE bool
+ReadableStreamDefaultControllerErrorIfNeeded(JSContext* cx,
+                                             Handle<ReadableStreamDefaultController*> controller,
+                                             HandleValue e)
+{
+    // Step 1: If controller.[[controlledReadableStream]].[[state]] is "readable",
+    //         perform ! ReadableStreamDefaultControllerError(controller, e).
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+    if (stream->readable())
+        return ReadableStreamControllerError(cx, controller, e);
+    return true;
+}
+
+// Streams spec, 3.9.8. ReadableStreamDefaultControllerGetDesiredSize ( controller )
+// and
+// Streams spec 3.12.13. ReadableByteStreamControllerGetDesiredSize ( controller )
+static MOZ_MUST_USE double
+ReadableStreamControllerGetDesiredSizeUnchecked(NativeObject* controller)
+{
+    // Steps 1-4 done at callsites, so only assert that they have been done.
+#if DEBUG
+    ReadableStream* stream = StreamFromController(controller);
+    MOZ_ASSERT(!(stream->errored() || stream->closed()));
+#endif // DEBUG
+
+    // Step 5: Return controller.[[strategyHWM]] − controller.[[queueTotalSize]].
+    double strategyHWM = controller->getFixedSlot(ControllerSlot_StrategyHWM).toNumber();
+    double queueSize = controller->getFixedSlot(QueueContainerSlot_TotalSize).toNumber();
+    return strategyHWM - queueSize;
+}
+
+// Streams spec, 3.10.3 new ReadableByteStreamController ( stream, underlyingSource,
+//                                                         highWaterMark )
+// Steps 3 - 16.
+static MOZ_MUST_USE ReadableByteStreamController*
+CreateReadableByteStreamController(JSContext* cx, Handle<ReadableStream*> stream,
+                                   HandleValue underlyingByteSource,
+                                   HandleValue highWaterMarkVal)
+{
+    Rooted<ReadableByteStreamController*> controller(cx);
+    controller = NewBuiltinClassInstance<ReadableByteStreamController>(cx);
+    if (!controller)
+        return nullptr;
+
+    // Step 3: Set this.[[controlledReadableStream]] to stream.
+    controller->setFixedSlot(ControllerSlot_Stream, ObjectValue(*stream));
+
+    // Step 4: Set this.[[underlyingByteSource]] to underlyingByteSource.
+    controller->setFixedSlot(ControllerSlot_UnderlyingSource, underlyingByteSource);
+
+    // Step 5: Set this.[[pullAgain]], and this.[[pulling]] to false.
+    controller->setFixedSlot(ControllerSlot_Flags, Int32Value(0));
+
+    // Step 6: Perform ! ReadableByteStreamControllerClearPendingPullIntos(this).
+    if (!ReadableByteStreamControllerClearPendingPullIntos(cx, controller))
+        return nullptr;
+
+    // Step 7: Perform ! ResetQueue(this).
+    if (!ResetQueue(cx, controller))
+        return nullptr;
+
+    // Step 8: Set this.[[started]] and this.[[closeRequested]] to false.
+    // These should be false by default, unchanged since step 5.
+    MOZ_ASSERT(ControllerFlags(controller) == 0);
+
+    // Step 9: Set this.[[strategyHWM]] to
+    //         ? ValidateAndNormalizeHighWaterMark(highWaterMark).
+    double highWaterMark;
+    if (!ValidateAndNormalizeHighWaterMark(cx, highWaterMarkVal, &highWaterMark))
+        return nullptr;
+    controller->setFixedSlot(ControllerSlot_StrategyHWM, NumberValue(highWaterMark));
+
+    // Step 10: Let autoAllocateChunkSize be
+    //          ? GetV(underlyingByteSource, "autoAllocateChunkSize").
+    RootedValue autoAllocateChunkSize(cx);
+    if (!GetProperty(cx, underlyingByteSource, cx->names().autoAllocateChunkSize,
+                     &autoAllocateChunkSize))
+    {
+        return nullptr;
+    }
+
+    // Step 11: If autoAllocateChunkSize is not undefined,
+    if (!autoAllocateChunkSize.isUndefined()) {
+        // Step a: If ! IsInteger(autoAllocateChunkSize) is false, or if
+        //         autoAllocateChunkSize ≤ 0, throw a RangeError exception.
+        if (!IsInteger(autoAllocateChunkSize) || autoAllocateChunkSize.toNumber() <= 0) {
+            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                      JSMSG_READABLEBYTESTREAMCONTROLLER_BAD_CHUNKSIZE);
+            return nullptr;
+    }
+    }
+
+    // Step 12: Set this.[[autoAllocateChunkSize]] to autoAllocateChunkSize.
+    controller->setFixedSlot(ByteControllerSlot_AutoAllocateSize, autoAllocateChunkSize);
+
+    // Step 13: Set this.[[pendingPullIntos]] to a new empty List.
+    if (!SetNewList(cx, controller, ByteControllerSlot_PendingPullIntos))
+        return nullptr;
+
+    // Step 14: Let controller be this (implicit).
+
+    // Step 15: Let startResult be
+    //          ? InvokeOrNoop(underlyingSource, "start", « this »).
+    RootedValue startResult(cx);
+    RootedValue controllerVal(cx, ObjectValue(*controller));
+    if (!InvokeOrNoop(cx, underlyingByteSource, cx->names().start, controllerVal, &startResult))
+        return nullptr;
+
+    // Step 16: Let startPromise be a promise resolved with startResult:
+    RootedObject startPromise(cx, PromiseObject::unforgeableResolve(cx, startResult));
+    if (!startPromise)
+        return nullptr;
+
+    RootedObject onStartFulfilled(cx, NewHandler(cx, ControllerStartHandler, controller));
+    if (!onStartFulfilled)
+        return nullptr;
+
+    RootedObject onStartRejected(cx, NewHandler(cx, ControllerStartFailedHandler, controller));
+    if (!onStartRejected)
+        return nullptr;
+
+    if (!JS::AddPromiseReactions(cx, startPromise, onStartFulfilled, onStartRejected))
+        return nullptr;
+
+    return controller;
+}
+
+bool
+ReadableByteStreamController::hasExternalSource() {
+    return ControllerFlags(this) & ControllerFlag_ExternalSource;
+}
+
+// Streams spec, 3.10.3.
+// new ReadableByteStreamController ( stream, underlyingByteSource,
+//                                    highWaterMark )
+bool
+ReadableByteStreamController::constructor(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    if (!ThrowIfNotConstructing(cx, args, "ReadableByteStreamController"))
+        return false;
+
+    // Step 1: If ! IsReadableStream(stream) is false, throw a TypeError exception.
+    HandleValue streamVal = args.get(0);
+    if (!Is<ReadableStream>(streamVal)) {
+        ReportArgTypeError(cx, "ReadableStreamDefaultController", "ReadableStream",
+                           args.get(0));
+        return false;
+    }
+
+    Rooted<ReadableStream*> stream(cx, &streamVal.toObject().as<ReadableStream>());
+
+    // Step 2: If stream.[[readableStreamController]] is not undefined, throw a
+    //         TypeError exception.
+    if (HasController(stream)) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAM_CONTROLLER_SET);
+        return false;
+    }
+
+    RootedObject controller(cx, CreateReadableByteStreamController(cx, stream, args.get(1),
+                                                                   args.get(2)));
+    if (!controller)
+        return false;
+
+    args.rval().setObject(*controller);
+    return true;
+}
+
+// Version of the ReadableByteStreamConstructor that's specialized for
+// handling external, embedding-provided, underlying sources.
+static MOZ_MUST_USE ReadableByteStreamController*
+CreateReadableByteStreamController(JSContext* cx, Handle<ReadableStream*> stream,
+                                   void* underlyingSource)
+{
+    Rooted<ReadableByteStreamController*> controller(cx);
+    controller = NewBuiltinClassInstance<ReadableByteStreamController>(cx);
+    if (!controller)
+        return nullptr;
+
+    // Step 3: Set this.[[controlledReadableStream]] to stream.
+    controller->setFixedSlot(ControllerSlot_Stream, ObjectValue(*stream));
+
+    // Step 4: Set this.[[underlyingByteSource]] to underlyingByteSource.
+    controller->setFixedSlot(ControllerSlot_UnderlyingSource, PrivateValue(underlyingSource));
+
+    // Step 5: Set this.[[pullAgain]], and this.[[pulling]] to false.
+    controller->setFixedSlot(ControllerSlot_Flags, Int32Value(ControllerFlag_ExternalSource));
+
+    // Step 6: Perform ! ReadableByteStreamControllerClearPendingPullIntos(this).
+    // Omitted.
+
+    // Step 7: Perform ! ResetQueue(this).
+    controller->setFixedSlot(QueueContainerSlot_TotalSize, Int32Value(0));
+
+    // Step 8: Set this.[[started]] and this.[[closeRequested]] to false.
+    // Step 9: Set this.[[strategyHWM]] to
+    //         ? ValidateAndNormalizeHighWaterMark(highWaterMark).
+    controller->setFixedSlot(ControllerSlot_StrategyHWM, Int32Value(0));
+
+    // Step 10: Let autoAllocateChunkSize be
+    //          ? GetV(underlyingByteSource, "autoAllocateChunkSize").
+    // Step 11: If autoAllocateChunkSize is not undefined,
+    // Step 12: Set this.[[autoAllocateChunkSize]] to autoAllocateChunkSize.
+    // Omitted.
+
+    // Step 13: Set this.[[pendingPullIntos]] to a new empty List.
+    if (!SetNewList(cx, controller, ByteControllerSlot_PendingPullIntos))
+        return nullptr;
+
+    // Step 14: Let controller be this (implicit).
+    // Step 15: Let startResult be
+    //          ? InvokeOrNoop(underlyingSource, "start", « this »).
+    // Omitted.
+
+    // Step 16: Let startPromise be a promise resolved with startResult:
+    RootedObject startPromise(cx, PromiseObject::unforgeableResolve(cx, UndefinedHandleValue));
+    if (!startPromise)
+        return nullptr;
+
+    RootedObject onStartFulfilled(cx, NewHandler(cx, ControllerStartHandler, controller));
+    if (!onStartFulfilled)
+        return nullptr;
+
+    RootedObject onStartRejected(cx, NewHandler(cx, ControllerStartFailedHandler, controller));
+    if (!onStartRejected)
+        return nullptr;
+
+    if (!JS::AddPromiseReactions(cx, startPromise, onStartFulfilled, onStartRejected))
+        return nullptr;
+
+    return controller;
+}
+
+static MOZ_MUST_USE ReadableStreamBYOBRequest*
+CreateReadableStreamBYOBRequest(JSContext* cx, Handle<ReadableByteStreamController*> controller,
+                                HandleObject view);
+
+// Streams spec, 3.10.4.1. get byobRequest
+static MOZ_MUST_USE bool
+ReadableByteStreamController_byobRequest_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableByteStreamController*> controller(cx);
+    controller = &args.thisv().toObject().as<ReadableByteStreamController>();
+
+    // Step 2: If this.[[byobRequest]] is undefined and this.[[pendingPullIntos]]
+    //         is not empty,
+    Value val = controller->getFixedSlot(ByteControllerSlot_BYOBRequest);
+    RootedValue byobRequest(cx, val);
+    val = controller->getFixedSlot(ByteControllerSlot_PendingPullIntos);
+    RootedNativeObject pendingPullIntos(cx, &val.toObject().as<NativeObject>());
+
+    if (byobRequest.isUndefined() && pendingPullIntos->getDenseInitializedLength() != 0) {
+        // Step a: Let firstDescriptor be the first element of this.[[pendingPullIntos]].
+        Rooted<PullIntoDescriptor*> firstDescriptor(cx);
+        firstDescriptor = PeekList<PullIntoDescriptor>(pendingPullIntos);
+
+        // Step b: Let view be ! Construct(%Uint8Array%,
+        //  « firstDescriptor.[[buffer]],
+        //  firstDescriptor.[[byteOffset]] + firstDescriptor.[[bytesFilled]],
+        //  firstDescriptor.[[byteLength]] − firstDescriptor.[[bytesFilled]] »).
+        RootedArrayBufferObject buffer(cx, firstDescriptor->buffer());
+        uint32_t bytesFilled = firstDescriptor->bytesFilled();
+        RootedObject view(cx, JS_NewUint8ArrayWithBuffer(cx, buffer,
+                                                         firstDescriptor->byteOffset() + bytesFilled,
+                                                         firstDescriptor->byteLength() - bytesFilled));
+        if (!view)
+            return false;
+
+        // Step c: Set this.[[byobRequest]] to
+        //         ! Construct(ReadableStreamBYOBRequest, « this, view »).
+        RootedObject request(cx, CreateReadableStreamBYOBRequest(cx, controller, view));
+        if (!request)
+            return false;
+        byobRequest = ObjectValue(*request);
+        controller->setFixedSlot(ByteControllerSlot_BYOBRequest, byobRequest);
+    }
+
+    // Step 3: Return this.[[byobRequest]].
+    args.rval().set(byobRequest);
+    return true;
+}
+
+static bool
+ReadableByteStreamController_byobRequest(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If IsReadableByteStreamController(this) is false, throw a TypeError
+    //         exception.
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableByteStreamController>,
+                                ReadableByteStreamController_byobRequest_impl>(cx, args);
+}
+
+// Streams spec, 3.10.4.2. get desiredSize
+// Combined with 3.8.4.1 above.
+
+static bool
+ReadableByteStreamController_desiredSize(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableByteStreamController(this) is false, throw a
+    //         TypeError exception.
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableByteStreamController>,
+                                ReadableStreamController_desiredSize_impl>(cx, args);
+}
+
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerClose(JSContext* cx, Handle<ReadableByteStreamController*> controller);
+
+// Streams spec, 3.10.4.3. close()
+static MOZ_MUST_USE bool
+ReadableByteStreamController_close_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableByteStreamController*> controller(cx);
+    controller = &args.thisv().toObject().as<ReadableByteStreamController>();
+
+    // Steps 2-3.
+    if (!VerifyControllerStateForClosing(cx, controller))
+        return false;
+
+    // Step 4: Perform ? ReadableByteStreamControllerClose(this).
+    if (!ReadableByteStreamControllerClose(cx, controller))
+        return false;
+    args.rval().setUndefined();
+    return true;
+}
+
+static bool
+ReadableByteStreamController_close(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableByteStreamController(this) is false, throw a
+    //         TypeError exception.
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableByteStreamController>,
+                                ReadableByteStreamController_close_impl>(cx, args);
+}
+
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerEnqueue(JSContext* cx,
+                                    Handle<ReadableByteStreamController*> controller,
+                                    HandleObject chunk);
+
+// Streams spec, 3.10.4.4. enqueue ( chunk )
+static MOZ_MUST_USE bool
+ReadableByteStreamController_enqueue_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableByteStreamController*> controller(cx);
+    controller = &args.thisv().toObject().as<ReadableByteStreamController>();
+    HandleValue chunkVal = args.get(0);
+
+    // Step 2: If this.[[closeRequested]] is true, throw a TypeError exception.
+    if (ControllerFlags(controller) & ControllerFlag_CloseRequested) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMCONTROLLER_CLOSED, "enqueue");
+        return false;
+    }
+
+    // Step 3: If this.[[controlledReadableStream]].[[state]] is not "readable",
+    //         throw a TypeError exception.
+    if (!StreamFromController(controller)->readable()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMCONTROLLER_NOT_READABLE, "enqueue");
+        return false;
+    }
+
+    // Step 4: If Type(chunk) is not Object, throw a TypeError exception.
+    // Step 5: If chunk does not have a [[ViewedArrayBuffer]] internal slot,
+    //         throw a TypeError exception.
+    if (!chunkVal.isObject() || !JS_IsArrayBufferViewObject(&chunkVal.toObject())) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLEBYTESTREAMCONTROLLER_BAD_CHUNK,
+                                  "ReadableByteStreamController#enqueue");
+        return false;
+    }
+    RootedObject chunk(cx, &chunkVal.toObject());
+
+    // Step 6: Return ! ReadableByteStreamControllerEnqueue(this, chunk).
+    if (!ReadableByteStreamControllerEnqueue(cx, controller, chunk))
+        return false;
+    args.rval().setUndefined();
+    return true;
+}
+
+static bool
+ReadableByteStreamController_enqueue(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableByteStreamController(this) is false, throw a
+    //         TypeError exception.
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableByteStreamController>,
+                                ReadableByteStreamController_enqueue_impl>(cx, args);
+}
+
+// Streams spec, 3.10.4.5. error ( e )
+static MOZ_MUST_USE bool
+ReadableByteStreamController_error_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableByteStreamController*> controller(cx);
+    controller = &args.thisv().toObject().as<ReadableByteStreamController>();
+    HandleValue e = args.get(0);
+
+    // Step 2: Let stream be this.[[controlledReadableStream]].
+    // Step 3: If stream.[[state]] is not "readable", throw a TypeError exception.
+    if (!StreamFromController(controller)->readable()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMCONTROLLER_NOT_READABLE, "error");
+        return false;
+    }
+
+    // Step 4: Perform ! ReadableByteStreamControllerError(this, e).
+    if (!ReadableStreamControllerError(cx, controller, e))
+        return false;
+    args.rval().setUndefined();
+    return true;
+}
+
+static bool
+ReadableByteStreamController_error(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableByteStreamController(this) is false, throw a
+    //         TypeError exception.
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableByteStreamController>,
+                                ReadableByteStreamController_error_impl>(cx, args);
+}
+
+static const JSPropertySpec ReadableByteStreamController_properties[] = {
+    JS_PSG("byobRequest", ReadableByteStreamController_byobRequest, 0),
+    JS_PSG("desiredSize", ReadableByteStreamController_desiredSize, 0),
+    JS_PS_END
+};
+
+static const JSFunctionSpec ReadableByteStreamController_methods[] = {
+    JS_FN("close",      ReadableByteStreamController_close,     0, 0),
+    JS_FN("enqueue",    ReadableByteStreamController_enqueue,   1, 0),
+    JS_FN("error",      ReadableByteStreamController_error,     1, 0),
+    JS_FS_END
+};
+
+static void
+ReadableByteStreamControllerFinalize(FreeOp* fop, JSObject* obj)
+{
+    ReadableByteStreamController& controller = obj->as<ReadableByteStreamController>();
+
+    if (controller.getFixedSlot(ControllerSlot_Flags).isUndefined())
+        return;
+
+    uint32_t flags = ControllerFlags(&controller);
+    if (!(flags & ControllerFlag_ExternalSource))
+        return;
+
+    uint8_t embeddingFlags = flags >> ControllerEmbeddingFlagsOffset;
+
+    void* underlyingSource = controller.getFixedSlot(ControllerSlot_UnderlyingSource).toPrivate();
+    obj->runtimeFromAnyThread()->readableStreamFinalizeCallback(underlyingSource, embeddingFlags);
+}
+
+static const ClassOps ReadableByteStreamControllerClassOps = {
+    nullptr,        /* addProperty */
+    nullptr,        /* delProperty */
+    nullptr,        /* getProperty */
+    nullptr,        /* setProperty */
+    nullptr,        /* enumerate */
+    nullptr,        /* resolve */
+    nullptr,        /* mayResolve */
+    ReadableByteStreamControllerFinalize,
+    nullptr,        /* call        */
+    nullptr,        /* hasInstance */
+    nullptr,        /* construct   */
+    nullptr,        /* trace   */
+};
+
+CLASS_SPEC(ReadableByteStreamController, 3, 9, ClassSpec::DontDefineConstructor,
+           JSCLASS_BACKGROUND_FINALIZE, &ReadableByteStreamControllerClassOps);
+
+// Streams spec, 3.10.5.1. [[PullSteps]] ()
+// Unified with 3.8.5.1 above.
+
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerHandleQueueDrain(JSContext* cx, HandleNativeObject controller);
+
+// Streams spec, 3.10.5.2. [[PullSteps]] ()
+static JSObject*
+ReadableByteStreamControllerPullSteps(JSContext* cx, HandleNativeObject controller)
+{
+    // Step 1: Let stream be this.[[controlledReadableStream]].
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+
+    // Step 2: MOZ_ASSERT: ! ReadableStreamHasDefaultReader(stream) is true.
+    MOZ_ASSERT(ReadableStreamHasDefaultReader(stream));
+
+    RootedValue val(cx);
+    // Step 3: If this.[[queueTotalSize]] > 0,
+    double queueTotalSize = controller->getFixedSlot(QueueContainerSlot_TotalSize).toNumber();
+    if (queueTotalSize > 0) {
+        // Step 3.a: MOZ_ASSERT: ! ReadableStreamGetNumReadRequests(_stream_) is 0.
+        MOZ_ASSERT(ReadableStreamGetNumReadRequests(stream) == 0);
+
+        RootedObject view(cx);
+
+        if (stream->mode() == JS::ReadableStreamMode::ExternalSource) {
+            val = controller->getFixedSlot(ControllerSlot_UnderlyingSource);
+            void* underlyingSource = val.toPrivate();
+
+            view = JS_NewUint8Array(cx, queueTotalSize);
+            if (!view)
+                return nullptr;
+
+            size_t bytesWritten;
+            {
+                JS::AutoCheckCannotGC noGC(cx);
+                bool dummy;
+                void* buffer = JS_GetArrayBufferViewData(view, &dummy, noGC);
+                auto cb = cx->runtime()->readableStreamWriteIntoReadRequestCallback;
+                MOZ_ASSERT(cb);
+                // TODO: use bytesWritten to correctly update the request's state.
+                cb(cx, stream, underlyingSource, stream->embeddingFlags(), buffer,
+                   queueTotalSize, &bytesWritten);
+            }
+
+            queueTotalSize = queueTotalSize - bytesWritten;
+        } else {
+            // Step 3.b: Let entry be the first element of this.[[queue]].
+            // Step 3.c: Remove entry from this.[[queue]], shifting all other elements
+            //           downward (so that the second becomes the first, and so on).
+            val = controller->getFixedSlot(QueueContainerSlot_Queue);
+            RootedNativeObject queue(cx, &val.toObject().as<NativeObject>());
+            Rooted<ByteStreamChunk*> entry(cx, ShiftFromList<ByteStreamChunk>(cx, queue));
+            MOZ_ASSERT(entry);
+
+            queueTotalSize = queueTotalSize - entry->byteLength();
+
+            // Step 3.f: Let view be ! Construct(%Uint8Array%, « entry.[[buffer]],
+            //                                   entry.[[byteOffset]], entry.[[byteLength]] »).
+            // (reordered)
+            RootedObject buffer(cx, entry->buffer());
+
+            uint32_t byteOffset = entry->byteOffset();
+            view = JS_NewUint8ArrayWithBuffer(cx, buffer, byteOffset, entry->byteLength());
+            if (!view)
+                return nullptr;
+        }
+
+        // Step 3.d: Set this.[[queueTotalSize]] to
+        //           this.[[queueTotalSize]] − entry.[[byteLength]].
+        // (reordered)
+        controller->setFixedSlot(QueueContainerSlot_TotalSize, Int32Value(queueTotalSize));
+
+        // Step 3.e: Perform ! ReadableByteStreamControllerHandleQueueDrain(this).
+        // (reordered)
+        if (!ReadableByteStreamControllerHandleQueueDrain(cx, controller))
+            return nullptr;
+
+        // Step 3.g: Return a promise resolved with ! CreateIterResultObject(view, false).
+        val.setObject(*view);
+        RootedObject iterResult(cx, CreateIterResultObject(cx, val, false));
+        if (!iterResult)
+            return nullptr;
+        val.setObject(*iterResult);
+
+        return PromiseObject::unforgeableResolve(cx, val);
+    }
+
+    // Step 4: Let autoAllocateChunkSize be this.[[autoAllocateChunkSize]].
+    val = controller->getFixedSlot(ByteControllerSlot_AutoAllocateSize);
+
+    // Step 5: If autoAllocateChunkSize is not undefined,
+    if (!val.isUndefined()) {
+        double autoAllocateChunkSize = val.toNumber();
+
+        // Step 5.a: Let buffer be Construct(%ArrayBuffer%, « autoAllocateChunkSize »).
+        RootedObject bufferObj(cx, JS_NewArrayBuffer(cx, autoAllocateChunkSize));
+
+        // Step 5.b: If buffer is an abrupt completion,
+        //           return a promise rejected with buffer.[[Value]].
+        if (!bufferObj)
+            return PromiseRejectedWithPendingError(cx);
+
+        RootedArrayBufferObject buffer(cx, &bufferObj->as<ArrayBufferObject>());
+
+        // Step 5.c: Let pullIntoDescriptor be Record {[[buffer]]: buffer.[[Value]],
+        //                                             [[byteOffset]]: 0,
+        //                                             [[byteLength]]: autoAllocateChunkSize,
+        //                                             [[bytesFilled]]: 0, [[elementSize]]: 1,
+        //                                             [[ctor]]: %Uint8Array%,
+        //                                             [[readerType]]: `"default"`}.
+        RootedObject pullIntoDescriptor(cx);
+        pullIntoDescriptor = PullIntoDescriptor::create(cx, buffer, 0,
+                                                        autoAllocateChunkSize, 0, 1,
+                                                        nullptr,
+                                                        ReaderType_Default);
+        if (!pullIntoDescriptor)
+            return PromiseRejectedWithPendingError(cx);
+
+        // Step 5.d: Append pullIntoDescriptor as the last element of this.[[pendingPullIntos]].
+        val = controller->getFixedSlot(ByteControllerSlot_PendingPullIntos);
+        RootedNativeObject pendingPullIntos(cx, &val.toObject().as<NativeObject>());
+        val = ObjectValue(*pullIntoDescriptor);
+        if (!AppendToList(cx, pendingPullIntos, val))
+            return nullptr;
+    }
+
+    // Step 6: Let promise be ! ReadableStreamAddReadRequest(stream).
+    Rooted<PromiseObject*> promise(cx, ReadableStreamAddReadRequest(cx, stream));
+    if (!promise)
+        return nullptr;
+
+    // Step 7: Perform ! ReadableByteStreamControllerCallPullIfNeeded(this).
+    if (!ReadableStreamControllerCallPullIfNeeded(cx, controller))
+        return nullptr;
+
+    // Step 8: Return promise.
+    return promise;
+}
+
+/**
+ * Unified implementation of ReadableStream controllers' [[PullSteps]] internal
+ * methods.
+ * Streams spec, 3.8.5.2. [[PullSteps]] ()
+ * and
+ * Streams spec, 3.10.5.2. [[PullSteps]] ()
+ */
+static MOZ_MUST_USE JSObject*
+ReadableStreamControllerPullSteps(JSContext* cx, HandleNativeObject controller)
+{
+    MOZ_ASSERT(IsReadableStreamController(controller));
+
+    if (controller->is<ReadableStreamDefaultController>())
+        return ReadableStreamDefaultControllerPullSteps(cx, controller);
+
+    return ReadableByteStreamControllerPullSteps(cx, controller);
+}
+
+
+static MOZ_MUST_USE ReadableStreamBYOBRequest*
+CreateReadableStreamBYOBRequest(JSContext* cx, Handle<ReadableByteStreamController*> controller,
+                                HandleObject view)
+{
+    MOZ_ASSERT(controller);
+    MOZ_ASSERT(JS_IsArrayBufferViewObject(view));
+
+    Rooted<ReadableStreamBYOBRequest*> request(cx);
+    request = NewBuiltinClassInstance<ReadableStreamBYOBRequest>(cx);
+    if (!request)
+        return nullptr;
+
+  // Step 1: Set this.[[associatedReadableByteStreamController]] to controller.
+  request->setFixedSlot(BYOBRequestSlot_Controller, ObjectValue(*controller));
+
+  // Step 2: Set this.[[view]] to view.
+  request->setFixedSlot(BYOBRequestSlot_View, ObjectValue(*view));
+
+  return request;
+}
+
+// Streams spec, 3.11.3. new ReadableStreamBYOBRequest ( controller, view )
+bool
+ReadableStreamBYOBRequest::constructor(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    HandleValue controllerVal = args.get(0);
+    HandleValue viewVal = args.get(1);
+
+    if (!ThrowIfNotConstructing(cx, args, "ReadableStreamBYOBRequest"))
+        return false;
+
+    // TODO: open PR against spec to add these checks.
+    // They're expected to have happened in code using requests.
+    if (!Is<ReadableByteStreamController>(controllerVal)) {
+        ReportArgTypeError(cx, "ReadableStreamBYOBRequest",
+                           "ReadableByteStreamController", args.get(0));
+        return false;
+    }
+
+    Rooted<ReadableByteStreamController*> controller(cx);
+    controller = &controllerVal.toObject().as<ReadableByteStreamController>();
+
+    if (!viewVal.isObject() || !JS_IsArrayBufferViewObject(&viewVal.toObject())) {
+        ReportArgTypeError(cx, "ReadableStreamBYOBRequest", "ArrayBuffer view",
+                           args.get(1));
+        return false;
+    }
+
+    RootedArrayBufferObject view(cx, &viewVal.toObject().as<ArrayBufferObject>());
+
+    RootedObject request(cx, CreateReadableStreamBYOBRequest(cx, controller, view));
+    if (!request)
+        return false;
+
+    args.rval().setObject(*request);
+    return true;
+}
+
+// Streams spec, 3.11.4.1 get view
+static MOZ_MUST_USE bool
+ReadableStreamBYOBRequest_view_impl(JSContext* cx, const CallArgs& args)
+{
+    // Step 2: Return this.[[view]].
+    NativeObject* request = &args.thisv().toObject().as<NativeObject>();
+    args.rval().set(request->getFixedSlot(BYOBRequestSlot_View));
+    return true;
+}
+
+static bool
+ReadableStreamBYOBRequest_view(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableStreamBYOBRequest(this) is false, throw a TypeError
+    //         exception.
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableStreamBYOBRequest>,
+                                ReadableStreamBYOBRequest_view_impl>(cx, args);
+}
+
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerRespond(JSContext* cx,
+                                    Handle<ReadableByteStreamController*> controller,
+                                    HandleValue bytesWrittenVal);
+
+// Streams spec, 3.11.4.2. respond ( bytesWritten )
+static MOZ_MUST_USE bool
+ReadableStreamBYOBRequest_respond_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableStreamBYOBRequest*> request(cx);
+    request = &args.thisv().toObject().as<ReadableStreamBYOBRequest>();
+    HandleValue bytesWritten = args.get(0);
+
+    // Step 2: If this.[[associatedReadableByteStreamController]] is undefined,
+    //         throw a TypeError exception.
+    RootedValue controllerVal(cx, request->getFixedSlot(BYOBRequestSlot_Controller));
+    if (controllerVal.isUndefined()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMBYOBREQUEST_NO_CONTROLLER, "respond");
+        return false;
+    }
+
+    // Step 3: Return ?
+    // ReadableByteStreamControllerRespond(this.[[associatedReadableByteStreamController]],
+    //                                     bytesWritten).
+    Rooted<ReadableByteStreamController*> controller(cx);
+    controller = &controllerVal.toObject().as<ReadableByteStreamController>();
+
+    if (!ReadableByteStreamControllerRespond(cx, controller, bytesWritten))
+        return false;
+
+    args.rval().setUndefined();
+    return true;
+}
+
+static bool
+ReadableStreamBYOBRequest_respond(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableStreamBYOBRequest(this) is false, throw a TypeError
+    //         exception.
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableStreamBYOBRequest>,
+                                ReadableStreamBYOBRequest_respond_impl>(cx, args);
+}
+
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerRespondWithNewView(JSContext* cx,
+                                               Handle<ReadableByteStreamController*> controller,
+                                               HandleObject view);
+
+// Streams spec, 3.11.4.3. respondWithNewView ( view )
+static MOZ_MUST_USE bool
+ReadableStreamBYOBRequest_respondWithNewView_impl(JSContext* cx, const CallArgs& args)
+{
+    Rooted<ReadableStreamBYOBRequest*> request(cx);
+    request = &args.thisv().toObject().as<ReadableStreamBYOBRequest>();
+    HandleValue viewVal = args.get(0);
+
+    // Step 2: If this.[[associatedReadableByteStreamController]] is undefined,
+    //         throw a TypeError exception.
+    RootedValue controllerVal(cx, request->getFixedSlot(BYOBRequestSlot_Controller));
+    if (controllerVal.isUndefined()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMBYOBREQUEST_NO_CONTROLLER, "respondWithNewView");
+        return false;
+    }
+
+    // Step 3: If Type(chunk) is not Object, throw a TypeError exception.
+    // Step 4: If view does not have a [[ViewedArrayBuffer]] internal slot, throw
+    //         a TypeError exception.
+    if (!viewVal.isObject() || !JS_IsArrayBufferViewObject(&viewVal.toObject())) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLEBYTESTREAMCONTROLLER_BAD_CHUNK,
+                                  "ReadableStreamBYOBRequest#respondWithNewView");
+        return false;
+    }
+
+    // Step 5: Return ?
+    // ReadableByteStreamControllerRespondWithNewView(this.[[associatedReadableByteStreamController]],
+    //                                                view).
+    Rooted<ReadableByteStreamController*> controller(cx);
+    controller = &controllerVal.toObject().as<ReadableByteStreamController>();
+    RootedObject view(cx, &viewVal.toObject());
+
+    if (!ReadableByteStreamControllerRespondWithNewView(cx, controller, view))
+        return false;
+
+    args.rval().setUndefined();
+    return true;
+}
+
+static bool
+ReadableStreamBYOBRequest_respondWithNewView(JSContext* cx, unsigned argc, Value* vp)
+{
+    // Step 1: If ! IsReadableStreamBYOBRequest(this) is false, throw a TypeError
+    //         exception.
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return CallNonGenericMethod<Is<ReadableStreamBYOBRequest>,
+                                ReadableStreamBYOBRequest_respondWithNewView_impl>(cx, args);
+}
+
+static const JSPropertySpec ReadableStreamBYOBRequest_properties[] = {
+    JS_PSG("view", ReadableStreamBYOBRequest_view, 0),
+    JS_PS_END
+};
+
+static const JSFunctionSpec ReadableStreamBYOBRequest_methods[] = {
+    JS_FN("respond",            ReadableStreamBYOBRequest_respond,            1, 0),
+    JS_FN("respondWithNewView", ReadableStreamBYOBRequest_respondWithNewView, 1, 0),
+    JS_FS_END
+};
+
+CLASS_SPEC(ReadableStreamBYOBRequest, 3, 2, ClassSpec::DontDefineConstructor, 0,
+           JS_NULL_CLASS_OPS);
+
+// Streams spec, 3.12.1. IsReadableStreamBYOBRequest ( x )
+// Implemented via is<ReadableStreamBYOBRequest>()
+
+// Streams spec, 3.12.2. IsReadableByteStreamController ( x )
+// Implemented via is<ReadableByteStreamController>()
+
+// Streams spec, 3.12.3. ReadableByteStreamControllerCallPullIfNeeded ( controller )
+// Unified with 3.9.2 above.
+
+static void
+ReadableByteStreamControllerInvalidateBYOBRequest(NativeObject* controller);
+
+// Streams spec, 3.12.4. ReadableByteStreamControllerClearPendingPullIntos ( controller )
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerClearPendingPullIntos(JSContext* cx, HandleNativeObject controller)
+{
+    MOZ_ASSERT(controller->is<ReadableByteStreamController>());
+
+    // Step 1: Perform ! ReadableByteStreamControllerInvalidateBYOBRequest(controller).
+    ReadableByteStreamControllerInvalidateBYOBRequest(controller);
+
+    // Step 2: Set controller.[[pendingPullIntos]] to a new empty List.
+    return SetNewList(cx, controller, ByteControllerSlot_PendingPullIntos);
+}
+
+// Streams spec, 3.12.5. ReadableByteStreamControllerClose ( controller )
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerClose(JSContext* cx, Handle<ReadableByteStreamController*> controller)
+{
+    // Step 1: Let stream be controller.[[controlledReadableStream]].
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+
+    // Step 2: Assert: controller.[[closeRequested]] is false.
+    MOZ_ASSERT(!(ControllerFlags(controller) & ControllerFlag_CloseRequested));
+
+    // Step 3: Assert: stream.[[state]] is "readable".
+    MOZ_ASSERT(stream->readable());
+
+    // Step 4: If controller.[[queueTotalSize]] > 0,
+    double queueTotalSize = controller->getFixedSlot(QueueContainerSlot_TotalSize).toNumber();
+    if (queueTotalSize > 0) {
+        // Step a: Set controller.[[closeRequested]] to true.
+        AddControllerFlags(controller, ControllerFlag_CloseRequested);
+
+        // Step b: Return
+        return true;
+    }
+
+    // Step 5: If controller.[[pendingPullIntos]] is not empty,
+    RootedValue val(cx, controller->getFixedSlot(ByteControllerSlot_PendingPullIntos));
+    RootedNativeObject pendingPullIntos(cx, &val.toObject().as<NativeObject>());
+    if (pendingPullIntos->getDenseInitializedLength() != 0) {
+        // Step a: Let firstPendingPullInto be the first element of
+        //         controller.[[pendingPullIntos]].
+        Rooted<PullIntoDescriptor*> firstPendingPullInto(cx);
+        firstPendingPullInto = PeekList<PullIntoDescriptor>(pendingPullIntos);
+
+        // Step b: If firstPendingPullInto.[[bytesFilled]] > 0,
+        if (firstPendingPullInto->bytesFilled() > 0) {
+            // Step i: Let e be a new TypeError exception. {
+            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                      JSMSG_READABLEBYTESTREAMCONTROLLER_CLOSE_PENDING_PULL);
+            RootedValue e(cx);
+            // Not much we can do about uncatchable exceptions, just bail.
+            if (!cx->getPendingException(&e))
+                return false;
+            // Step ii: Perform ! ReadableByteStreamControllerError(controller, e).
+            if (!ReadableStreamControllerError(cx, controller, e))
+                return false;
+
+            // Step iii: Throw e.
+            return false;
+        }
+    }
+
+    // Step 6: Perform ! ReadableStreamClose(stream).
+    return ReadableStreamCloseInternal(cx, stream);
+}
+
+static MOZ_MUST_USE JSObject*
+ReadableByteStreamControllerConvertPullIntoDescriptor(JSContext* cx,
+                                                      Handle<PullIntoDescriptor*> pullIntoDescriptor);
+
+// Streams spec, 3.12.6. ReadableByteStreamControllerCommitPullIntoDescriptor ( stream, pullIntoDescriptor )
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerCommitPullIntoDescriptor(JSContext* cx, Handle<ReadableStream*> stream,
+                                                     Handle<PullIntoDescriptor*> pullIntoDescriptor)
+{
