Great work! This is really impressive stuff, and it's applaudable that you saw this project through to the very end!

I'm actually seeing some hitching in the car demo on my reasonably fast dev machine, particularly when I'm interacting with the bridge (computing contact forces of a bunch of bodies connected with joints is a lot of work): http://piqnt.com/planck.js/Car

Have you looked at perf yet? How does the performance of your rewrite planck.js compare to a compiled emscripten version?

Would you ever consider using something like ASM.js or WebAssembly? Physics libraries are definitely one of those performance-critical applications where this kind of stuff actually matters a whole lot.