This is a port of the excellent box2d-lite library by Erin Catto in JavaScript. box2d-js is a complete re-write of box2d-lite in JavaScript and is not cross-compiled from its original C++ sources.
The library aims to be a source for understanding physics in games and might as well be used for small browser-based games. Therefore it's kept as simple as possible and doesn't include optimizations that would make the code hard to understand.
Some modifications have been made to the original implementation for simplicity reasons.
The test bed (index.html) is available at GitHub Pages.