There is something about Javascript inheritance that I can't understand

Question

I was reading Introduction to Object-Oriented JavaScript from Mozilla Developer Network, time to learn so serious Javascript before start using node.js.

Anyway, inheritance thing seems so obscure to me. Copy and paste from the documentation:

// define the Person Class
function Person() {}

Person.prototype.walk = function(){
  alert ('I am walking!');
};
Person.prototype.sayHello = function(){
  alert ('hello');
};

This is easy, but things get complicated with Student inheritance. Does anyone else think that the following three statements do essentially the same thing?

// define the Student class
function Student() {
  // Call the parent constructor
  Person.call(this);
}

// inherit Person
Student.prototype = new Person();

// correct the constructor pointer because it points to Person
Student.prototype.constructor = Student;

I understand the first one (calling the parent constructor) because is so similar to Java, PHP and so on. But then problems begin.

Why there is the need of calling Student.prototype ~~and Student.prototype.constructor~~?

A clear explanation is needed. Why this code:

// define the Student class
function Student() {
  // Call the parent constructor
  Person.call(this);
}

var student1 = new Student();

is not enough for inheritance to work?

EDIT: regarding the constructor thing, it has been already answered here.

possible duplicate of [Why is it necessary to set the prototype constructor?](http://stackoverflow.com/questions/8453887/why-is-it-necessary-to-set-the-prototype-constructor) — pimvdb, Aug 31 '12 at 14:43
"Object-Oriented JavaScript" doesn't exists, Javascript is Prototype Oriented and you must adapt your Object-Oriented approaches to Prototypes. — Chiguireitor, Aug 31 '12 at 14:47
@Chiguireitor i didn't write the documentation title, you know... — gremo, Aug 31 '12 at 14:48
@pimvdb you're partially right. My question overlaps that posted by you for the constructor thing. But it's not exactly the same: i'd like to know why the second statement is needed. — gremo, Aug 31 '12 at 14:51

score 2 · Accepted Answer · answered Aug 31 '12 at 14:58

This doesn't work:

function Student() {
  Person.call(this);
}

var student1 = new Student();

because Person's prototype properties are not available on Student instances. Person.call(this) merely calls Person with a specific this value (namely the Student instance). But the Person function is completely empty - so here it doesn't do anything. There is no relation between Student and Person other than the useless Person call.

To get Person's functions the .prototype assignment is necessary.

Before:

<a Student instance>
  its prototype: Student.prototype, with all student functions
    its prototype: Object.prototype

After:

<a Student instance>
  its prototype: <a Person instance>, with all student functions if you add them
    its prototype: Person.prototype, with all person functions
      its prototype: Object.prototype

score 1 · Answer 2 · answered Aug 31 '12 at 14:50

If you're using Node.js, I would suggest looking into ECMAScript 5 methods for object creation. MDN has a page here which might be a good guide. See also John Resig's overview of the new method. Prototypal inheritance is hard to get your head around when coming from a classical OO language like Java. Good luck!

There is something about Javascript inheritance that I can't understand

2 Answers2