Creates a promise that can be resolved or reject from outside of its scope

getControlledPromise.ts

/* eslint-disable @typescript-eslint/no-explicit-any */
type Reject = (reason?: any) => void
type Resolve<T> = (val: T | PromiseLike<T>) => void

export interface ControlledPromise<T> extends Promise<T> {
  reject: Reject
  resolve: Resolve<T>
}

function noop() {}

/**
 * Returns a controlled promise, a promise with two additional properties:
 * 'resolve' and 'reject'
 */
export function getControlledPromise<T = unknown>() {
  let resolve: Resolve<T> = noop
  let reject: Reject = noop

  const bag = new Promise((res, rej) => {
    resolve = res
    reject = rej
  }) as ControlledPromise<T>

  bag.resolve = resolve
  bag.reject = reject

  return bag as ControlledPromise<T>
}

// ...

async function main() {
  const controlledPromise = getControlledPromise()

  setTimeout(() => controlledPromise.resolve('potato'), 3000)

  const result = await controlledPromise

  console.log(result) // prints "potato" after three seconds
}

main()