Using SelectAll checkbox in grid

<asp:TemplateColumn>
         <HeaderTemplate>
          <INPUT id="cbCheckAll" onclick="CheckItems();" type="checkbox" enabled="false">          
         </HeaderTemplate>
         <ItemTemplate>
           <INPUT type=checkbox value='<%# Container.dataitem("VendorCode") %>' name=cbdgVendorCode>
            </ItemTemplate>
        </asp:TemplateColumn>


function CheckItems() ////  Check & Uncheck the check boxes in the datagrid
{
 try
  {
   chkField = eval(document.getElementsByName("cbdgVendorCode"));
   if (document.getElementById("cbCheckAll").checked)
   val= true;
   else
   val=false;
   for (i = 0; i < chkField.length ; i++)
   {
   chkField[i].checked = val ;
   //alert(chkField[i].value);
   }
  
  }
 catch(e) {}
}