Subject.js

class Subject {
  constructor(state) {
    this.state = state;
    this.observers = [];
  }

  getState() {
    return this.state;
  }

  setState(state) {
    this.state = state;

    this.notifyObservers(this.state);
  }

  registerObserver(observer) {
    this.observers.push(observer);
  }

  unregisterObserver(observer) {
    this.observers = this.observers.filter(obs => obs !== observer);
  }

  notifyObservers(data) {
    this.observers.forEach(observer => observer.notify(data));
  }
}