diff -r 000000000000 -r 6474c204b198 js/src/jit-test/tests/debug/Frame-onStep-iterators.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jit-test/tests/debug/Frame-onStep-iterators.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,20 @@
+var g = newGlobal();
+var dbg = new Debugger;
+var gw = dbg.addDebuggee(g);
+var log;
+var a = [];
+
+dbg.onDebuggerStatement = function (frame) {
+  log += 'd';
+  frame.onStep = function () {
+    // This handler must not wipe out the debuggee's value in JSContext::iterValue.
+    log += 's';
+    // This will use JSContext::iterValue in the debugger.
+    for (let i of a)
+      log += 'i';
+  };
+};
+
+log = '';
+g.eval("debugger; for (let i of [1,2,3]) print(i);");
+assertEq(!!log.match(/^ds*$/), true);