Check if instance has inherited function in prototype chain

Question

If I have an inheritance method set up like this:

function A() {}


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

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

function C() {
    B.call(this, {});
}

C.prototype = Object.create(B);
C.prototype.constructor = C;


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

Is it possible to confirm, that the instance c has A in it's prototype chain, or has the information been lost somewhere?

Suren Srapyan · Answer 1 · 2016-08-10T07:45:59.033

1

No !!!. In your example

C.prototype = Object.create(C);
C.prototype.constructor = C;

You have a prototype of C instances set to C

If you change to C.prototype = Object.create(B);

you will see this

But this doesn't mean that your c is instanceof A. Why ? Because you have set the prototype to an Function not to an Object

If you will change your code to this, that its prototype will be an object

function A() {}


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

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

function C() {
    B.call(this, {});
}

C.prototype = Object.create(new B());
C.prototype.constructor = C;


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

In this case your c has A in it's prototype chain.

You can test it with c instanceof B and c instanceof A in the console

edited Aug 10 '16 at 07:45

answered Aug 10 '16 at 07:38

Suren Srapyan

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Try `console.log(c instanceof A)` – thefourtheye Aug 10 '16 at 07:41
@thefourtheye don't hurry – Suren Srapyan Aug 10 '16 at 07:44
What if `A` or `B` accepts a parameter? How would you establish the prototype chain? – thefourtheye Aug 10 '16 at 07:45
@thefourtheye what is the problem in parameters? – Suren Srapyan Aug 10 '16 at 07:46
Sorry the C.prototype = Object.create(C) was a typo. Thanks for the answer, I'll try it out! – LongInt Aug 10 '16 at 07:51
This is not the correct way to fix this. Let me find a dupe. – thefourtheye Aug 10 '16 at 07:52
See if [this answer](http://stackoverflow.com/a/22316035/1903116) helps and [this](http://stackoverflow.com/a/17393153/1903116) – thefourtheye Aug 10 '16 at 08:02
@thefourtheye Thanks! So using Object.create(F.prototype) vs Object.create(new F()) is better because it creates less objects? – LongInt Aug 10 '16 at 09:23

Check if instance has inherited function in prototype chain

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