You have a point, I shouldn't have said "no concurrency".

You are restricted to what ammounts to coroutining, which is just one of the many concurrency techniques out there, none of which you can teach with javascript.

You wouldn't be able to show many of the other possibilities of concurrent programming.

Callback hell? What is this 2010? Callback hell is a non-issue in modern JS code with promises, and async/await.

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.