Promise.any()

The Promise.any() static method takes an iterable of promises as input and returns a single Promise. This returned promise fulfills when any of the input's promises fulfills, with this first fulfillment value. It rejects when all of the input's promises reject (including when an empty iterable is passed), with an AggregateError containing an array of rejection reasons.

Syntax

Promise.any(iterable)

Parameters

iterable
- : An iterable (such as an Array) of promises.

Return value

A Promise that is:

Already rejected, if the iterable passed is empty.
Asynchronously fulfilled, when any of the promises in the given iterable fulfills. The fulfillment value is the fulfillment value of the first promise that was fulfilled.
Asynchronously rejected, when all of the promises in the given iterable reject. The rejection reason is an AggregateError containing an array of rejection reasons in its errors property. The errors are in the order of the promises passed, regardless of completion order. If the iterable passed is non-empty but contains no pending promises, the returned promise is still asynchronously (instead of synchronously) rejected.

Description

The Promise.any() method is one of the promise concurrency methods. This method is useful for returning the first promise that fulfills. It short-circuits after a promise fulfills, so it does not wait for the other promises to complete once it finds one.

Unlike Promise.all, which returns an array of fulfillment values, we only get one fulfillment value (assuming at least one promise fulfills). This can be beneficial if we need only one promise to fulfill but we do not care which one does. Note another difference: this method rejects upon receiving an empty iterable, since, truthfully, the iterable contains no items that fulfill. You may compare Promise.any() and Promise.all() with Array.prototype.some and Array.prototype.every.

Also, unlike Promise.race, which returns the first settled value (either fulfillment or rejection), this method returns the first fulfilled value. This method ignores all rejected promises up until the first promise that fulfills.

Examples

Using Promise.any()

Promise.any() fulfills with the first promise to fulfill, even if a promise rejects first. This is in contrast to Promise.race, which fulfills or rejects with the first promise to settle.

const pErr = new Promise((resolve, reject) => {
  reject("Always fails");
});

const pSlow = new Promise((resolve, reject) => {
  setTimeout(resolve, 500, "Done eventually");
});

const pFast = new Promise((resolve, reject) => {
  setTimeout(resolve, 100, "Done quick");
});

Promise.any([pErr, pSlow, pFast]).then((value) => {
  console.log(value);
  // pFast fulfills first
});
// Logs:
// Done quick

Rejections with AggregateError

Promise.any() rejects with an AggregateError if no promise fulfills.

const failure = new Promise((resolve, reject) => {
  reject("Always fails");
});

Promise.any([failure]).catch((err) => {
  console.log(err);
});
// AggregateError: No Promise in Promise.any was resolved

Displaying the first image loaded

In this example, we have a function that fetches an image and returns a blob. We use Promise.any() to fetch a couple of images and display the first one available (i.e. whose promise has resolved).

async function fetchAndDecode(url, description) {
  const res = await fetch(url);
  if (!res.ok) {
    throw new Error(`HTTP error! status: ${res.status}`);
  }
  const data = await res.blob();
  return [data, description];
}

const coffee = fetchAndDecode("coffee.jpg", "Coffee");
const tea = fetchAndDecode("tea.jpg", "Tea");

Promise.any([coffee, tea])
  .then(([blob, description]) => {
    const objectURL = URL.createObjectURL(blob);
    const image = document.createElement("img");
    image.src = objectURL;
    image.alt = description;
    document.body.appendChild(image);
  })
  .catch((e) => {
    console.error(e);
  });