The Tor Browser / file revision

 // When a recursive function has many frames on the stack, onStep may be set or

 // not independently on each frame.

 var g = newGlobal();

 g.eval("function f(x) {\n" +

        "    if (x > 0)\n" +

        "        f(x - 1);\n" +

        "    else\n" +

        "        debugger;\n" +

        "    return x;\n" +

        "}");

 var dbg = Debugger(g);

 var seen = [0, 0, 0, 0, 0, 0, 0, 0];

 function step() {

     seen[this.arguments[0]] = 1;

 }

 dbg.onEnterFrame = function (frame) {

     // Turn on stepping for even-numbered frames.

     var x = frame.arguments[0];

     if (x % 2 === 0)

         frame.onStep = step;

 };

 dbg.onDebuggerStatement = function (frame) {

     // This is called with 8 call frames on the stack, 7 down to 0.

     // At this point we should have seen all the even-numbered frames.

     assertEq(seen.join(""), "10101010");

     // Now reset seen to see which frames fire onStep on the way out.

     seen = [0, 0, 0, 0, 0, 0, 0, 0];

 };

 g.f(7);

 assertEq(seen.join(""), "10101010");