How do I return the accumulated results of multiple (parallel) asynchronous function calls in a loop?

Question

I have a function foo which makes multiple (parallel) asynchronous calls in a loop. I need to somehow wait until the results of all of the calls are available.

Answer 1

Don't use Promise.all! That fails the entire operation if any one of your promises fails!

Unless you're okay with that prospect, you'd be way better off doing something like this:

function sleep(ms) {
  return new Promise((resolve, reject) => {
    console.log(`starting ${ms}`);
    setTimeout(() => {
      if (ms > 1000) {
        console.log(`Threw out ${ms} because it took too long!`);
        reject(ms);
      } else {
        console.log(`done ${ms}`);
        resolve(ms);
      }
    }, ms);
  });
}

(async () => {
  console.log('aPromise, bPromise, cPromise executed concurrently as promises are in an array');
  const start = new Date();
  const aPromise = sleep(2000);
  const bPromise = sleep(500);
  const cPromise = sleep(5);
  
  try {
    const [a, b, c] = [await aPromise, await bPromise, await cPromise];
    // The code below this line will only run when all 3 promises are fulfilled:
    console.log(`slept well - got ${a} ${b} ${c} in ${new Date()-start}ms`);
  } catch (err) {
    console.log(`slept rough in ${err}ms`);
  }
})();