// Created on savesnippets.com · https://savesnippets.com/X3fuafvhoukW9d
type Deferred<T> = {
  promise: Promise<T>;
  resolve: (value: T | PromiseLike<T>) => void;
  reject:  (reason?: unknown) => void;
};

export function deferred<T>(): Deferred<T> {
  let resolve!: Deferred<T>['resolve'];
  let reject!:  Deferred<T>['reject'];
  const promise = new Promise<T>((res, rej) => { resolve = res; reject = rej; });
  return { promise, resolve, reject };
}

// Wait for a one-shot event
const d = deferred<MessageEvent>();
worker.addEventListener('message', d.resolve, { once: true });
const msg = await d.promise;