diff -r 000000000000 -r 6474c204b198 js/src/jit-test/tests/basic/testNewArrayCount.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jit-test/tests/basic/testNewArrayCount.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,12 @@
+function testNewArrayCount()
+{
+  function count(a) { var n = 0; for (var p in a) n++; return n; }
+  var a = [];
+  for (var i = 0; i < 5; i++)
+    a = [0];
+  assertEq(count(a), 1);
+  for (var i = 0; i < 5; i++)
+    a = [0, , 2];
+  assertEq(count(a), 2);
+}
+testNewArrayCount();