Javascript has concurrency (which async/await and promises are).
It doesn’t have parallelism at the language level (but see, e.g., the Web Workers API).
Yes and no.
At the “ECMAScript”-level, JavaScript has no built-in concurrency, but every serious JavaScript implementation enables proper concurrency via the async primitives and the internal scheduling.
Put simply, if you know how to write async code properly, your JavaScript code can achieve high concurrency!
Highly concurrent code need not execute in parallel. Concurrency enables parallelism; concurrency does NOT mandate parallelism.
On top of that, it actually is possible in JavaScript environments like Node.js to write code that can, in fact, run in parallel.
Promise.all([new Promise(t => setTimeout(t, 3000)), new Promise(t => setTimeout(t, 3000))]).then(() => console.log('done'))
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?).
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.
- 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!