Promise.all return value

Question

I'm trying to build an array of objects where some of the fields need remote data. It all works with awaits inside the loop but I'm interested in optimising this. I wrote the following proof-of-concept:

async function p1(x) { // the first remote data
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(`p1-${x}`);
    }, 250);
  });
}

async function p2(x) { // the second remote data
  return new Promise((resolve) => {
    setTimeout( () => {
      resolve(`p2-${x}`);
    }, 250);
  });
}

async function t(n) {
  return { a: await p1(n), b: await p2(n), c: n };
}

async function r() {
  const results = new Map();
  for (let n = 0; n < 2; n++) {
    results.set(n, t(n));
  }
  await Promise.all(results.values());
  console.log('in function', results);
  return results;
}

(async () => {
  const answers = await r();
  console.log('answers', JSON.stringify(answers));
})().catch((e) => {
  console.log(e);
});

This is the output from running node (13.x):

in function Map(2) {
  0 => Promise { { a: 'p1-0', b: 'p2-0', c: 0 } },
  1 => Promise { { a: 'p1-1', b: 'p2-1', c: 1 } }
}
answers {}

So my function r() seems to be doing the right thing but the value returned is empty. What am I not understanding?

If this isn't the best way to do what I'm trying to do, then I'm open to suggestions! — Julian, Feb 26 '20 at 08:56
Only problem is that you can't meaningfully stringify a Map like that — CertainPerformance, Feb 26 '20 at 08:56
You're returning the Map instead of the result of `await Promise.all` — Khaled Osman, Feb 26 '20 at 08:56
Because your attempt to stringify the Map is the only problem with the code — CertainPerformance, Feb 26 '20 at 09:03
OK, now I know I am getting the map back OK, but the map contains resolved promises. How do I get at the values without another await in a loop? — Julian, Feb 26 '20 at 09:09

Promise.all return value

0 Answers0