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!

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 :)