Box2DWeb supports most of the API from the original C++ Box2D except chain shapes. :/
It is the most widely used Javascript Box2D. If you need the API documentation for Box2DWeb, check out Box2DFlash. http://www.box2dflash.org/docs/2.1a/reference/
Box2DWeb is auto generated from Box2DFlash using a compiler. So the API is the same.
I doubt Box2DWeb will get any update in the future anymore as Box2DFlash has shown no activities anymore. You can see the author's rational on why he decided not to write a direct Box2D --> Box2DWeb port.
https://code.google.com/p/box2dweb/wiki/Roadmap