+    // Step 1: MOZ_ASSERT: stream.[[state]] is not "errored".
+    MOZ_ASSERT(!stream->errored());
+
+    // Step 2: Let done be false.
+    bool done = false;
+
+    // Step 3: If stream.[[state]] is "closed",
+    if (stream->closed()) {
+        // Step a: MOZ_ASSERT: pullIntoDescriptor.[[bytesFilled]] is 0.
+        MOZ_ASSERT(pullIntoDescriptor->bytesFilled() == 0);
+
+        // Step b: Set done to true.
+        done = true;
+    }
+
+    // Step 4: Let filledView be
+    //         ! ReadableByteStreamControllerConvertPullIntoDescriptor(pullIntoDescriptor).
+    RootedObject filledView(cx);
+    filledView = ReadableByteStreamControllerConvertPullIntoDescriptor(cx, pullIntoDescriptor);
+    if (!filledView)
+        return false;
+
+    // Step 5: If pullIntoDescriptor.[[readerType]] is "default",
+    uint32_t readerType = pullIntoDescriptor->readerType();
+    RootedValue filledViewVal(cx, ObjectValue(*filledView));
+    if (readerType == ReaderType_Default) {
+        // Step a: Perform ! ReadableStreamFulfillReadRequest(stream, filledView, done).
+        if (!ReadableStreamFulfillReadOrReadIntoRequest(cx, stream, filledViewVal, done))
+            return false;
+    } else {
+        // Step 6: Otherwise,
+        // Step a: MOZ_ASSERT: pullIntoDescriptor.[[readerType]] is "byob".
+        MOZ_ASSERT(readerType == ReaderType_BYOB);
+
+        // Step b: Perform ! ReadableStreamFulfillReadIntoRequest(stream, filledView, done).
+        if (!ReadableStreamFulfillReadOrReadIntoRequest(cx, stream, filledViewVal, done))
+            return false;
+    }
+
+    return true;
+}
+
+// Streams spec, 3.12.7. ReadableByteStreamControllerConvertPullIntoDescriptor ( pullIntoDescriptor )
+static MOZ_MUST_USE JSObject*
+ReadableByteStreamControllerConvertPullIntoDescriptor(JSContext* cx,
+                                                      Handle<PullIntoDescriptor*> pullIntoDescriptor)
+{
+    // Step 1: Let bytesFilled be pullIntoDescriptor.[[bytesFilled]].
+    uint32_t bytesFilled = pullIntoDescriptor->bytesFilled();
+
+    // Step 2: Let elementSize be pullIntoDescriptor.[[elementSize]].
+    uint32_t elementSize = pullIntoDescriptor->elementSize();
+
+    // Step 3: Assert: bytesFilled <= pullIntoDescriptor.[[byteLength]].
+    MOZ_ASSERT(bytesFilled <= pullIntoDescriptor->byteLength());
+
+    // Step 4: Assert: bytesFilled mod elementSize is 0.
+    MOZ_ASSERT(bytesFilled % elementSize == 0);
+
+    // Step 5: Return ! Construct(pullIntoDescriptor.[[ctor]],
+    //                            pullIntoDescriptor.[[buffer]],
+    //                            pullIntoDescriptor.[[byteOffset]],
+    //                            bytesFilled / elementSize).
+    RootedObject ctor(cx, pullIntoDescriptor->ctor());
+    if (!ctor) {
+        if (!GetBuiltinConstructor(cx, JSProto_Uint8Array, &ctor))
+            return nullptr;
+    }
+    RootedObject buffer(cx, pullIntoDescriptor->buffer());
+    uint32_t byteOffset = pullIntoDescriptor->byteOffset();
+    FixedConstructArgs<3> args(cx);
+    args[0].setObject(*buffer);
+    args[1].setInt32(byteOffset);
+    args[2].setInt32(bytesFilled / elementSize);
+    return JS_New(cx, ctor, args);
+}
+
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerEnqueueChunkToQueue(JSContext* cx,
+                                                Handle<ReadableByteStreamController*> controller,
+                                                HandleObject buffer, uint32_t byteOffset,
+                                                uint32_t byteLength);
+
+static MOZ_MUST_USE ArrayBufferObject*
+TransferArrayBuffer(JSContext* cx, HandleObject buffer);
+
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(JSContext* cx,
+                                                                 Handle<ReadableByteStreamController*> controller);
+
+// Streams spec, 3.12.8. ReadableByteStreamControllerEnqueue ( controller, chunk )
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerEnqueue(JSContext* cx,
+                                    Handle<ReadableByteStreamController*> controller,
+                                    HandleObject chunk)
+{
+    // Step 1: Let stream be controller.[[controlledReadableStream]].
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+
+    // Step 2: Assert: controller.[[closeRequested]] is false.
+    MOZ_ASSERT(!(ControllerFlags(controller) & ControllerFlag_CloseRequested));
+
+    // Step 3: Assert: stream.[[state]] is "readable".
+    MOZ_ASSERT(stream->readable());
+
+    // To make enqueuing chunks via JSAPI nicer, we want to be able to deal
+    // with ArrayBuffer objects in addition to ArrayBuffer views here.
+    // This cannot happen when enqueuing happens via
+    // ReadableByteStreamController_enqueue because that throws if invoked
+    // with anything but an ArrayBuffer view.
+
+    Rooted<ArrayBufferObject*> buffer(cx);
+    uint32_t byteOffset;
+    uint32_t byteLength;
+
+    if (chunk->is<ArrayBufferObject>()) {
+        // Steps 4-6 for ArrayBuffer objects.
+        buffer = &chunk->as<ArrayBufferObject>();
+        byteOffset = 0;
+        byteLength = buffer->byteLength();
+    } else {
+        // Step 4: Let buffer be chunk.[[ViewedArrayBuffer]].
+        bool dummy;
+        JSObject* bufferObj = JS_GetArrayBufferViewBuffer(cx, chunk, &dummy);
+        if (!bufferObj)
+            return false;
+        buffer = &bufferObj->as<ArrayBufferObject>();
+
+        // Step 5: Let byteOffset be chunk.[[ByteOffset]].
+        byteOffset = JS_GetArrayBufferViewByteOffset(chunk);
+
+        // Step 6: Let byteLength be chunk.[[ByteLength]].
+        byteLength = JS_GetArrayBufferViewByteLength(chunk);
+    }
+
+    // Step 7: Let transferredBuffer be ! TransferArrayBuffer(buffer).
+    RootedArrayBufferObject transferredBuffer(cx, TransferArrayBuffer(cx, buffer));
+    if (!transferredBuffer)
+        return false;
+
+    // Step 8: If ! ReadableStreamHasDefaultReader(stream) is true
+    if (ReadableStreamHasDefaultReader(stream)) {
+        // Step a: If ! ReadableStreamGetNumReadRequests(stream) is 0,
+        if (ReadableStreamGetNumReadRequests(stream) == 0) {
+            // Step i: Perform
+            //         ! ReadableByteStreamControllerEnqueueChunkToQueue(controller,
+            //                                                           transferredBuffer,
+            //                                                           byteOffset,
+            //                                                           byteLength).
+            if (!ReadableByteStreamControllerEnqueueChunkToQueue(cx, controller, transferredBuffer,
+                                                                 byteOffset, byteLength))
+            {
+                return false;
+            }
+        } else {
+            // Step b: Otherwise,
+            // Step i: Assert: controller.[[queue]] is empty.
+#if DEBUG
+            RootedValue val(cx, controller->getFixedSlot(QueueContainerSlot_Queue));
+            RootedNativeObject queue(cx, &val.toObject().as<NativeObject>());
+            MOZ_ASSERT(queue->getDenseInitializedLength() == 0);
+#endif // DEBUG
+
+            // Step ii: Let transferredView be
+            //          ! Construct(%Uint8Array%, transferredBuffer, byteOffset, byteLength).
+            RootedObject transferredView(cx, JS_NewUint8ArrayWithBuffer(cx, transferredBuffer,
+                                                                        byteOffset, byteLength));
+            if (!transferredView)
+                return false;
+
+            // Step iii: Perform ! ReadableStreamFulfillReadRequest(stream, transferredView, false).
+            RootedValue chunk(cx, ObjectValue(*transferredView));
+            if (!ReadableStreamFulfillReadOrReadIntoRequest(cx, stream, chunk, false))
+                return false;
+        }
+    } else if (ReadableStreamHasBYOBReader(stream)) {
+        // Step 9: Otherwise,
+        // Step a: If ! ReadableStreamHasBYOBReader(stream) is true,
+        // Step i: Perform
+        //         ! ReadableByteStreamControllerEnqueueChunkToQueue(controller,
+        //                                                           transferredBuffer,
+        //                                                           byteOffset,
+        //                                                           byteLength).
+        if (!ReadableByteStreamControllerEnqueueChunkToQueue(cx, controller, transferredBuffer,
+                                                             byteOffset, byteLength))
+        {
+            return false;
+        }
+
+        // Step ii: Perform ! ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(controller).
+        if (!ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(cx, controller))
+            return false;
+    } else {
+        // Step b: Otherwise,
+        // Step i: Assert: ! IsReadableStreamLocked(stream) is false.
+        MOZ_ASSERT(!stream->locked());
+
+        // Step ii: Perform
+        //          ! ReadableByteStreamControllerEnqueueChunkToQueue(controller,
+        //                                                            transferredBuffer,
+        //                                                            byteOffset,
+        //                                                            byteLength).
+        if (!ReadableByteStreamControllerEnqueueChunkToQueue(cx, controller, transferredBuffer,
+                                                            byteOffset, byteLength))
+        {
+            return false;
+        }
+    }
+
+    return true;
+}
+
+// Streams spec, 3.12.9.
+// ReadableByteStreamControllerEnqueueChunkToQueue ( controller, buffer,
+//                                                   byteOffset, byteLength )
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerEnqueueChunkToQueue(JSContext* cx,
+                                                Handle<ReadableByteStreamController*> controller,
+                                                HandleObject buffer, uint32_t byteOffset,
+                                                uint32_t byteLength)
+{
+    MOZ_ASSERT(controller->is<ReadableByteStreamController>(), "must operate on ReadableByteStreamController");
+
+    // Step 1: Append Record {[[buffer]]: buffer,
+    //                        [[byteOffset]]: byteOffset,
+    //                        [[byteLength]]: byteLength}
+    //         as the last element of controller.[[queue]].
+    RootedValue val(cx, controller->getFixedSlot(QueueContainerSlot_Queue));
+    RootedNativeObject queue(cx, &val.toObject().as<NativeObject>());
+
+    Rooted<ByteStreamChunk*> chunk(cx);
+    chunk = ByteStreamChunk::create(cx, buffer, byteOffset, byteLength);
+    if (!chunk)
+        return false;
+
+    RootedValue chunkVal(cx, ObjectValue(*chunk));
+    if (!AppendToList(cx, queue, chunkVal))
+        return false;
+
+    // Step 2: Add byteLength to controller.[[queueTotalSize]].
+    double queueTotalSize = controller->getFixedSlot(QueueContainerSlot_TotalSize).toNumber();
+    controller->setFixedSlot(QueueContainerSlot_TotalSize,
+                             NumberValue(queueTotalSize + byteLength));
+
+    return true;
+}
+
+// Streams spec, 3.12.10. ReadableByteStreamControllerError ( controller, e )
+// Unified with 3.9.6 above.
+
+// Streams spec, 3.12.11. ReadableByteStreamControllerFillHeadPullIntoDescriptor ( controler, size, pullIntoDescriptor )
+static void
+ReadableByteStreamControllerFillHeadPullIntoDescriptor(ReadableByteStreamController* controller, uint32_t size,
+                                                       Handle<PullIntoDescriptor*> pullIntoDescriptor)
+{
+    // Step 1: Assert: either controller.[[pendingPullIntos]] is empty, or the
+    //         first element of controller.[[pendingPullIntos]] is pullIntoDescriptor.
+#if DEBUG
+    Value val = controller->getFixedSlot(ByteControllerSlot_PendingPullIntos);
+    NativeObject* pendingPullIntos = &val.toObject().as<NativeObject>();
+    MOZ_ASSERT(pendingPullIntos->getDenseInitializedLength() == 0 ||
+               &pendingPullIntos->getDenseElement(0).toObject() == pullIntoDescriptor);
+#endif // DEBUG
+
+    // Step 2: Perform ! ReadableByteStreamControllerInvalidateBYOBRequest(controller).
+    ReadableByteStreamControllerInvalidateBYOBRequest(controller);
+
+    // Step 3: Set pullIntoDescriptor.[[bytesFilled]] to pullIntoDescriptor.[[bytesFilled]] + size.
+    pullIntoDescriptor->setBytesFilled(pullIntoDescriptor->bytesFilled() + size);
+}
+
+// Streams spec, 3.12.12. ReadableByteStreamControllerFillPullIntoDescriptorFromQueue ( controller, pullIntoDescriptor )
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerFillPullIntoDescriptorFromQueue(JSContext* cx,
+                                                            Handle<ReadableByteStreamController*> controller,
+                                                            Handle<PullIntoDescriptor*> pullIntoDescriptor,
+                                                            bool* ready)
+{
+    *ready = false;
+
+    // Step 1: Let elementSize be pullIntoDescriptor.[[elementSize]].
+    uint32_t elementSize = pullIntoDescriptor->elementSize();
+
+    // Step 2: Let currentAlignedBytes be pullIntoDescriptor.[[bytesFilled]] −
+    //         (pullIntoDescriptor.[[bytesFilled]] mod elementSize).
+    uint32_t bytesFilled = pullIntoDescriptor->bytesFilled();
+    uint32_t currentAlignedBytes = bytesFilled - (bytesFilled % elementSize);
+
+    // Step 3: Let maxBytesToCopy be min(controller.[[queueTotalSize]],
+    //         pullIntoDescriptor.[[byteLength]] − pullIntoDescriptor.[[bytesFilled]]).
+    uint32_t byteLength = pullIntoDescriptor->byteLength();
+
+    // The queue size could be negative or overflow uint32_t. We cannot
+    // validly have a maxBytesToCopy value that'd overflow uint32_t, though,
+    // so just clamp to that.
+    Value sizeVal = controller->getFixedSlot(QueueContainerSlot_TotalSize);
+    uint32_t queueTotalSize = JS::ToUint32(sizeVal.toNumber());
+    uint32_t maxBytesToCopy = std::min(queueTotalSize, byteLength - bytesFilled);
+
+    // Step 4: Let maxBytesFilled be pullIntoDescriptor.[[bytesFilled]] + maxBytesToCopy.
+    uint32_t maxBytesFilled = bytesFilled + maxBytesToCopy;
+
+    // Step 5: Let maxAlignedBytes be maxBytesFilled − (maxBytesFilled mod elementSize).
+    uint32_t maxAlignedBytes = maxBytesFilled - (maxBytesFilled % elementSize);
+
+    // Step 6: Let totalBytesToCopyRemaining be maxBytesToCopy.
+    uint32_t totalBytesToCopyRemaining = maxBytesToCopy;
+
+    // Step 7: Let ready be false (implicit).
+
+    // Step 8: If maxAlignedBytes > currentAlignedBytes,
+    if (maxAlignedBytes > currentAlignedBytes) {
+        // Step a: Set totalBytesToCopyRemaining to maxAlignedBytes −
+        //         pullIntoDescriptor.[[bytesFilled]].
+        totalBytesToCopyRemaining = maxAlignedBytes - bytesFilled;
+
+        // Step b: Let ready be true.
+        *ready = true;
+    }
+
+    if (ControllerFlags(controller) & ControllerFlag_ExternalSource) {
+        // TODO: it probably makes sense to eagerly drain the underlying source.
+        // We have a buffer lying around anyway, whereas the source might be
+        // able to free or reuse buffers once their content is copied into
+        // our buffer.
+        if (!ready)
+            return true;
+
+        Value val = controller->getFixedSlot(ControllerSlot_UnderlyingSource);
+        void* underlyingSource = val.toPrivate();
+
+        RootedArrayBufferObject targetBuffer(cx, pullIntoDescriptor->buffer());
+        Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+
+        size_t bytesWritten;
+        {
+            JS::AutoCheckCannotGC noGC(cx);
+            bool dummy;
+            uint8_t* buffer = JS_GetArrayBufferData(targetBuffer, &dummy, noGC);
+            buffer += bytesFilled;
+            auto cb = cx->runtime()->readableStreamWriteIntoReadRequestCallback;
+            MOZ_ASSERT(cb);
+            cb(cx, stream, underlyingSource, stream->embeddingFlags(), buffer,
+               totalBytesToCopyRemaining, &bytesWritten);
+            pullIntoDescriptor->setBytesFilled(bytesFilled + bytesWritten);
+        }
+
+        queueTotalSize -= bytesWritten;
+        controller->setFixedSlot(QueueContainerSlot_TotalSize, Int32Value(queueTotalSize));
+
+        return true;
+    }
+
+    // Step 9: Let queue be controller.[[queue]].
+    RootedValue val(cx, controller->getFixedSlot(QueueContainerSlot_Queue));
+    RootedNativeObject queue(cx, &val.toObject().as<NativeObject>());
+
+    // Step 10: Repeat the following steps while totalBytesToCopyRemaining > 0,
+    Rooted<ByteStreamChunk*> headOfQueue(cx);
+    while (totalBytesToCopyRemaining > 0) {
+        MOZ_ASSERT(queue->getDenseInitializedLength() != 0);
+
+        // Step a: Let headOfQueue be the first element of queue.
+        headOfQueue = PeekList<ByteStreamChunk>(queue);
+
+        // Step b: Let bytesToCopy be min(totalBytesToCopyRemaining,
+        //                                headOfQueue.[[byteLength]]).
+        uint32_t byteLength = headOfQueue->byteLength();
+        uint32_t bytesToCopy = std::min(totalBytesToCopyRemaining, byteLength);
+
+        // Step c: Let destStart be pullIntoDescriptor.[[byteOffset]] +
+        //         pullIntoDescriptor.[[bytesFilled]].
+        uint32_t destStart = pullIntoDescriptor->byteOffset() + bytesFilled;
+
+        // Step d: Perform ! CopyDataBlockBytes(pullIntoDescriptor.[[buffer]].[[ArrayBufferData]],
+        //                                      destStart,
+        //                                      headOfQueue.[[buffer]].[[ArrayBufferData]],
+        //                                      headOfQueue.[[byteOffset]],
+        //                                      bytesToCopy).
+        RootedArrayBufferObject sourceBuffer(cx, headOfQueue->buffer());
+        uint32_t sourceOffset = headOfQueue->byteOffset();
+        RootedArrayBufferObject targetBuffer(cx, pullIntoDescriptor->buffer());
+        ArrayBufferObject::copyData(targetBuffer, destStart, sourceBuffer, sourceOffset,
+                                    bytesToCopy);
+
+        // Step e: If headOfQueue.[[byteLength]] is bytesToCopy,
+        if (byteLength == bytesToCopy) {
+            // Step i: Remove the first element of queue, shifting all other elements
+            //         downward (so that the second becomes the first, and so on).
+            headOfQueue = ShiftFromList<ByteStreamChunk>(cx, queue);
+            MOZ_ASSERT(headOfQueue);
+        } else {
+            // Step f: Otherwise,
+            // Step i: Set headOfQueue.[[byteOffset]] to headOfQueue.[[byteOffset]] +
+            //         bytesToCopy.
+            headOfQueue->SetByteOffset(sourceOffset + bytesToCopy);
+
+            // Step ii: Set headOfQueue.[[byteLength]] to headOfQueue.[[byteLength]] −
+            //          bytesToCopy.
+            headOfQueue->SetByteLength(byteLength - bytesToCopy);
+        }
+
+        // Step g: Set controller.[[queueTotalSize]] to
+        //         controller.[[queueTotalSize]] − bytesToCopy.
+        queueTotalSize = controller->getFixedSlot(QueueContainerSlot_TotalSize).toNumber();
+        queueTotalSize -= bytesToCopy;
+        controller->setFixedSlot(QueueContainerSlot_TotalSize, NumberValue(queueTotalSize));
+
+        // Step h: Perform ! ReadableByteStreamControllerFillHeadPullIntoDescriptor(controller,
+        //                                                                          bytesToCopy,
+        //                                                                          pullIntoDescriptor).
+        ReadableByteStreamControllerFillHeadPullIntoDescriptor(controller, bytesToCopy,
+                                                               pullIntoDescriptor);
+        bytesFilled += bytesToCopy;
+        MOZ_ASSERT(bytesFilled == pullIntoDescriptor->bytesFilled());
+
+        // Step i: Set totalBytesToCopyRemaining to totalBytesToCopyRemaining − bytesToCopy.
+        totalBytesToCopyRemaining -= bytesToCopy;
+    }
+
+    // Step 11: If ready is false,
+    if (!*ready) {
+        // Step a: Assert: controller.[[queueTotalSize]] is 0.
+        MOZ_ASSERT(controller->getFixedSlot(QueueContainerSlot_TotalSize).toNumber() == 0);
+
+        // Step b: Assert: pullIntoDescriptor.[[bytesFilled]] > 0.
+        MOZ_ASSERT(bytesFilled > 0, "should have filled some bytes");
+
+        // Step c: Assert: pullIntoDescriptor.[[bytesFilled]] <
+        //         pullIntoDescriptor.[[elementSize]].
+        MOZ_ASSERT(bytesFilled < elementSize);
+    }
+
+    // Step 12: Return ready.
+    return true;
+}
+
+// Streams spec 3.12.13. ReadableByteStreamControllerGetDesiredSize ( controller )
+// Unified with 3.9.8 above.
+
+// Streams spec, 3.12.14. ReadableByteStreamControllerHandleQueueDrain ( controller )
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerHandleQueueDrain(JSContext* cx, HandleNativeObject controller)
+{
+    MOZ_ASSERT(controller->is<ReadableByteStreamController>());
+
+    // Step 1: Assert: controller.[[controlledReadableStream]].[[state]] is "readable".
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+    MOZ_ASSERT(stream->readable());
+
+    // Step 2: If controller.[[queueTotalSize]] is 0 and
+    //         controller.[[closeRequested]] is true,
+    double totalSize = controller->getFixedSlot(QueueContainerSlot_TotalSize).toNumber();
+    bool closeRequested = ControllerFlags(controller) & ControllerFlag_CloseRequested;
+    if (totalSize == 0 && closeRequested) {
+      // Step a: Perform ! ReadableStreamClose(controller.[[controlledReadableStream]]).
+      return ReadableStreamCloseInternal(cx, stream);
+    }
+
+    // Step 3: Otherwise,
+    // Step a: Perform ! ReadableByteStreamControllerCallPullIfNeeded(controller).
+    return ReadableStreamControllerCallPullIfNeeded(cx, controller);
+}
+
+// Streams spec 3.12.15. ReadableByteStreamControllerInvalidateBYOBRequest ( controller )
+static void
+ReadableByteStreamControllerInvalidateBYOBRequest(NativeObject* controller)
+{
+    MOZ_ASSERT(controller->is<ReadableByteStreamController>());
+
+    // Step 1: If controller.[[byobRequest]] is undefined, return.
+    Value byobRequestVal = controller->getFixedSlot(ByteControllerSlot_BYOBRequest);
+    if (byobRequestVal.isUndefined())
+        return;
+
+    NativeObject* byobRequest = &byobRequestVal.toObject().as<NativeObject>();
+    // Step 2: Set controller.[[byobRequest]].[[associatedReadableByteStreamController]]
+    //         to undefined.
+    byobRequest->setFixedSlot(BYOBRequestSlot_Controller, UndefinedValue());
+
+    // Step 3: Set controller.[[byobRequest]].[[view]] to undefined.
+    byobRequest->setFixedSlot(BYOBRequestSlot_View, UndefinedValue());
+
+    // Step 4: Set controller.[[byobRequest]] to undefined.
+    controller->setFixedSlot(ByteControllerSlot_BYOBRequest, UndefinedValue());
+}
+
+static MOZ_MUST_USE PullIntoDescriptor*
+ReadableByteStreamControllerShiftPendingPullInto(JSContext* cx, HandleNativeObject controller);
+
+// Streams spec 3.12.16. ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue ( controller )
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(JSContext* cx,
+                                                                 Handle<ReadableByteStreamController*> controller)
+{
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+
+    // Step 1: Assert: controller.[[closeRequested]] is false.
+    MOZ_ASSERT(!(ControllerFlags(controller) & ControllerFlag_CloseRequested));
+
+    // Step 2: Repeat the following steps while controller.[[pendingPullIntos]]
+    //         is not empty,
+    RootedValue val(cx, controller->getFixedSlot(ByteControllerSlot_PendingPullIntos));
+    RootedNativeObject pendingPullIntos(cx, &val.toObject().as<NativeObject>());
+    Rooted<PullIntoDescriptor*> pullIntoDescriptor(cx);
+    while (pendingPullIntos->getDenseInitializedLength() != 0) {
+        // Step a: If controller.[[queueTotalSize]] is 0, return.
+        double queueTotalSize = controller->getFixedSlot(QueueContainerSlot_TotalSize).toNumber();
+        if (queueTotalSize == 0)
+            return true;
+
+        // Step b: Let pullIntoDescriptor be the first element of
+        //         controller.[[pendingPullIntos]].
+        pullIntoDescriptor = PeekList<PullIntoDescriptor>(pendingPullIntos);
+
+        // Step c: If ! ReadableByteStreamControllerFillPullIntoDescriptorFromQueue(controller, pullIntoDescriptor)
+        //         is true,
+        bool ready;
+        if (!ReadableByteStreamControllerFillPullIntoDescriptorFromQueue(cx, controller,
+                                                                         pullIntoDescriptor,
+                                                                         &ready))
+        {
+            return false;
+        }
+        if (ready) {
+            // Step i: Perform ! ReadableByteStreamControllerShiftPendingPullInto(controller).
+            if (!ReadableByteStreamControllerShiftPendingPullInto(cx, controller))
+                return false;
+
+            // Step ii: Perform ! ReadableByteStreamControllerCommitPullIntoDescriptor(controller.[[controlledReadableStream]],
+            //                                                                         pullIntoDescriptor).
+            if (!ReadableByteStreamControllerCommitPullIntoDescriptor(cx, stream,
+                                                                      pullIntoDescriptor))
+            {
+                return false;
+            }
+        }
+    }
+
+    return true;
+}
+
+// Streams spec, 3.12.17. ReadableByteStreamControllerPullInto ( controller, view )
+static MOZ_MUST_USE JSObject*
+ReadableByteStreamControllerPullInto(JSContext* cx,
+                                     Handle<ReadableByteStreamController*> controller,
+                                     Handle<ArrayBufferViewObject*> view)
+{
+    MOZ_ASSERT(controller->is<ReadableByteStreamController>());
+
+    // Step 1: Let stream be controller.[[controlledReadableStream]].
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+
+    // Step 2: Let elementSize be 1.
+    uint32_t elementSize = 1;
+
+    RootedObject ctor(cx);
+    // Step 4: If view has a [[TypedArrayName]] internal slot (i.e., it is not a
+    //         DataView),
+    if (view->is<TypedArrayObject>()) {
+        JSProtoKey protoKey = StandardProtoKeyOrNull(view);
+        MOZ_ASSERT(protoKey);
+
+        if (!GetBuiltinConstructor(cx, protoKey, &ctor))
+            return nullptr;
+        elementSize = 1 << TypedArrayShift(view->as<TypedArrayObject>().type());
+    } else {
+        // Step 3: Let ctor be %DataView% (reordered).
+        if (!GetBuiltinConstructor(cx, JSProto_DataView, &ctor))
+            return nullptr;
+    }
+
+    // Step 5: Let pullIntoDescriptor be Record {[[buffer]]: view.[[ViewedArrayBuffer]],
+    //                                           [[byteOffset]]: view.[[ByteOffset]],
+    //                                           [[byteLength]]: view.[[ByteLength]],
+    //                                           [[bytesFilled]]: 0,
+    //                                           [[elementSize]]: elementSize,
+    //                                           [[ctor]]: ctor,
+    //                                           [[readerType]]: "byob"}.
+    bool dummy;
+    RootedArrayBufferObject buffer(cx, &JS_GetArrayBufferViewBuffer(cx, view, &dummy)
+                                       ->as<ArrayBufferObject>());
+    if (!buffer)
+        return nullptr;
+
+    uint32_t byteOffset = JS_GetArrayBufferViewByteOffset(view);
+    uint32_t byteLength = JS_GetArrayBufferViewByteLength(view);
+    Rooted<PullIntoDescriptor*> pullIntoDescriptor(cx);
+    pullIntoDescriptor = PullIntoDescriptor::create(cx, buffer, byteOffset, byteLength, 0,
+                                                    elementSize, ctor,
+                                                    ReaderType_BYOB);
+    if (!pullIntoDescriptor)
+        return nullptr;
+
+    // Step 6: If controller.[[pendingPullIntos]] is not empty,
+    RootedValue val(cx, controller->getFixedSlot(ByteControllerSlot_PendingPullIntos));
+    RootedNativeObject pendingPullIntos(cx, &val.toObject().as<NativeObject>());
+    if (pendingPullIntos->getDenseInitializedLength() != 0) {
+        // Step a: Set pullIntoDescriptor.[[buffer]] to
+        //         ! TransferArrayBuffer(pullIntoDescriptor.[[buffer]]).
+        RootedArrayBufferObject transferredBuffer(cx, TransferArrayBuffer(cx, buffer));
+        if (!transferredBuffer)
+            return nullptr;
+        pullIntoDescriptor->setBuffer(transferredBuffer);
+
+        // Step b: Append pullIntoDescriptor as the last element of
+        //         controller.[[pendingPullIntos]].
+        val = ObjectValue(*pullIntoDescriptor);
+        if (!AppendToList(cx, pendingPullIntos, val))
+            return nullptr;
+
+        // Step c: Return ! ReadableStreamAddReadIntoRequest(stream).
+        return ReadableStreamAddReadIntoRequest(cx, stream);
+    }
+
+    // Step 7: If stream.[[state]] is "closed",
+    if (stream->closed()) {
+        // Step a: Let emptyView be ! Construct(ctor, pullIntoDescriptor.[[buffer]],
+        //                                            pullIntoDescriptor.[[byteOffset]], 0).
+        FixedConstructArgs<3> args(cx);
+        args[0].setObject(*buffer);
+        args[1].setInt32(byteOffset);
+        args[2].setInt32(0);
+        RootedObject emptyView(cx, JS_New(cx, ctor, args));
+        if (!emptyView)
+            return nullptr;
+
+        // Step b: Return a promise resolved with
+        //         ! CreateIterResultObject(emptyView, true).
+        RootedValue val(cx, ObjectValue(*emptyView));
+        RootedObject iterResult(cx, CreateIterResultObject(cx, val, true));
+        if (!iterResult)
+            return nullptr;
+        val = ObjectValue(*iterResult);
+        return PromiseObject::unforgeableResolve(cx, val);
+    }
+
+    // Step 8: If controller.[[queueTotalSize]] > 0,
+    double queueTotalSize = controller->getFixedSlot(QueueContainerSlot_TotalSize).toNumber();
+    if (queueTotalSize > 0) {
+        // Step a: If ! ReadableByteStreamControllerFillPullIntoDescriptorFromQueue(controller,
+        //                                                                          pullIntoDescriptor)
+        //         is true,
+        bool ready;
+        if (!ReadableByteStreamControllerFillPullIntoDescriptorFromQueue(cx, controller,
+                                                                         pullIntoDescriptor, &ready))
+        {
+            return nullptr;
+        }
+
+        if (ready) {
+            // Step i: Let filledView be
+            //         ! ReadableByteStreamControllerConvertPullIntoDescriptor(pullIntoDescriptor).
+            RootedObject filledView(cx);
+            filledView = ReadableByteStreamControllerConvertPullIntoDescriptor(cx,
+                                                                               pullIntoDescriptor);
+            if (!filledView)
+                return nullptr;
+
+            // Step ii: Perform ! ReadableByteStreamControllerHandleQueueDrain(controller).
+            if (!ReadableByteStreamControllerHandleQueueDrain(cx, controller))
+                return nullptr;
+
+            // Step iii: Return a promise resolved with
+            //           ! CreateIterResultObject(filledView, false).
+            val = ObjectValue(*filledView);
+            RootedObject iterResult(cx, CreateIterResultObject(cx, val, false));
+            if (!iterResult)
+                return nullptr;
+            val = ObjectValue(*iterResult);
+            return PromiseObject::unforgeableResolve(cx, val);
+        }
+
+        // Step b: If controller.[[closeRequested]] is true,
+        if (ControllerFlags(controller) & ControllerFlag_CloseRequested) {
+            // Step i: Let e be a TypeError exception.
+            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                      JSMSG_READABLESTREAMCONTROLLER_CLOSED, "read");
+
+            // Not much we can do about uncatchable exceptions, just bail.
+            RootedValue e(cx);
+            if (!GetAndClearException(cx, &e))
+                return nullptr;
+
+            // Step ii: Perform ! ReadableByteStreamControllerError(controller, e).
+            if (!ReadableStreamControllerError(cx, controller, e))
+                return nullptr;
+
+            // Step iii: Return a promise rejected with e.
+            return PromiseObject::unforgeableReject(cx, e);
+        }
+    }
+
+    // Step 9: Set pullIntoDescriptor.[[buffer]] to
+    //         ! TransferArrayBuffer(pullIntoDescriptor.[[buffer]]).
+    RootedArrayBufferObject transferredBuffer(cx, TransferArrayBuffer(cx, buffer));
+    if (!transferredBuffer)
+        return nullptr;
+    pullIntoDescriptor->setBuffer(transferredBuffer);
+
+    // Step 10: Append pullIntoDescriptor as the last element of
+    //          controller.[[pendingPullIntos]].
+    val = ObjectValue(*pullIntoDescriptor);
+    if (!AppendToList(cx, pendingPullIntos, val))
+        return nullptr;
+
+    // Step 11: Let promise be ! ReadableStreamAddReadIntoRequest(stream).
+    Rooted<PromiseObject*> promise(cx, ReadableStreamAddReadIntoRequest(cx, stream));
+    if (!promise)
+        return nullptr;
+
+    // Step 12: Perform ! ReadableByteStreamControllerCallPullIfNeeded(controller).
+    if (!ReadableStreamControllerCallPullIfNeeded(cx, controller))
+        return nullptr;
+
+    // Step 13: Return promise.
+    return promise;
+}
+
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerRespondInternal(JSContext* cx,
+                                            Handle<ReadableByteStreamController*> controller,
+                                            double bytesWritten);
+
+// Streams spec 3.12.18. ReadableByteStreamControllerRespond( controller, bytesWritten )
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerRespond(JSContext* cx,
+                                    Handle<ReadableByteStreamController*> controller,
+                                    HandleValue bytesWrittenVal)
+{
+    MOZ_ASSERT(controller->is<ReadableByteStreamController>());
+
+    // Step 1: Let bytesWritten be ? ToNumber(bytesWritten).
+    double bytesWritten;
+    if (!ToNumber(cx, bytesWrittenVal, &bytesWritten))
+        return false;
+
+    // Step 2: If ! IsFiniteNonNegativeNumber(bytesWritten) is false,
+    if (bytesWritten < 0 || mozilla::IsNaN(bytesWritten) || mozilla::IsInfinite(bytesWritten)) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_NUMBER_MUST_BE_FINITE_NON_NEGATIVE, "bytesWritten");
+        return false;
+    }
+
+    // Step 3: Assert: controller.[[pendingPullIntos]] is not empty.
+#if DEBUG
+    Value val = controller->getFixedSlot(ByteControllerSlot_PendingPullIntos);
+    RootedNativeObject pendingPullIntos(cx, &val.toObject().as<NativeObject>());
+    MOZ_ASSERT(pendingPullIntos->getDenseInitializedLength() != 0);
+#endif // DEBUG
+
+    // Step 4: Perform ? ReadableByteStreamControllerRespondInternal(controller, bytesWritten).
+    return ReadableByteStreamControllerRespondInternal(cx, controller, bytesWritten);
+}
+
+// Streams spec 3.12.19. ReadableByteStreamControllerRespondInClosedState( controller, firstDescriptor )
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerRespondInClosedState(JSContext* cx,
+                                                   Handle<ReadableByteStreamController*> controller,
+                                                   Handle<PullIntoDescriptor*> firstDescriptor)
+{
+    // Step 1: Set firstDescriptor.[[buffer]] to
+    //         ! TransferArrayBuffer(firstDescriptor.[[buffer]]).
+    RootedArrayBufferObject buffer(cx, firstDescriptor->buffer());
+    RootedArrayBufferObject transferredBuffer(cx, TransferArrayBuffer(cx, buffer));
+    if (!transferredBuffer)
+        return false;
+    firstDescriptor->setBuffer(transferredBuffer);
+
+    // Step 2: Assert: firstDescriptor.[[bytesFilled]] is 0.
+    MOZ_ASSERT(firstDescriptor->bytesFilled() == 0);
+
+    // Step 3: Let stream be controller.[[controlledReadableStream]].
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+
+    // Step 4: If ReadableStreamHasBYOBReader(stream) is true,
+    if (ReadableStreamHasBYOBReader(stream)) {
+        // Step a: Repeat the following steps while
+        //         ! ReadableStreamGetNumReadIntoRequests(stream) > 0,
+        Rooted<PullIntoDescriptor*> descriptor(cx);
+        while (ReadableStreamGetNumReadRequests(stream) > 0) {
+            // Step i: Let pullIntoDescriptor be
+            //         ! ReadableByteStreamControllerShiftPendingPullInto(controller).
+            descriptor = ReadableByteStreamControllerShiftPendingPullInto(cx, controller);
+            if (!descriptor)
+                return false;
+
+            // Step ii: Perform !
+            //          ReadableByteStreamControllerCommitPullIntoDescriptor(stream,
+            //                                                               pullIntoDescriptor).
+            if (!ReadableByteStreamControllerCommitPullIntoDescriptor(cx, stream, descriptor))
+                return false;
+        }
+    }
+
+    return true;
+}
+
+// Streams spec 3.12.20.
+// ReadableByteStreamControllerRespondInReadableState( controller, bytesWritten, pullIntoDescriptor )
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerRespondInReadableState(JSContext* cx,
+                                                   Handle<ReadableByteStreamController*> controller,
+                                                   uint32_t bytesWritten,
+                                                   Handle<PullIntoDescriptor*> pullIntoDescriptor)
