ObserverDesignPattern.js

const subject = {
  observers: [],
  subscribe(fn) {
    this.observers.push(fn)
  },
  notify(arg) {
    this.observers.forEach(fn => fn(arg));
  }
}

const joe = {
  update(message) {
    console.log(`Joe receive ${message}`)
  },
  hiking() {
    console.log('Joe go hike')
  }
}

const jay = {
  update(message) {
    console.log(`Jay read ${message}`)
  }
}
// --------------

subject.subscribe(joe.update);
subject.subscribe(joe.hiking);
subject.subscribe(jay.update);
subject.notify('news paper');
// => Joe receive news paper
// => Joe go hike
// => Jay read news paper