JavaScript Promise.allSettled()

Summary: in this tutorial, you’ll learn about the Promise.allSettled() method to compose promises.

Introduction to the Promise.allSettled() method

ES2020 introduced the Promise.allSettled() method that accepts a list of Promises and returns a new promise that resolves after all the input promises have settled, either resolved or rejected.

The following shows the syntax of the Promise.allSettled() method:

Promise.allSettled(iterable);Code language: JavaScript (javascript)

The iterable contains the promises. The Promise.allSettled() returns a pending promise that will be asynchronously fulfilled once every input promise has settled.

The Promise.allSettled() method returns a promise that resolves to an array of objects that each describes the result of the input promise.

Each object has two properties: status and value (or reason).

• The status can be either fulfilled or rejected.
• The value if case the promise is fulfilled or reason) if the promise is rejected.

The following diagram illustrates how the Promise.allSettled() method works:

In this diagram:

• The promise1 rejects to the error at t1.
• The promise2 resolves to a value at t2.
• The Promise.allSettled() method resolves to an array containing objects that describe the statuses and outcomes of the promise1 and promise2.

JavaScript Promise.allSettled() example

The following example uses the Promise.allSettled() to wait for all the input Promises to settle:

const p1 = new Promise((resolve, reject) => {
    setTimeout(() => {
        console.log('The first promise has resolved');
        resolve(10);
    }, 1 * 1000);

});

const p2 = new Promise((resolve, reject) => {
    setTimeout(() => {
        console.log('The second promise has rejected');
        reject(20);
    }, 2 * 1000);
});

Promise.allSettled([p1, p2])
    .then((result) => {
        console.log(result);
    });
Code language: JavaScript (javascript)

Output:

How it works:

• The first promise p1 resolves to the value 10 after one second
• The second promise p2 rejects for a reason with a value 20 after two seconds.
• The Promise.allSettled() returns a promise that resolves to the result array that has two elements. The first element is an object resolved by the p1 promise and the second one is another object which is rejected by the p2 promise.

Summary

• The Promise.allSettled() method accepts an iterable of promises and returns a new promise that resolves when every input promise has settled with an array of objects that describes the result of each promise in the iterable object.