+{
+    // Step 1: If pullIntoDescriptor.[[bytesFilled]] + bytesWritten > pullIntoDescriptor.[[byteLength]],
+    //         throw a RangeError exception.
+    uint32_t bytesFilled = pullIntoDescriptor->bytesFilled();
+    uint32_t byteLength = pullIntoDescriptor->byteLength();
+    if (bytesFilled + bytesWritten > byteLength) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLEBYTESTREAMCONTROLLER_INVALID_BYTESWRITTEN);
+        return false;
+    }
+
+    // Step 2: Perform ! ReadableByteStreamControllerFillHeadPullIntoDescriptor(controller,
+    //                                                                          bytesWritten,
+    //                                                                          pullIntoDescriptor).
+    ReadableByteStreamControllerFillHeadPullIntoDescriptor(controller, bytesWritten,
+                                                           pullIntoDescriptor);
+    bytesFilled += bytesWritten;
+
+    // Step 3: If pullIntoDescriptor.[[bytesFilled]] <
+    //         pullIntoDescriptor.[[elementSize]], return.
+    uint32_t elementSize = pullIntoDescriptor->elementSize();
+    if (bytesFilled < elementSize)
+        return true;
+
+    // Step 4: Perform ! ReadableByteStreamControllerShiftPendingPullInto(controller).
+    if (!ReadableByteStreamControllerShiftPendingPullInto(cx, controller))
+        return false;
+
+    // Step 5: Let remainderSize be pullIntoDescriptor.[[bytesFilled]] mod
+    //         pullIntoDescriptor.[[elementSize]].
+    uint32_t remainderSize = bytesFilled % elementSize;
+
+    // Step 6: If remainderSize > 0,
+    RootedArrayBufferObject buffer(cx, pullIntoDescriptor->buffer());
+    if (remainderSize > 0) {
+        // Step a: Let end be pullIntoDescriptor.[[byteOffset]] +
+        //         pullIntoDescriptor.[[bytesFilled]].
+        uint32_t end = pullIntoDescriptor->byteOffset() + bytesFilled;
+
+        // Step b: Let remainder be ? CloneArrayBuffer(pullIntoDescriptor.[[buffer]],
+        //                                             end − remainderSize,
+        //                                             remainderSize, %ArrayBuffer%).
+        // TODO: this really, really should just use a slot to store the remainder.
+        RootedObject remainderObj(cx, JS_NewArrayBuffer(cx, remainderSize));
+        if (!remainderObj)
+            return false;
+        RootedArrayBufferObject remainder(cx, &remainderObj->as<ArrayBufferObject>());
+        ArrayBufferObject::copyData(remainder, 0, buffer, end - remainderSize, remainderSize);
+
+        // Step c: Perform ! ReadableByteStreamControllerEnqueueChunkToQueue(controller,
+        //                                                                   remainder, 0,
+        //                                                                   remainder.[[ByteLength]]).
+        // Note: `remainderSize` is equivalent to remainder.[[ByteLength]].
+        if (!ReadableByteStreamControllerEnqueueChunkToQueue(cx, controller, remainder, 0,
+                                                             remainderSize))
+        {
+            return false;
+        }
+    }
+
+    // Step 7: Set pullIntoDescriptor.[[buffer]] to
+    //         ! TransferArrayBuffer(pullIntoDescriptor.[[buffer]]).
+    RootedArrayBufferObject transferredBuffer(cx, TransferArrayBuffer(cx, buffer));
+    if (!transferredBuffer)
+        return false;
+    pullIntoDescriptor->setBuffer(transferredBuffer);
+
+    // Step 8: Set pullIntoDescriptor.[[bytesFilled]] to pullIntoDescriptor.[[bytesFilled]] −
+    //         remainderSize.
+    pullIntoDescriptor->setBytesFilled(bytesFilled - remainderSize);
+
+    // Step 9: Perform ! ReadableByteStreamControllerCommitPullIntoDescriptor(controller.[[controlledReadableStream]],
+    //                                                                        pullIntoDescriptor).
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+    if (!ReadableByteStreamControllerCommitPullIntoDescriptor(cx, stream, pullIntoDescriptor))
+        return false;
+
+    // Step 10: Perform ! ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(controller).
+    return ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(cx, controller);
+}
+
+// Streams spec, 3.12.21. ReadableByteStreamControllerRespondInternal ( controller, bytesWritten )
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerRespondInternal(JSContext* cx,
+                                            Handle<ReadableByteStreamController*> controller,
+                                            double bytesWritten)
+{
+    // Step 1: Let firstDescriptor be the first element of controller.[[pendingPullIntos]].
+    RootedValue val(cx, controller->getFixedSlot(ByteControllerSlot_PendingPullIntos));
+    RootedNativeObject pendingPullIntos(cx, &val.toObject().as<NativeObject>());
+    Rooted<PullIntoDescriptor*> firstDescriptor(cx);
+    firstDescriptor = PeekList<PullIntoDescriptor>(pendingPullIntos);
+
+    // Step 2: Let stream be controller.[[controlledReadableStream]].
+    Rooted<ReadableStream*> stream(cx, StreamFromController(controller));
+
+    // Step 3: If stream.[[state]] is "closed",
+    if (stream->closed()) {
+        // Step a: If bytesWritten is not 0, throw a TypeError exception.
+        if (bytesWritten != 0) {
+            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                      JSMSG_READABLESTREAMBYOBREQUEST_RESPOND_CLOSED);
+            return false;
+        }
+
+        // Step b: Perform
+        //         ! ReadableByteStreamControllerRespondInClosedState(controller, firstDescriptor).
+        return ReadableByteStreamControllerRespondInClosedState(cx, controller, firstDescriptor);
+    }
+
+    // Step 4: Otherwise,
+    // Step a: Assert: stream.[[state]] is "readable".
+    MOZ_ASSERT(stream->readable());
+
+    // Step b: Perform ? ReadableByteStreamControllerRespondInReadableState(controller,
+    //                                                                      bytesWritten,
+    //                                                                      firstDescriptor).
+    return ReadableByteStreamControllerRespondInReadableState(cx, controller, bytesWritten,
+                                                              firstDescriptor);
+}
+
+// Streams spec, 3.12.22. ReadableByteStreamControllerRespondWithNewView ( controller, view )
+static MOZ_MUST_USE bool
+ReadableByteStreamControllerRespondWithNewView(JSContext* cx,
+                                               Handle<ReadableByteStreamController*> controller,
+                                               HandleObject view)
+{
+    // Step 1: Assert: controller.[[pendingPullIntos]] is not empty.
+    RootedValue val(cx, controller->getFixedSlot(ByteControllerSlot_PendingPullIntos));
+    RootedNativeObject pendingPullIntos(cx, &val.toObject().as<NativeObject>());
+    MOZ_ASSERT(pendingPullIntos->getDenseInitializedLength() != 0);
+
+    // Step 2: Let firstDescriptor be the first element of controller.[[pendingPullIntos]].
+    Rooted<PullIntoDescriptor*> firstDescriptor(cx);
+    firstDescriptor = PeekList<PullIntoDescriptor>(pendingPullIntos);
+
+    // Step 3: If firstDescriptor.[[byteOffset]] + firstDescriptor.[[bytesFilled]]
+    //         is not view.[[ByteOffset]], throw a RangeError exception.
+    uint32_t byteOffset = uint32_t(JS_GetArrayBufferViewByteOffset(view));
+    if (firstDescriptor->byteOffset() + firstDescriptor->bytesFilled() != byteOffset) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLEBYTESTREAMCONTROLLER_INVALID_VIEW_OFFSET);
+        return false;
+    }
+
+    // Step 4: If firstDescriptor.[[byteLength]] is not view.[[ByteLength]],
+    //         throw a RangeError exception.
+    uint32_t byteLength = JS_GetArrayBufferViewByteLength(view);
+    if (firstDescriptor->byteLength() != byteLength) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLEBYTESTREAMCONTROLLER_INVALID_VIEW_SIZE);
+        return false;
+    }
+
+    // Step 5: Set firstDescriptor.[[buffer]] to view.[[ViewedArrayBuffer]].
+    bool dummy;
+    RootedArrayBufferObject buffer(cx,
+                                   &AsArrayBuffer(JS_GetArrayBufferViewBuffer(cx, view, &dummy)));
+    if (!buffer)
+        return false;
+    firstDescriptor->setBuffer(buffer);
+
+    // Step 6: Perform ? ReadableByteStreamControllerRespondInternal(controller,
+    //                                                               view.[[ByteLength]]).
+    return ReadableByteStreamControllerRespondInternal(cx, controller, byteLength);
+}
+
+// Streams spec, 3.12.23. ReadableByteStreamControllerShiftPendingPullInto ( controller )
+static MOZ_MUST_USE PullIntoDescriptor*
+ReadableByteStreamControllerShiftPendingPullInto(JSContext* cx, HandleNativeObject controller)
+{
+    MOZ_ASSERT(controller->is<ReadableByteStreamController>());
+
+    // Step 1: Let descriptor be the first element of controller.[[pendingPullIntos]].
+    // Step 2: Remove descriptor from controller.[[pendingPullIntos]], shifting
+    //         all other elements downward (so that the second becomes the first,
+    //         and so on).
+    RootedValue val(cx, controller->getFixedSlot(ByteControllerSlot_PendingPullIntos));
+    RootedNativeObject pendingPullIntos(cx, &val.toObject().as<NativeObject>());
+    Rooted<PullIntoDescriptor*> descriptor(cx);
+    descriptor = ShiftFromList<PullIntoDescriptor>(cx, pendingPullIntos);
+    MOZ_ASSERT(descriptor);
+
+    // Step 3: Perform ! ReadableByteStreamControllerInvalidateBYOBRequest(controller).
+    ReadableByteStreamControllerInvalidateBYOBRequest(controller);
+
+    // Step 4: Return descriptor.
+    return descriptor;
+}
+
+// Streams spec, 3.12.24. ReadableByteStreamControllerShouldCallPull ( controller )
+// Unified with 3.9.3 above.
+
+// Streams spec, 6.1.2. new ByteLengthQueuingStrategy({ highWaterMark })
+bool
+js::ByteLengthQueuingStrategy::constructor(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    RootedObject strategy(cx, NewBuiltinClassInstance<ByteLengthQueuingStrategy>(cx));
+    if (!strategy)
+        return false;
+
+    RootedObject argObj(cx, ToObject(cx, args.get(0)));
+    if (!argObj)
+      return false;
+
+    RootedValue highWaterMark(cx);
+    if (!GetProperty(cx, argObj, argObj, cx->names().highWaterMark, &highWaterMark))
+      return false;
+
+    if (!SetProperty(cx, strategy, cx->names().highWaterMark, highWaterMark))
+      return false;
+
+    args.rval().setObject(*strategy);
+    return true;
+}
+
+// Streams spec 6.1.3.1. size ( chunk )
+bool
+ByteLengthQueuingStrategy_size(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    // Step 1: Return ? GetV(chunk, "byteLength").
+    return GetProperty(cx, args.get(0), cx->names().byteLength, args.rval());
+}
+
+static const JSPropertySpec ByteLengthQueuingStrategy_properties[] = {
+    JS_PS_END
+};
+
+static const JSFunctionSpec ByteLengthQueuingStrategy_methods[] = {
+    JS_FN("size", ByteLengthQueuingStrategy_size, 1, 0),
+    JS_FS_END
+};
+
+CLASS_SPEC(ByteLengthQueuingStrategy, 1, 0, 0, 0, JS_NULL_CLASS_OPS);
+
+// Streams spec, 6.2.2. new CountQueuingStrategy({ highWaterMark })
+bool
+js::CountQueuingStrategy::constructor(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    Rooted<CountQueuingStrategy*> strategy(cx, NewBuiltinClassInstance<CountQueuingStrategy>(cx));
+    if (!strategy)
+        return false;
+
+    RootedObject argObj(cx, ToObject(cx, args.get(0)));
+    if (!argObj)
+      return false;
+
+    RootedValue highWaterMark(cx);
+    if (!GetProperty(cx, argObj, argObj, cx->names().highWaterMark, &highWaterMark))
+      return false;
+
+    if (!SetProperty(cx, strategy, cx->names().highWaterMark, highWaterMark))
+      return false;
+
+    args.rval().setObject(*strategy);
+    return true;
+}
+
+// Streams spec 6.2.3.1. size ( chunk )
+bool
+CountQueuingStrategy_size(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    // Step 1: Return 1.
+    args.rval().setInt32(1);
+    return true;
+}
+
+static const JSPropertySpec CountQueuingStrategy_properties[] = {
+    JS_PS_END
+};
+
+static const JSFunctionSpec CountQueuingStrategy_methods[] = {
+    JS_FN("size", CountQueuingStrategy_size, 0, 0),
+    JS_FS_END
+};
+
+CLASS_SPEC(CountQueuingStrategy, 1, 0, 0, 0, JS_NULL_CLASS_OPS);
+
+#undef CLASS_SPEC
+
+// Streams spec, 6.3.1. DequeueValue ( container ) nothrow
+inline static MOZ_MUST_USE bool
+DequeueValue(JSContext* cx, HandleNativeObject container, MutableHandleValue chunk)
+{
+    // Step 1: Assert: container has [[queue]] and [[queueTotalSize]] internal
+    //         slots.
+    MOZ_ASSERT(IsReadableStreamController(container));
+
+    // Step 2: Assert: queue is not empty.
+    RootedValue val(cx, container->getFixedSlot(QueueContainerSlot_Queue));
+    RootedNativeObject queue(cx, &val.toObject().as<NativeObject>());
+    MOZ_ASSERT(queue->getDenseInitializedLength() > 0);
+
+    // Step 3. Let pair be the first element of queue.
+    // Step 4. Remove pair from queue, shifting all other elements downward
+    //         (so that the second becomes the first, and so on).
+    Rooted<QueueEntry*> pair(cx, ShiftFromList<QueueEntry>(cx, queue));
+    MOZ_ASSERT(pair);
+
+    // Step 5: Set container.[[queueTotalSize]] to
+    //         container.[[queueTotalSize]] − pair.[[size]].
+    // Step 6: If container.[[queueTotalSize]] < 0, set
+    //         container.[[queueTotalSize]] to 0.
+    //         (This can occur due to rounding errors.)
+    double totalSize = container->getFixedSlot(QueueContainerSlot_TotalSize).toNumber();
+
+    totalSize -= pair->size();
+    if (totalSize < 0)
+        totalSize = 0;
+    container->setFixedSlot(QueueContainerSlot_TotalSize, NumberValue(totalSize));
+
+    // Step 7: Return pair.[[value]].
+    chunk.set(pair->value());
+    return true;
+}
+
+// Streams spec, 6.3.2. EnqueueValueWithSize ( container, value, size ) throws
+static MOZ_MUST_USE bool
+EnqueueValueWithSize(JSContext* cx, HandleNativeObject container, HandleValue value,
+                     HandleValue sizeVal)
+{
+    // Step 1: Assert: container has [[queue]] and [[queueTotalSize]] internal
+    //         slots.
+    MOZ_ASSERT(IsReadableStreamController(container));
+
+    // Step 2: Let size be ? ToNumber(size).
+    double size;
+    if (!ToNumber(cx, sizeVal, &size))
+        return false;
+
+    // Step 3: If ! IsFiniteNonNegativeNumber(size) is false, throw a RangeError
+    //         exception.
+    if (size < 0 || mozilla::IsNaN(size) || mozilla::IsInfinite(size)) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_NUMBER_MUST_BE_FINITE_NON_NEGATIVE, "size");
+        return false;
+    }
+
+    // Step 4: Append Record {[[value]]: value, [[size]]: size} as the last element
+    //         of container.[[queue]].
+    RootedValue val(cx, container->getFixedSlot(QueueContainerSlot_Queue));
+    RootedNativeObject queue(cx, &val.toObject().as<NativeObject>());
+
+    QueueEntry* entry = QueueEntry::create(cx, value, size);
+    if (!entry)
+        return false;
+    val = ObjectValue(*entry);
+    if (!AppendToList(cx, queue, val))
+        return false;
+
+    // Step 5: Set container.[[queueTotalSize]] to
+    //         container.[[queueTotalSize]] + size.
+    double totalSize = container->getFixedSlot(QueueContainerSlot_TotalSize).toNumber();
+    container->setFixedSlot(QueueContainerSlot_TotalSize, NumberValue(totalSize + size));
+
+    return true;
+}
+
+// Streams spec, 6.3.3. PeekQueueValue ( container ) nothrow
+// Used by WritableStream.
+// static MOZ_MUST_USE Value
+// PeekQueueValue(NativeObject* container)
+// {
+//     // Step 1: Assert: container has [[queue]] and [[queueTotalSize]] internal
+//     //         slots.
+//     MOZ_ASSERT(IsReadableStreamController(container));
+
+//     // Step 2: Assert: queue is not empty.
+//     Value val = container->getFixedSlot(QueueContainerSlot_Queue);
+//     NativeObject* queue = &val.toObject().as<NativeObject>();
+//     MOZ_ASSERT(queue->getDenseInitializedLength() > 0);
+
+//     // Step 3: Let pair be the first element of container.[[queue]].
+//     QueueEntry* pair = PeekList<QueueEntry>(queue);
+
+//     // Step 4: Return pair.[[value]].
+//     return pair->value();
+// }
+
+/**
+ * Streams spec, 6.3.4. ResetQueue ( container ) nothrow
+ */
+inline static MOZ_MUST_USE bool
+ResetQueue(JSContext* cx, HandleNativeObject container)
+{
+    // Step 1: Assert: container has [[queue]] and [[queueTotalSize]] internal
+    //         slots.
+    MOZ_ASSERT(IsReadableStreamController(container));
+
+    // Step 2: Set container.[[queue]] to a new empty List.
+    if (!SetNewList(cx, container, QueueContainerSlot_Queue))
+        return false;
+
+    // Step 3: Set container.[[queueTotalSize]] to 0.
+    container->setFixedSlot(QueueContainerSlot_TotalSize, NumberValue(0));
+
+    return true;
+}
+
+
+/**
+ * Streams spec, 6.4.1. InvokeOrNoop ( O, P, args )
+ */
+inline static MOZ_MUST_USE bool
+InvokeOrNoop(JSContext* cx, HandleValue O, HandlePropertyName P, HandleValue arg,
+             MutableHandleValue rval)
+{
+    // Step 1: Assert: P is a valid property key (omitted).
+    // Step 2: If args was not passed, let args be a new empty List (omitted).
+    // Step 3: Let method be ? GetV(O, P).
+    RootedValue method(cx);
+    if (!GetProperty(cx, O, P, &method))
+        return false;
+
+    // Step 4: If method is undefined, return.
+    if (method.isUndefined())
+        return true;
+
+    // Step 5: Return ? Call(method, O, args).
+    return Call(cx, method, O, arg, rval);
+}
+
+/**
+ * Streams spec, 6.4.3. PromiseInvokeOrNoop ( O, P, args )
+ * Specialized to one arg, because that's what all stream related callers use.
+ */
+static MOZ_MUST_USE JSObject*
+PromiseInvokeOrNoop(JSContext* cx, HandleValue O, HandlePropertyName P, HandleValue arg)
+{
+    // Step 1: Assert: O is not undefined.
+    MOZ_ASSERT(!O.isUndefined());
+
+    // Step 2: Assert: ! IsPropertyKey(P) is true (implicit).
+    // Step 3: Assert: args is a List (omitted).
+
+    // Step 4: Let returnValue be InvokeOrNoop(O, P, args).
+    // Step 5: If returnValue is an abrupt completion, return a promise
+    //         rejected with returnValue.[[Value]].
+    RootedValue returnValue(cx);
+    if (!InvokeOrNoop(cx, O, P, arg, &returnValue))
+        return PromiseRejectedWithPendingError(cx);
+
+    // Step 6: Otherwise, return a promise resolved with returnValue.[[Value]].
+    return PromiseObject::unforgeableResolve(cx, returnValue);
+}
+
+/**
+ * Streams spec, 6.4.4 TransferArrayBuffer ( O )
+ */
+static MOZ_MUST_USE ArrayBufferObject*
+TransferArrayBuffer(JSContext* cx, HandleObject buffer)
+{
+    // Step 1 (implicit).
+
+    // Step 2.
+    MOZ_ASSERT(buffer->is<ArrayBufferObject>());
+
+    // Step 3.
+    MOZ_ASSERT(!JS_IsDetachedArrayBufferObject(buffer));
+
+    // Step 5 (reordered).
+    uint32_t size = buffer->as<ArrayBufferObject>().byteLength();
+
+    // Steps 4, 6.
+    void* contents = JS_StealArrayBufferContents(cx, buffer);
+    if (!contents)
+        return nullptr;
+    MOZ_ASSERT(JS_IsDetachedArrayBufferObject(buffer));
+
+    // Step 7.
+    RootedObject transferredBuffer(cx, JS_NewArrayBufferWithContents(cx, size, contents));
+    if (!transferredBuffer)
+        return nullptr;
+    return &transferredBuffer->as<ArrayBufferObject>();
+}
+
+// Streams spec, 6.4.5. ValidateAndNormalizeHighWaterMark ( highWaterMark )
+static MOZ_MUST_USE bool
+ValidateAndNormalizeHighWaterMark(JSContext* cx, HandleValue highWaterMarkVal, double* highWaterMark)
+{
+    // Step 1: Set highWaterMark to ? ToNumber(highWaterMark).
+    if (!ToNumber(cx, highWaterMarkVal, highWaterMark))
+        return false;
+
+    // Step 2: If highWaterMark is NaN, throw a TypeError exception.
+    // Step 3: If highWaterMark < 0, throw a RangeError exception.
+    if (mozilla::IsNaN(*highWaterMark) || *highWaterMark < 0) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_STREAM_INVALID_HIGHWATERMARK);
+        return false;
+    }
+
+    // Step 4: Return highWaterMark.
+    return true;
+}
+
+// Streams spec, 6.4.6. ValidateAndNormalizeQueuingStrategy ( size, highWaterMark )
+static MOZ_MUST_USE bool
+ValidateAndNormalizeQueuingStrategy(JSContext* cx, HandleValue size,
+                                    HandleValue highWaterMarkVal, double* highWaterMark)
+{
+    // Step 1: If size is not undefined and ! IsCallable(size) is false, throw a
+    //         TypeError exception.
+    if (!size.isUndefined() && !IsCallable(size)) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_NOT_FUNCTION,
+                                  "ReadableStream argument options.size");
+        return false;
+    }
+
+    // Step 2: Let highWaterMark be ? ValidateAndNormalizeHighWaterMark(highWaterMark).
+    if (!ValidateAndNormalizeHighWaterMark(cx, highWaterMarkVal, highWaterMark))
+        return false;
+
+    // Step 3: Return Record {[[size]]: size, [[highWaterMark]]: highWaterMark}.
+    return true;
+}
+
+MOZ_MUST_USE bool
+js::ReadableStreamReaderCancel(JSContext* cx, HandleObject readerObj, HandleValue reason)
+{
+    MOZ_ASSERT(IsReadableStreamReader(readerObj));
+    RootedNativeObject reader(cx, &readerObj->as<NativeObject>());
+    MOZ_ASSERT(StreamFromReader(reader));
+    return ReadableStreamReaderGenericCancel(cx, reader, reason);
+}
+
+MOZ_MUST_USE bool
+js::ReadableStreamReaderReleaseLock(JSContext* cx, HandleObject readerObj)
+{
+    MOZ_ASSERT(IsReadableStreamReader(readerObj));
+    RootedNativeObject reader(cx, &readerObj->as<NativeObject>());
+    MOZ_ASSERT(ReadableStreamGetNumReadRequests(StreamFromReader(reader)) == 0);
+    return ReadableStreamReaderGenericRelease(cx, reader);
+}
+
+MOZ_MUST_USE bool
+ReadableStream::enqueue(JSContext* cx, Handle<ReadableStream*> stream, HandleValue chunk)
+{
+    Rooted<ReadableStreamDefaultController*> controller(cx);
+    controller = &ControllerFromStream(stream)->as<ReadableStreamDefaultController>();
+
+    MOZ_ASSERT(!(ControllerFlags(controller) & ControllerFlag_CloseRequested));
+    MOZ_ASSERT(stream->readable());
+
+    return ReadableStreamDefaultControllerEnqueue(cx, controller, chunk);
+}
+
+MOZ_MUST_USE bool
+ReadableStream::enqueueBuffer(JSContext* cx, Handle<ReadableStream*> stream,
+                              Handle<ArrayBufferObject*> chunk)
+{
+    Rooted<ReadableByteStreamController*> controller(cx);
+    controller = &ControllerFromStream(stream)->as<ReadableByteStreamController>();
+
+    MOZ_ASSERT(!(ControllerFlags(controller) & ControllerFlag_CloseRequested));
+    MOZ_ASSERT(stream->readable());
+
+    return ReadableByteStreamControllerEnqueue(cx, controller, chunk);
+}
+
+void
+ReadableStream::desiredSize(bool* hasSize, double* size) const
+{
+    if (errored()) {
+        *hasSize = false;
+        return;
+    }
+
+    *hasSize = true;
+
+    if (closed()) {
+        *size = 0;
+        return;
+    }
+
+    NativeObject* controller = ControllerFromStream(this);
+    *size = ReadableStreamControllerGetDesiredSizeUnchecked(controller);
+}
+
+/*static */ bool
+ReadableStream::getExternalSource(JSContext* cx, Handle<ReadableStream*> stream, void** source)
+{
+    MOZ_ASSERT(stream->mode() == JS::ReadableStreamMode::ExternalSource);
+    if (stream->locked()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_READABLESTREAM_LOCKED);
+        return false;
+    }
+    if (!stream->readable()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMCONTROLLER_NOT_READABLE,
+                                  "ReadableStreamGetExternalUnderlyingSource");
+        return false;
+    }
+
+    auto controller = &ControllerFromStream(stream)->as<ReadableByteStreamController>();
+    AddControllerFlags(controller, ControllerFlag_SourceLocked);
+    *source = controller->getFixedSlot(ControllerSlot_UnderlyingSource).toPrivate();
+    return true;
+}
+
+void
+ReadableStream::releaseExternalSource()
+{
+    MOZ_ASSERT(mode() == JS::ReadableStreamMode::ExternalSource);
+    MOZ_ASSERT(locked());
+    auto controller = ControllerFromStream(this);
+    MOZ_ASSERT(ControllerFlags(controller) & ControllerFlag_SourceLocked);
+    RemoveControllerFlags(controller, ControllerFlag_SourceLocked);
+}
+
+uint8_t
+ReadableStream::embeddingFlags() const
+{
+    uint8_t flags = ControllerFlags(ControllerFromStream(this)) >> ControllerEmbeddingFlagsOffset;
+    MOZ_ASSERT_IF(flags, mode() == JS::ReadableStreamMode::ExternalSource);
+    return flags;
+}
+
+// Streams spec, 3.10.4.4. steps 1-3
+// and
+// Streams spec, 3.12.8. steps 8-9
+//
+// Adapted to handling updates signaled by the embedding for streams with
+// external underlying sources.
+//
+// The remaining steps of those two functions perform checks and asserts that
+// don't apply to streams with external underlying sources.
+MOZ_MUST_USE bool
+ReadableStream::updateDataAvailableFromSource(JSContext* cx, Handle<ReadableStream*> stream,
+                                              uint32_t availableData)
+{
+    Rooted<ReadableByteStreamController*> controller(cx);
+    controller = &ControllerFromStream(stream)->as<ReadableByteStreamController>();
+
+    // Step 2: If this.[[closeRequested]] is true, throw a TypeError exception.
+    if (ControllerFlags(controller) & ControllerFlag_CloseRequested) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMCONTROLLER_CLOSED, "enqueue");
+        return false;
+    }
+
+    // Step 3: If this.[[controlledReadableStream]].[[state]] is not "readable",
+    //         throw a TypeError exception.
+    if (!StreamFromController(controller)->readable()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMCONTROLLER_NOT_READABLE, "enqueue");
+        return false;
+    }
+
+    RemoveControllerFlags(controller, ControllerFlag_Pulling | ControllerFlag_PullAgain);
+
+#if DEBUG
+    uint32_t oldAvailableData = controller->getFixedSlot(QueueContainerSlot_TotalSize).toInt32();
+#endif // DEBUG
+    controller->setFixedSlot(QueueContainerSlot_TotalSize, Int32Value(availableData));
+
+    // Step 8.a: If ! ReadableStreamGetNumReadRequests(stream) is 0,
+    // Reordered because for externally-sourced streams it applies regardless
+    // of reader type.
+    if (ReadableStreamGetNumReadRequests(stream) == 0)
+        return true;
+
+    // Step 8: If ! ReadableStreamHasDefaultReader(stream) is true
+    if (ReadableStreamHasDefaultReader(stream)) {
+        // Step b: Otherwise,
+        // Step i: Assert: controller.[[queue]] is empty.
+        MOZ_ASSERT(oldAvailableData == 0);
+
+        // Step ii: Let transferredView be
+        //          ! Construct(%Uint8Array%, transferredBuffer, byteOffset, byteLength).
+        JSObject* viewObj = JS_NewUint8Array(cx, availableData);
+        Rooted<ArrayBufferViewObject*> transferredView(cx, &viewObj->as<ArrayBufferViewObject>());
+        if (!transferredView)
+            return false;
+
+        Value val = controller->getFixedSlot(ControllerSlot_UnderlyingSource);
+        void* underlyingSource = val.toPrivate();
+
+        size_t bytesWritten;
+        {
+            JS::AutoCheckCannotGC noGC(cx);
+            bool dummy;
+            void* buffer = JS_GetArrayBufferViewData(transferredView, &dummy, noGC);
+            auto cb = cx->runtime()->readableStreamWriteIntoReadRequestCallback;
+            MOZ_ASSERT(cb);
+            // TODO: use bytesWritten to correctly update the request's state.
+            cb(cx, stream, underlyingSource, stream->embeddingFlags(), buffer,
+               availableData, &bytesWritten);
+        }
+
+        // Step iii: Perform ! ReadableStreamFulfillReadRequest(stream, transferredView, false).
+        RootedValue chunk(cx, ObjectValue(*transferredView));
+        if (!ReadableStreamFulfillReadOrReadIntoRequest(cx, stream, chunk, false))
+            return false;
+
+        controller->setFixedSlot(QueueContainerSlot_TotalSize,
+                                 Int32Value(availableData - bytesWritten));
+    } else if (ReadableStreamHasBYOBReader(stream)) {
+        // Step 9: Otherwise,
+        // Step a: If ! ReadableStreamHasBYOBReader(stream) is true,
+        // Step i: Perform
+        // (Not needed for external underlying sources.)
+
+        // Step ii: Perform ! ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(controller).
+        if (!ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue(cx, controller))
+            return false;
+    } else {
+        // Step b: Otherwise,
+        // Step i: Assert: ! IsReadableStreamLocked(stream) is false.
+        MOZ_ASSERT(!stream->locked());
+
+        // Step ii: Perform
+        //          ! ReadableByteStreamControllerEnqueueChunkToQueue(controller,
+        //                                                            transferredBuffer,
+        //                                                            byteOffset,
+        //                                                            byteLength).
+        // (Not needed for external underlying sources.)
+    }
+
+    return true;
+}
+
+MOZ_MUST_USE bool
+ReadableStream::close(JSContext* cx, Handle<ReadableStream*> stream)
+{
+    RootedNativeObject controllerObj(cx, ControllerFromStream(stream));
+    if (!VerifyControllerStateForClosing(cx, controllerObj))
+        return false;
+
+    if (controllerObj->is<ReadableStreamDefaultController>()) {
+        Rooted<ReadableStreamDefaultController*> controller(cx);
+        controller = &controllerObj->as<ReadableStreamDefaultController>();
+        return ReadableStreamDefaultControllerClose(cx, controller);
+    }
+
+    Rooted<ReadableByteStreamController*> controller(cx);
+    controller = &controllerObj->as<ReadableByteStreamController>();
+    return ReadableByteStreamControllerClose(cx, controller);
+}
+
+MOZ_MUST_USE bool
+ReadableStream::error(JSContext* cx, Handle<ReadableStream*> stream, HandleValue reason)
+{
+    // Step 3: If stream.[[state]] is not "readable", throw a TypeError exception.
+    if (!stream->readable()) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAMCONTROLLER_NOT_READABLE, "error");
+        return false;
+    }
+
+    // Step 4: Perform ! ReadableStreamDefaultControllerError(this, e).
+    RootedNativeObject controller(cx, ControllerFromStream(stream));
+    return ReadableStreamControllerError(cx, controller, reason);
+}
+
+MOZ_MUST_USE bool
+ReadableStream::tee(JSContext* cx, Handle<ReadableStream*> stream, bool cloneForBranch2,
+                    MutableHandle<ReadableStream*> branch1Stream,
+                    MutableHandle<ReadableStream*> branch2Stream)
+{
+    return ReadableStreamTee(cx, stream, false, branch1Stream, branch2Stream);
+}
+
+MOZ_MUST_USE NativeObject*
+ReadableStream::getReader(JSContext* cx, Handle<ReadableStream*> stream,
+                          JS::ReadableStreamReaderMode mode)
+{
+    if (mode == JS::ReadableStreamReaderMode::Default)
+        return CreateReadableStreamDefaultReader(cx, stream);
+    return CreateReadableStreamBYOBReader(cx, stream);
+}
+
+JS_FRIEND_API(JSObject*)
+js::UnwrapReadableStream(JSObject* obj)
+{
+    if (JSObject* unwrapped = CheckedUnwrap(obj))
+        return unwrapped->is<ReadableStream>() ? unwrapped : nullptr;
+    return nullptr;
+}
diff --git a/js/src/builtin/Stream.h b/js/src/builtin/Stream.h
new file mode 100644
index 0000000000..0726e78361
--- /dev/null
+++ b/js/src/builtin/Stream.h
@@ -0,0 +1,170 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef builtin_Stream_h
+#define builtin_Stream_h
+
+#include "builtin/Promise.h"
+#include "vm/NativeObject.h"
+
+
+namespace js {
+
+class AutoSetNewObjectMetadata;
+
+class ReadableStream : public NativeObject
+{
+  public:
+    static ReadableStream* createDefaultStream(JSContext* cx, HandleValue underlyingSource,
+                                               HandleValue size, HandleValue highWaterMark,
+                                               HandleObject proto = nullptr);
+    static ReadableStream* createByteStream(JSContext* cx, HandleValue underlyingSource,
+                                            HandleValue highWaterMark,
+                                            HandleObject proto = nullptr);
+    static ReadableStream* createExternalSourceStream(JSContext* cx, void* underlyingSource,
+                                                      uint8_t flags, HandleObject proto = nullptr);
+
+    bool readable() const;
+    bool closed() const;
+    bool errored() const;
+    bool disturbed() const;
+
+    bool locked() const;
+
+    void desiredSize(bool* hasSize, double* size) const;
+
+    JS::ReadableStreamMode mode() const;
+
+    static MOZ_MUST_USE bool close(JSContext* cx, Handle<ReadableStream*> stream);
+    static MOZ_MUST_USE JSObject* cancel(JSContext* cx, Handle<ReadableStream*> stream,
+                                         HandleValue reason);
+    static MOZ_MUST_USE bool error(JSContext* cx, Handle<ReadableStream*> stream,
+                                   HandleValue error);
+
+    static MOZ_MUST_USE NativeObject* getReader(JSContext* cx, Handle<ReadableStream*> stream,
+                                                JS::ReadableStreamReaderMode mode);
+
+    static MOZ_MUST_USE bool tee(JSContext* cx,
+                                 Handle<ReadableStream*> stream, bool cloneForBranch2,
+                                 MutableHandle<ReadableStream*> branch1Stream,
+                                 MutableHandle<ReadableStream*> branch2Stream);
+
+    static MOZ_MUST_USE bool enqueue(JSContext* cx, Handle<ReadableStream*> stream,
+                                     HandleValue chunk);
+    static MOZ_MUST_USE bool enqueueBuffer(JSContext* cx, Handle<ReadableStream*> stream,
+                                           Handle<ArrayBufferObject*> chunk);
+    static MOZ_MUST_USE bool getExternalSource(JSContext* cx, Handle<ReadableStream*> stream,
+                                               void** source);
+    void releaseExternalSource();
+    uint8_t embeddingFlags() const;
+    static MOZ_MUST_USE bool updateDataAvailableFromSource(JSContext* cx,
+                                                           Handle<ReadableStream*> stream,
+                                                           uint32_t availableData);
+
+    enum State {
+         Readable  = 1 << 0,
+         Closed    = 1 << 1,
+         Errored   = 1 << 2,
+         Disturbed = 1 << 3
+    };
+
+  private:
+    static MOZ_MUST_USE ReadableStream* createStream(JSContext* cx, HandleObject proto = nullptr);
+
+  public:
+    static bool constructor(JSContext* cx, unsigned argc, Value* vp);
+    static const ClassSpec classSpec_;
+    static const Class class_;
+    static const ClassSpec protoClassSpec_;
+    static const Class protoClass_;
+};
+
+class ReadableStreamDefaultReader : public NativeObject
+{
+  public:
+    static MOZ_MUST_USE JSObject* read(JSContext* cx, Handle<ReadableStreamDefaultReader*> reader);
+
+    static bool constructor(JSContext* cx, unsigned argc, Value* vp);
+    static const ClassSpec classSpec_;
+    static const Class class_;
+    static const ClassSpec protoClassSpec_;
+    static const Class protoClass_;
+};
+
+class ReadableStreamBYOBReader : public NativeObject
+{
+  public:
+    static MOZ_MUST_USE JSObject* read(JSContext* cx, Handle<ReadableStreamBYOBReader*> reader,
+                                       Handle<ArrayBufferViewObject*> view);
+
+    static bool constructor(JSContext* cx, unsigned argc, Value* vp);
+    static const ClassSpec classSpec_;
+    static const Class class_;
+    static const ClassSpec protoClassSpec_;
+    static const Class protoClass_;
+};
+
+bool ReadableStreamReaderIsClosed(const JSObject* reader);
+
+MOZ_MUST_USE bool ReadableStreamReaderCancel(JSContext* cx, HandleObject reader,
+                                             HandleValue reason);
+
+MOZ_MUST_USE bool ReadableStreamReaderReleaseLock(JSContext* cx, HandleObject reader);
+
+class ReadableStreamDefaultController : public NativeObject
+{
+  public:
+    static bool constructor(JSContext* cx, unsigned argc, Value* vp);
+    static const ClassSpec classSpec_;
+    static const Class class_;
+    static const ClassSpec protoClassSpec_;
+    static const Class protoClass_;
+};
+
+class ReadableByteStreamController : public NativeObject
+{
+  public:
+    bool hasExternalSource();
+
+    static bool constructor(JSContext* cx, unsigned argc, Value* vp);
+    static const ClassSpec classSpec_;
+    static const Class class_;
+    static const ClassSpec protoClassSpec_;
+    static const Class protoClass_;
+};
+
+class ReadableStreamBYOBRequest : public NativeObject
+{
+  public:
+    static bool constructor(JSContext* cx, unsigned argc, Value* vp);
+    static const ClassSpec classSpec_;
+    static const Class class_;
+    static const ClassSpec protoClassSpec_;
+    static const Class protoClass_;
+};
+
+class ByteLengthQueuingStrategy : public NativeObject
+{
+  public:
+    static bool constructor(JSContext* cx, unsigned argc, Value* vp);
+    static const ClassSpec classSpec_;
+    static const Class class_;
+    static const ClassSpec protoClassSpec_;
+    static const Class protoClass_;
+};
+
+class CountQueuingStrategy : public NativeObject
+{
+  public:
+    static bool constructor(JSContext* cx, unsigned argc, Value* vp);
+    static const ClassSpec classSpec_;
+    static const Class class_;
+    static const ClassSpec protoClassSpec_;
+    static const Class protoClass_;
+};
+
+} // namespace js
+
+#endif /* builtin_Stream_h */
diff --git a/js/src/builtin/TestingFunctions.cpp b/js/src/builtin/TestingFunctions.cpp
index 2608733853..f7af8b6fe9 100644
--- a/js/src/builtin/TestingFunctions.cpp
+++ b/js/src/builtin/TestingFunctions.cpp
@@ -1441,6 +1441,14 @@ RejectPromise(JSContext* cx, unsigned argc, Value* vp)
     return result;
 }
 
