Edit in JSFiddle

var test1 = document.getElementsByClassName('test1');
var test2 = document.getElementsByClassName('test2');

for(var i = 0; i < test1.length; i++){
	test1[i].onclick = function(){alert(i)}
}

function t2(num){
	return function(){alert(num);}
}

for(var i = 0; i < test2.length; i++){
	test2[i].onclick = t2(i)
}

<div>
  <input class="test1" type="button" value="1 - 1">
  <input class="test1" type="button" value="1 - 2">
  <input class="test1" type="button" value="1 - 3">
  <input class="test1" type="button" value="1 - 4">
</div><br/>
<div>
  <input class="test2" type="button" value="2 - 1">
  <input class="test2" type="button" value="2 - 2">
  <input class="test2" type="button" value="2 - 3">
  <input class="test2" type="button" value="2 - 4">
</div>