Select/Deselect all checkboxes inside Gridview

function check_all_check_boxes(chk)
{
   var grid_employee = document.getElementById('<%=grid_view_employee_details.ClientID %>');

   if(grid_employee!=null)
   {
      for (row_index = 1; row_index <=grid_employee.rows.length-1; row_index++)
      {
         grid_employee.rows[row_index].cells[0].getElementsByTagName("input")[0].checked = chk.checked;
      }
   }
}