// mind blown!  custom property setters and getters in JavaScript!

function Fancy() {
  var thing;

  Object.defineProperty(this, 'thing', {
    get: function(){
      return thing;
    },
    set: function(value){
      thing = value + '!';
      console.log("thing changed to '" + thing + '".');
    }
  });
}

var fancy = new Fancy();
fancy.thing = 'herp';
fancy.thing = 'derp';
console.log('and the value was really saved as "' + fancy.thing + '".');

// output:
// thing changed to 'herp!".
// thing changed to 'derp!".
// and the value was really saved as "derp!".