+static bool
+StreamsAreEnabled(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setBoolean(cx->options().streams());
+    return true;
+}
+
 static unsigned finalizeCount = 0;
 
 static void
@@ -4144,6 +4152,10 @@ JS_FN_HELP("rejectPromise", RejectPromise, 2, 0,
 "rejectPromise(promise, reason)",
 "  Reject a Promise by calling the JSAPI function JS::RejectPromise."),
 
+JS_FN_HELP("streamsAreEnabled", StreamsAreEnabled, 0, 0,
+"streamsAreEnabled()",
+"  Returns a boolean indicating whether WHATWG Streams are enabled for the current compartment."),
+
     JS_FN_HELP("makeFinalizeObserver", MakeFinalizeObserver, 0, 0,
 "makeFinalizeObserver()",
 "  Get a special object whose finalization increases the counter returned\n"
diff --git a/js/src/builtin/TypedArray.js b/js/src/builtin/TypedArray.js
index 57f6d738ca..a8510faa71 100644
--- a/js/src/builtin/TypedArray.js
+++ b/js/src/builtin/TypedArray.js
@@ -2,8 +2,6 @@
  * 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 "TypedObjectConstants.h"
-
 function ViewedArrayBufferIfReified(tarray) {
     assert(IsTypedArray(tarray), "non-typed array asked for its buffer");
 
