function Person(name, surname) {
	this.name = name;
	this.surname = surname;
	
	Object.preventExtensions(this);
}

function Developer(name, surname, knownLanguage) {
	Person.apply(this, arguments);
	this.knownLanguage =  knownLanguage;
}

var dev = new Developer("Mario", "Rossi", "JavaScript");
console.log(dev.knownLanguage);		// wynik:		undefined