Javascript inheriting instance attributes?

Question

function A(){
  // sort of like this
  // this.arr = [];
}

function B(){
  // not here though
  // this.arr = [];
}

B.prototype = new A(); // all B instances would share this prototypes attributes

// I want all B instances to have an arr attributes (array)
// of their own but I want the parent class to define it

var b1 = new B();
var b2 = new B();

b1.arr.push(1);
b2.arr.push(2);

b1.arr.length === 1; // should be true
b2.arr.length === 1; // should be true

I want write code in A that would define a arr variable for each instance of a child class B such that arr is a new object for each B instance. I could make this happen by setting this.arr = [] in the B constructor, but is this possible to accomplish with code written onto A instead?

Loïc Faure-Lacroix · Accepted Answer · 2015-05-19 15:54:44Z

There is a big problem in your idea of inheritance in Javascript. In javascript, you really have objects and that's pretty much it. A constructor is a method that is called with new and create a new object.

This part of code isn't really right as you're creating a prototype using the object created from A... That said, you're not really using the prototype A. As inheritance per say doesn't really exist, you'll have to implement something that will call every constructors that are required to create object B.

B.prototype = new A();

You should use this method instead:

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

In this case, you'll have all the prototypes of A in B... But you'll still have to call the constructor of A if it is necessary.

function A() {
   this.arr = []
}

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

Using more complex library, you could end up with inheritance and something similar to super in python. If you're really into that, you could also create a structure that will automatically call each constructor of each sub prototypes.

maioman · Accepted Answer · 2015-05-19 15:53:24Z

0

maybe this is what you're looking for:

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

With Object.create() you can create an object with a specific prototype, and I believe this is what you're trying to do.

edited May 19, 2015 at 15:53

answered May 19, 2015 at 15:50

maioman

18.8k4 gold badges39 silver badges46 bronze badges

Comments

roxxypoxxy · Accepted Answer · 2018-05-15 10:48:01Z

0

One way I can think of that to work is:

function A(){
   this.arr = [];
}

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

edited May 15, 2018 at 10:48

answered May 19, 2015 at 15:57

roxxypoxxy

3,1212 gold badges24 silver badges28 bronze badges

1 Comment

Loïc Faure-Lacroix Over a year ago

It's not necessary to call new A(). As you're calling the constructor of A in the constructor of B, it will be as if you called the constructor twice. In the prototype of B you really only need to copy the prototype of A with Object.create.

Collectives™ on Stack Overflow

Javascript inheriting instance attributes?

3 Answers 3

Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related