观察者模式

class Subject {
  constructor() {
    this.items = [];
  }
  registerObserver(observer) {
    this.items.push(observer);
  }
  notifyObservers() {
    for (let i = 0; i < this.items.length; i++) {
      this.items[i].update(i);
    }
  }
}

class Observer {
  constructor(subject) {
    subject.registerObserver(this);
  }
  update(index) {
    console.log(`receive${index}`);
  }
}

const s = new Subject();
new Observer(s);
new Observer(s);

s.notifyObservers();