diff -r 000000000000 -r 6474c204b198 js/src/jit-test/tests/debug/Frame-onStep-02.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jit-test/tests/debug/Frame-onStep-02.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,27 @@
+// Setting frame.onStep to undefined turns off single-stepping.
+
+var g = newGlobal();
+g.a = 0;
+g.eval("function f() {\n" +
+       "    a++;\n" +
+       "    a++;\n" +
+       "    a++;\n" +
+       "    a++;\n" +
+       "    return a;\n" +
+       "}\n");
+
+var dbg = Debugger(g);
+var seen = [0, 0, 0, 0, 0];
+dbg.onEnterFrame = function (frame) {
+    seen[g.a] = 1;
+    frame.onStep = function () {
+        seen[g.a] = 1;
+        if (g.a === 2) {
+            frame.onStep = undefined;
+            assertEq(frame.onStep, undefined);
+        }
+    };
+}
+
+g.f();
+assertEq(seen.join(","), "1,1,1,0,0");