Question

I am reading this: http://killdream.github.com/blog/2011/10/understanding-javascript-oop/index.html

and i’ve encountered some code that i can’t understand:

function Person(first_name, last_name) {
    this.first_name = first_name
    this.last_name  = last_name
}

// Defines the `name' getter/setter
Object.defineProperty(Person.prototype, 'name', { get: get_full_name
                                                , set: set_full_name
                                                , configurable: true
                                                , enumerable:   true })

Why is he using Object.defineProperty on Person.prototype and not simply on Person?
Why not simply include name in the definition or make Person.name = bla...?

(EDIT: SOLVED)
also, why am i seeing this endless loop of prototype reference??

Answer 1

If he’d use Object.defineProperty on Person then you’d define a property on that function and not on the instances you create with new Person.

I.e. given Object.defineProperty(Person, ...) you were able to do

Person.name = 'foo';

but that would not really help in this situation.

On the other hand, properties of the prototype are shared by all instances and therefore it makes sense to define this property on the prototype.

Here Person is a constructor function which is supposed to be called with the new keyword. It probably will help to read about what new is doing, in order to understand why Person.prototype has to be extended.
The gist: When called with new, inside the function, this will refer to an empty object inheriting from Person.prototype.

Regarding the second part: Each function has a prototype property, and each prototype has a constructor property referring to its “parent” function. Thus you have a self referencing structure:

Person.prototype.constructor === Person;

What you showed is not the prototype chain, which can be revealed by inspecting __proto__.

I could simulate the same with:

var a = {};
a.b = {a: a};

and I could access a.b.a.b.a.b.a.b.a.b.a indefinitely.