If you are running on chrome, you can simply use the bind function,
this.tickIntervalId = setInterval(this.Tick.bind(this), 1000/60); //Update at 60 fps
or if you are using the popular underscore utility
this.tickIntervalId = setInterval(_.bind(this.Tick,this), 1000/60); //Update at 60 fps
The problem is that your Tick function gets called in a global context. Which means that, the 'this' pointer is the global 'this' which is not what you need. Hence you need to bind the 'this' context so that when the Tick function gets called from a global context, the original 'this' pointer is still saved.