Because the entire point is to learn and not actually to ship a product, I wrote my own. The biggest libraries I use that I didn't write are HowlerJS for audio, PixiJS for graphics, and React for UI. The actual "game engine" is a very primitive one I wrote myself. You can see it here: https://github.com/ablakey/kestrel/blob/master/src/Engine.ts

One of the best things about NIH-ing my own engine is that I don't need to make it generic, meaning one less layer of abstraction to complicate implementation and debugging.