How to observe property change returned by object's method

I'm working on login system and need to watch if user is logged in / out. The value which determines user login status is returned in following way:

Parse.User.current().id // returns id when logged in
Parse.User.current() // returns null when logged out

expected functionality (pseudocode):

watch( Parse.User.current().id, function (old_val, new_val) {
  console.log( 'User id has changed from: ', old_val, ' to: ', new_val );
});
// expected result: User id has changed from null to H4tn34vw0

I would like to keep wide browser support, like ie9+. I considered Object.observe() but it can watch object properties, not values returned by methods if i'm correct. Also using setInterval() and checking the object every second may not be a good idea for performance.