Javascript observing

Index.js

const observer = new Observer()

let u1 = new User({name: 'jan'}, observer)
let u2 = new User({name: 'piet'}, observer)
let u3 = new User({name: 'klaas'}, observer)
let u4 = new User({name: 'kees'}, observer)

u2.sendMessage('Hi everybody!')

// output:
// "jan got a message from piet: Hi everybody!"
// "piet sent a message: Hi everybody!"
// "klaas got a message from piet: Hi everybody!"
// "kees got a message from piet: Hi everybody!"

// https://codepen.io/kobuxz/pen/VrJaJj

Observer.js

/**
 * All observables get collected here.
 * notify executes the function in the type
 * variable for all attached objects.
 */
class Observer {
  constructor () {
    this.observables = []
  }
  
  attach (obj) {
    this.observables.push(obj)
  }
  
  notify (type, obj, user) {
    this.observables.forEach((val, key, arr) => {
      val[type](obj, user)
    })
  }
}

User.js

/**
 * In the constructor the class gets attached 
 * to the observer class. 
 * When sendMessage gets executed 
 * all classes attached to the same 
 * observer gets notified.
 */
class User {
  
  constructor (obj, observer) {
    this.data = obj
    this.observer = observer
    this.observer.attach(this)
  }
  
  notify () {
    this.observer.notify(this)
  }
  
  getMessage (msg, user) {
    if (this.data.name !== user.data.name) {
      console.log(`${this.data.name} got a message from ${user.data.name}: ${msg}`)  
    } else {
      console.log(`${this.data.name} sent a message: ${msg}`)
    }
    
  }
  
  sendMessage (msg) {
    this.observer.notify('getMessage', msg, this)
  }
}