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I'm playing with javascript prototype. I'm new to it so I've got a small question.

I'm using this article as a guide.

I've got a Product defined and a Book defined. what is the purpose of Book.prototype.constructor = Book(); this. I can't figure out. I'm able to call parent constructor with and without it both successfully.

Book.prototype = new Product;
Book.prototype.constructor = Book; // What's the purpose of this

Here's my jsFiddle link

I'm playing with javascript prototype. I'm new to it so I've got a small question.

I'm using this article as a guide.

I've got a Product defined and a Book defined. what is the purpose of Book.prototype.constructor = Book(); this. I can't figure out. I'm able to call parent constructor with and without it both successfully.

Book.prototype = new Product;
Book.prototype.constructor = Book; // What's the purpose of this

Here's my jsFiddle link

• javascript

Share Improve this question Follow edited Apr 3, 2011 at 11:44 Headshota asked Apr 3, 2011 at 11:23 HeadshotaHeadshota 21.4k1313 gold badges6262 silver badges8282 bronze badges 4

• It should not be "Book()", but just "Book". – Pointy Commented Apr 3, 2011 at 11:37
• It works both with Book, and Book(). =) can you tell me what is the difference? because I don't know. thanks. – Headshota Commented Apr 3, 2011 at 11:40
• 3 The difference is that the "constructor" property of the Book prototype (and thus, by default, that of all instances of Book) will not be defined if you initialize it to "Book()", because calling Book() returns nothing. When you set it to just "Book", you're making it a reference to the function object. – Pointy Commented Apr 3, 2011 at 12:13
• Here is another blog post discussing the issue. – Pointy Commented Apr 3, 2011 at 12:15

Add a ment  | 

3 Answers 3

Sorted by: Reset to default 9

First off, new Product() creates an object with all prototype variables of the Product function. So, by setting Book.prototype = new Product(), Book inherits all prototype variables of Product.

You might think you could also say: Book.prototype = Product.prototype, but that solution doesn't work as expected. The prototype of Book bees a pointer to the prototype of Product, and therefore it's not a copy. If you would change something to the prototype of Book, it is actually changed in the prototype of Product, and that's not what you want.

Shortly, new Product() creates a copy of all prototype variables.

---

But there is a problem with this method too. If you would create a new Book now, the Product constructor is called, instead of the Book constructor. To solve that, you need to set the constructor correctly again, and it'll work:

Book.prototype.constructor = Book;
// Note: it's not Book(), but Book, it's a reference to the function

When a constructor (which is just a function) is called with the new operator, a new object is created and the constructor's this keyword is set to a reference to that new object. The constructor function's prototype has a constructor property that points to the function.

The new now object's internal [[prototype]] property points to the constructor's public prototype. Objects don't inherit from their own public prototype.

In your code:

function Product(dName) {
    this.displayName = dName;   
}

function Book(dName, isbn) {
    //Properties
    this.ISBN = "";

    // Constructor
    Product.call(this, dName);
    this.ISBN = isbn;

    // Methods
    this.getName = function() {
        return this.displayName + ': ' + this.ISBN;
    };
}

Book.prototype = new Product();

That sets Book's prototype to a new instance of Product (i.e. an object with Product.prototype as its internal [[prototype]]).

Book.prototype.constructor = Book(); // What's the purpose of this

You should assign a reference, not call Book, so remove the ().

Instances of Book inherit their constructor property from Book.prototype. But since it's an instance of Product, it inherits constructor from Product.prototype and so references Product. Remove the () so that Book.prototype.constructor references Book. Now instances of Book will have a constructor property inherited from Book.prototype that references Book.

It's a pretty useless property as it can be easily overwritten, as a result using the constructor property to determine if an object is an instance of something is not done very often. The instanceOf operator is also flawed, but there aren't many reasons to use either.

var b = new Book("Book", "993438403994");

alert(b.getName());

Oh I just realized based on my helpers here =).

It's not a must to do

Book.prototype.constructor = Book;

the Book constructor will be called anyway. but if Book will be extended in the future I will not be able to call it's constructor from it's child class without it.

Am I correct? if not correct me please.

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