Implement Promise.race()

Question

Accepted Answer

**Approach:** Iterate and attach `then(resolve, reject)` to outer promise; first settlement wins.

```javascript
function promiseRace(iterable) {
  const promises = [...iterable];
  return new Promise((resolve, reject) => {
    for (const p of promises) Promise.resolve(p).then(resolve, reject);
  });
}

// Demo
promiseRace([
  new Promise(r => setTimeout(()=>r('slow'), 100)),
  new Promise(r => setTimeout(()=>r('fast'), 10))
]).then(v => console.log(v)); // fast
promiseRace([Promise.reject('oops'), Promise.resolve(1)])
  .catch(e => console.log('race rejected', e)); // oops
console.log(promiseRace([1, Promise.resolve(2)])); // Promise that resolves to 1
function withTimeout(promise, ms) {
  return promiseRace([promise, new Promise((_, rej) => setTimeout(()=>rej(new Error('timeout')), ms))]);
}
withTimeout(new Promise(r=>setTimeout(()=>r('ok'), 50)), 100).then(v=>console.log('ok', v));
```
Empty iterable returns forever-pending promise per spec.

Implement Promise.race()

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Solution ready — 2 min read

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Discussion (0)

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.