Can't return correct value from js function

I've got a function that take a data from firebase db and it must return it from the function:

function f(){
    var adr = window.residentInstance.address;
    var ref = window.db.ref().child('SwapQueries').child(adr); 
    var res;
    ref.once('value', function(snapshot){res = snapshot.val()});
    return res;
}

The problem is js is totally asynchronous and I've found no way to wait until promise is done, so how am I supposed to solve this problem?

via Keltar Helviett