diff -r 000000000000 -r 6474c204b198 js/public/PropertyKey.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/public/PropertyKey.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,95 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * 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/. */
+
+/* JS::PropertyKey implementation. */
+
+#ifndef js_PropertyKey_h
+#define js_PropertyKey_h
+
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+
+namespace JS {
+
+class PropertyKey;
+
+namespace detail {
+
+extern JS_PUBLIC_API(bool)
+ToPropertyKeySlow(JSContext *cx, HandleValue v, PropertyKey *key);
+
+} // namespace detail
+
+/*
+ * A PropertyKey is a key used to access some property on an object.  It is a
+ * natural way to represent a property accessed using a JavaScript value.
+ *
+ * PropertyKey can represent indexes, named properties, and ES6 symbols.  The
+ * latter aren't implemented in SpiderMonkey yet, but PropertyKey carves out
+ * space for them.
+ */
+class PropertyKey
+{
+    Value v;
+    friend JS_PUBLIC_API(bool) detail::ToPropertyKeySlow(JSContext *cx, HandleValue v, PropertyKey *key);
+
+  public:
+    explicit PropertyKey(uint32_t index) : v(PrivateUint32Value(index)) {}
+
+    /*
+     * An index is a string property name whose characters exactly spell out an
+     * unsigned 32-bit integer in decimal: "0", "1", "2", ...., "4294967294",
+     * "4294967295".
+     */
+    bool isIndex(uint32_t *index) {
+        // The implementation here assumes that private uint32_t are stored
+        // using the int32_t representation.  This is purely an implementation
+        // detail: embedders must not rely upon this!
+        if (!v.isInt32())
+            return false;
+        *index = v.toPrivateUint32();
+        return true;
+    }
+
+    /*
+     * A name is a string property name which is *not* an index.  Note that by
+     * the ECMAScript language grammar, any dotted property access |obj.prop|
+     * will access a named property.
+     */
+    bool isName(JSString **str) {
+        uint32_t dummy;
+        if (isIndex(&dummy))
+            return false;
+        *str = v.toString();
+        return true;
+    }
+
+    /*
+     * A symbol is a property name that's a Symbol, a particular kind of object
+     * in ES6.  It is the only kind of property name that's not a string.
+     *
+     * SpiderMonkey doesn't yet implement symbols, but we're carving out API
+     * space for them in advance.
+     */
+    bool isSymbol() {
+        return false;
+    }
+};
+
+inline bool
+ToPropertyKey(JSContext *cx, HandleValue v, PropertyKey *key)
+{
+    if (v.isInt32() && v.toInt32() >= 0) {
+        *key = PropertyKey(uint32_t(v.toInt32()));
+        return true;
+    }
+
+    return detail::ToPropertyKeySlow(cx, v, key);
+}
+
+} // namespace JS
+
+#endif /* js_PropertyKey_h */