> My problem: I do not understand his code.

but the fact is if your workmate is writing unreadable and unmaintainable code, hes the problem, not you.

That's a stretch, especially since OP also says:

Or anyone elses.

Idiomatic new js/typescript is a completely different beast from old javascript. For all intents and purposes, it's a different language entirely. No one would be expected to pick it up in a minute. Sounds to me like this guy has decades old knowledge and doesn't know where to learn the new stuff, since the target is moving from one year to the next.

You yourself are suggesting today, that he learns libraries that are no longer necessary, like lodash.