Game programming with Javascript?

I like to start off teaching Javascript, because everyone with a computer has all the tools you need: a browser and a text editor. Its pretty easy to have kids change the screen color, make pop-ups appear, take user input and munge it around. But kids want to program games, so I'm considering Flash. In the meantime, here is a full-fledged Javascript game we may try to edit.

This is a blog post rather than a lesson plan, as we haven't tried it out yet!