How to detect Capslock is On/Off using Javascript?

<asp:TextBox ID="TextBox1" onkeypress="capsDetect(arguments[0]);" runat="server"></asp:TextBox>

<script language="javascript" type="text/javascript">
var capsError = 'WARNING:\n\nCaps Lock is enabled\n\nThis field is case sensitive';

function capsDetect( e ) {
if( !e ) { e = window.event; } if( !e ) { MWJ_say_Caps( false ); return; }
//what (case sensitive in good browsers) key was pressed
var theKey = e.which ? e.which : ( e.keyCode ? e.keyCode : ( e.charCode ? e.charCode : 0 ) );
//was the shift key was pressed
var theShift = e.shiftKey || ( e.modifiers && ( e.modifiers & 4 ) ); //bitWise AND
//if upper case, check if shift is not pressed. if lower case, check if shift is pressed
MWJ_say_Caps( ( theKey > 64 && theKey < 91 && !theShift ) || ( theKey > 96 && theKey < 123 && theShift ) );
}
function MWJ_say_Caps( oC ) {
if( typeof( capsError ) == 'string' ) { if( oC ) { alert( capsError ); } } else { capsError( oC ); }
}
</script>