Promise.any
Learn to capture the first fulfilled promise with Promise.any.
Promise.any
Promise.any returns the first promise that fulfils. It ignores rejections and waits until one succeeds. This makes it ideal for picking the fastest answer among several redundant endpoints.
const winner = await Promise.any([
fetch('/primary'),
fetch('/backup')
]);If every promise rejects, it throws an AggregateError; its .errors array holds each rejection reason.
AggregateError
When every promise rejects, Promise.any throws an AggregateError. This special error collects each rejection reason in its .errors array.
try {
await Promise.any([p1, p2]);
} catch (e) {
console.log(e.errors);
}So you can see why each endpoint failed in one place, which makes debugging easier.
Exercise Pick the first success from three endpoints
Define three promises: primary, backup1 and backup2. primary resolves with 'primary' after 100ms (use setTimeout). backup1 resolves with 'backup1' after 50ms. backup2 rejects immediately via Promise.reject. Use Promise.any to capture the first success into winner and log it with console.log.
Show example solution
const primary = new Promise(r => setTimeout(r, 100, 'primary'));
const backup1 = new Promise(r => setTimeout(r, 50, 'backup1'));
const backup2 = Promise.reject('unavailable');
const winner = await Promise.any([primary, backup1, backup2]);
console.log(winner);Test yourself
-
Catch the first success
const fastest = await Promise.____([fetchA(), fetchB(), fetchC()]);Show answer
const fastest = await Promise.any([fetchA(), fetchB(), fetchC()]); -
Three promises are passed to Promise.any and all three reject. What happens?
- An AggregateError is thrown
- undefined is returned
- The first rejection reason is returned
- An empty array is returned
Show answer
An AggregateError is thrown — Correct! When all promises reject, Promise.any throws an AggregateError.
-
Match Promise.any behaviours
- first fulfil
- returns value
- all reject
- AggregateError
- rejections
- ignored
- backup endpoints
- first wins