ASP.Net GridView Get Header Cell Text when clicked using JavaScript

<script type = "text/javascript">
    window.onload = function () {
        var grid = document.getElementById("<%=GridView1.ClientID %>");
        var headerCells = grid.getElementsByTagName("th");
        for (var i = 0; i < headerCells.length; i++) {
            headerCells[i].onclick = function () {
                alert(this.innerHTML);
            };
        }
    }
</script>