Edit in JSFiddle

var btns = document.querySelectorAll('button');

for (var i = 0; i < btns.length; i++) {
  btns[i].addEventListener('click', (function(num) {
    return function() {
      alert(num);
    };
  })(i), false);
}

<div>
  <button type="button">0</button>
  <button type="button">1</button>
  <button type="button">2</button>
  <button type="button">3</button>
  <button type="button">4</button>
</div>