@@ -1781,7 +1779,7 @@ function ArrayBufferSlice(start, end) {
         ThrowTypeError(JSMSG_TYPED_ARRAY_DETACHED);
 
     // Steps 19-21.
-    ArrayBufferCopyData(new_, O, first | 0, newLen | 0, isWrapped);
+    ArrayBufferCopyData(new_, 0, O, first | 0, newLen | 0, isWrapped);
 
     // Step 22.
     return new_;
@@ -1863,7 +1861,7 @@ function SharedArrayBufferSlice(start, end) {
         ThrowTypeError(JSMSG_SHORT_SHARED_ARRAY_BUFFER_RETURNED, newLen, actualLen);
 
     // Steps 16-18.
-    SharedArrayBufferCopyData(new_, O, first | 0, newLen | 0, isWrapped);
+    SharedArrayBufferCopyData(new_, 0, O, first | 0, newLen | 0, isWrapped);
 
     // Step 19.
     return new_;
diff --git a/js/src/builtin/Utilities.js b/js/src/builtin/Utilities.js
index 51c5a574fd..1cc15ed68e 100644
--- a/js/src/builtin/Utilities.js
+++ b/js/src/builtin/Utilities.js
@@ -23,6 +23,7 @@
 */
 
 #include "SelfHostingDefines.h"
+#include "TypedObjectConstants.h"
 
 // Assertions and debug printing, defined here instead of in the header above
 // to make `assert` invisible to C++.
diff --git a/js/src/js.msg b/js/src/js.msg
index e651b10137..b8aa77902d 100644
--- a/js/src/js.msg
+++ b/js/src/js.msg
@@ -613,11 +613,39 @@ MSG_DEF(JSMSG_RETURN_NOT_CALLABLE,     0, JSEXN_TYPEERR, "property 'return' of i
 MSG_DEF(JSMSG_ITERATOR_NO_THROW,       0, JSEXN_TYPEERR, "iterator does not have a 'throw' method")
 
 // Async Iteration
-MSG_DEF(JSMSG_FOR_AWAIT_NOT_OF,        0, JSEXN_TYPEERR, "'for await' loop should be used with 'of'")
+MSG_DEF(JSMSG_FOR_AWAIT_NOT_OF,        0, JSEXN_SYNTAXERR, "'for await' loop should be used with 'of'")
 MSG_DEF(JSMSG_NOT_AN_ASYNC_GENERATOR,  0, JSEXN_TYPEERR, "Not an async generator")
 MSG_DEF(JSMSG_NOT_AN_ASYNC_ITERATOR,   0, JSEXN_TYPEERR, "Not an async from sync iterator")
 MSG_DEF(JSMSG_GET_ASYNC_ITER_RETURNED_PRIMITIVE, 0, JSEXN_TYPEERR, "[Symbol.asyncIterator]() returned a non-object value")
 
+// ReadableStream
+MSG_DEF(JSMSG_READABLESTREAM_UNDERLYINGSOURCE_TYPE_WRONG,0, JSEXN_RANGEERR,"'underlyingSource.type' must be \"bytes\" or undefined.")
+MSG_DEF(JSMSG_READABLESTREAM_INVALID_READER_MODE,        0, JSEXN_RANGEERR,"'mode' must be \"byob\" or undefined.")
+MSG_DEF(JSMSG_NUMBER_MUST_BE_FINITE_NON_NEGATIVE, 1, JSEXN_RANGEERR, "'{0}' must be a finite, non-negative number.")
+MSG_DEF(JSMSG_READABLEBYTESTREAMCONTROLLER_INVALID_BYTESWRITTEN, 0, JSEXN_RANGEERR, "'bytesWritten' exceeds remaining length.")
+MSG_DEF(JSMSG_READABLEBYTESTREAMCONTROLLER_INVALID_VIEW_SIZE, 0, JSEXN_RANGEERR, "view size does not match requested data.")
+MSG_DEF(JSMSG_READABLEBYTESTREAMCONTROLLER_INVALID_VIEW_OFFSET, 0, JSEXN_RANGEERR, "view offset does not match requested position.")
+MSG_DEF(JSMSG_READABLESTREAM_NOT_LOCKED,                 1, JSEXN_TYPEERR, "'{0}' may only be called on a locked stream.")
+MSG_DEF(JSMSG_READABLESTREAM_LOCKED,                     0, JSEXN_TYPEERR, "A Reader may only be created for an unlocked ReadableStream.")
+MSG_DEF(JSMSG_READABLESTREAM_NOT_BYTE_STREAM_CONTROLLER, 1, JSEXN_TYPEERR, "{0} requires a ReadableByteStreamController.")
+MSG_DEF(JSMSG_READABLESTREAM_NOT_DEFAULT_CONTROLLER,     1, JSEXN_TYPEERR, "{0} requires a ReadableStreamDefaultController.")
+MSG_DEF(JSMSG_READABLESTREAM_CONTROLLER_SET,             0, JSEXN_TYPEERR, "The ReadableStream already has a controller defined.")
+MSG_DEF(JSMSG_READABLESTREAMREADER_NOT_OWNED,            1, JSEXN_TYPEERR, "The ReadableStream reader method '{0}' may only be called on a reader owned by a stream.")
+MSG_DEF(JSMSG_READABLESTREAMREADER_NOT_EMPTY,            1, JSEXN_TYPEERR, "The ReadableStream reader method '{0}' may not be called on a reader with read requests.")
+MSG_DEF(JSMSG_READABLESTREAMBYOBREADER_READ_EMPTY_VIEW,  0, JSEXN_TYPEERR, "ReadableStreamBYOBReader.read() was passed an empty TypedArrayBuffer view.")
+MSG_DEF(JSMSG_READABLESTREAMREADER_RELEASED,             0, JSEXN_TYPEERR, "The ReadableStream reader was released.")
+MSG_DEF(JSMSG_READABLESTREAMCONTROLLER_CLOSED,           1, JSEXN_TYPEERR, "'{0}' called on a stream already closing.")
+MSG_DEF(JSMSG_READABLESTREAMCONTROLLER_NOT_READABLE,     1, JSEXN_TYPEERR, "'{0}' may only be called on a stream in the 'readable' state.")
+MSG_DEF(JSMSG_READABLEBYTESTREAMCONTROLLER_BAD_CHUNKSIZE,0, JSEXN_RANGEERR, "ReadableByteStreamController requires a positive integer or undefined for 'autoAllocateChunkSize'.")
+MSG_DEF(JSMSG_READABLEBYTESTREAMCONTROLLER_BAD_CHUNK,    1, JSEXN_TYPEERR, "{0} passed a bad chunk.")
+MSG_DEF(JSMSG_READABLEBYTESTREAMCONTROLLER_CLOSE_PENDING_PULL, 0, JSEXN_TYPEERR, "The ReadableByteStreamController cannot be closed while the buffer is being filled.")
+MSG_DEF(JSMSG_READABLESTREAMBYOBREQUEST_NO_CONTROLLER,   1, JSEXN_TYPEERR, "ReadableStreamBYOBRequest method '{0}' called on a request with no controller.")
+MSG_DEF(JSMSG_READABLESTREAMBYOBREQUEST_RESPOND_CLOSED,  0, JSEXN_TYPEERR, "ReadableStreamBYOBRequest method 'respond' called with non-zero number of bytes with a closed controller.")
+MSG_DEF(JSMSG_READABLESTREAM_METHOD_NOT_IMPLEMENTED,     1, JSEXN_TYPEERR, "ReadableStream method {0} not yet implemented")
+
+// Other Stream-related
+MSG_DEF(JSMSG_STREAM_INVALID_HIGHWATERMARK,             0, JSEXN_RANGEERR, "'highWaterMark' must be a non-negative, non-NaN number.")
+
 // BigInt
 MSG_DEF(JSMSG_BIGINT_TO_NUMBER, 0, JSEXN_TYPEERR, "can't convert BigInt to number")
 MSG_DEF(JSMSG_NUMBER_TO_BIGINT, 0, JSEXN_RANGEERR, "can't convert non-finite number to BigInt")
diff --git a/js/src/jsapi-tests/moz.build b/js/src/jsapi-tests/moz.build
index 35bc69b85b..1e5f43aa5a 100644
--- a/js/src/jsapi-tests/moz.build
+++ b/js/src/jsapi-tests/moz.build
@@ -126,6 +126,11 @@ if CONFIG['ENABLE_ION']:
         'testJitRValueAlloc.cpp',
     ]
 
+if CONFIG['ENABLE_STREAMS']:
+    UNIFIED_SOURCES += [
+        'testReadableStream.cpp',
+    ]
+
 DEFINES['EXPORT_JS_API'] = True
 
