Step One: Watch Mary Rose Cook Live Codes Space Invaders from Front-Trends. (The second worst conference name ever?)
Step Two: Fork this gist.
Step Three: Respond to this question in your fork: What is one approach you can take from this Mary's code and implement in your project?
I like how she used window.onload. That's cool. Her collision detection will also come in handy for our game, as tomatoes need to know when they splat against our face. Her tick function also seems like somthing we can use. A center property combined with a size property will probably help us to track where our face is, rather than tracking an x and y. Keyboarder is pretty sweet also. We would like sound with our game also, so her sound functions look appealing. Basically, almost everything she uses seems like it could apply to our game, sans the patrol and shooting.
Step Four: Totally Optional: take a look at some of the other forks and comment if the spirit moves you.