[danik.js]

Фу setAttribute('onclick'), фу clval без var.

<style>
  div.a {
    padding:5px;
    margin: 5px;
    background-color: red;
    float: left;
  }
</style>
<div class="a">1</div>
<div class="a">2</div>
<div class="a">3</div>
<div class="a">4</div>
<div class="a">5</div>
<script>
  (function () {
    var divs = document.querySelectorAll("div.a");
    var handler = function() {
        // var index = this.dataset.index;
        var index = this.getAttribute('data-index');
        alert(index);
    }
    for (var i = 0; i < divs.length; i++) {
        // divs[i].dataset.index = i;
        divs[i].setAttribute('data-index', i);
        divs[i].onclick = handler;
    }
  }());
</script>

ES5:

<style>
  div.a {
    padding:5px;
    margin: 5px;
    background-color: red;
    float: left;
  }
</style>
<div class="a">1</div>
<div class="a">2</div>
<div class="a">3</div>
<div class="a">4</div>
<div class="a">5</div>
<script>
  (function () {
    var divs = document.querySelectorAll("div.a");
    var handler = function() {
        var index = Array.prototype.indexOf.call(divs, this);
        alert(index);
    }
    for (var i = 0; i < divs.length; i++) {
        divs[i].onclick = handler;
    }
  }());
</script>