 LOCAL_INCLUDES += [
diff --git a/js/src/jsapi-tests/testIntTypesABI.cpp b/js/src/jsapi-tests/testIntTypesABI.cpp
index 066e7ad1b8..420a07c7f8 100644
--- a/js/src/jsapi-tests/testIntTypesABI.cpp
+++ b/js/src/jsapi-tests/testIntTypesABI.cpp
@@ -29,6 +29,7 @@
 #include "js/RequiredDefines.h"
 #include "js/RootingAPI.h"
 #include "js/SliceBudget.h"
+#include "js/Stream.h"
 #include "js/StructuredClone.h"
 #include "js/TracingAPI.h"
 #include "js/TrackedOptimizationInfo.h"
diff --git a/js/src/jsapi-tests/testReadableStream.cpp b/js/src/jsapi-tests/testReadableStream.cpp
new file mode 100644
index 0000000000..f1d373d5d2
--- /dev/null
+++ b/js/src/jsapi-tests/testReadableStream.cpp
@@ -0,0 +1,676 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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 "jsapi.h"
+
+#include "jsapi-tests/tests.h"
+
+using namespace JS;
+
+char test_buffer_data[] = "1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
+
+static JSObject*
+NewDefaultStream(JSContext* cx, HandleObject source = nullptr, HandleFunction size = nullptr,
+                 double highWaterMark = 1, HandleObject proto = nullptr)
+{
+    RootedObject stream(cx, NewReadableDefaultStreamObject(cx, source, size, highWaterMark,
+                                                           proto));
+    MOZ_ASSERT_IF(stream, IsReadableStream(stream));
+    return stream;
+}
+
+static JSObject*
+NewByteStream(JSContext* cx, double highWaterMark = 0, HandleObject proto = nullptr)
+{
+    RootedObject source(cx, JS_NewPlainObject(cx));
+    MOZ_ASSERT(source);
+
+    RootedObject stream(cx, NewReadableByteStreamObject(cx, source, highWaterMark, proto));
+    MOZ_ASSERT_IF(stream, IsReadableStream(stream));
+    return stream;
+}
+
+static bool dataRequestCBCalled = false;
+static void
+DataRequestCB(JSContext* cx, HandleObject stream, void* underlyingSource, uint8_t flags,
+              size_t desiredSize)
+{
+    MOZ_ASSERT(!dataRequestCBCalled, "Invalid test setup");
+    dataRequestCBCalled = true;
+}
+
+static bool writeIntoRequestBufferCBCalled = false;
+static void
+WriteIntoRequestBufferCB(JSContext* cx, HandleObject stream, void* underlyingSource, uint8_t flags,
+                         void* buffer, size_t length, size_t* bytesWritten)
+{
+    MOZ_ASSERT(!writeIntoRequestBufferCBCalled, "Invalid test setup");
+    MOZ_ASSERT(length <= sizeof(test_buffer_data));
+    memcpy(buffer, test_buffer_data, length);
+    writeIntoRequestBufferCBCalled = true;
+    *bytesWritten = length;
+}
+
+static bool cancelStreamCBCalled = false;
+static Value cancelStreamReason;
+static Value
+CancelStreamCB(JSContext* cx, HandleObject stream, void* underlyingSource, uint8_t flags,
+               HandleValue reason)
+{
+    MOZ_ASSERT(!cancelStreamCBCalled, "Invalid test setup");
+    cancelStreamCBCalled = true;
+    cancelStreamReason = reason;
+    return reason;
+}
+
+static bool streamClosedCBCalled = false;
+static Value streamClosedReason;
+static void
+StreamClosedCB(JSContext* cx, HandleObject stream, void* underlyingSource, uint8_t flags)
+{
+    MOZ_ASSERT(!streamClosedCBCalled, "Invalid test setup");
+    streamClosedCBCalled = true;
+}
+
+static bool streamErroredCBCalled = false;
+static Value streamErroredReason;
+static void
+StreamErroredCB(JSContext* cx, HandleObject stream, void* underlyingSource, uint8_t flags,
+                HandleValue reason)
+{
+    MOZ_ASSERT(!streamErroredCBCalled, "Invalid test setup");
+    streamErroredCBCalled = true;
+    streamErroredReason = reason;
+}
+
+static bool finalizeStreamCBCalled = false;
+static void* finalizedStreamUnderlyingSource;
+static void
+FinalizeStreamCB(void* underlyingSource, uint8_t flags)
+{
+    MOZ_ASSERT(!finalizeStreamCBCalled, "Invalid test setup");
+    finalizeStreamCBCalled = true;
+    finalizedStreamUnderlyingSource = underlyingSource;
+}
+
+static void
+ResetCallbacks()
+{
+    dataRequestCBCalled = false;
+    writeIntoRequestBufferCBCalled = false;
+    cancelStreamReason = UndefinedValue();
+    cancelStreamCBCalled = false;
+    streamClosedCBCalled = false;
+    streamErroredCBCalled = false;
+    finalizeStreamCBCalled = false;
+}
+
+static bool
+GetIterResult(JSContext* cx, HandleObject promise, MutableHandleValue value, bool* done)
+{
+    RootedObject iterResult(cx, &GetPromiseResult(promise).toObject());
+
+    bool found;
+    if (!JS_HasProperty(cx, iterResult, "value", &found))
+        return false;
+    MOZ_ASSERT(found);
+    if (!JS_HasProperty(cx, iterResult, "done", &found))
+        return false;
+    MOZ_ASSERT(found);
+
+    RootedValue doneVal(cx);
+    if (!JS_GetProperty(cx, iterResult, "value", value))
+        return false;
+    if (!JS_GetProperty(cx, iterResult, "done", &doneVal))
+        return false;
+
+    *done = doneVal.toBoolean();
+    MOZ_ASSERT_IF(*done, value.isUndefined());
+
+    return true;
+}
+
+static JSObject*
+GetReadChunk(JSContext* cx, HandleObject readRequest)
+{
+    MOZ_ASSERT(GetPromiseState(readRequest) == PromiseState::Fulfilled);
+    RootedValue resultVal(cx, GetPromiseResult(readRequest));
+    MOZ_ASSERT(resultVal.isObject());
+    RootedObject result(cx, &resultVal.toObject());
+    RootedValue chunkVal(cx);
+    JS_GetProperty(cx, result, "value", &chunkVal);
+    return &chunkVal.toObject();
+}
+
+BEGIN_TEST(testReadableStream_NewReadableStream)
+{
+    RootedObject stream(cx, NewDefaultStream(cx));
+    CHECK(stream);
+    CHECK(ReadableStreamGetMode(stream) == ReadableStreamMode::Default);
+    return true;
+}
+END_TEST(testReadableStream_NewReadableStream)
+
+BEGIN_TEST(testReadableStream_NewReadableByteStream)
+{
+    RootedObject stream(cx, NewByteStream(cx));
+    CHECK(stream);
+    CHECK(ReadableStreamGetMode(stream) == ReadableStreamMode::Byte);
+    return true;
+}
+END_TEST(testReadableStream_NewReadableByteStream)
+
+BEGIN_TEST(testReadableStream_ReadableStreamGetReaderDefault)
+{
+    RootedObject stream(cx, NewDefaultStream(cx));
+    CHECK(stream);
+
+    RootedObject reader(cx, ReadableStreamGetReader(cx, stream, ReadableStreamReaderMode::Default));
+    CHECK(reader);
+    CHECK(IsReadableStreamDefaultReader(reader));
+    CHECK(ReadableStreamIsLocked(stream));
+    CHECK(!ReadableStreamReaderIsClosed(reader));
+
+    return true;
+}
+END_TEST(testReadableStream_ReadableStreamGetReaderDefault)
+
+BEGIN_TEST(testReadableStream_ReadableStreamGetReaderBYOB)
+{
+    RootedObject stream(cx, NewByteStream(cx));
+    CHECK(stream);
+
+    RootedObject reader(cx, ReadableStreamGetReader(cx, stream, ReadableStreamReaderMode::BYOB));
+    CHECK(reader);
+    CHECK(IsReadableStreamBYOBReader(reader));
+    CHECK(ReadableStreamIsLocked(stream));
+    CHECK(!ReadableStreamReaderIsClosed(reader));
+
+    return true;
+}
+END_TEST(testReadableStream_ReadableStreamGetReaderBYOB)
+
+BEGIN_TEST(testReadableStream_ReadableStreamTee)
+{
+    RootedObject stream(cx, NewDefaultStream(cx));
+    CHECK(stream);
+
+    RootedObject leftStream(cx);
+    RootedObject rightStream(cx);
+    CHECK(ReadableStreamTee(cx, stream, &leftStream, &rightStream));
+    CHECK(ReadableStreamIsLocked(stream));
+    CHECK(leftStream);
+    CHECK(IsReadableStream(leftStream));
+    CHECK(rightStream);
+    CHECK(IsReadableStream(rightStream));
+
+    return true;
+}
+END_TEST(testReadableStream_ReadableStreamTee)
+
+BEGIN_TEST(testReadableStream_ReadableStreamEnqueue)
+{
+    RootedObject stream(cx, NewDefaultStream(cx));
+    CHECK(stream);
+
+    RootedObject chunk(cx, JS_NewPlainObject(cx));
+    CHECK(chunk);
+    RootedValue chunkVal(cx, ObjectValue(*chunk));
+    CHECK(ReadableStreamEnqueue(cx, stream, chunkVal));
+
+    return true;
+}
+END_TEST(testReadableStream_ReadableStreamEnqueue)
+
+BEGIN_TEST(testReadableStream_ReadableByteStreamEnqueue)
+{
+    RootedObject stream(cx, NewDefaultStream(cx));
+    CHECK(stream);
+
+    RootedObject chunk(cx, JS_NewUint8Array(cx, 42));
+    CHECK(chunk);
+    CHECK(!ReadableByteStreamEnqueueBuffer(cx, stream, chunk));
+    CHECK(JS_IsExceptionPending(cx));
+
+    return true;
+}
+END_TEST(testReadableStream_ReadableByteStreamEnqueue)
+
+BEGIN_TEST(testReadableStream_ReadableStreamDefaultReaderRead)
+{
+    RootedObject stream(cx, NewDefaultStream(cx));
+    CHECK(stream);
+    RootedObject reader(cx, ReadableStreamGetReader(cx, stream, ReadableStreamReaderMode::Default));
+    CHECK(reader);
+
+    RootedObject request(cx, ReadableStreamDefaultReaderRead(cx, reader));
+    CHECK(request);
+    CHECK(IsPromiseObject(request));
+    CHECK(GetPromiseState(request) == PromiseState::Pending);
+
+    RootedObject chunk(cx, JS_NewPlainObject(cx));
+    CHECK(chunk);
+    RootedValue chunkVal(cx, ObjectValue(*chunk));
+    CHECK(ReadableStreamEnqueue(cx, stream, chunkVal));
+
+    CHECK(GetReadChunk(cx, request) == chunk);
+
+    return true;
+}
+END_TEST(testReadableStream_ReadableStreamDefaultReaderRead)
+
+BEGIN_TEST(testReadableStream_ReadableByteStreamDefaultReaderRead)
+{
+    RootedObject stream(cx, NewByteStream(cx));
+    CHECK(stream);
+
+    RootedObject reader(cx, ReadableStreamGetReader(cx, stream, ReadableStreamReaderMode::Default));
+    CHECK(reader);
+
+    RootedObject request(cx, ReadableStreamDefaultReaderRead(cx, reader));
+    CHECK(request);
+    CHECK(IsPromiseObject(request));
+    CHECK(GetPromiseState(request) == PromiseState::Pending);
+
+    size_t length = sizeof(test_buffer_data);
+    RootedObject buffer(cx, JS_NewArrayBufferWithExternalContents(cx, length, test_buffer_data));
+    CHECK(buffer);
+    RootedObject chunk(cx, JS_NewUint8ArrayWithBuffer(cx, buffer, 0, length));
+    CHECK(chunk);
+    bool isShared;
+    CHECK(!JS_IsDetachedArrayBufferObject(buffer));
+
+    CHECK(ReadableByteStreamEnqueueBuffer(cx, stream, chunk));
+
+    CHECK(JS_IsDetachedArrayBufferObject(buffer));
+    RootedObject readChunk(cx, GetReadChunk(cx, request));
+    CHECK(JS_IsUint8Array(readChunk));
+    void* readBufferData;
+    {
+        JS::AutoCheckCannotGC autoNoGC(cx);
+        readBufferData = JS_GetArrayBufferViewData(readChunk, &isShared, autoNoGC);
+    }
+    CHECK(readBufferData);
+    CHECK(!memcmp(test_buffer_data, readBufferData, length));
+
+    return true;
+}
+END_TEST(testReadableStream_ReadableByteStreamDefaultReaderRead)
+
+BEGIN_TEST(testReadableStream_ReadableByteStreamBYOBReaderRead)
+{
+    RootedObject stream(cx, NewByteStream(cx));
+    CHECK(stream);
+
+    RootedObject reader(cx, ReadableStreamGetReader(cx, stream, ReadableStreamReaderMode::BYOB));
+    CHECK(reader);
+
+    size_t length = sizeof(test_buffer_data);
+    RootedObject targetArray(cx, JS_NewUint8Array(cx, length));
+    bool isShared;
+
+    RootedObject request(cx, ReadableStreamBYOBReaderRead(cx, reader, targetArray));
+    CHECK(request);
+    CHECK(IsPromiseObject(request));
+    CHECK(GetPromiseState(request) == PromiseState::Pending);
+    CHECK(JS_IsDetachedArrayBufferObject(JS_GetArrayBufferViewBuffer(cx, targetArray, &isShared)));
+
+    RootedObject buffer(cx, JS_NewArrayBufferWithExternalContents(cx, length, test_buffer_data));
+    CHECK(buffer);
+    CHECK(!JS_IsDetachedArrayBufferObject(buffer));
+
+    CHECK(ReadableByteStreamEnqueueBuffer(cx, stream, buffer));
+
+    CHECK(JS_IsDetachedArrayBufferObject(buffer));
+    RootedObject readChunk(cx, GetReadChunk(cx, request));
+    CHECK(JS_IsUint8Array(readChunk));
+    void* readBufferData;
+    {
+        JS::AutoCheckCannotGC autoNoGC(cx);
+        readBufferData = JS_GetArrayBufferViewData(readChunk, &isShared, autoNoGC);
+    }
+    CHECK(readBufferData);
+    CHECK(!memcmp(test_buffer_data, readBufferData, length));
+    // TODO: eliminate the memcpy that happens here.
+//    CHECK(readBufferData == test_buffer_data);
+
+    return true;
+}
+END_TEST(testReadableStream_ReadableByteStreamBYOBReaderRead)
+
+BEGIN_TEST(testReadableStream_ReadableStreamDefaultReaderClose)
+{
+    SetReadableStreamCallbacks(cx, &DataRequestCB, &WriteIntoRequestBufferCB,
+                                             &CancelStreamCB, &StreamClosedCB, &StreamErroredCB,
+                                             &FinalizeStreamCB);
+    RootedObject stream(cx, NewDefaultStream(cx));
+    CHECK(stream);
+    RootedObject reader(cx, ReadableStreamGetReader(cx, stream, ReadableStreamReaderMode::Default));
+    CHECK(reader);
+
+    RootedObject request(cx, ReadableStreamDefaultReaderRead(cx, reader));
+    CHECK(request);
+    CHECK(IsPromiseObject(request));
+    CHECK(GetPromiseState(request) == PromiseState::Pending);
+
+    CHECK(ReadableStreamClose(cx, stream));
+
+    bool done;
+    RootedValue value(cx);
+    CHECK(GetPromiseState(request) == PromiseState::Fulfilled);
+    CHECK(GetIterResult(cx, request, &value, &done));
+    CHECK(value.isUndefined());
+    CHECK(done);
+
+    // The callbacks are only invoked for external streams.
+    CHECK(!streamClosedCBCalled);
+
+    return true;
+}
+END_TEST(testReadableStream_ReadableStreamDefaultReaderClose)
+
+BEGIN_TEST(testReadableStream_ReadableStreamDefaultReaderError)
+{
+    ResetCallbacks();
+    SetReadableStreamCallbacks(cx, &DataRequestCB, &WriteIntoRequestBufferCB,
+                                             &CancelStreamCB, &StreamClosedCB, &StreamErroredCB,
+                                             &FinalizeStreamCB);
+    RootedObject stream(cx, NewDefaultStream(cx));
+    CHECK(stream);
+    RootedObject reader(cx, ReadableStreamGetReader(cx, stream, ReadableStreamReaderMode::Default));
+    CHECK(reader);
+
+    RootedObject request(cx, ReadableStreamDefaultReaderRead(cx, reader));
+    CHECK(request);
+    CHECK(IsPromiseObject(request));
+    CHECK(GetPromiseState(request) == PromiseState::Pending);
+
+    CHECK(ReadableStreamIsLocked(stream));
+    CHECK(ReadableStreamIsReadable(stream));
+    RootedValue error(cx, Int32Value(42));
+    CHECK(ReadableStreamError(cx, stream, error));
+
+    CHECK(GetPromiseState(request) == PromiseState::Rejected);
+    RootedValue reason(cx, GetPromiseResult(request));
+    CHECK(reason.isInt32());
+    CHECK(reason.toInt32() == 42);
+
+    // The callbacks are only invoked for external streams.
+    CHECK(!streamErroredCBCalled);
+
+    return true;
+}
+END_TEST(testReadableStream_ReadableStreamDefaultReaderError)
+
+static JSObject*
+NewExternalSourceStream(JSContext* cx, void* underlyingSource,
+                        RequestReadableStreamDataCallback dataRequestCallback,
+                        WriteIntoReadRequestBufferCallback writeIntoReadRequestCallback,
+                        CancelReadableStreamCallback cancelCallback,
+                        ReadableStreamClosedCallback closedCallback,
+                        ReadableStreamErroredCallback erroredCallback,
+                        ReadableStreamFinalizeCallback finalizeCallback)
+{
+    SetReadableStreamCallbacks(cx, dataRequestCallback, writeIntoReadRequestCallback,
+                                             cancelCallback, closedCallback, erroredCallback,
+                                             finalizeCallback);
+    RootedObject stream(cx, NewReadableExternalSourceStreamObject(cx, underlyingSource));
+    MOZ_ASSERT_IF(stream, IsReadableStream(stream));
+    return stream;
+}
+
+BEGIN_TEST(testReadableStream_CreateReadableByteStreamWithExternalSource)
+{
+    ResetCallbacks();
+
+    RootedObject stream(cx, NewExternalSourceStream(cx, &test_buffer_data, &DataRequestCB,
+                                                    &WriteIntoRequestBufferCB, &CancelStreamCB,
+                                                    &StreamClosedCB, &StreamErroredCB,
+                                                    &FinalizeStreamCB));
+    CHECK(stream);
+    CHECK(ReadableStreamGetMode(stream) == JS::ReadableStreamMode::ExternalSource);
+    void* underlyingSource;
+    CHECK(ReadableStreamGetExternalUnderlyingSource(cx, stream, &underlyingSource));
+    CHECK(underlyingSource == &test_buffer_data);
+    CHECK(ReadableStreamIsLocked(stream));
+    ReadableStreamReleaseExternalUnderlyingSource(stream);
+
+    return true;
+}
+END_TEST(testReadableStream_CreateReadableByteStreamWithExternalSource)
+
+BEGIN_TEST(testReadableStream_ExternalSourceCancel)
+{
+    ResetCallbacks();
+
+    RootedObject stream(cx, NewExternalSourceStream(cx, &test_buffer_data, &DataRequestCB,
+                                                    &WriteIntoRequestBufferCB, &CancelStreamCB,
+                                                    &StreamClosedCB, &StreamErroredCB,
+                                                    &FinalizeStreamCB));
+    CHECK(stream);
+    RootedValue reason(cx, Int32Value(42));
+    CHECK(ReadableStreamCancel(cx, stream, reason));
+    CHECK(cancelStreamCBCalled);
+    CHECK(cancelStreamReason == reason);
+
+    return true;
+}
+END_TEST(testReadableStream_ExternalSourceCancel)
+
+BEGIN_TEST(testReadableStream_ExternalSourceGetReader)
+{
+    ResetCallbacks();
+
+    RootedObject stream(cx, NewExternalSourceStream(cx, &test_buffer_data, &DataRequestCB,
+                                                    &WriteIntoRequestBufferCB, &CancelStreamCB,
+                                                    &StreamClosedCB, &StreamErroredCB,
+                                                    &FinalizeStreamCB));
+    CHECK(stream);
+
+    RootedValue streamVal(cx, ObjectValue(*stream));
+    CHECK(JS_SetProperty(cx, global, "stream", streamVal));
+    RootedValue rval(cx);
+    EVAL("stream.getReader()", &rval);
+    CHECK(rval.isObject());
+    RootedObject reader(cx, &rval.toObject());
+    CHECK(IsReadableStreamDefaultReader(reader));
+
+    return true;
+}
+END_TEST(testReadableStream_ExternalSourceGetReader)
+
+BEGIN_TEST(testReadableStream_ExternalSourceUpdateAvailableData)
+{
+    ResetCallbacks();
+
+    RootedObject stream(cx, NewExternalSourceStream(cx, &test_buffer_data, &DataRequestCB,
+                                                    &WriteIntoRequestBufferCB, &CancelStreamCB,
+                                                    &StreamClosedCB, &StreamErroredCB,
+                                                    &FinalizeStreamCB));
+    CHECK(stream);
+
+    ReadableStreamUpdateDataAvailableFromSource(cx, stream, 1024);
+
+    return true;
+}
+END_TEST(testReadableStream_ExternalSourceUpdateAvailableData)
+
+struct ReadFromExternalSourceFixture : public JSAPITest
+{
+    virtual ~ReadFromExternalSourceFixture() {}
+
+    bool readWithoutDataAvailable(const char* evalSrc, const char* evalSrc2,
+                                  uint32_t writtenLength)
+    {
+        ResetCallbacks();
+        definePrint();
+
+        RootedObject stream(cx, NewExternalSourceStream(cx, &test_buffer_data, &DataRequestCB,
+                                                        &WriteIntoRequestBufferCB,
+                                                        &CancelStreamCB,
+                                                        &StreamClosedCB, &StreamErroredCB,
+                                                        &FinalizeStreamCB));
+        CHECK(stream);
+        js::RunJobs(cx);
+        void* underlyingSource;
+        CHECK(ReadableStreamGetExternalUnderlyingSource(cx, stream, &underlyingSource));
+        CHECK(underlyingSource == &test_buffer_data);
+        CHECK(ReadableStreamIsLocked(stream));
+        ReadableStreamReleaseExternalUnderlyingSource(stream);
+
+        RootedValue streamVal(cx, ObjectValue(*stream));
+        CHECK(JS_SetProperty(cx, global, "stream", streamVal));
+
+        RootedValue rval(cx);
+        EVAL(evalSrc, &rval);
+        CHECK(dataRequestCBCalled);
+        CHECK(!writeIntoRequestBufferCBCalled);
+        CHECK(rval.isObject());
+        RootedObject promise(cx, &rval.toObject());
+        CHECK(IsPromiseObject(promise));
+        CHECK(GetPromiseState(promise) == PromiseState::Pending);
+
+        size_t length = sizeof(test_buffer_data);
+        ReadableStreamUpdateDataAvailableFromSource(cx, stream, length);
+
+        CHECK(writeIntoRequestBufferCBCalled);
+        CHECK(GetPromiseState(promise) == PromiseState::Fulfilled);
+        RootedValue iterVal(cx);
+        bool done;
+        if (!GetIterResult(cx, promise, &iterVal, &done))
+            return false;
+
+        CHECK(!done);
+        RootedObject chunk(cx, &iterVal.toObject());
+        CHECK(JS_IsUint8Array(chunk));
+
+        {
+            JS::AutoCheckCannotGC noGC(cx);
+            bool dummy;
+            void* buffer = JS_GetArrayBufferViewData(chunk, &dummy, noGC);
+            CHECK(!memcmp(buffer, test_buffer_data, writtenLength));
+        }
+
+        dataRequestCBCalled = false;
+        writeIntoRequestBufferCBCalled = false;
+        EVAL(evalSrc2, &rval);
+        CHECK(dataRequestCBCalled);
+        CHECK(!writeIntoRequestBufferCBCalled);
+
+        return true;
+    }
+
+    bool readWithDataAvailable(const char* evalSrc, uint32_t writtenLength) {
+        ResetCallbacks();
+        definePrint();
+
+        RootedObject stream(cx, NewExternalSourceStream(cx, &test_buffer_data, &DataRequestCB,
+                                                        &WriteIntoRequestBufferCB, &CancelStreamCB,
+                                                        &StreamClosedCB, &StreamErroredCB,
+                                                        &FinalizeStreamCB));
+        CHECK(stream);
+        void* underlyingSource;
+        CHECK(ReadableStreamGetExternalUnderlyingSource(cx, stream, &underlyingSource));
+        CHECK(underlyingSource == &test_buffer_data);
+        CHECK(ReadableStreamIsLocked(stream));
+        ReadableStreamReleaseExternalUnderlyingSource(stream);
+
+        size_t length = sizeof(test_buffer_data);
+        ReadableStreamUpdateDataAvailableFromSource(cx, stream, length);
+
+        RootedValue streamVal(cx, ObjectValue(*stream));
+        CHECK(JS_SetProperty(cx, global, "stream", streamVal));
+
+        RootedValue rval(cx);
+        EVAL(evalSrc, &rval);
+        CHECK(writeIntoRequestBufferCBCalled);
+        CHECK(rval.isObject());
+        RootedObject promise(cx, &rval.toObject());
+        CHECK(IsPromiseObject(promise));
+        CHECK(GetPromiseState(promise) == PromiseState::Fulfilled);
+        RootedValue iterVal(cx);
+        bool done;
+        if (!GetIterResult(cx, promise, &iterVal, &done))
+            return false;
+
+        CHECK(!done);
+        RootedObject chunk(cx, &iterVal.toObject());
+        CHECK(JS_IsUint8Array(chunk));
+
+        {
+            JS::AutoCheckCannotGC noGC(cx);
+            bool dummy;
+            void* buffer = JS_GetArrayBufferViewData(chunk, &dummy, noGC);
+            CHECK(!memcmp(buffer, test_buffer_data, writtenLength));
+        }
+
+        return true;
+    }
+};
+
+BEGIN_FIXTURE_TEST(ReadFromExternalSourceFixture,
+                   testReadableStream_ExternalSourceReadDefaultWithoutDataAvailable)
+{
+    return readWithoutDataAvailable("r = stream.getReader(); r.read()", "r.read()", sizeof(test_buffer_data));
+}
+END_FIXTURE_TEST(ReadFromExternalSourceFixture,
+                 testReadableStream_ExternalSourceReadDefaultWithoutDataAvailable)
+
+BEGIN_FIXTURE_TEST(ReadFromExternalSourceFixture,
+                   testReadableStream_ExternalSourceCloseWithPendingRead)
+{
+    CHECK(readWithoutDataAvailable("r = stream.getReader(); request0 = r.read(); "
+                                   "request1 = r.read(); request0", "r.read()",
+                                   sizeof(test_buffer_data)));
+
+    RootedValue val(cx);
+    CHECK(JS_GetProperty(cx, global, "request1", &val));
+    CHECK(val.isObject());
+    RootedObject request(cx, &val.toObject());
+    CHECK(IsPromiseObject(request));
+    CHECK(GetPromiseState(request) == PromiseState::Pending);
+
+    CHECK(JS_GetProperty(cx, global, "stream", &val));
+    RootedObject stream(cx, &val.toObject());
+    ReadableStreamClose(cx, stream);
+
+    val = GetPromiseResult(request);
+    MOZ_ASSERT(val.isObject());
+    RootedObject result(cx, &val.toObject());
+
+    JS_GetProperty(cx, result, "done", &val);
+    CHECK(val.isBoolean());
+    CHECK(val.toBoolean() == true);
+
+    JS_GetProperty(cx, result, "value", &val);
+    CHECK(val.isUndefined());
+    return true;
+}
+END_FIXTURE_TEST(ReadFromExternalSourceFixture,
+                 testReadableStream_ExternalSourceCloseWithPendingRead)
+
+BEGIN_FIXTURE_TEST(ReadFromExternalSourceFixture,
+                   testReadableStream_ExternalSourceReadDefaultWithDataAvailable)
+{
+    return readWithDataAvailable("r = stream.getReader(); r.read()", sizeof(test_buffer_data));
+}
+END_FIXTURE_TEST(ReadFromExternalSourceFixture,
+                 testReadableStream_ExternalSourceReadDefaultWithDataAvailable)
+
+BEGIN_FIXTURE_TEST(ReadFromExternalSourceFixture,
+                   testReadableStream_ExternalSourceReadBYOBWithoutDataAvailable)
+{
+    return readWithoutDataAvailable("r = stream.getReader({mode: 'byob'}); r.read(new Uint8Array(63))", "r.read(new Uint8Array(10))", 10);
+}
+END_FIXTURE_TEST(ReadFromExternalSourceFixture,
+                 testReadableStream_ExternalSourceReadBYOBWithoutDataAvailable)
+
+BEGIN_FIXTURE_TEST(ReadFromExternalSourceFixture,
+                   testReadableStream_ExternalSourceReadBYOBWithDataAvailable)
+{
+    return readWithDataAvailable("r = stream.getReader({mode: 'byob'}); r.read(new Uint8Array(10))", 10);
+}
+END_FIXTURE_TEST(ReadFromExternalSourceFixture,
+                 testReadableStream_ExternalSourceReadBYOBWithDataAvailable)
diff --git a/js/src/jsapi-tests/tests.cpp b/js/src/jsapi-tests/tests.cpp
index b75fa6602d..61e79eab6d 100644
--- a/js/src/jsapi-tests/tests.cpp
+++ b/js/src/jsapi-tests/tests.cpp
@@ -81,6 +81,10 @@ JSObject* JSAPITest::createGlobal(JSPrincipals* principals)
     /* Create the global object. */
     JS::RootedObject newGlobal(cx);
     JS::CompartmentOptions options;
+#ifdef ENABLE_STREAMS
+    options.creationOptions().setStreamsEnabled(true);
+#endif
+    printf("enabled\n");
     options.behaviors().setVersion(JSVERSION_LATEST);
     newGlobal = JS_NewGlobalObject(cx, getGlobalClass(), principals, JS::FireOnNewGlobalHook,
                                    options);
diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index 0fd82f7c2d..9067ca3d3d 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -46,6 +46,7 @@
 #include "builtin/MapObject.h"
 #include "builtin/Promise.h"
 #include "builtin/RegExp.h"
+#include "builtin/Stream.h"
 #include "builtin/SymbolObject.h"
 #ifdef ENABLE_BINARYDATA
 # include "builtin/TypedObject.h"
@@ -5095,7 +5096,6 @@ CallOriginalPromiseThenImpl(JSContext* cx, JS::HandleObject promiseObj,
             return false;
     }
     return true;
-
 }
 
