std/async/deferred.ts

// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
// This module is browser compatible.

// 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.

/** @deprecated (will be removed in 0.209.0) Use {@linkcode Promise.withResolvers} instead. */
export interface Deferred<T> extends Promise<T> {
  readonly state: "pending" | "fulfilled" | "rejected";
  resolve(value?: T | PromiseLike<T>): void;
  // deno-lint-ignore no-explicit-any
  reject(reason?: any): void;
}

/**
 * @deprecated (will be removed in 0.209.0) Use {@linkcode Promise.withResolvers} instead.
 *
 * Creates a Promise with the `reject` and `resolve` functions placed as methods
 * on the promise object itself.
 *
 * @example
 * ```typescript
 * import { deferred } from "https://deno.land/std@$STD_VERSION/async/deferred.ts";
 *
 * const p = deferred<number>();
 * // ...
 * p.resolve(42);
 * ```
 */
export function deferred<T>(): Deferred<T> {
  let methods;
  let state = "pending";
  const promise = new Promise<T>((resolve, reject) => {
    methods = {
      async resolve(value: T | PromiseLike<T>) {
        await value;
        state = "fulfilled";
        resolve(value);
      },
      // deno-lint-ignore no-explicit-any
      reject(reason?: any) {
        state = "rejected";
        reject(reason);
      },
    };
  });
  Object.defineProperty(promise, "state", { get: () => state });
  return Object.assign(promise, methods) as Deferred<T>;
}