Referencing server controls from Javascript

< script language="javascript" type="text/javascript" >
function incrementValue() {
var txtToIncr = document.getElementById('< %= txtToIncr.ClientID % >');
var CurrentValue = 0;
if (txtToIncr.value != '')
CurrentValue = parseInt(txtToIncr.value);
txtToIncr.value = CurrentValue + 1;
}
< /script >