const minusButton = document.querySelector("#button1");

const plusButton = document.querySelector("#button2");

const output = document.querySelector("#output");

​

const minusButtonClick$ = Rx.Observable.fromEvent(button1, "click");

const plusButtonClick$ = Rx.Observable.fromEvent(button2, "click");

​

Rx.Observable.merge(

    minusButtonClick$.map(_ => -1),

    plusButtonClick$.map(_ => +1))

  .startWith(0)

  .scan((aggr, curr) => aggr + curr, 0)

  .subscribe(console.log);