New Promise Methods: allSettled & any

Introduction

We covered basic and advanced promises in the last two blog posts. There are two reasonably new operators/methods to promises that can make life easier. Let's go over them.

AllSettled

ES2020 or ES11 introduced promise.allSettled so it is fairly new and should be used with caution. Check the browsers you are planning to support.

allSettled returns a promise when all the promises provided to it have either resolved or rejects. The return is an array of objects where each object describes the outcome of input promises.

allSettled and promise.all have a minor difference.

promise.all rejects with the first rejection of any of the promises given as input. So if we provide five promises to promise.all and two of them fail, promise.all will reject with the result of the very first failure.

promise.allSettled on the other hand will wait for all promises to finish and provide the result of each promise provided as input (be it resolved or rejected). Use promise.allSettled when the async promises are not dependent on each other, and you can retry only the ones that failed. If your course of action depends on all async tasks completing successfully before moving on, use promise.all.

const promise1 = Promise.resolve("Parwinder");
const promise2 = new Promise((resolve) => {
    setTimeout(() => {
        resolve("Lauren");
    }, 2000);
});
const promise3 = Promise.reject("Robert");
const promise4 = Promise.resolve("Eliu");

Promise.allSettled([promise1, promise2, promise3, promise4]).then((data) => {
    console.log(data);
});

Once all four promises above resolve/reject, allSettled will pass the result to the callback in then. The log will output:

[{
  status: "fulfilled",
  value: "Parwinder"
}, {
  status: "fulfilled",
  value: "Lauren"
}, {
  reason: "Robert",
  status: "rejected"
}, {
  status: "fulfilled",
  value: "Eliu"
}]

Any

🚨 Promise.any is currently in stage 3 of the TC39 proposal (candidate stage). While it will most likely make it to the next release of ECMAScript, there are no guarantees. Please use it with extra caution.

Promise.any works with an iterable of promises. It returns a single promise that resolves with the value from the first successful promise in the iterable. If none of the promises in the iterable succeeds, it returns an AggregateError (a new subclass of Error). AggregateError is used to group individual errors from all promises input.

Promise.any is the exact opposite of Promise.all

const promise1 = Promise.resolve("Parwinder");
const promise2 = new Promise((resolve) => {
    setTimeout(() => {
        resolve("Lauren");
    }, 2000);
});
const promise3 = Promise.reject("Robert");
const promise4 = Promise.resolve("Eliu");

Promise.any([promise1, promise2, promise3, promise4]).then((data) => {
    console.log(data); // Parwinder (first successful promise)
});

In case of rejection from all promises

const promise1 = Promise.reject("Parwinder");
const promise2 = new Promise((resolve) => {
    setTimeout(() => {
        reject("Lauren");
    }, 2000);
});
const promise3 = Promise.reject("Robert");
const promise4 = Promise.reject("Eliu");

Promise.any([promise1, promise2, promise3, promise4]).then((data) => {
    console.log(data); // "AggregateError: No Promise in Promise.any was resolved"
});