Javascript code to check/uncheck all checkboxes from header check box

<script type="text/javascript">
var TotalChkBx;
var Counter;
window.onload = function()
{
    TotalChkBx = parseInt('<%= this.GridView3.Rows.Count %>');
    Counter = 0;
}
function SelectAll(CheckBox)
{
    var TargetBaseControl = document.getElementById('<%= this.GridView3.ClientID %>');
    var TargetChildControl = "chkselect";
    var Inputs = TargetBaseControl.getElementsByTagName("input");
    for(var n = 0; n < Inputs.length; ++n)
    if(Inputs[n].type == 'checkbox' && Inputs[n].id.indexOf(TargetChildControl,0) >= 0)
    Inputs[n].checked = CheckBox.checked;
    Counter = CheckBox.checked ? TotalChkBx : 0;
}
function ChildClick(CheckBox, HCheckBox)
{
    var HeaderCheckBox = document.getElementById(HCheckBox);
    if(CheckBox.checked && Counter < TotalChkBx)
    Counter++;
    else if(Counter > 0)
    Counter--;
    if(Counter < TotalChkBx)
    HeaderCheckBox.checked = false;
    else if(Counter == TotalChkBx)
    HeaderCheckBox.checked = true;
}
</script>
onclick="javascript:SelectAll(this);"