❗️Что будет на выходе:

class SimpleObservable {
  constructor(subscribeFn) {
    this.subscribeFn = subscribeFn;
  }
  
  subscribe(observer) {
    return this.subscribeFn(observer);
  }
}

const obs = new SimpleObservable(observer => {
  observer.next('first');
  observer.next('second');
  observer.complete();
});

const results = [];
obs.subscribe({
  next: val => results.push(val),
  complete: () => results.push('done')
});

console.log(results.join('-'));

Ответ: first-second-done

JavaScript test | #JavaScript