By concurrency I mean expressing in the language that a computation or operation is independent, for instance by putting it in a thread or making it lazy, and all the programming techniques that this enables: active objects, order finding (which plays very well with paralellism btw), etc.

Parallelism I see as a hardware feature that can make programs faster.

I thought about the "callback hell", this is a bit more like coroutines, a quite limited form of concurrency (like non-preemptive threads). In this sense Js is very limited in what programming techniques can be taught.