Javascript Funtion to Select All the Check Box in a Column of a Grid

<script type="text/javascript" language="javascript">
function SelectAll(CheckBox) {

// Give the Grid ID in the below line
TotalChkBx = parseInt('<%= this.grdDispatchGrid.Rows.Count %>');

var TargetBaseControl = document.getElementById('<%= this.grdDispatchGrid.ClientID %>');

// Give the ColumnId of a grid which must be checked

var TargetChildControl = "ChkApporove";

var Inputs = TargetBaseControl.getElementsByTagName("input");

for (var iCount = 0; iCount < Inputs.length; ++iCount)
{

if (Inputs[iCount].type == 'checkbox' && Inputs[iCount].id.indexOf(TargetChildControl, 0) >= 0)

Inputs[iCount].checked = CheckBox.checked;
}

}    </script>


<asp:TemplateField>
<ItemTemplate>
<asp:CheckBox ID="ChkApporove" runat="server" />
</ItemTemplate>
<HeaderTemplate>
<asp:CheckBox ID="chkSelectAll" runat="server" Text="All" onclick="SelectAll(this);" />
</HeaderTemplate>
</asp:TemplateField>