 JS_PUBLIC_API(JSObject*)
@@ -5126,7 +5126,7 @@ JS::AddPromiseReactions(JSContext* cx, JS::HandleObject promiseObj,
  * Unforgeable version of Promise.all for internal use.
  *
  * Takes a dense array of Promise objects and returns a promise that's
- * resolved with an array of resolution values when all those promises ahve
+ * resolved with an array of resolution values when all those promises have
  * been resolved, or rejected with the rejection value of the first rejected
  * promise.
  *
@@ -5141,6 +5141,368 @@ JS::GetWaitForAllPromise(JSContext* cx, const JS::AutoObjectVector& promises)
     return js::GetWaitForAllPromise(cx, promises);
 }
 
+JS_PUBLIC_API(JSObject*)
+JS::NewReadableDefaultStreamObject(JSContext* cx,
+                                   JS::HandleObject underlyingSource /* = nullptr */,
+                                   JS::HandleFunction size /* = nullptr */,
+                                   double highWaterMark /* = 1 */,
+                                   JS::HandleObject proto /* = nullptr */)
+{
+    MOZ_ASSERT(!cx->runtime()->isAtomsCompartment(cx->compartment()));
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+
+    RootedObject source(cx, underlyingSource);
+    if (!source) {
+        source = NewBuiltinClassInstance<PlainObject>(cx);
+        if (!source)
+            return nullptr;
+    }
+    RootedValue sourceVal(cx, ObjectValue(*source));
+    RootedValue sizeVal(cx, size ? ObjectValue(*size) : UndefinedValue());
+    RootedValue highWaterMarkVal(cx, NumberValue(highWaterMark));
+    return ReadableStream::createDefaultStream(cx, sourceVal, sizeVal, highWaterMarkVal, proto);
+}
+
+JS_PUBLIC_API(JSObject*)
+JS::NewReadableByteStreamObject(JSContext* cx,
+                                JS::HandleObject underlyingSource /* = nullptr */,
+                                double highWaterMark /* = 1 */,
+                                JS::HandleObject proto /* = nullptr */)
+{
+    MOZ_ASSERT(!cx->runtime()->isAtomsCompartment(cx->compartment()));
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+
+    RootedObject source(cx, underlyingSource);
+    if (!source) {
+        source = NewBuiltinClassInstance<PlainObject>(cx);
+        if (!source)
+            return nullptr;
+    }
+    RootedValue sourceVal(cx, ObjectValue(*source));
+    RootedValue highWaterMarkVal(cx, NumberValue(highWaterMark));
+    return ReadableStream::createByteStream(cx, sourceVal, highWaterMarkVal, proto);
+}
+
+extern JS_PUBLIC_API(void)
+JS::SetReadableStreamCallbacks(JSContext* cx,
+                               JS::RequestReadableStreamDataCallback dataRequestCallback,
+                               JS::WriteIntoReadRequestBufferCallback writeIntoReadRequestCallback,
+                               JS::CancelReadableStreamCallback cancelCallback,
+                               JS::ReadableStreamClosedCallback closedCallback,
+                               JS::ReadableStreamErroredCallback erroredCallback,
+                               JS::ReadableStreamFinalizeCallback finalizeCallback)
+{
+    MOZ_ASSERT(dataRequestCallback);
+    MOZ_ASSERT(writeIntoReadRequestCallback);
+    MOZ_ASSERT(cancelCallback);
+    MOZ_ASSERT(closedCallback);
+    MOZ_ASSERT(erroredCallback);
+    MOZ_ASSERT(finalizeCallback);
+
+    JSRuntime* rt = cx->runtime();
+
+    MOZ_ASSERT(!rt->readableStreamDataRequestCallback);
+    MOZ_ASSERT(!rt->readableStreamWriteIntoReadRequestCallback);
+    MOZ_ASSERT(!rt->readableStreamCancelCallback);
+    MOZ_ASSERT(!rt->readableStreamClosedCallback);
+    MOZ_ASSERT(!rt->readableStreamErroredCallback);
+    MOZ_ASSERT(!rt->readableStreamFinalizeCallback);
+
+    rt->readableStreamDataRequestCallback = dataRequestCallback;
+    rt->readableStreamWriteIntoReadRequestCallback = writeIntoReadRequestCallback;
+    rt->readableStreamCancelCallback = cancelCallback;
+    rt->readableStreamClosedCallback = closedCallback;
+    rt->readableStreamErroredCallback = erroredCallback;
+    rt->readableStreamFinalizeCallback = finalizeCallback;
+}
+
+JS_PUBLIC_API(bool)
+JS::HasReadableStreamCallbacks(JSContext* cx)
+{
+    return cx->runtime()->readableStreamDataRequestCallback;
+}
+
+JS_PUBLIC_API(JSObject*)
+JS::NewReadableExternalSourceStreamObject(JSContext* cx, void* underlyingSource,
+                                          uint8_t flags /* = 0 */,
+                                          HandleObject proto /* = nullptr */)
+{
+    MOZ_ASSERT(!cx->runtime()->isAtomsCompartment(cx->compartment()));
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+
+#ifdef DEBUG
+    JSRuntime* rt = cx->runtime();
+    MOZ_ASSERT(rt->readableStreamDataRequestCallback);
+    MOZ_ASSERT(rt->readableStreamWriteIntoReadRequestCallback);
+    MOZ_ASSERT(rt->readableStreamCancelCallback);
+    MOZ_ASSERT(rt->readableStreamClosedCallback);
+    MOZ_ASSERT(rt->readableStreamErroredCallback);
+    MOZ_ASSERT(rt->readableStreamFinalizeCallback);
+#endif // DEBUG
+
+    return ReadableStream::createExternalSourceStream(cx, underlyingSource, flags, proto);
+}
+
+JS_PUBLIC_API(uint8_t)
+JS::ReadableStreamGetEmbeddingFlags(const JSObject* stream)
+{
+    return stream->as<ReadableStream>().embeddingFlags();
+}
+
+JS_PUBLIC_API(bool)
+JS::IsReadableStream(const JSObject* obj)
+{
+    return obj->is<ReadableStream>();
+}
+
+JS_PUBLIC_API(bool)
+JS::IsReadableStreamReader(const JSObject* obj)
+{
+    return obj->is<ReadableStreamDefaultReader>() || obj->is<ReadableStreamBYOBReader>();
+}
+
+JS_PUBLIC_API(bool)
+JS::IsReadableStreamDefaultReader(const JSObject* obj)
+{
+    return obj->is<ReadableStreamDefaultReader>();
+}
+
+JS_PUBLIC_API(bool)
+JS::IsReadableStreamBYOBReader(const JSObject* obj)
+{
+    return obj->is<ReadableStreamBYOBReader>();
+}
+
+JS_PUBLIC_API(bool)
+JS::ReadableStreamIsReadable(const JSObject* stream)
+{
+    return stream->as<ReadableStream>().readable();
+}
+
+JS_PUBLIC_API(bool)
+JS::ReadableStreamIsLocked(const JSObject* stream)
+{
+    return stream->as<ReadableStream>().locked();
+}
+
+JS_PUBLIC_API(bool)
+JS::ReadableStreamIsDisturbed(const JSObject* stream)
+{
+    return stream->as<ReadableStream>().disturbed();
+}
+
+JS_PUBLIC_API(JSObject*)
+JS::ReadableStreamCancel(JSContext* cx, HandleObject streamObj, HandleValue reason)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+    assertSameCompartment(cx, streamObj);
+    assertSameCompartment(cx, reason);
+
+    Rooted<ReadableStream*> stream(cx, &streamObj->as<ReadableStream>());
+    return ReadableStream::cancel(cx, stream, reason);
+}
+
+JS_PUBLIC_API(JS::ReadableStreamMode)
+JS::ReadableStreamGetMode(const JSObject* stream)
+{
+    return stream->as<ReadableStream>().mode();
+}
+
+JS_PUBLIC_API(JSObject*)
+JS::ReadableStreamGetReader(JSContext* cx, HandleObject streamObj, ReadableStreamReaderMode mode)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+    assertSameCompartment(cx, streamObj);
+
+    Rooted<ReadableStream*> stream(cx, &streamObj->as<ReadableStream>());
+    return ReadableStream::getReader(cx, stream, mode);
+}
+
+JS_PUBLIC_API(bool)
+JS::ReadableStreamGetExternalUnderlyingSource(JSContext* cx, HandleObject streamObj, void** source)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+    assertSameCompartment(cx, streamObj);
+
+    Rooted<ReadableStream*> stream(cx, &streamObj->as<ReadableStream>());
+    return ReadableStream::getExternalSource(cx, stream, source);
+}
+
+JS_PUBLIC_API(void)
+JS::ReadableStreamReleaseExternalUnderlyingSource(JSObject* stream)
+{
+    stream->as<ReadableStream>().releaseExternalSource();
+}
+
+JS_PUBLIC_API(bool)
+JS::ReadableStreamUpdateDataAvailableFromSource(JSContext* cx, JS::HandleObject streamObj,
+                                                uint32_t availableData)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+    assertSameCompartment(cx, streamObj);
+
+    Rooted<ReadableStream*> stream(cx, &streamObj->as<ReadableStream>());
+    return ReadableStream::updateDataAvailableFromSource(cx, stream, availableData);
+}
+
+JS_PUBLIC_API(bool)
+JS::ReadableStreamTee(JSContext* cx, HandleObject streamObj,
+                      MutableHandleObject branch1Obj, MutableHandleObject branch2Obj)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+    assertSameCompartment(cx, streamObj);
+
+    Rooted<ReadableStream*> stream(cx, &streamObj->as<ReadableStream>());
+    Rooted<ReadableStream*> branch1Stream(cx);
+    Rooted<ReadableStream*> branch2Stream(cx);
+
+    if (!ReadableStream::tee(cx, stream, false, &branch1Stream, &branch2Stream))
+        return false;
+
+    branch1Obj.set(branch1Stream);
+    branch2Obj.set(branch2Stream);
+
+    return true;
+}
+
+JS_PUBLIC_API(void)
+JS::ReadableStreamGetDesiredSize(JSObject* streamObj, bool* hasValue, double* value)
+{
+    streamObj->as<ReadableStream>().desiredSize(hasValue, value);
+}
+
+JS_PUBLIC_API(bool)
+JS::ReadableStreamClose(JSContext* cx, HandleObject streamObj)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+    assertSameCompartment(cx, streamObj);
+
+    Rooted<ReadableStream*> stream(cx, &streamObj->as<ReadableStream>());
+    return ReadableStream::close(cx, stream);
+}
+
+JS_PUBLIC_API(bool)
+JS::ReadableStreamEnqueue(JSContext* cx, HandleObject streamObj, HandleValue chunk)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+    assertSameCompartment(cx, streamObj);
+    assertSameCompartment(cx, chunk);
+
+    Rooted<ReadableStream*> stream(cx, &streamObj->as<ReadableStream>());
+    if (stream->mode() != JS::ReadableStreamMode::Default) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAM_NOT_DEFAULT_CONTROLLER,
+                                  "JS::ReadableStreamEnqueue");
+        return false;
+    }
+    return ReadableStream::enqueue(cx, stream, chunk);
+}
+
+JS_PUBLIC_API(bool)
+JS::ReadableByteStreamEnqueueBuffer(JSContext* cx, HandleObject streamObj, HandleObject chunkObj)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+    assertSameCompartment(cx, streamObj);
+    assertSameCompartment(cx, chunkObj);
+
+    Rooted<ReadableStream*> stream(cx, &streamObj->as<ReadableStream>());
+    if (stream->mode() != JS::ReadableStreamMode::Byte) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLESTREAM_NOT_BYTE_STREAM_CONTROLLER,
+                                  "JS::ReadableByteStreamEnqueueBuffer");
+        return false;
+    }
+
+    Rooted<ArrayBufferObject*> buffer(cx);
+    if (chunkObj->is<ArrayBufferViewObject>()) {
+        bool dummy;
+        buffer = &JS_GetArrayBufferViewBuffer(cx, chunkObj, &dummy)->as<ArrayBufferObject>();
+    } else if (chunkObj->is<ArrayBufferObject>()) {
+        buffer = &chunkObj->as<ArrayBufferObject>();
+    } else {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                                  JSMSG_READABLEBYTESTREAMCONTROLLER_BAD_CHUNK,
+                                  "JS::ReadableByteStreamEnqueueBuffer");
+        return false;
+    }
+
+    return ReadableStream::enqueueBuffer(cx, stream, buffer);
+}
+
+JS_PUBLIC_API(bool)
+JS::ReadableStreamError(JSContext* cx, HandleObject streamObj, HandleValue error)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+    assertSameCompartment(cx, streamObj);
+    assertSameCompartment(cx, error);
+
+    Rooted<ReadableStream*> stream(cx, &streamObj->as<ReadableStream>());
+    return js::ReadableStream::error(cx, stream, error);
+}
+
+JS_PUBLIC_API(bool)
+JS::ReadableStreamReaderIsClosed(const JSObject* reader)
+{
+    return js::ReadableStreamReaderIsClosed(reader);
+}
+
+JS_PUBLIC_API(bool)
+JS::ReadableStreamReaderCancel(JSContext* cx, HandleObject reader, HandleValue reason)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+    assertSameCompartment(cx, reader);
+    assertSameCompartment(cx, reason);
+
+    return js::ReadableStreamReaderCancel(cx, reader, reason);
+}
+
+JS_PUBLIC_API(bool)
+JS::ReadableStreamReaderReleaseLock(JSContext* cx, HandleObject reader)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+    assertSameCompartment(cx, reader);
+
+    return js::ReadableStreamReaderReleaseLock(cx, reader);
+}
+
+JS_PUBLIC_API(JSObject*)
+JS::ReadableStreamDefaultReaderRead(JSContext* cx, HandleObject readerObj)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+    assertSameCompartment(cx, readerObj);
+
+    Rooted<ReadableStreamDefaultReader*> reader(cx, &readerObj->as<ReadableStreamDefaultReader>());
+    return js::ReadableStreamDefaultReader::read(cx, reader);
+}
+
+JS_PUBLIC_API(JSObject*)
+JS::ReadableStreamBYOBReaderRead(JSContext* cx, HandleObject readerObj, HandleObject viewObj)
+{
+    AssertHeapIsIdle(cx);
+    CHECK_REQUEST(cx);
+    assertSameCompartment(cx, readerObj);
+    assertSameCompartment(cx, viewObj);
+
+    Rooted<ReadableStreamBYOBReader*> reader(cx, &readerObj->as<ReadableStreamBYOBReader>());
+    Rooted<ArrayBufferViewObject*> view(cx, &viewObj->as<ArrayBufferViewObject>());
+    return js::ReadableStreamBYOBReader::read(cx, reader, view);
+}
+
 JS_PUBLIC_API(void)
 JS::SetAsyncTaskCallbacks(JSContext* cx, JS::StartAsyncTaskCallback start,
                           JS::FinishAsyncTaskCallback finish)
diff --git a/js/src/jsapi.h b/js/src/jsapi.h
index f80d2602e6..951b612502 100644
--- a/js/src/jsapi.h
+++ b/js/src/jsapi.h
@@ -35,6 +35,7 @@
 #include "js/Principals.h"
 #include "js/Realm.h"
 #include "js/RootingAPI.h"
+#include "js/Stream.h"
 #include "js/TracingAPI.h"
 #include "js/UniquePtr.h"
 #include "js/Utility.h"
@@ -1161,6 +1162,16 @@ class JS_PUBLIC_API(ContextOptions) {
         return *this;
     }
 
+    bool streams() const { return streams_; }
+    ContextOptions& setStreams(bool flag) {
+        streams_ = flag;
+        return *this;
+    }
+    ContextOptions& toggleStreams() {
+        streams_ = !streams_;
+        return *this;
+    }
+
     bool nativeRegExp() const { return nativeRegExp_; }
     ContextOptions& setNativeRegExp(bool flag) {
         nativeRegExp_ = flag;
@@ -1243,6 +1254,7 @@ class JS_PUBLIC_API(ContextOptions) {
     bool strictMode_ : 1;
     bool extraWarnings_ : 1;
     bool arrayProtoValues_ : 1;
+    bool streams_ : 1;
 };
 
 JS_PUBLIC_API(ContextOptions&)
diff --git a/js/src/jsfriendapi.h b/js/src/jsfriendapi.h
index b4c4e15801..c463019dac 100644
--- a/js/src/jsfriendapi.h
+++ b/js/src/jsfriendapi.h
@@ -1787,6 +1787,9 @@ UnwrapArrayBufferView(JSObject* obj);
 extern JS_FRIEND_API(JSObject*)
 UnwrapSharedArrayBuffer(JSObject* obj);
 
+extern JS_FRIEND_API(JSObject*)
+UnwrapReadableStream(JSObject* obj);
+
 
 namespace detail {
 
@@ -2005,6 +2008,12 @@ JS_IsArrayBufferViewObject(JSObject* obj);
 extern JS_FRIEND_API(uint32_t)
 JS_GetArrayBufferViewByteLength(JSObject* obj);
 
+/**
+ * More generic name for JS_GetTypedArrayByteOffset to cover DataViews as well
+ */
+extern JS_FRIEND_API(uint32_t)
+JS_GetArrayBufferViewByteOffset(JSObject* obj);
+
 /*
  * Return a pointer to the start of the data referenced by a typed array. The
  * data is still owned by the typed array, and should not be modified on
diff --git a/js/src/jsnum.cpp b/js/src/jsnum.cpp
index fd23e6ccd5..beff701bd0 100644
--- a/js/src/jsnum.cpp
+++ b/js/src/jsnum.cpp
@@ -1003,6 +1003,13 @@ static const JSFunctionSpec number_methods[] = {
     JS_FS_END
 };
 
+bool
+js::IsInteger(const Value& val)
+{
+    return val.isInt32() ||
+           (mozilla::IsFinite(val.toDouble()) && JS::ToInteger(val.toDouble()) == val.toDouble());
+}
+
 // ES6 draft ES6 15.7.3.12
 static bool
 Number_isInteger(JSContext* cx, unsigned argc, Value* vp)
@@ -1013,9 +1020,7 @@ Number_isInteger(JSContext* cx, unsigned argc, Value* vp)
         return true;
     }
     Value val = args[0];
-    args.rval().setBoolean(val.isInt32() ||
-                           (mozilla::IsFinite(val.toDouble()) &&
-                            JS::ToInteger(val.toDouble()) == val.toDouble()));
+    args.rval().setBoolean(js::IsInteger(val));
     return true;
 }
 
diff --git a/js/src/jsnum.h b/js/src/jsnum.h
index ee07d0a35d..7c96bfb652 100644
--- a/js/src/jsnum.h
+++ b/js/src/jsnum.h
@@ -72,6 +72,10 @@ Int32ToString(ExclusiveContext* cx, int32_t i);
 extern JSAtom*
 Int32ToAtom(ExclusiveContext* cx, int32_t si);
 
+// ES6 15.7.3.12
+extern bool
+IsInteger(const Value& val);
+
 /*
  * Convert an integer or double (contained in the given value) to a string and
  * append to the given buffer.
diff --git a/js/src/jsprototypes.h b/js/src/jsprototypes.h
index 52d0d8ea4a..9822a47b6c 100644
--- a/js/src/jsprototypes.h
+++ b/js/src/jsprototypes.h
@@ -108,6 +108,17 @@ IF_SAB(real,imaginary)(Atomics, InitAtomicsClass, OCLASP(Atomics)) \
     imaginary(WasmMemory,       dummy,                  dummy) \
     imaginary(WasmTable,        dummy,                  dummy) \
     real(Promise,               InitViaClassSpec,       OCLASP(Promise)) \
+    real(ReadableStream,        InitViaClassSpec,       &js::ReadableStream::class_) \
+    real(ReadableStreamDefaultReader,         InitViaClassSpec, &js::ReadableStreamDefaultReader::class_) \
+    real(ReadableStreamBYOBReader,            InitViaClassSpec, &js::ReadableStreamBYOBReader::class_) \
+    real(ReadableStreamDefaultController,     InitViaClassSpec, &js::ReadableStreamDefaultController::class_) \
+    real(ReadableByteStreamController,        InitViaClassSpec, &js::ReadableByteStreamController::class_) \
+    real(ReadableStreamBYOBRequest,           InitViaClassSpec, &js::ReadableStreamBYOBRequest::class_) \
+    imaginary(WritableStream,   dummy,                  dummy) \
+    imaginary(WritableStreamDefaultWriter,    dummy,    dummy) \
+    imaginary(WritableStreamDefaultController,dummy,    dummy) \
+    real(ByteLengthQueuingStrategy,           InitViaClassSpec, &js::ByteLengthQueuingStrategy::class_) \
+    real(CountQueuingStrategy,  InitViaClassSpec,       &js::CountQueuingStrategy::class_) \
 
