diff -r 000000000000 -r 6474c204b198 js/src/tests/ecma_3/Function/regress-313570.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/tests/ecma_3/Function/regress-313570.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,30 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+//-----------------------------------------------------------------------------
+var BUGNUMBER = 313570;
+var summary = 'length of objects whose prototype chain includes a function';
+var actual = '';
+var expect = '';
+
+printBugNumber(BUGNUMBER);
+printStatus (summary);
+
+function tmp() {}
+tmp.prototype = function(a, b, c) {};
+var obj = new tmp();
+
+// arity
+expect = 3;
+actual = obj.length;
+reportCompare(expect, actual, summary + ': arity');
+
+// immutable
+obj.length = 10;
+
+expect = 3;
+actual = obj.length;
+reportCompare(expect, actual, summary + ': immutable');
+