JavaScriptAsyncIntermediate

Promise.all()

Runs promises in parallel and resolves with all results (or rejects fast).

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Promise.all([promise1, promise2, ...])
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What it does

Overview

Runs promises in parallel and resolves with all results (or rejects fast).

Promise.all() accepts an iterable of promises and returns a single Promise that resolves to an array of results. It is the primary tool for aggregating the results of multiple asynchronous operations that can run concurrently. Performance-wise, it reduces total execution time to the duration of the longest promise rather than the sum of all durations. However, it implements a 'fail-fast' mechanism: if any input promise rejects, the returned promise immediately rejects with that reason, discarding results from other pending promises. If an empty iterable is passed, it returns a synchronously resolved promise. Non-promise values in the iterable are automatically wrapped via Promise.resolve() and included in the final array. IE not supported.

Quick reference

Syntax

Promise.all([promise1, promise2, ...])

Inputs

Parameters

iterableIterable<Promise> · Promises (or values) to await in parallel.

See it in practice

Examples

1

Parallel Independent Requests

const p1 = Promise.resolve(3);
const p2 = 42;
const p3 = new Promise((resolve) => setTimeout(resolve, 100, 'foo'));

Promise.all([p1, p2, p3]).then(values => {
  console.log(values);
});
Output:
[3, 42, "foo"]

Aggregates multiple values, including non-promise constants, into a single result array in the order they were provided.

2

Handling Fail-Fast Rejections

const p1 = Promise.resolve('Success');
const p2 = Promise.reject('Error: Failed');
const p3 = new Promise((resolve) => setTimeout(resolve, 500, 'Slow Success'));

Promise.all([p1, p2, p3])
  .then(values => console.log(values))
  .catch(error => console.error(error));
Output:
"Error: Failed"

If any promise in the array rejects, the entire operation fails immediately regardless of other pending promises.

3

Mapping Data to Concurrent Promises

const ids = [1, 2, 3];
const fetchData = (id) => Promise.resolve(`Data for ${id}`);

const requests = ids.map(id => fetchData(id));

Promise.all(requests).then(results => {
  console.log(results);
});
Output:
["Data for 1", "Data for 2", "Data for 3"]

Common pattern for transforming an array of data into an array of concurrent asynchronous operations.

Debug faster

Common Errors

1

LogicError

Cause: Expecting partial results when a rejection occurs.

Fix: Use Promise.allSettled() if you need the results of all promises regardless of whether some fail.

Promise.all([p1, failingP2]).catch(err => 'Still need p1 result');
2

TypeError

Cause: Passing a non-iterable value (like a plain object) as the argument.

Fix: Ensure the input is an Array or another iterable type.

Promise.all({ task1: p1, task2: p2 });

Runtime support

Compatibility

Node.js, BrowserAll modern browsersES6 (2015)

Source: MDN Web Docs

Common questions

Frequently Asked Questions

Runs promises in parallel and resolves with all results (or rejects fast).

iterable: Promises (or values) to await in parallel.

LogicError: Use Promise.allSettled() if you need the results of all promises regardless of whether some fail. TypeError: Ensure the input is an Array or another iterable type.