Promise.withResolvers() polyfill

promise-with-resolvers-polyfill.js

if (typeof Promise.withResolvers === 'undefined') {
  Promise.withResolvers = function () {
    let resolve, reject
    const promise = new Promise((res, rej) => {
      resolve = res
      reject = rej
    })
    return { promise, resolve, reject }
  }
}

// Usage:
// const { promise, resolve, reject } = Promise.withResolvers()
// console.log(promise, resolve, reject) // Promise { <pending> } [Function (anonymous)] [Function (anonymous)]
// ... Do something async and then call resolve or reject!

@lmammino this is not working:

Object.assign(Promise, {
	withResolvers: () => {
		let resolve;
		let reject;

		const promise = new Promise((resolve, reject) => {
			resolve = resolve;
			reject = reject;
		});

		return {
			promise,
			resolve,
			reject
		};
}});

const a = Promise.withResolvers()
a.promise.then(() => console.log('resolved'))
a.resolve();

With this error:

VM717:20 Uncaught TypeError: a.resolve is not a function
    at <anonymous>:20:3

But I see that it is because of the name that I am reassigning the variable inside the function, not the one outside. It's totally my fault. Sorry about that.

A TypeScript version:

if (typeof Promise.withResolvers === "undefined") {
  Promise.withResolvers = <T>() => {
    let resolve: (value: T | PromiseLike<T>) => void;
    let reject: (reason?: unknown) => void;
    const promise = new Promise<T>((res, rej) => {
      resolve = res;
      reject = rej;
    });
    return { promise, resolve: resolve!, reject: reject! };
  };
}

@Stadly you can put the ! on the declaration, rather than on the return object.

if (typeof Promise.withResolvers === "undefined") {
  Promise.withResolvers = <T>() => {
    let resolve!: (value: T | PromiseLike<T>) => void;
    let reject!: (reason?: unknown) => void;
    const promise = new Promise<T>((res, rej) => {
      resolve = res;
      reject = rej;
    });
    return { promise, resolve, reject };
  };
}

Thanks, @WORMSS! I was not aware :)