It's a simulation of an X server done with JavaScript. The README.md explains it in a bit more detail: https://github.com/magcius/xplain#demo-framework--xserverjs

Basically, I emulate what an X server would do, semantically, without implementing any of the actual wire protocol. The codebase is, I hope, pretty well commented and designed as something to learn from.

https://github.com/magcius/xplain/blob/gh-pages/src/server/s...