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Promise.all

Promise.all

Introduction

Promise.all waits for all promises to fulfill. If any promise rejects, the entire Promise.all rejects immediately.

Basic Usage

const promises = [
  fetch('/api/users').then(r => r.json()),
  fetch('/api/posts').then(r => r.json()),
  fetch('/api/comments').then(r => r.json())
];

try {
  const [users, posts, comments] = await Promise.all(promises);
  console.log('All data loaded:', { users, posts, comments });
} catch (error) {
  console.error('One request failed:', error);
}

Fail-Fast Behavior

const p1 = Promise.resolve('OK');
const p2 = Promise.reject(new Error('Failed!'));
const p3 = Promise.resolve('Also OK'); // Never reached

Promise.all([p1, p2, p3])
  .then(results => console.log(results))
  .catch(err => console.log(err.message)); // 'Failed!'
// Promise.all rejects IMMEDIATELY when p2 rejects
// p3 is still pending — its result is lost

When to Use

Loading multiple independent data sets in parallel
Performing parallel operations where all must succeed
Waiting for several async setup steps before proceeding

Summary

Resolves with array of results when all promises fulfill
Rejects immediately if any promise rejects
Results maintain the order of the input array
Fail-fast behavior: one failure cancels the whole operation

Further Reading

MDN: Promise.all