Calling Parent's Constructor In JavaScript OOP

Question

I have created two objects:

Mammal
Cat

Cat extends Mammal. Both objects have constructor which takes one parameter called config. I am trying to overwrite Mammals constructor in Cat's constructor but I am getting strange results:

function Mammal(config) {
    this.config = config;
    console.log(this.config);
}

function Cat(config) {
    // call parent constructor
    Mammal.call(this, config);
}
Cat.prototype = new Mammal();

var felix = new Cat({
    "name": "Felix"
});

This prints in the console:

undefined fiddle.jshell.net/_display/:23
Object {name: "Felix"}

Why is the parent constructor called twice? And why, when it's called the first time, this.config is undefined? I am assigning the property. Could you help me fix this code?

http://jsfiddle.net/DS7zA/

See the question What is the reason to use the 'new' keyword here? and its answers stating that one should not… — Bergi
– Bergi, Commented Jun 7, 2013 at 10:21

Alnitak · Accepted Answer · 2013-06-07 10:51:55Z

1

It's called twice because you call it with Cat.prototype = new Mammal(). You're creating the prototype by copying from an instance of Mammal, rather than from the "prototypical" Mammal.

The correct line would be:

Cat.prototype = Object.create(Mammal.prototype);

edited Jun 7, 2013 at 10:51

answered Jun 7, 2013 at 9:57

Alnitak

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1 Comment

Richard Knop Over a year ago

Thanks... This prototype style inheritance is confusing me a lot. I'm not used to it yet.

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