JavaScript Promise.all()

Summary: in this tutorial, you will learn how to use the Promise.all() static method to aggregate results from multiple asynchronous operations.

Introduction to the JavaScript Promise.all() method

The Promise.all() static method takes an iterable of promises:

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

The Promise.all() method returns a single promise that resolves when all the input promises have been resolved. The returned promise resolves to an array of the results of the input promises:

In this diagram, the promise1 resolves to a value v1 at t1 and the promise2 resolves to a value v2 at t2. Hence, the Promise.all(promise1, promise2) returns a promise that resolves to an array containing the results of the promise1 and promise2 [v1, v2] at t2.

In other words, the Promise.all() waits for all the input promises to resolve and returns a new promise that resolves to an array containing the results of the input promises.

If one of the input promises is rejected, the Promise.all() method immediately returns a promise that is rejected with an error of the first rejected promise:

In this diagram, the promise2 rejects at t1 with an error. Therefore, the Promise.all() returns a new promise that is immediately rejected with the same error. Also, the Promise.all() doesn’t care about other input promises, whether they will be resolved or rejected.

In practice, the Promise.all() is useful to aggregate the results from multiple asynchronous operations.

JavaScript Promise.all() method examples

Let’s take some examples to understand how the Promise.all() method works.

1) Resolved promises example

The following promises resolve to 10, 20, and 30 after 1, 2, and 3 seconds. We use the setTimeout() to simulate the asynchronous operations:

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 resolved');
    resolve(20);
  }, 2 * 1000);
});
const p3 = new Promise((resolve, reject) => {
  setTimeout(() => {
    console.log('The third promise has resolved');
    resolve(30);
  }, 3 * 1000);
});

Promise.all([p1, p2, p3]).then((results) => {
  const total = results.reduce((p, c) => p + c);

  console.log(`Results: ${results}`);
  console.log(`Total: ${total}`);
});
Code language: JavaScript (javascript)

Output

The first promise has resolved
The second promise has resolved
The third promise has resolved
Results: 10,20,30
Total: 60

When all promises have been resolved, the values from these promises are passed into the callback of the then() method as an array.

Inside the callback, we use the Array’s reduce() method to calculate the total value and use the console.log to display the array of values as well as the total.

2) Rejected promises example

The Promise.all() returns a Promise that is rejected if any of the input promises are rejected.

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('Failed');
    }, 2 * 1000);
});
const p3 = new Promise((resolve, reject) => {
    setTimeout(() => {
        console.log('The third promise has resolved');
        resolve(30);
    }, 3 * 1000);
});


Promise.all([p1, p2, p3])
    .then(console.log) // never execute
    .catch(console.log);
Code language: JavaScript (javascript)

Output:

The first promise has resolved
The second promise has rejected
Failed
The third promise has resolved

In this example, we have three promises: the first one is resolved after 1 second, the second is rejected after 2 seconds, and the third one is resolved after 3 seconds.

As a result, the returned promise is rejected because the second promise is rejected. The catch() method is executed to display the reason for the rejected promise.

Summary

• The Promise.all() method accepts a list of promises and returns a new promise that resolves to an array of results of the input promises if all the input promises are resolved, or rejected with an error of the first rejected promise.
• Use the Promise.all() method to aggregate results from multiple asynchronous operations.

Quiz