Checked All checkbox during Grid View Load while Page Load using Javascript Code

Checked All checkbox during Grid View Load while Page Load using Javascript Code with the following code :

<HeaderTemplate>
<input id="chkAll" onclick="javascript:SelectAllChkboxes(this);" runat="server" type="checkbox" />
</HeaderTemplate>
   <script type="text/javascript">

function SelectAllChkboxes(spanChks) {

var oItem = spanChks.children;
var theBox = (spanChks.type == "checkbox") ? spanChks : spanChks.children.item[0];
xState = theBox.checked;
elme = theBox.form.elements;
for (i = 0; i < elme.length; i++)
if (elme[i].type == "checkbox" && elme[i].id != theBox.id) {
if (elme[i].checked != xState)
elme[i].click();
}

}
</script>