Canvas Bouncing Balls

Just for try some canvas animation, I've created a Pelota (ball for non-spanish speakers) object that creates a ball on a canvas and makes it bounce around it.

For solving when do I have to clear the canvas, I've given an id attribute to each instance of Pelota class. So each time init() method is executed, it checks which instance is running, and if it's id is 1 (the first object) clears the canvas.