 #define JS_FOR_EACH_PROTOTYPE(macro) JS_FOR_PROTOTYPES(macro,macro)
 
diff --git a/js/src/moz.build b/js/src/moz.build
index 5429888a2c..d42cf59730 100644
--- a/js/src/moz.build
+++ b/js/src/moz.build
@@ -92,6 +92,7 @@ EXPORTS.js += [
     '../public/Result.h',
     '../public/RootingAPI.h',
     '../public/SliceBudget.h',
+    '../public/Stream.h',
     '../public/StructuredClone.h',
     '../public/SweepingAPI.h',
     '../public/TraceKind.h',
@@ -384,6 +385,7 @@ main_deunified_sources = [
 SOURCES += [
     'builtin/BigInt.cpp',
     'builtin/RegExp.cpp',
+    'builtin/Stream.cpp',
     'frontend/Parser.cpp',
     'gc/StoreBuffer.cpp',
     'jsarray.cpp',
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index a77e128a52..1f89e38482 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -318,6 +318,7 @@ static bool enableUnboxedArrays = false;
 static bool enableSharedMemory = SHARED_MEMORY_DEFAULT;
 static bool enableWasmAlwaysBaseline = false;
 static bool enableArrayProtoValues = true;
+static bool enableStreams = false;
 static bool printTiming = false;
 static const char* jsCacheDir = nullptr;
 static const char* jsCacheAsmJSPath = nullptr;
@@ -7565,6 +7566,7 @@ SetContextOptions(JSContext* cx, const OptionParser& op)
     enableUnboxedArrays = op.getBoolOption("unboxed-arrays");
     enableWasmAlwaysBaseline = op.getBoolOption("wasm-always-baseline");
     enableArrayProtoValues = !op.getBoolOption("no-array-proto-values");
+    enableStreams = op.getBoolOption("enable-streams");
 
     JS::ContextOptionsRef(cx).setBaseline(enableBaseline)
                              .setIon(enableIon)
@@ -7573,7 +7575,8 @@ SetContextOptions(JSContext* cx, const OptionParser& op)
                              .setWasmAlwaysBaseline(enableWasmAlwaysBaseline)
                              .setNativeRegExp(enableNativeRegExp)
                              .setUnboxedArrays(enableUnboxedArrays)
-                             .setArrayProtoValues(enableArrayProtoValues);
+                             .setArrayProtoValues(enableArrayProtoValues)
+                             .setStreams(enableStreams);
 
     if (op.getBoolOption("wasm-check-bce"))
         jit::JitOptions.wasmAlwaysCheckBounds = true;
@@ -7845,7 +7848,8 @@ SetWorkerContextOptions(JSContext* cx)
                              .setWasmAlwaysBaseline(enableWasmAlwaysBaseline)
                              .setNativeRegExp(enableNativeRegExp)
                              .setUnboxedArrays(enableUnboxedArrays)
-                             .setArrayProtoValues(enableArrayProtoValues);
+                             .setArrayProtoValues(enableArrayProtoValues)
+                             .setStreams(enableStreams);
     cx->setOffthreadIonCompilationEnabled(offthreadCompilation);
     cx->profilingScripts = enableCodeCoverage || enableDisassemblyDumps;
 
@@ -8019,6 +8023,7 @@ main(int argc, char** argv, char** envp)
         || !op.addBoolOption('\0', "wasm-always-baseline", "Enable wasm baseline compiler when possible")
         || !op.addBoolOption('\0', "wasm-check-bce", "Always generate wasm bounds check, even redundant ones.")
         || !op.addBoolOption('\0', "no-array-proto-values", "Remove Array.prototype.values")
+        || !op.addBoolOption('\0', "enable-streams", "Enable WHATWG Streams")
 #ifdef ENABLE_SHARED_ARRAY_BUFFER
         || !op.addStringOption('\0', "shared-memory", "on/off",
                                "SharedArrayBuffer and Atomics "
diff --git a/js/src/vm/ArrayBufferObject.cpp b/js/src/vm/ArrayBufferObject.cpp
index db1d7c798f..6ace034adb 100644
--- a/js/src/vm/ArrayBufferObject.cpp
+++ b/js/src/vm/ArrayBufferObject.cpp
@@ -1255,15 +1255,16 @@ ArrayBufferObject::finalize(FreeOp* fop, JSObject* obj)
 }
 
 /* static */ void
-ArrayBufferObject::copyData(Handle<ArrayBufferObject*> toBuffer,
-                            Handle<ArrayBufferObject*> fromBuffer,
-                            uint32_t fromIndex, uint32_t count)
+ArrayBufferObject::copyData(Handle<ArrayBufferObject*> toBuffer, uint32_t toIndex,
+                            Handle<ArrayBufferObject*> fromBuffer, uint32_t fromIndex,
+                            uint32_t count)
 {
     MOZ_ASSERT(toBuffer->byteLength() >= count);
+    MOZ_ASSERT(toBuffer->byteLength() >= toIndex + count);
     MOZ_ASSERT(fromBuffer->byteLength() >= fromIndex);
     MOZ_ASSERT(fromBuffer->byteLength() >= fromIndex + count);
 
-    memcpy(toBuffer->dataPointer(), fromBuffer->dataPointer() + fromIndex, count);
+    memcpy(toBuffer->dataPointer() + toIndex, fromBuffer->dataPointer() + fromIndex, count);
 }
 
 /* static */ void
@@ -1890,6 +1891,17 @@ JS_GetArrayBufferViewByteLength(JSObject* obj)
            : obj->as<TypedArrayObject>().byteLength();
 }
 
+JS_FRIEND_API(uint32_t)
+JS_GetArrayBufferViewByteOffset(JSObject* obj)
+{
+    obj = CheckedUnwrap(obj);
+    if (!obj)
+        return 0;
+    return obj->is<DataViewObject>()
+           ? obj->as<DataViewObject>().byteOffset()
+           : obj->as<TypedArrayObject>().byteOffset();
+}
+
 JS_FRIEND_API(JSObject*)
 JS_GetObjectAsArrayBufferView(JSObject* obj, uint32_t* length, bool* isSharedMemory, uint8_t** data)
 {
diff --git a/js/src/vm/ArrayBufferObject.h b/js/src/vm/ArrayBufferObject.h
index 4ff7962cfb..f4010c6c77 100644
--- a/js/src/vm/ArrayBufferObject.h
+++ b/js/src/vm/ArrayBufferObject.h
@@ -261,9 +261,9 @@ class ArrayBufferObject : public ArrayBufferObjectMaybeShared
     template<typename T>
     static bool createTypedArrayFromBuffer(JSContext* cx, unsigned argc, Value* vp);
 
-    static void copyData(Handle<ArrayBufferObject*> toBuffer,
-                         Handle<ArrayBufferObject*> fromBuffer,
-                         uint32_t fromIndex, uint32_t count);
+    static void copyData(Handle<ArrayBufferObject*> toBuffer, uint32_t toIndex,
+                         Handle<ArrayBufferObject*> fromBuffer, uint32_t fromIndex,
+                         uint32_t count);
 
     static void trace(JSTracer* trc, JSObject* obj);
     static void objectMoved(JSObject* obj, const JSObject* old);
diff --git a/js/src/vm/CommonPropertyNames.h b/js/src/vm/CommonPropertyNames.h
index f05a4db9be..ebfad89fa0 100644
--- a/js/src/vm/CommonPropertyNames.h
+++ b/js/src/vm/CommonPropertyNames.h
@@ -18,6 +18,8 @@
     macro(apply, apply, "apply") \
     macro(args, args, "args") \
     macro(arguments, arguments, "arguments") \
+    macro(AcquireReadableStreamBYOBReader, AcquireReadableStreamBYOBReader, "AcquireReadableStreamBYOBReader") \
+    macro(AcquireReadableStreamDefaultReader, AcquireReadableStreamDefaultReader, "AcquireReadableStreamDefaultReader") \
     macro(ArrayBufferSpecies, ArrayBufferSpecies, "ArrayBufferSpecies") \
     macro(ArrayIterator, ArrayIterator, "Array Iterator") \
     macro(ArrayIteratorNext, ArrayIteratorNext, "ArrayIteratorNext") \
@@ -35,6 +37,7 @@
     macro(AsyncGeneratorFunction, AsyncGeneratorFunction, "AsyncGeneratorFunction") \
     macro(AsyncWrapped, AsyncWrapped, "AsyncWrapped") \
     macro(async, async, "async") \
+    macro(autoAllocateChunkSize, autoAllocateChunkSize, "autoAllocateChunkSize") \
     macro(await, await, "await") \
     macro(bigint64, bigint64, "bigint64") \
     macro(biguint64, biguint64, "biguint64") \
@@ -48,6 +51,7 @@
     macro(buffer, buffer, "buffer") \
     macro(builder, builder, "builder") \
     macro(by, by, "by") \
+    macro(byob, byob, "byob") \
     macro(byteAlignment, byteAlignment, "byteAlignment") \
     macro(byteLength, byteLength, "byteLength") \
     macro(byteOffset, byteOffset, "byteOffset") \
@@ -59,6 +63,7 @@
     macro(callee, callee, "callee") \
     macro(caller, caller, "caller") \
     macro(callFunction, callFunction, "callFunction") \
+    macro(cancel, cancel, "cancel") \
     macro(case, case_, "case") \
     macro(caseFirst, caseFirst, "caseFirst") \
     macro(catch, catch_, "catch") \
@@ -180,6 +185,7 @@
     macro(has, has, "has") \
     macro(hasOwn, hasOwn, "hasOwn") \
     macro(hasOwnProperty, hasOwnProperty, "hasOwnProperty") \
+    macro(highWaterMark, highWaterMark, "highWaterMark") \
     macro(hour, hour, "hour") \
     macro(hourCycle, hourCycle, "hourCycle") \
     macro(if, if_, "if") \
@@ -250,6 +256,7 @@
     macro(minusSign, minusSign, "minusSign") \
     macro(minute, minute, "minute") \
     macro(missingArguments, missingArguments, "missingArguments") \
+    macro(mode, mode, "mode") \
     macro(module, module, "module") \
     macro(Module, Module, "Module") \
     macro(ModuleInstantiate, ModuleInstantiate, "ModuleInstantiate") \
@@ -312,7 +319,52 @@
     macro(prototype, prototype, "prototype") \
     macro(proxy, proxy, "proxy") \
     macro(public, public_, "public") \
+    macro(pull, pull, "pull") \
     macro(raw, raw, "raw") \
+    macro(ReadableByteStreamControllerGetDesiredSize, \
+          ReadableByteStreamControllerGetDesiredSize, \
+          "ReadableByteStreamControllerGetDesiredSize") \
+    macro(ReadableByteStreamController_close, \
+          ReadableByteStreamController_close, \
+          "ReadableByteStreamController_close") \
+    macro(ReadableByteStreamController_enqueue, \
+          ReadableByteStreamController_enqueue, \
+          "ReadableByteStreamController_enqueue") \
+    macro(ReadableByteStreamController_error, \
+          ReadableByteStreamController_error, \
+          "ReadableByteStreamController_error") \
+    macro(ReadableStreamBYOBReader_cancel, \
+          ReadableStreamBYOBReader_cancel, \
+          "ReadableStreamBYOBReader_cancel") \
+    macro(ReadableStreamBYOBReader_read, \
+          ReadableStreamBYOBReader_read, \
+          "ReadableStreamBYOBReader_read") \
+    macro(ReadableStreamBYOBReader_releaseLock, \
+          ReadableStreamBYOBReader_releaseLock, \
+          "ReadableStreamBYOBReader_releaseLock") \
+    macro(ReadableStream_cancel, ReadableStream_cancel, "ReadableStream_cancel") \
+    macro(ReadableStreamDefaultControllerGetDesiredSize, \
+          ReadableStreamDefaultControllerGetDesiredSize, \
+          "ReadableStreamDefaultControllerGetDesiredSize") \
+    macro(ReadableStreamDefaultController_close, \
+          ReadableStreamDefaultController_close, \
+          "ReadableStreamDefaultController_close") \
+    macro(ReadableStreamDefaultController_enqueue, \
+          ReadableStreamDefaultController_enqueue, \
+          "ReadableStreamDefaultController_enqueue") \
+    macro(ReadableStreamDefaultController_error, \
+          ReadableStreamDefaultController_error, \
+          "ReadableStreamDefaultController_error") \
+    macro(ReadableStreamDefaultReader_cancel, \
+          ReadableStreamDefaultReader_cancel, \
+          "ReadableStreamDefaultReader_cancel") \
+    macro(ReadableStreamDefaultReader_read, \
+          ReadableStreamDefaultReader_read, \
+          "ReadableStreamDefaultReader_read") \
+    macro(ReadableStreamDefaultReader_releaseLock, \
+          ReadableStreamDefaultReader_releaseLock, \
+          "ReadableStreamDefaultReader_releaseLock") \
+    macro(ReadableStreamTee, ReadableStreamTee, "ReadableStreamTee") \
     macro(reason, reason, "reason") \
     macro(RegExpFlagsGetter, RegExpFlagsGetter, "RegExpFlagsGetter") \
     macro(RegExpStringIterator, RegExpStringIterator, "RegExp String Iterator") \
@@ -347,6 +399,7 @@
     macro(StarGeneratorNext, StarGeneratorNext, "StarGeneratorNext") \
     macro(StarGeneratorReturn, StarGeneratorReturn, "StarGeneratorReturn") \
     macro(StarGeneratorThrow, StarGeneratorThrow, "StarGeneratorThrow") \
+    macro(start, start, "start") \
     macro(startTimestamp, startTimestamp, "startTimestamp") \
     macro(state, state, "state") \
     macro(static, static_, "static") \
diff --git a/js/src/vm/GlobalObject.cpp b/js/src/vm/GlobalObject.cpp
index b7d3344b3e..1321be9e9b 100644
--- a/js/src/vm/GlobalObject.cpp
+++ b/js/src/vm/GlobalObject.cpp
@@ -23,6 +23,7 @@
 #include "builtin/Promise.h"
 #include "builtin/RegExp.h"
 #include "builtin/SelfHostingDefines.h"
+#include "builtin/Stream.h"
 #include "builtin/SymbolObject.h"
 #include "builtin/TypedObject.h"
 #include "builtin/WeakMapObject.h"
@@ -98,6 +99,16 @@ GlobalObject::skipDeselectedConstructor(JSContext* cx, JSProtoKey key)
       case JSProto_WebAssembly:
         return !wasm::HasSupport(cx);
 
+      case JSProto_ReadableStream:
+      case JSProto_ReadableStreamDefaultReader:
+      case JSProto_ReadableStreamBYOBReader:
+      case JSProto_ReadableStreamDefaultController:
+      case JSProto_ReadableByteStreamController:
+      case JSProto_ReadableStreamBYOBRequest:
+      case JSProto_ByteLengthQueuingStrategy:
+      case JSProto_CountQueuingStrategy:
+        return !cx->options().streams();
+
 #ifdef ENABLE_SHARED_ARRAY_BUFFER
       case JSProto_Atomics:
       case JSProto_SharedArrayBuffer:
diff --git a/js/src/vm/Runtime.cpp b/js/src/vm/Runtime.cpp
index 053b7c44b0..c5deff940c 100644
--- a/js/src/vm/Runtime.cpp
+++ b/js/src/vm/Runtime.cpp
@@ -156,6 +156,12 @@ JSRuntime::JSRuntime(JSRuntime* parentRuntime)
     startAsyncTaskCallback(nullptr),
     finishAsyncTaskCallback(nullptr),
     promiseTasksToDestroy(mutexid::PromiseTaskPtrVector),
+    readableStreamDataRequestCallback(nullptr),
+    readableStreamWriteIntoReadRequestCallback(nullptr),
+    readableStreamCancelCallback(nullptr),
+    readableStreamClosedCallback(nullptr),
+    readableStreamErroredCallback(nullptr),
+    readableStreamFinalizeCallback(nullptr),
     exclusiveAccessLock(mutexid::RuntimeExclusiveAccess),
 #ifdef DEBUG
     mainThreadHasExclusiveAccess(false),
diff --git a/js/src/vm/Runtime.h b/js/src/vm/Runtime.h
index 962f0e1a1e..13523844c1 100644
--- a/js/src/vm/Runtime.h
+++ b/js/src/vm/Runtime.h
@@ -655,6 +655,13 @@ struct JSRuntime : public JS::shadow::Runtime,
     JS::FinishAsyncTaskCallback finishAsyncTaskCallback;
     js::ExclusiveData<js::PromiseTaskPtrVector> promiseTasksToDestroy;
 
+    JS::RequestReadableStreamDataCallback readableStreamDataRequestCallback;
+    JS::WriteIntoReadRequestBufferCallback readableStreamWriteIntoReadRequestCallback;
+    JS::CancelReadableStreamCallback readableStreamCancelCallback;
+    JS::ReadableStreamClosedCallback readableStreamClosedCallback;
+    JS::ReadableStreamErroredCallback readableStreamErroredCallback;
+    JS::ReadableStreamFinalizeCallback readableStreamFinalizeCallback;
+
   private:
     /*
      * Lock taken when using per-runtime or per-zone data that could otherwise
diff --git a/js/src/vm/SelfHosting.cpp b/js/src/vm/SelfHosting.cpp
index e9c72d1bf1..e006846752 100644
--- a/js/src/vm/SelfHosting.cpp
+++ b/js/src/vm/SelfHosting.cpp
@@ -36,6 +36,7 @@
 #include "builtin/Promise.h"
 #include "builtin/Reflect.h"
 #include "builtin/SelfHostingDefines.h"
+#include "builtin/Stream.h"
 #include "builtin/TypedObject.h"
 #include "builtin/WeakSetObject.h"
 #include "gc/Marking.h"
@@ -71,7 +72,6 @@ using JS::AutoCheckCannotGC;
 using mozilla::IsInRange;
 using mozilla::Maybe;
 using mozilla::PodMove;
-using mozilla::Maybe;
 
 static void
 selfHosting_WarningReporter(JSContext* cx, JSErrorReport* report)
@@ -1098,9 +1098,9 @@ static bool
 intrinsic_ArrayBufferCopyData(JSContext* cx, unsigned argc, Value* vp)
 {
     CallArgs args = CallArgsFromVp(argc, vp);
-    MOZ_ASSERT(args.length() == 5);
+    MOZ_ASSERT(args.length() == 6);
 
-    bool isWrapped = args[4].toBoolean();
+    bool isWrapped = args[5].toBoolean();
     Rooted<T*> toBuffer(cx);
     if (!isWrapped) {
         toBuffer = &args[0].toObject().as<T>();
@@ -1114,11 +1114,12 @@ intrinsic_ArrayBufferCopyData(JSContext* cx, unsigned argc, Value* vp)
         }
         toBuffer = toBufferObj.as<T>();
     }
-    Rooted<T*> fromBuffer(cx, &args[1].toObject().as<T>());
-    uint32_t fromIndex = uint32_t(args[2].toInt32());
-    uint32_t count = uint32_t(args[3].toInt32());
+    uint32_t toIndex = uint32_t(args[1].toInt32());
+    Rooted<T*> fromBuffer(cx, &args[2].toObject().as<T>());
+    uint32_t fromIndex = uint32_t(args[3].toInt32());
+    uint32_t count = uint32_t(args[4].toInt32());
 
-    T::copyData(toBuffer, fromBuffer, fromIndex, count);
+    T::copyData(toBuffer, toIndex, fromBuffer, fromIndex, count);
 
     args.rval().setUndefined();
     return true;
@@ -2426,14 +2427,14 @@ static const JSFunctionSpec intrinsic_functions[] = {
                     intrinsic_PossiblyWrappedArrayBufferByteLength<ArrayBufferObject>, 1,0,
                     IntrinsicPossiblyWrappedArrayBufferByteLength),
     JS_FN("ArrayBufferCopyData",
-          intrinsic_ArrayBufferCopyData<ArrayBufferObject>,             5,0),
+          intrinsic_ArrayBufferCopyData<ArrayBufferObject>,             6,0),
 
     JS_FN("SharedArrayBufferByteLength",
           intrinsic_ArrayBufferByteLength<SharedArrayBufferObject>,     1,0),
     JS_FN("PossiblyWrappedSharedArrayBufferByteLength",
           intrinsic_PossiblyWrappedArrayBufferByteLength<SharedArrayBufferObject>, 1,0),
     JS_FN("SharedArrayBufferCopyData",
-          intrinsic_ArrayBufferCopyData<SharedArrayBufferObject>,       5,0),
+          intrinsic_ArrayBufferCopyData<SharedArrayBufferObject>,       6,0),
 
     JS_FN("IsUint8TypedArray",        intrinsic_IsUint8TypedArray,      1,0),
     JS_FN("IsInt8TypedArray",         intrinsic_IsInt8TypedArray,       1,0),
@@ -2489,6 +2490,9 @@ static const JSFunctionSpec intrinsic_functions[] = {
     JS_FN("CallWeakSetMethodIfWrapped",
           CallNonGenericSelfhostedMethod<Is<WeakSetObject>>, 2, 0),
 
+    JS_FN("IsReadableStreamBYOBRequest",
+          intrinsic_IsInstanceOfBuiltin<ReadableStreamBYOBRequest>, 1, 0),
+
     // See builtin/TypedObject.h for descriptors of the typedobj functions.
     JS_FN("NewOpaqueTypedObject",           js::NewOpaqueTypedObject, 1, 0),
     JS_FN("NewDerivedTypedObject",          js::NewDerivedTypedObject, 3, 0),
diff --git a/js/src/vm/SharedArrayObject.cpp b/js/src/vm/SharedArrayObject.cpp
index 8e55001d13..44fe3b790d 100644
--- a/js/src/vm/SharedArrayObject.cpp
+++ b/js/src/vm/SharedArrayObject.cpp
@@ -331,15 +331,16 @@ SharedArrayBufferObject::addSizeOfExcludingThis(JSObject* obj, mozilla::MallocSi
 }
 
 /* static */ void
-SharedArrayBufferObject::copyData(Handle<SharedArrayBufferObject*> toBuffer,
-                                  Handle<SharedArrayBufferObject*> fromBuffer,
-                                  uint32_t fromIndex, uint32_t count)
+SharedArrayBufferObject::copyData(Handle<SharedArrayBufferObject*> toBuffer, uint32_t toIndex,
+                                  Handle<SharedArrayBufferObject*> fromBuffer, uint32_t fromIndex,
+                                  uint32_t count)
 {
     MOZ_ASSERT(toBuffer->byteLength() >= count);
+    MOZ_ASSERT(toBuffer->byteLength() >= toIndex + count);
     MOZ_ASSERT(fromBuffer->byteLength() >= fromIndex);
     MOZ_ASSERT(fromBuffer->byteLength() >= fromIndex + count);
 
-    jit::AtomicOperations::memcpySafeWhenRacy(toBuffer->dataPointerShared(),
+    jit::AtomicOperations::memcpySafeWhenRacy(toBuffer->dataPointerShared() + toIndex,
                                               fromBuffer->dataPointerShared() + fromIndex,
                                               count);
 }
diff --git a/js/src/vm/SharedArrayObject.h b/js/src/vm/SharedArrayObject.h
index 05db688184..19048336cb 100644
--- a/js/src/vm/SharedArrayObject.h
+++ b/js/src/vm/SharedArrayObject.h
@@ -146,9 +146,9 @@ class SharedArrayBufferObject : public ArrayBufferObjectMaybeShared
     static void addSizeOfExcludingThis(JSObject* obj, mozilla::MallocSizeOf mallocSizeOf,
                                        JS::ClassInfo* info);
 
-    static void copyData(Handle<SharedArrayBufferObject*> toBuffer,
-                         Handle<SharedArrayBufferObject*> fromBuffer,
-                         uint32_t fromIndex, uint32_t count);
+    static void copyData(Handle<SharedArrayBufferObject*> toBuffer, uint32_t toIndex,
+                         Handle<SharedArrayBufferObject*> fromBuffer, uint32_t fromIndex,
+                         uint32_t count);
 
     SharedArrayRawBuffer* rawBufferObject() const;
 
diff --git a/js/xpconnect/src/XPCJSContext.cpp b/js/xpconnect/src/XPCJSContext.cpp
index 8ecc14b650..cf7785e34c 100644
--- a/js/xpconnect/src/XPCJSContext.cpp
+++ b/js/xpconnect/src/XPCJSContext.cpp
@@ -1438,6 +1438,8 @@ ReloadPrefsCallback(const char* pref, void* data)
     bool werror = Preferences::GetBool(JS_OPTIONS_DOT_STR "werror");
 
     bool extraWarnings = Preferences::GetBool(JS_OPTIONS_DOT_STR "strict");
+    
+    bool streams = Preferences::GetBool(JS_OPTIONS_DOT_STR "streams");
 
     bool unboxedObjects = Preferences::GetBool(JS_OPTIONS_DOT_STR "unboxed_objects");
     
@@ -1463,7 +1465,8 @@ ReloadPrefsCallback(const char* pref, void* data)
                              .setDumpStackOnDebuggeeWouldRun(dumpStackOnDebuggeeWouldRun)
                              .setWerror(werror)
                              .setExtraWarnings(extraWarnings)
-                             .setArrayProtoValues(arrayProtoValues);
+                             .setArrayProtoValues(arrayProtoValues)
+                             .setStreams(streams);
 
     JS_SetParallelParsingEnabled(cx, parallelParsing);
     JS_SetOffthreadIonCompilationEnabled(cx, offthreadIonCompilation);