enable callback function

I tried a copy-paste of the example code, and the callback wasn't working. 

The reason was, the `draw` function (besides missing an argument) was using `this.draw.bind(this)`, which is calling itself. The problem is that this is the equivalent of calling `bind()` with *no arguments*.

So no matter what was passed in (`draw(completeFn)`), it would never get called when the counter was 0. It was like there were no arguments at all.