Alternative for Promise.allSettled

Question

I am currently using Promise.allSettled to wait for all my promises to finish(irrespective of whether they resolve or get rejected).

Since my project is compliant to Node v12.3.1 I am unable to use this? What other simple alternatives can I use.

Sample code:

async function runner() {
    let promises = [];
    for(let i=0; i<10; i++) {
        promises.push(fetcher())
    }
    await Promise.allSettled(promises).then(([results]) => console.log(results.length));
    console.log('continue work...');
}

Note: Promise.allSettled is available from Node version >12.9.
Adding shims is also not an option.

Answer 1

There's a small polyfill trick that you can do manually to simulate the effects of Promise.allSettled.

Here's the snippet.

if (!Promise.allSettled) {
  Promise.allSettled = promises =>
    Promise.all(
      promises.map((promise, i) =>
        promise
          .then(value => ({
            status: "fulfilled",
            value,
          }))
          .catch(reason => ({
            status: "rejected",
            reason,
          }))
      )
    );
}

Promise.allSettled(promises).then(console.log);

That means map all of the promises, then return the results, either successful or rejected.

An alternative, if you do not want the object-like nature of Promise.all, the following snippet may help. This is very simple, you just need to add .catch method here.

const promises = [
  fetch('/something'),
  fetch('/something'),
  fetch('/something'),
].map(p => p.catch(e => e)); // this will prevent the promise from breaking out, but there will be no 'result-object', unlike the first solution.

await Promise.all(promises);