[ksa]

вызов namedOne.fullName() должен быть без скобок

Вариант "без скобок"...

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