Read server side variable in Javascript in ASP.Net

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

var TNo = '<%= Request.QueryString["TNo"] %>';
window.open('BRFUHistory.aspx?TNo='+TNo+'','CH','location=0,status=1,scrollbars=1,width=480,height=300');
}
</script>