Proxies

var handler = {
  get: function(target, propName, receiver) {
    if (propName === "id") {
      return 3;
    }
    return Reflect.get(target, propName, receiver);
  }
};
var x = {}; // a vanilla object
var p = new Proxy(x, handler);
p.id // returns 3
x.id // returns undefined