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diff --git a/js/src/builtin/streams/MiscellaneousOperations.h b/js/src/builtin/streams/MiscellaneousOperations.h
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+++ b/js/src/builtin/streams/MiscellaneousOperations.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Miscellaneous operations. */
+
+#ifndef builtin_streams_MiscellaneousOperations_h
+#define builtin_streams_MiscellaneousOperations_h
+
+#include "mozilla/Attributes.h"  // MOZ_MUST_USE
+
+#include "jstypes.h"           // JS_PUBLIC_API
+#include "js/CallArgs.h"       // JS::CallArgs
+#include "js/RootingAPI.h"     // JS::{,Mutable}Handle
+#include "js/Value.h"          // JS::Value
+#include "vm/JSObject.h"       // JSObject
+#include "vm/PromiseObject.h"  // js::PromiseObject
+
+struct JS_PUBLIC_API JSContext;
+
+namespace js {
+
+class PropertyName;
+
+extern MOZ_MUST_USE PromiseObject* PromiseRejectedWithPendingError(
+    JSContext* cx);
+
+inline MOZ_MUST_USE bool ReturnPromiseRejectedWithPendingError(
+    JSContext* cx, const JS::CallArgs& args) {
+  PromiseObject* promise = PromiseRejectedWithPendingError(cx);
+  if (!promise) {
+    return false;
+  }
+
+  args.rval().setObject(*promise);
+  return true;
+}
+
+/**
+ * Streams spec, 6.3.1.
+ *      CreateAlgorithmFromUnderlyingMethod ( underlyingObject, methodName,
+ *                                            algoArgCount, extraArgs )
+ *
+ * This function only partly implements the standard algorithm. We do not
+ * actually create a new JSFunction completely encapsulating the new algorithm.
+ * Instead, this just gets the specified method and checks for errors. It's the
+ * caller's responsibility to make sure that later, when the algorithm is
+ * "performed", the appropriate steps are carried out.
+ */
+extern MOZ_MUST_USE bool CreateAlgorithmFromUnderlyingMethod(
+    JSContext* cx, JS::Handle<JS::Value> underlyingObject,
+    const char* methodNameForErrorMessage, JS::Handle<PropertyName*> methodName,
+    JS::MutableHandle<JS::Value> method);
+
+/**
+ * Streams spec, 6.3.2. InvokeOrNoop ( O, P, args )
+ * As it happens, all callers pass exactly one argument.
+ */
+extern MOZ_MUST_USE bool InvokeOrNoop(JSContext* cx, JS::Handle<JS::Value> O,
+                                      JS::Handle<PropertyName*> P,
+                                      JS::Handle<JS::Value> arg,
+                                      JS::MutableHandle<JS::Value> rval);
+
+/**
+ * Streams spec, 6.3.7. ValidateAndNormalizeHighWaterMark ( highWaterMark )
+ */
+extern MOZ_MUST_USE bool ValidateAndNormalizeHighWaterMark(
+    JSContext* cx, JS::Handle<JS::Value> highWaterMarkVal,
+    double* highWaterMark);
+
+/**
+ * Streams spec, 6.3.8. MakeSizeAlgorithmFromSizeFunction ( size )
+ */
+extern MOZ_MUST_USE bool MakeSizeAlgorithmFromSizeFunction(
+    JSContext* cx, JS::Handle<JS::Value> size);
+
+template <class T>
+inline bool IsMaybeWrapped(const JS::Handle<JS::Value> v) {
+  return v.isObject() && v.toObject().canUnwrapAs<T>();
+}
+
+}  // namespace js
+
+#endif  // builtin_streams_MiscellaneousOperations_h