Edit in JSFiddle

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

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