denoland-deno/std/async/deferred.ts

// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
// TODO(ry) It'd be better to make Deferred a class that inherits from
// Promise, rather than an interface. This is possible in ES2016, however
// typescript produces broken code when targeting ES5 code.
// See https://github.com/Microsoft/TypeScript/issues/15202
// At the time of writing, the github issue is closed but the problem remains.
export interface Deferred<T> extends Promise<T> {
  resolve: (value?: T | PromiseLike<T>) => void;
  // deno-lint-ignore no-explicit-any
  reject: (reason?: any) => void;
}

/** Creates a Promise with the `reject` and `resolve` functions
 * placed as methods on the promise object itself. It allows you to do:
 *
 *     const p = deferred<number>();
 *     // ...
 *     p.resolve(42);
 */
export function deferred<T>(): Deferred<T> {
  let methods;
  const promise = new Promise<T>((resolve, reject): void => {
    methods = { resolve, reject };
  });
  return Object.assign(promise, methods) as Deferred<T>;
}