DesignPatters: Observer (javascript)

observer.js

function User() {
  this.subscribers = []
}

User.prototype.subscribe = function(subscriber) {
  this.subscribers.push(subscriber)
}

User.prototype.unsubscribe = function(subscriber) {
  this.subscribers = this.subscribers.filter((item) => {
    return item !== subscriber
  })
}

User.prototype.walk = function(meters) {
  this.subscribers.forEach((subscriber) => {
    subscriber.call(this, meters)
  })
}

function run() {
  let user = new User()
  let subscriber1 = function(meters) {
    console.log('subscriber1', 'user walk', meters, 'meters')
  }
  user.subscribe(subscriber1)
  
  let subscriber2 = function(meters) {
    console.log('subscriber2', 'user walk', meters, 'meters')
  }
  user.subscribe(subscriber2)
  
  user.walk(5)
  
  user.unsubscribe(subscriber1)
  
  user.walk(10)
}