--- a/js/src/jsobj.cpp
+++ b/js/src/jsobj.cpp
@@ -3171,16 +3171,30 @@ js::ToPrimitiveSlow(JSContext* cx, JSTyp
         if (vp.isObject())
             return ReportCantConvert(cx, JSMSG_TOPRIMITIVE_RETURNED_OBJECT, obj, preferredType);
         return true;
     }
 
     return OrdinaryToPrimitive(cx, obj, preferredType, vp);
 }
 
+/* ES6 draft rev 28 (2014 Oct 14) 7.1.14 */
+bool
+js::ToPropertyKeySlow(JSContext* cx, HandleValue argument, MutableHandleId result)
+{
+    MOZ_ASSERT(argument.isObject());
+
+    // Steps 1-2.
+    RootedValue key(cx, argument);
+    if (!ToPrimitiveSlow(cx, JSTYPE_STRING, &key))
+        return false;
+
+    // Steps 3-4.
+    return ValueToId<CanGC>(cx, key, result);
+}
 
 /* * */
 
 bool
 js::IsDelegate(JSContext* cx, HandleObject obj, const js::Value& v, bool* result)
 {
     if (v.isPrimitive()) {
         *result = false;

--- a/js/src/jsobjinlines.h
+++ b/js/src/jsobjinlines.h
@@ -578,27 +578,27 @@ HasNoToPrimitiveMethodPure(JSObject* obj
     JSObject* pobj;
     PropertyResult prop;
     if (!LookupPropertyPure(cx, obj, id, &pobj, &prop))
         return false;
 
     return !prop;
 }
 
+extern bool
+ToPropertyKeySlow(JSContext* cx, HandleValue argument, MutableHandleId result);
+
 /* ES6 draft rev 28 (2014 Oct 14) 7.1.14 */
-inline bool
+MOZ_ALWAYS_INLINE bool
 ToPropertyKey(JSContext* cx, HandleValue argument, MutableHandleId result)
 {
-    // Steps 1-2.
-    RootedValue key(cx, argument);
-    if (!ToPrimitive(cx, JSTYPE_STRING, &key))
-        return false;
+    if (MOZ_LIKELY(argument.isPrimitive()))
+        return ValueToId<CanGC>(cx, argument, result);
 
-    // Steps 3-4.
-    return ValueToId<CanGC>(cx, key, result);
+    return ToPropertyKeySlow(cx, argument, result);
 }
 
 /*
  * Return true if this is a compiler-created internal function accessed by
  * its own object. Such a function object must not be accessible to script
  * or embedding code.
  */
 inline bool