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diff --git a/js/public/HelperThreadAPI.h b/js/public/HelperThreadAPI.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/. */
+
+/*
+ * API for supplying an external thread pool to run internal work off the main
+ * thread.
+ */
+
+#ifndef js_HelperThreadAPI_h
+#define js_HelperThreadAPI_h
+
+#include <stddef.h>  // size_t
+
+#include "jstypes.h"  // JS_PUBLIC_API
+
+namespace JS {
+
+// Argument passed to the task callback to indicate whether we're invoking it
+// because a new task was added by the JS engine or because we're on a helper
+// thread that just finished a task and there are other tasks pending.
+enum class DispatchReason { NewTask, FinishedTask };
+
+/**
+ * Set callback to dispatch a tasks to an external thread pool.
+ *
+ * When the task runs it should call JS::RunHelperThreadTask.
+ */
+using HelperThreadTaskCallback = void (*)(DispatchReason reason);
+extern JS_PUBLIC_API void SetHelperThreadTaskCallback(
+    HelperThreadTaskCallback callback, size_t threadCount, size_t stackSize);
+
+// Function to call from external thread pool to run a helper thread task.
+extern JS_PUBLIC_API void RunHelperThreadTask();
+
+}  // namespace JS
+
+#endif  // js_HelperThreadAPI_h