emptinessandform,
function NamedOne(first, last) {
this.firstName = first;
this.lastName = last;
this.fullName = this.firstName + ' ' + this.lastName;
}
var namedOne = new NamedOne("Ваня", "Иванов");
namedOne.firstName = "Петя";
namedOne.lastName = "Петров";
namedOne.fullName = namedOne.firstName + ' ' + namedOne.lastName;
alert(namedOne.fullName);