I initially tried just that. Implementing the canvas drawing api turned out to be too slow with the Spidermonkey js engine, which is what I had to use. If I could use a jit version of Spidermonkey or iOS's native Javascript engine it likely would have been fast enough. Apple doesn't let you run jit compilers though, or link to their Javascript engine.

After finding that out I moved to implementing a higher level api. It's similar to the cocos2d-iphone(http://www.cocos2d-iphone.org) framework and, indeed, is a layer on top of cocos2d.