It shows how Observable in RxJS works.

observable.js

let o = new Observable(observer => {
  const token = doAsyncThing((err, value) => {
    if (err) {
      observer.error(err);
    } else {
      observer.next(value);
      observer.complete();
    }
  });
  return () => {
    cancelAsyncThing(token);
  };
});

Reference:https://www.youtube.com/watch?v=KOOT7BArVHQ