code: function () {
      var elapsedTime = (this.nextFrame().timestamp - this.currentFrame().timestamp) / 1000;
      if (elapsedTime < 0) {
        elapsedTime = this.timeBetweenLoops; //arbitrary pause at slightly less than 30 fps.
      }
      console.assert(!isNaN(elapsedTime));
      return elapsedTime;
    }
docs: returns ms