object.observe.js

var person = { shoeSize: 43, name: 'nekman' };

Object.observe(person, function(changes) {
  var change = changes[0];
  console.log('Changed property "%s" from %s to %s.', 
      change.name, 
      change.oldValue,
      change.object[change.name]
  ); 
});

// change a object property...
person.shoeSize = 44;