Javascript dynamic getter, setter using defineProperty

gistfile1.js

var User = (function () {
    function User (id, nam) {
        var self = this;

        this.id = id;
        this.nam = nam;

        this.__data = {};

        for(var p in self) {
            if(p === '__data') {
                return;
            }

            self.__data[p] = self[p];

            (function(field_name) {
                Object.defineProperty (self, field_name, {
                    get: function () { 
                        console.log('GET', field_name);
                        return self.__data[field_name];
                    },
                    set: function (new_value) {
                        console.log('SET', field_name, new_value);
                        self.__data[field_name] = new_value;
                        self.__data['modified'] = (new Date()).getTime();
                    }
                });
            })(p);
        }
    };

    return User;
}) ();

var user1 = new User(1, "Paul");
var user2 = new User(2, "Mark");

console.log(user1.id);

user1.id = 5;

console.log(user1.id);