Hi,
you need to include below code,
<scriptsrc="/sap/public/bc/ui2/services/sap/ui2/srvc/error.js"></script>
<scriptsrc="/sap/public/bc/ui2/services/sap/ui2/srvc/utils.js"></script>
<scriptsrc="/sap/public/bc/ui2/shell-api/sap/ui2/shell/startup.js"></script>
<script>
var oUser = sap.ui2.shell.getUser();
oUser.load({}, function(){
var fullName = oUser.getFullName();
var userID = oUser.getId();
alert(fullName);
alert(userID);
},
function(){
alert ('user fetching failed');
});
</script>
refer Accessing System and User Information - User Interface Add-On for SAP NetWeaver - SAP Library
Regards,
Chandra