A JavaScript with TypeScript JSDoc solution for a deferred promise that can be externally resolved or rejected.

Deferred.mjs

// @ts-check

/**
 * A deferred promise that can be externally resolved or rejected.
 * @template [Resolves=void] What the promise resolves.
 */
export default class Deferred {
  constructor() {
    /** The promise. */
    this.promise = /** @type {Promise<Resolves>} */ (
      new Promise((resolve, reject) => {
        /** Resolves the promise. */
        this.resolve = resolve;

        /** Rejects the promise. */
        this.reject = reject;
      })
    );
  }
}

example-1.mjs

// @ts-check

import Deferred from "./Deferred.mjs";

const deferred = new Deferred();

setTimeout(() => {
  deferred.resolve();
}, 1000);

// Has a void type.
const foo = await deferred.promise;

example-2.mjs

// @ts-check

import Deferred from "./Deferred.mjs";

const deferred = /** @type {Deferred<boolean>} */ (new Deferred());

setTimeout(() => {
  deferred.resolve(
    // TS only allows a boolean here.
    true
  );
}, 1000);

// Has a boolean type.
const foo = await deferred.promise;

An alternative approach (where the Deferred class extends Promise for slightly better ergonomics at the cost of more complexity) can be seen at https://gist.github.com/jaydenseric/2e9db5ab90bd92a3cf0a48f3ce87effa .