Create a thenable Deffered object in TypeScript

createDeferred.ts

// A simple take at implementing Deferred as "thenable" object in TypeScript 
// https://dev.to/noseratio/we-can-make-any-javascript-object-await-able-with-then-method-1apl
// https://gist.github.com/noseratio/da756d5bd1360cafa6eabc4d475a9ab7

type Resolve<T> = (v: T) => void;
type Reject = (e: Error) => void;

interface IDeferred<T> {
  resolve: Resolve<T>;
  reject: Reject;
  then(onResolved: Resolve<T>, onRejected: Reject): Promise<unknown>;
}

function createDeferred<T = void>(): IDeferred<T> {
  let resolve: Resolve<T> | undefined, reject: Reject | undefined;

  const promise = new Promise<T>((...args) => 
    [resolve, reject] = args);

  return Object.freeze(<IDeferred<T>>{
    resolve: resolve!,
    reject: reject!,
    then: (...args) => promise.then(...args)
  });
}

async function main() {
  const d = createDeferred();
  setTimeout(() => d.resolve(), 1000);
  await d;
  alert("done");
}

main().catch(e => alert(e.message));