const addButton = document.getElementById('addButton');

const minusButton = document.getElementById('minusButton');

const state = document.getElementById('state');

​

const addClick = Rx.Observable.fromEvent(addButton, 'click').mapTo(1);

const minusClick = Rx.Observable.fromEvent(minusButton, 'click').mapTo(-1);

​

​

const numberState = Rx.Observable.empty()

  .startWith(0)

  .merge(addClick, minusClick)

  .scan((origin, next) => origin + next, 0)

numberState

  .subscribe({

    next: (value) => { state.innerHTML = value;},

    error: (err) => { console.log('Error: ' + err); },

    complete: () => { console.log('complete'); }

  });