function User(username, password) {
    Object.defineProperties(this, {
        id: {
            get: function() {
                return 1;
            },
            enumerable: true
        },
        username: {
            get: function() {
                return username;
            },
            enumerable: true
        },
        password: {
            get: function() {
                return password;
            }
        }
    });
}

var pippo = new User('pippo', 123456);
var pluto = new User('pluto', 'ciao!11');