so it looks to me that `fillSquare` is defined here but ultimately ends up as a method on `ctx`. The use of `this` here leaves me a bit confused... who will `this` be?

after taking a cursory look at how method definition works in javascript, it looks as though there are a few appealing alternatives to defining this "method" at the top level, then attaching it to the object afterwards. I found [this article](http://www.phpied.com/3-ways-to-define-a-javascript-class/) pretty informative.