The Tor Browser / file revision

 // Setting onStep does not affect later calls to the same function.

 // (onStep is per-frame, not per-function.)

 var g = newGlobal();

 g.a = 1;

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

        "    var x = 2 * a;\n" +

        "    return x * x;\n" +

        "}\n");

 var dbg = Debugger(g);

 var log = '';

 dbg.onEnterFrame = function (frame) {

     log += '+';

     frame.onStep = function () {

         if (log.charAt(log.length - 1) != 's')

             log += 's';

     };

 };

 g.f(1);

 log += '|';

 g.f(2);

 log += '|';

 dbg.onEnterFrame = undefined;

 g.f(3);

 assertEq(log, '+s|+s|');