Javascript API - OndataUpdated

Summary

Javascript API - Restriction

Content

 

I refer below Javascript API code. PortReference is contact custom attribute text field. 

 

I want execute below code when there is any change in portreference field (not for other fields).  I have tried to add if condition, but I don't know exact syntax. How to restrict? 

 

function ondataupdated(obj) {

var c = window.external.Contact;

var portreference = c.GetCustomFieldByName("CO$PortReference");

var newPortreference = portreference.toUpperCase();

c.SetCustomFieldByName("CO$PortReference",newPortreference);

}