Function to Select or Deselect all checkbox in a gridview through javascript

<script language="javascript" type="text/javascript" >

function SelectAll(SelBtn)
     {
          var gvET = document.getElementById("GridView1"); 
             var rCount = gvET.rows.length;
             var btnSel = SelBtn.value;
             var rowIdx;
             // check the Page Count is greater than 1 then rowindex will start from 2 else 1
             if(document.getElementById("hidgridpagecount").value>1)
             {
                  rowIdx=2;
             }
             else
             {
                  rowIdx=1;
             }
             for (rowIdx; rowIdx<=rCount-1; rowIdx++)
             {
                  var rowElement = gvET.rows[rowIdx];
                  var chkBox = rowElement.cells[0].firstChild;
                  if (btnSel == 'Select All')
                  {
                       chkBox.checked = true;
                  }
                  else
                  {
                       chkBox.checked = false;
                  }
             }
             if (btnSel == 'Select All')
             {
                  document.getElementById('btnSelAll_Top').value ="Deselect All";
                  document.getElementById('btnSelAll_Bot').value ="Deselect All";
             }
             else
             {
                  document.getElementById('btnSelAll_Top').value ="Select All";
                  document.getElementById('btnSelAll_Bot').value ="Select All";
             }
             return false;
     }
</script>