JS Bin // source https://jsbin.com/bovexi

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>
<body>
  <div id="mount">
    <button id="decrement">-</button>    
    <label id="count"></label>
    <button id="increment">+</button>
  </div>
<script src="https://unpkg.com/@reactivex/rxjs@5.0.3/dist/global/rx.js"></script>
<script id="jsbin-javascript">
document.addEventListener('DOMContentLoaded', () => {
  let increment = document.querySelector('#increment'),
      decrement = document.querySelector('#decrement'),
      count = document.querySelector('#count');

let increment$ = Rx.Observable
    .fromEvent(increment, 'click')
    .map(e => 1);

let decrement$ = Rx.Observable
    .fromEvent(decrement, 'click')
    .map(e => -1);

let count$ = Rx.Observable
    .empty()
    .defaultIfEmpty(0)
    .merge(increment$, decrement$)
    .scan((acc, curr) => acc + curr, 0)

count$.subscribe(data => {
    count.textContent = data
  })
})
</script>

let increment$ = Rx.Observable
    .fromEvent(increment, 'click')
    .map(e => 1);

let decrement$ = Rx.Observable
    .fromEvent(decrement, 'click')
    .map(e => -1);

let count$ = Rx.Observable
    .empty()
    .defaultIfEmpty(0)
    .merge(increment$, decrement$)
    .scan((acc, curr) => acc + curr, 0)

count$.subscribe(data => {
    count.textContent = data
  })
})

</script></body>
</html>

Updated missing url https://unpkg.com/@reactivex/rxjs@5.0.3/dist/global/Rx.js to https://unpkg.com/@reactivex/rxjs@5.0.3/dist/global/rx.js

https://unpkg.com/@reactivex/rxjs@5.0.3/dist/global/Rx.js