js inheritance and prototype assignment

Question

I am learning about JS prototyping and inheritance, I had learned that the right way to do it is by:

function A(){}
A.prototype.doSomething=function(){}
function B(){}
B.prototype = new A();
console.log( (new B()) instanceof A);//true
console.log( (new B()) instanceof B);//true

As you can see I am setting new instance of A into B but as you can see it works great with

function A(){}
A.prototype.doSomething=function(){}
function B(){}
B.prototype = A.prototype;
console.log( (new B()) instanceof A);//true
console.log( (new B()) instanceof B);//true

but here: http://ejohn.org/apps/learn/#76

they claimed that prototype assignment is wrong,I don't understand why?

try see mdn about instanceof, when you assign prototype, in second case, you don't get B inherit A you get B and A inherit from some other — Grundy
– Grundy, Commented Jan 27, 2015 at 14:02
You have a syntax error in 2nd line. Did you mean A.prototype.doSomething = function(){}? — pawel
– pawel, Commented Jan 27, 2015 at 14:14
Doing B.prototype = A.prototype; you will have ((new B() instanceof A) == true but also ((new A()) instanceof B) == true, does as Grundy suggested. The site you posted fail in the instanceof test due Ninja.prototype = { dance: Person.prototype.dance }; this break the prototype chain. — Prusse
– Prusse, Commented Jan 27, 2015 at 14:32
it breaks it beacuse Ninja.prototype = { dance: Person.prototype.dance }; is reassign the general object into prototype and overrides the assignement — Wazime
– Wazime, Commented Jan 27, 2015 at 15:00

Wazime · Accepted Answer · 2015-01-27 15:31:58Z

1

here is the reason in the first example:

console.log( (new B()) instanceof A);//true

But

console.log( (new A()) instanceof B);//true

So this is wrong usage....
The right way is to do it in the next way:

function Parent(){}
function Child(){}
Child.prototype = Object.create(Parent.prototype);

answered Jan 27, 2015 at 15:31

Wazime

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