OOP inheritance and objects instaces

Question

I working out the inheritance and OOPS in JavaScript. I created 'A' object and 'B' object and inherited all the properties of 'B' to 'A'.

While I check the newly created object instance by using 'instanceof', I am getting both objects are true.
But I have created the object using 'B' constructor only.

function A(){
    this.a = "test1",
    this.b = "test2"
}

function B(){
    this.c = "test3",
    this.d = "test4"
}

B.prototype = new A();

var userObj = new B();

console.log(userObj instanceof B); // true
console.log(userObj instanceof A); // true, How this can be true?

Naeem Shaikh · Accepted Answer · 2015-02-06 11:44:54Z

4

In JavaScript, the inheritance is prototype-based. That means that there are no classes. Instead, an object inherits from another object.

What you have done here is, object B inherits from object A because of this:

B.prototype = new A();

So ,now, userObj (instance of object B) of-course becomes instance of object A, (thats what inheritance is all about, right?)

So thats the reason

console.log(userObj instanceof B); // true
console.log(userObj instanceof A); //  Also true?

edited Feb 6, 2015 at 11:44

answered Jan 27, 2015 at 10:36

Naeem Shaikh

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Comments

AL-zami · Accepted Answer · 2015-01-27 10:57:10Z

1

Prototype object has a built in property called constructor.First see this image from chorme console than i will discuss

enter image description here

In your code B.prototype = new A(); =>this statement sets B's prototype to a instance of A.So B's prototype object has it's constructor property assigned to A();

console.log(userObj.constructor // it will show A.But it is not true

Remember it is not userObj's constructor.The constructor you get is from prototype obj.

console.log(userObj instanceof A); // true, How this can be true?

As userObj is a instance of B.And B inherits from A.So userObj is also a instance of A.But you can manipulate the constructor property set to B using

B.prototype.constructor=B;

answered Jan 27, 2015 at 10:57

AL-zami

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Comments

Deepak Kumar Padhy · Accepted Answer · 2015-01-27 10:46:29Z

0

console.log(userObj instanceof A); you will surely get true. This is as per the JavaScript standard.

When you are creating a object of B which inherits all the properties of A , the object contains a link __proto__ which points to A's Prototype.

So instanceof will find the A's constructor in the prototypical chaining of the Object B.

That's why it returns true.

edited Jan 27, 2015 at 10:46

answered Jan 27, 2015 at 10:38

Deepak Kumar Padhy

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