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Promise.all()

The Promise.all() method takes an iterable of promises as an input, and returns a single Promise that resolves to an array of the results of the input promises. This returned promise will fulfill when all of the input's promises have fulfilled, or if the input iterable contains no promises. It rejects immediately upon any of the input promises rejecting or non-promises throwing an error, and will reject with this first rejection message / error.

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Syntax

Promise.all(iterable);

Parameters

iterable

An iterable object such as an Array.

Return value

  • An already fulfilled Promise if the iterable passed is empty.
  • An asynchronously fulfilled Promise if the iterable passed contains no promises. Note, Google Chrome 58 returns an already fulfilled promise in this case.
  • A pending Promise in all other cases. This returned promise is then fulfilled/rejected asynchronously (as soon as the queue is empty) when all the promises in the given iterable have fulfilled, or if any of the promises reject. See the example about "Asynchronicity or synchronicity of Promise.all" below. Returned values will be in order of the Promises passed, regardless of completion order.

Description

This method can be useful for aggregating the results of multiple promises. It is typically used when there are multiple related asynchronous tasks that the overall code relies on to work successfully — all of whom we want to fulfill before the code execution continues.

Promise.all() will reject immediately upon any of the input promises rejecting. In comparison, the promise returned by Promise.allSettled() will wait for all input promises to complete, regardless of whether or not one rejects. Consequently, it will always return the final result of every promise and function from the input iterable.

Note: The order of the promise array is preserved upon completion of this method.

Fulfillment

The returned promise is fulfilled with an array containing all the fulfilled values (including non-promise values) in the iterable passed as the argument.

  • If an empty iterable is passed, then the promise returned by this method is fulfilled synchronously. The fulfilled value is an empty array.
  • If a nonempty iterable is passed, and all of the promises fulfill, or are not promises, then the promise returned by this method is fulfilled asynchronously.

Rejection

If any of the passed-in promises reject, Promise.all asynchronously rejects with the value of the promise that rejected, whether or not the other promises have settled.

Examples

Using Promise.all

Promise.all waits for all fulfillments (or the first rejection).

const p1 = Promise.resolve(3);
const p2 = 1337;
const p3 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("foo");
  }, 100);
});

Promise.all([p1, p2, p3]).then((values) => {
  console.log(values); // [3, 1337, "foo"]
});

If the iterable contains non-promise values, they will be ignored, but still counted in the returned promise array value (if the promise is fulfilled):

// this will be counted as if the iterable passed is empty, so it gets fulfilled
const p = Promise.all([1, 2, 3]);
// this will be counted as if the iterable passed contains only the resolved promise with value "444", so it gets fulfilled
const p2 = Promise.all([1, 2, 3, Promise.resolve(444)]);
// this will be counted as if the iterable passed contains only the rejected promise with value "555", so it gets rejected
const p3 = Promise.all([1, 2, 3, Promise.reject(555)]);

// using setTimeout we can execute code after the queue is empty
setTimeout(() => {
  console.log(p);
  console.log(p2);
  console.log(p3);
});

// logs
// Promise { <state>: "fulfilled", <value>: Array[3] }
// Promise { <state>: "fulfilled", <value>: Array[4] }
// Promise { <state>: "rejected", <reason>: 555 }

Asynchronicity or synchronicity of Promise.all

This following example demonstrates the asynchronicity (or synchronicity, if the iterable passed is empty) of Promise.all:

// we are passing as argument an array of promises that are already resolved,
// to trigger Promise.all as soon as possible
const resolvedPromisesArray = [Promise.resolve(33), Promise.resolve(44)];

const p = Promise.all(resolvedPromisesArray);
// immediately logging the value of p
console.log(p);

// using setTimeout we can execute code after the queue is empty
setTimeout(() => {
  console.log('the queue is now empty');
  console.log(p);
});

// logs, in order:
// Promise { <state>: "pending" }
// the queue is now empty
// Promise { <state>: "fulfilled", <value>: Array[2] }

The same thing happens if Promise.all rejects:

const mixedPromisesArray = [Promise.resolve(33), Promise.reject(44)];
const p = Promise.all(mixedPromisesArray);
console.log(p);
setTimeout(() => {
  console.log('the queue is now empty');
  console.log(p);
});

// logs
// Promise { <state>: "pending" }
// the queue is now empty
// Promise { <state>: "rejected", <reason>: 44 }

But, Promise.all resolves synchronously if and only if the iterable passed is empty:

const p = Promise.all([]); // will be immediately resolved
const p2 = Promise.all([1337, "hi"]); // non-promise values will be ignored, but the evaluation will be done asynchronously
console.log(p);
console.log(p2)
setTimeout(() => {
  console.log('the queue is now empty');
  console.log(p2);
});

// logs
// Promise { <state>: "fulfilled", <value>: Array[0] }
// Promise { <state>: "pending" }
// the queue is now empty
// Promise { <state>: "fulfilled", <value>: Array[2] }

Promise.all fail-fast behavior

Promise.all is rejected if any of the elements are rejected. For example, if you pass in four promises that resolve after a timeout and one promise that rejects immediately, then Promise.all will reject immediately.

const p1 = new Promise((resolve, reject) => {
  setTimeout(() => resolve('one'), 1000);
});
const p2 = new Promise((resolve, reject) => {
  setTimeout(() => resolve('two'), 2000);
});
const p3 = new Promise((resolve, reject) => {
  setTimeout(() => resolve('three'), 3000);
});
const p4 = new Promise((resolve, reject) => {
  setTimeout(() => resolve('four'), 4000);
});
const p5 = new Promise((resolve, reject) => {
  reject(new Error('reject'));
});

// Using .catch:
Promise.all([p1, p2, p3, p4, p5])
  .then((values) => {
    console.log(values);
  })
  .catch((error) => {
    console.error(error.message)
  });

// From console:
// "reject"

It is possible to change this behavior by handling possible rejections:

const p1 = new Promise((resolve, reject) => {
  setTimeout(() => resolve('p1_delayed_resolution'), 1000);
});

const p2 = new Promise((resolve, reject) => {
  reject(new Error('p2_immediate_rejection'));
});

Promise.all([
  p1.catch((error) => error),
  p2.catch((error) => error),
]).then((values) => {
  console.log(values[0]); // "p1_delayed_resolution"
  console.error(values[1]); // "Error: p2_immediate_rejection"
})

Specifications

Browser compatibility

Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet Deno Node.js
all
32
12
29
No
19
8
4.4.3
32
29
19
8
2.0
1.0
0.12.0

See also

© 2005–2022 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all