See my "minimal" C++ translation in my other post here. There's not much to add. For JavaScript start with their code, but add the allocation, just replace allocations with var a = new Uint32Array(height * width); and b the same. Add the timing (1), put in HTML and you're done. It's easy, just a few minutes for anybody who works with that (and this site should be filled with the competent developers AFAIK).

1) https://developer.mozilla.org/en-US/docs/Web/API/Performance