for await of VS Promise.all

Question

Is there any difference between this:

const promises = await Promise.all(items.map(e => somethingAsync(e)));
for (c         


        

Answer 1

Actually, using the for await syntax does fire the promises all at once.

The small piece of code proves it:

const sleep = s => {
  return new Promise(resolve => {
    setTimeout(resolve, s * 1000);
  });
}

const somethingAsync = async t => {
  await sleep(t);
  return t;
}

(async () => {
  const items = [1, 2, 3, 4];
  const now = Date.now();
  for await (const res of items.map(e => somethingAsync(e))) {
    console.log(res);
  }
  console.log("time: ", (Date.now() - now) / 1000);
})();

stdout: time: 4.001

But the inside of the loop doesn't act "as a callback". If I reverse the array, all the logs appear at once. I suppose that the promises are fired at once and the runtime just waits for the first one to resolve to go to the next iteration.

EDIT: Actually, using for await is bad practice when we use it with something other than an asynchronous iterator, best is to use Promise.all, according to @Bergi in his answer.