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

function Student(name, surname, subjectOfStudy) {
	Person.apply(this, arguments);
	this.subjectOfStudy =  subjectOfStudy;
}

function DevStudent(name, surname, knownLanguage, subjectOfStudy) {
	Developer.call(this, name, surname, knownLanguage);
	Student.call(this, name, surname, subjectOfStudy);
}

var johnSmith = new DevStudent("Jan", "Kowalski", "C#", "JavaScript");

console.log(johnSmith.knownLanguage);		// wynik: C#
console.log(johnSmith.subjectOfStudy);		// wynik: JavaScript
