Overview

Promise.all takes an iterable of promises and returns a single promise that:

• Resolves with an array of all fulfilled values (in input order) once every promise settles successfully.
• Rejects immediately with the reason of the first promise that rejects.

Implement promiseAll(promises) that behaves identically to the native Promise.all, including handling non-promise values in the input array.

Constraints

• Input is always an array (may contain promises, thenables, or plain values).
• Resolved values must preserve the original index order, not settlement order.
• Must reject with the first rejection reason — ignore later rejections.
• An empty array input should resolve immediately with [].
• Do not use Promise.all, Promise.allSettled, or Promise.race internally.

Examples

// Resolved example.
const p0 = Promise.resolve(3);
const p1 = 42;
const p2 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('foo');
  }, 100);
});

await promiseAll([p0, p1, p2]); // [3, 42, 'foo']

// Rejection example.
const p0 = Promise.resolve(30);
const p1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    reject('An error occurred!');
  }, 100);
});

try {
  await promiseAll([p0, p1]);
} catch (err) {
  console.log(err); // 'An error occurred!'
}

Notes

• Non-promise values should be treated as Promise.resolve(value).
• The order of resolution does not matter — only the final array order must match input order.
• Think carefully about the counter: when do you know all promises are done?

Solution

function promiseAll(promises) {
  return new Promise((resolve, reject) => {
    if (promises.length === 0) {
      resolve([]);
      return;
    }

    const results = new Array(promises.length);
    let settled = 0;

    promises.forEach((item, index) => {
      Promise.resolve(item).then(
        (value) => {
          results[index] = value;
          settled += 1;
          if (settled === promises.length) {
            resolve(results);
          }
        },
        (reason) => {
          reject(reason);
        },
      );
    });
  });
}

Resources

• Promise.all — MDN
• Promises/A+ Spec
• JavaScript Promise Combinators