What is Promise.all? Please implement Promise.all

What is Promise.all()?

Promise.all(iterable) takes an iterable of Promises and returns a new Promise:

• If all Promises fulfill, it resolves with an array of all results in the original order
• If any Promise rejects, it immediately rejects with that error (fail-fast)

Basic Usage

const p1 = Promise.resolve(1);
const p2 = Promise.resolve(2);
const p3 = Promise.resolve(3);

Promise.all([p1, p2, p3]).then(values => {
  console.log(values); // [1, 2, 3]
});

// Send multiple API requests in parallel (much faster than sequential)
async function loadAll() {
  const [users, posts, comments] = await Promise.all([
    fetch('/api/users').then(r => r.json()),
    fetch('/api/posts').then(r => r.json()),
    fetch('/api/comments').then(r => r.json()),
  ]);
  console.log(users, posts, comments);
}

Fail-Fast Example

const p1 = Promise.resolve('success');
const p2 = Promise.reject(new Error('failed'));
const p3 = Promise.resolve('also success');

Promise.all([p1, p2, p3])
  .then(values => console.log(values))        // won't execute
  .catch(err => console.error(err.message));  // 'failed'

Implementing Promise.all()

function myPromiseAll(promises) {
  return new Promise((resolve, reject) => {
    if (!promises.length) return resolve([]);

    const results = [];
    let completed = 0;

    promises.forEach((promise, index) => {
      Promise.resolve(promise)
        .then(value => {
          results[index] = value;  // preserve original order
          completed++;
          if (completed === promises.length) {
            resolve(results);
          }
        })
        .catch(reject);  // reject immediately on any failure
    });
  });
}

Comparison with Related Methods

Method	Behavior
Promise.all()	Resolves when ALL fulfill; rejects immediately if any rejects
Promise.allSettled()	Waits for ALL to settle; returns each outcome
Promise.any()	Resolves when ANY fulfills; rejects only if ALL reject
Promise.race()	Settles with the first to complete (success or failure)