Setting a fields value to null and not just "" (empty string).

I too an interested in this - I have a code that interacts with two fields - and you can only put data in one of the fields. So upon entering field B adobe checks to see if field A is null - if it is not an alert pops up and says "You can only have data in one field, shall I erase the data in the other field". Now, I can just put an empty string (value = "") but then that is not the same thing as null and so my code breaks down and even when the cell is empty (contains a zero length string "") the error message still runs. So I need to make it null and not just "" - how can I do this?
(by the way if it matters, it is a number field). Here is the code:
 
var FedDol = this.getField("IncomeTaxWitheldFederalPercent");
 
//Check to see if the other field is null
if (FedDol.value != null)
 
//If it is null run the alert
var nRslt = app.alert("Only one federal value may be entered - either dollars or percentage.\n\n" + "Shall I erase the percentage value in order for you to add a dollar amount?",2,2,"Warning");
 
//If the users selects 'yes' make the other field null
if(nRslt == 4)
FedDol.rawvalue == null;
— AndrewAlb