You can use http://notesninjas.com/#JSFieldExist to test to see if a field exists on a form, but sometimes you need to check to see if a variable exists.
Using
if (varname) alert("exists")
doesn't work.
You need to use typeof.
if (typeof(varname)!="undefined") alert("exists") else alert("doesnt exist')
you can also test for number string bookean object or null
From http://www.javascriptkit.com/javatutors/determinevar2.shtm