fork-join.mjs

import * as assert from 'assert/strict';

async function forkJoin(objOrArrOfPromises) {
  let promises;
  if (Array.isArray(objOrArrOfPromises)) {
    promises = objOrArrOfPromises;
  } else {
    promises = Object.values(objOrArrOfPromises);
  }
  const settled = await Promise.allSettled(promises);
  if (settled.some((p) => p.status === 'rejected')) {
    const reasons = settled.flatMap(
      (p) => p.status === 'rejected' ? [p.reason] : []);
    throw new AggregateError(reasons, 'One or more Promises were rejected');
  }
  
  // All Promises were fulfilled
  const values = settled.flatMap(
    (p) => p.status === 'fulfilled' ? [p.value] : []);

  if (Array.isArray(objOrArrOfPromises)) {
    return values;
  } else {
    const keys = Object.keys(objOrArrOfPromises);
    return Object.fromEntries(keys.map((key, i) => [key, values[i]]));
  }
}

assert.deepEqual(
  await forkJoin([Promise.resolve('a'), Promise.resolve('b'), Promise.resolve('c')]),
  ['a', 'b', 'c']
);

assert.deepEqual(
  await forkJoin({a: Promise.resolve(1), b: Promise.resolve(2), c: Promise.resolve(3)}),
  {a: 1, b: 2, c: 3}
);