http://fightcodegame.com/ is one I played a while ago.

Edit: I've got a bot ranked #237 still, a year on from its creation.

Thanks for the question. The comments below have a couple similar games, but we built JavaScript Battle because we felt it was unique. Let us know what you think!

Vindinium, gameplay-wise, is definitely the most similar. (vindinium.org)

Code & Conquer looks like vaporware at the moment, sadly :(