Implement Promise.all() from scratch

Question

Accepted Answer

**Approach:** Wrap each with `Promise.resolve`, track results array + remaining count. Resolve when count 0. Reject immediately on any rejection.

```javascript
function promiseAll(iterable) {
  const promises = [...iterable];
  if (promises.length === 0) return Promise.resolve([]);
  return new Promise((resolve, reject) => {
    const results = new Array(promises.length);
    let remaining = promises.length;
    promises.forEach((p, i) => {
      Promise.resolve(p).then(
        val => { results[i] = val; remaining--; if (remaining === 0) resolve(results); },
        err => reject(err)
      );
    });
  });
}

// Demo
promiseAll([1, Promise.resolve(2), new Promise(r => setTimeout(()=>r(3), 50))])
  .then(v => console.log(v)); // [1,2,3]
promiseAll([Promise.resolve(1), Promise.reject('oops'), Promise.resolve(3)])
  .catch(e => console.log('rejected', e)); // rejected oops
promiseAll([]).then(v => console.log('empty', v)); // empty []
promiseAll([{ then: cb => cb(99) }]).then(v => cons

Implement Promise.all() from scratch

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KEEP THE
STREAK ALIVE.

Understand the problem

Attempt it yourself

Study the solution

Solution ready — 2 min read

Join the discussion

Discussion (0)

KEEP THESTREAK ALIVE.

KEEP THE
STREAK ALIVE.