// http://nick.balestra.ch/2016/Understanding-the-observable-type/

const Observable = function(producer) {

  this.subscribe = producer

}

​

// http://nick.balestra.ch/2016/creating-observables/

Observable.fromEvent = function(element, event){

  const producer = observer => {

​

    const eventHandler = e => {

      observer.next(e)

    }

​

    try {

      element.addEventListener(event, eventHandler)

    } catch(err) {

      observer.error(err)

    }

​

    return {

      unsubscribe() {

        element.removeEventListener(event, eventHandler)

      }

    }

  }

​

  return new Observable(producer)

}

​

​

Observable.prototype.map = function(transformation){

  const originalProducer = this.subscribe;

​

  const newProducer = function(observer){

    originalProducer({

      next (value) {observer.next(transformation(value))},

      error (err) {observer.error(err)},

      complete () {observer.complete()}

    })

  }

​

  return new Observable(newProducer)

}

​

Observable.prototype.mapTo = function(value){

  return this.map(() => value)

}

​

​

const increaseButton  = document.getElementById('increase')

const action$ = Observable.fromEvent(increaseButton, 'click')

  .mapTo({ type: 'INCREASE'})

​

const click$ = action$.subscribe({

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

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

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

})

​