Waits until a specified number of promises is resolved/fulfilled

waitForNFulfilled.js

export async function waitForNFulfilled (n, ...promises) {
  const { length } = promises;

  if (n >= length)
    return Promise.all(promises);

  const leftPromises = [];
  const tallyPromises = [];

  // Promises can resolve on the same tick, so we need to mark the fulfilled ones in parallel
  async function tallySettled (promise) {
    try {
      await promise;
      // Dynamic, so passing the index to the tally function would not work
      tallyPromises[leftPromises.indexOf(promise)] = true;
    } catch {}
  }

  for (let i = 0; i < length; ++i) {
    leftPromises.push(tallySettled(promises[i]));
    tallyPromises.push(false);
  }

  const promisesFulfilled = [];

  for (;;) {
    // eslint-disable-next-line no-await-in-loop
    await Promise.race(leftPromises);

    for (let i = leftPromises.length - 1; i >= 0; --i) {
      if (tallyPromises[i]) {
        const [ fulfilledPromise ] = leftPromises.splice(i, 1);

        tallyPromises.splice(i, 1);

        if ((promisesFulfilled.push(fulfilledPromise)) === n)
          return Promise.all(promisesFulfilled);

        // TODO: throw an aggregate error when the count cannot be achieved anymore
        // number of rejections + n > number of promises
      }
    }
  }
}

waitForNSettled.js

export async function waitForNSettled (n, ...promises) {
  const { length } = promises;

  if (n >= length)
    return Promise.allSettled(promises);

  const leftPromises = [];
  const tallyPromises = [];

  // Promises can resolve on the same tick, so we need to mark the resolved ones in parallel
  async function tallySettled (promise) {
    try {
      await promise;
    } catch {}

    // Dynamic, so passing the index to the tally function would not work
    tallyPromises[leftPromises.indexOf(promise)] = true;
  }

  for (let i = 0; i < length; ++i) {
    leftPromises.push(tallySettled(promises[i]));
    tallyPromises.push(false);
  }

  const promisesSettled = [];

  for (;;) {
    // eslint-disable-next-line no-await-in-loop
    await Promise.race(leftPromises);

    for (let i = leftPromises.length - 1; i >= 0; --i) {
      if (tallyPromises[i]) {
        const [ resolvedPromise ] = leftPromises.splice(i, 1);

        tallyPromises.splice(i, 1);

        if ((promisesSettled.push(resolvedPromise)) === n)
          return Promise.allSettled(promisesSettled);
      }
    }
  }
}