const Observable = function(producer) {

  this.subscribe = producer

}

​

const helloWorldProducer = function(next, err=()=>{}, complete=()=>{}) {

  const observer = typeof next !== 'function' ?

    next : 

    { next, err, complete }

​

  observer.next('Hello')

  observer.next('World')

  observer.complete()

}

​

const observableHelloWorld = new Observable(helloWorldProducer)

​

​

observableHelloWorld.subscribe(

  (value) => console.log(value)

)