Proxies

Proxy.js

var p = Proxy.create({
  get: function(proxy, name) {        // intercepts property access
    return 'Hello, '+ name;
  },
  set: function(proxy, name, value) { // intercepts property assignments
    alert(name +'='+ value);
    return true;
  }
});

alert(p.world); // alerts 'Hello, world'
p.foo = 'bar';  // alerts foo=bar