[capableweb]

JavaScript does have concurrency! Try the following:

    Promise.all([new Promise(t => setTimeout(t, 3000)), new Promise(t => setTimeout(t, 3000))]).then(() => console.log('done'))

You'll notice it prints `done` after 3 seconds, not after 6.

It just happened to be executed one by one but the VM handles the switching for us.

What you're talking about is parallelism, which JavaScript indeed does lack and you'd use cluster or workers for that. That's when things happen at the same time, outsourced to different cores. This is what JavaScript cannot do (yet?).

[grupthink]

Is that concurrency? The CPU is not switching between tasks; rather, it's scheduling 2 callbacks to fire after a 3000ms delay.

If you could perform 2 tasks (e.g. console.log(Array(1e8).fill(0).map((a, i) => i)), and have both run to completion without a significant delay between the two tasks, that'd be impressively concurrent.

[vore]

Concurrency is not parallelism:

- Concurrency is making overlapping progress on two or more tasks.

- Parallelism is making simultaneous progress on two or more tasks.

Concurrent tasks can run in parallel, but they can also run not in parallel and still make overlapping progress by either cooperative or preemptive scheduling. You can imagine that in some larger tasks you have these 3 second sleeps, but all tasks are able to make overlapping progress without blocking the other ones -- that's concurrency!

[avinassh]

There is a great talk by Rob Pike: Concurrency is not Parallelism - https://www.youtube.com/watch?v=oV9rvDllKEg&t=23s