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diff --git a/js/public/ForOfIterator.h b/js/public/ForOfIterator.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/. */
+
+/*
+ * A convenience class that makes it easy to perform the operations of a for-of
+ * loop.
+ */
+
+#ifndef js_ForOfIterator_h
+#define js_ForOfIterator_h
+
+#include "mozilla/Attributes.h"  // MOZ_MUST_USE, MOZ_STACK_CLASS
+
+#include <stdint.h>  // UINT32_MAX, uint32_t
+
+#include "jstypes.h"  // JS_PUBLIC_API
+
+#include "js/RootingAPI.h"  // JS::{Handle,Rooted}
+#include "js/Value.h"       // JS::Value, JS::{,Mutable}Handle<JS::Value>
+
+struct JS_PUBLIC_API JSContext;
+class JS_PUBLIC_API JSObject;
+
+namespace JS {
+
+/**
+ * A convenience class for imitating a JS for-of loop. Typical usage:
+ *
+ *     JS::ForOfIterator it(cx);
+ *     if (!it.init(iterable)) {
+ *       return false;
+ *     }
+ *     JS::Rooted<JS::Value> val(cx);
+ *     while (true) {
+ *       bool done;
+ *       if (!it.next(&val, &done)) {
+ *         return false;
+ *       }
+ *       if (done) {
+ *         break;
+ *       }
+ *       if (!DoStuff(cx, val)) {
+ *         return false;
+ *       }
+ *     }
+ */
+class MOZ_STACK_CLASS JS_PUBLIC_API ForOfIterator {
+ protected:
+  JSContext* cx_;
+
+  // Use the ForOfPIC on the global object (see vm/GlobalObject.h) to try to
+  // optimize iteration across arrays.
+  //
+  //  Case 1: Regular Iteration
+  //      iterator - pointer to the iterator object.
+  //      nextMethod - value of |iterator|.next.
+  //      index - fixed to NOT_ARRAY (== UINT32_MAX)
+  //
+  //  Case 2: Optimized Array Iteration
+  //      iterator - pointer to the array object.
+  //      nextMethod - the undefined value.
+  //      index - current position in array.
+  //
+  // The cases are distinguished by whether |index == NOT_ARRAY|.
+  Rooted<JSObject*> iterator;
+  Rooted<Value> nextMethod;
+
+  static constexpr uint32_t NOT_ARRAY = UINT32_MAX;
+
+  uint32_t index = NOT_ARRAY;
+
+  ForOfIterator(const ForOfIterator&) = delete;
+  ForOfIterator& operator=(const ForOfIterator&) = delete;
+
+ public:
+  explicit ForOfIterator(JSContext* cx)
+      : cx_(cx), iterator(cx), nextMethod(cx) {}
+
+  enum NonIterableBehavior { ThrowOnNonIterable, AllowNonIterable };
+
+  /**
+   * Initialize the iterator.  If AllowNonIterable is passed then if getting
+   * the @@iterator property from iterable returns undefined init() will just
+   * return true instead of throwing.  Callers must then check
+   * valueIsIterable() before continuing with the iteration.
+   */
+  MOZ_MUST_USE bool init(
+      Handle<Value> iterable,
+      NonIterableBehavior nonIterableBehavior = ThrowOnNonIterable);
+
+  /**
+   * Get the next value from the iterator.  If false *done is true
+   * after this call, do not examine val.
+   */
+  MOZ_MUST_USE bool next(MutableHandle<Value> val, bool* done);
+
+  /**
+   * Close the iterator.
+   * For the case that completion type is throw.
+   */
+  void closeThrow();
+
+  /**
+   * If initialized with throwOnNonCallable = false, check whether
+   * the value is iterable.
+   */
+  bool valueIsIterable() const { return iterator; }
+
+ private:
+  inline bool nextFromOptimizedArray(MutableHandle<Value> val, bool* done);
+};
+
+}  // namespace JS
+
+#endif  // js_ForOfIterator_h