Promise.prototype.flatten function

• Spec in progress

Champions

• Work in progress

Status

This proposal is stage 0 in the TC39 Process.

Inspired By

https://twitter.com/DavidWells/status/1119729914876284928

Examples

While try -> catch works well, it introduces a lot of noise into software that can be avoided.

Consider:

async function test(promise1, promise2, promise3) {
  let val1, val2, val3;

  try {
    val1 = await promise1;
  } catch (ex) {
    // ignore exceptions
  }

  try {
    [val2, val3] = await Promise.all([promise2, promise3]);
  } catch (ex) {
    throw ex; // throw to caller
  }

  return val1 + val2 + val3;
}

Above contains a fair amount of boilerplate for a relatively simple logic flow. Now let's look at an alternative using flatten:

async function test(promise1, promise2, promise3) {
  const [, val1] = await promise1.flatten(); // ignore exceptions
  const [err, [val2, val3] = []] = await Promise.all([promise2, promise3]).flatten();

  if (err) throw err; // throw to caller

  return val1 + val2 + val3;
}

Under the covers both examples accomplish the same, but flatten can reduce code footprint and increase readability.

Polyfill

function flatten() {
  return this.then(ret => [undefined, ret]).catch(err => [err, undefined]);
}

Doesn't get much simpler than this. Tests included.

Discussion

https://twitter.com/Aaron_Silvas/status/1120721934730137601