Promise.race() returns a promise that settles as soon as any of the input promises settles—whether it resolves or rejects. The first promise to complete "wins the race" and determines the outcome.
🧠 Understanding Promise.race
Atom 1: Purpose
Promise.race returns a promise that settles as soon as any input promise settles — resolved or rejected.
First one wins — race ends immediately.
Atom 2: Inputs
- Takes an iterable (usually an array) of promises or values
- Wrap each with
Promise.resolve()to normalize non-promises
Atom 3: Core Logic
- Attach
.then(resolve)and.catch(reject)to each input - As soon as any one settles , resolve/reject outer promise
- Ignore later results
Atom 4: Edge Case
- If input is empty , the returned promise never settles
✅ Implementation: promiseRace
function promiseRace(iterable) { return new Promise((resolve, reject) => { for (const item of iterable) { Promise.resolve(item) .then(resolve) .catch(reject); } }); }
🧪 Example:
const p1 = new Promise((res) => setTimeout(() => res("one"), 500)); const p2 = new Promise((res) => setTimeout(() => res("two"), 100)); promiseRace([p1, p2]).then(console.log); // → "two"
⚠️ Example with Rejection:
const p1 = new Promise((_, rej) => setTimeout(() => rej("fail"), 50)); const p2 = new Promise((res) => setTimeout(() => res("win"), 100)); promiseRace([p1, p2]) .then(console.log) .catch(console.error); // → "fail"
Quick Quiz
Test your understanding with 3 quick questions
Q1What determines whether Promise.race resolves or rejects?
Q2What happens when Promise.race is passed an empty array?
Q3Which is a common use case for Promise.race?