This is very impressive! The most advanced JS 2d rigid body physics engine to date! You can do a lot of optimization though, probably double the speed. Tip: V8 loves objects and hates parsing strings.

Your functional spaghetti style is a bit weird though. I hate using the "find in file" function in my editor. I wanted to "rip" some of the functionality, but it's so entangled I would probably need all your other functions too to make it work. But maybe that's intended :P

Many people, me included, would be willing to pay for an engine like this. I tried your "Game physics for beginners" but I'm too stupid to understand all the articles from math professors. I could probably learn, but I rather pay someone who understands.

But for me to use your engine, the API needs to be much easier! Something like this:

  var box1 = new Body(size, position);
  var world = new World({gravity: 9.8});
  world.add(box1);
  world.render();

  console.log('New position: ' + box1.position);