Callback a list of promises in the order in which they complete.

deal-with-promises-as-they-resolve.js

/**
 * Used like:
 * dealWithPromisesAsTheyResolve([
 *   new Promise((res, rej) => setTimeout(res, 2000, 2000)),
 *   new Promise((res, rej) => setTimeout(res, 1000, 1000)),
 *   new Promise((res, rej) => setTimeout(res, 4000, 4000)),
 *   new Promise((res, rej) => setTimeout(res,    0,    0)),
 *   new Promise((res, rej) => setTimeout(rej, 3000, 3000)),
 * ], num => console.log(num), err => console.log(`error: ${err}`));
 *
 * Will output:
 *   0
 *   1000
 *   2000
 *   error: 3000
 *   4000
 */

async function dealWithPromisesAsTheyResolve(promises, resolveCallback, rejectCallback) {
  var _promises = new Map();
  promises.forEach(promise => _promises.set(
    promise,
    promise
      .then(value => [null, value, promise])
      .catch(error => [error, null, promise])
  ));

  while (_promises.size > 0) {
    let [error, value, promise] = await Promise.race(_promises.values());
    _promises.delete(promise);
    if (error) {
      rejectCallback(error);
    } else {
      resolveCallback(value);
    }
  }
}