Problem extending class with javascript object prototype

Question

I have got this problem... B is a base class, and A is a derived class... Event though A is derived from B, various objects of A points to the same object of B.

I know i have assigned an object of B to the prototype of A to make A child of B.

But different objects of A, they should have different address space to hold the variables, right? Can you anyone correct this?

    function B(){

        this.obj = {};
    }

    function A(){

    }

    A.prototype = new B();

    var a = new A();
    var b = new A();
    var c = new A();

    console.log(a.obj == b.obj); //prints true
    console.log(a.obj === b.obj); //prints true

    a.obj.name = "stackoverflow";
    console.log(b.obj.name); //prints stackoverflow

What change should I make in this code so that gives me following result.

a.obj === b.obj  //must be false

a instanceof A;  //must be true
a instanceof B;  //must be true

What exactly is it you're asking? What are the problems with the code you've posted or the difference in expected behavior? — Slavo
– Slavo, Commented Jun 29, 2011 at 10:26

Wladimir Palant · Accepted Answer · 2011-06-29 10:34:55Z

4

That's why you shouldn't have mutable values (particularly objects or arrays) on a prototype - the same value will be shared across all object instances and can be changed in any of them. Here you can avoid the problem by using Object.create that won't call the constructor of B when creating the prototype:

A.prototype = Object.create(B.prototype);

Constructor of A should then call the constructor of B for each new object:

function A() {
  B.call(this);
}

For browsers that don't support Object.create() you can emulate it as mentioned on http://javascript.crockford.com/prototypal.html.

answered Jun 29, 2011 at 10:34

Wladimir Palant

57.8k12 gold badges100 silver badges129 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Evert · Accepted Answer · 2011-06-29 10:32:19Z

2

Values are assigned by reference, and since all instances of A use the same instance of B as their prototype, they all refer to the same 'B'.

So this is exactly what's expected here. One way to solve this, is to add (for instance) a 'initialize' method the B 'class', which you could then call from within the A constructor.

You can also not use 'new B()' to define the prototype, and use Object.create instead. Object.create does not call B's constructor, but you can then call the parent constructor from A.

function A() {
   B.call(this);
}

answered Jun 29, 2011 at 10:32

Evert

101k18 gold badges136 silver badges221 bronze badges

1 Comment

Software Enthusiastic Over a year ago

I tried this, my second requirement doesn't work here... That is, a instanceof B == true

Community · Accepted Answer · 2017-05-23 11:55:53Z

1

This is a part of prototypes in Javascript, I suggest you read this excellent thread.

edited May 23, 2017 at 11:55

CommunityBot

11 silver badge

answered Jun 29, 2011 at 10:31

Dhaivat Pandya

6,5264 gold badges32 silver badges44 bronze badges

Collectives™ on Stack Overflow

Problem extending class with javascript object prototype

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related