Searching Employee Name via InternalId

I'm new to scripting and NetSuite so excuse my ignorance.  I have searched both NetSuite help and the user group, but did not find anything relevant to what I am trying to accomplish.  The script below is using the nlapiGetUser to lookup the employee firstname and lastname.  I am currently just trying to display that information in an alert (eventually storing it in a custom field).  The alert is returning undefined for both the firstname and lastname.  What am I missing?  Thanks for looking and any suggestions!

var userName = nlapiGetUser();
var filters = new Array();
filters[0] = new nlobjSearchFilter('internalid', null, 'is', userName, null);