[tsigel]

function NamedOne(first, last) {
    this.firstName = first;
    this.lastName = last;
    Object.defineProperty(this, "fullName", {
        get: function () {
            return this.firstName + " " + this.lastName;
        }
    });;
}
var namedOne = new NamedOne("Ваня","Иванов");
namedOne.firstName = "Петя";
alert(namedOne.fullName);