const Observable = function(producer) {

  this.subscribe = producer

}

​

const helloWorldProducer = function(observer) {

  observer.next('Hello')

  observer.next('World')

  observer.complete()

}

​

const observableHelloWorld = new Observable(helloWorldProducer)

​

const observer = {

  next(value) { console.log(value) },

  error(err) { console.log('Error: ', err) },

  complete() { console.log('Done') }

}

​

observableHelloWorld.subscribe(observer)