Something wrong when reset the javascript constructor

Question

I am trying to understand inheritance in javascript. I wrote a basic example to implement a simple inheritance between the parent object and a child. But I think there is something wrong when resetting the constructor.

function Mammal(){
}
function Dog(){

}
Mammal.prototype.walk = function(){
    console.log('walking..walking....');
}
Dog.prototype.bark = function(){
    console.log('wof wof!');
}

Dog.prototype = Object.create(Mammal.prototype);
Dog.prototype.constructor = Dog;
let dogInstance = new Dog();
dogInstance.walk();
dogInstance.bark();

Answer 1

Problem here Dog.prototype = Object.create(Mammal.prototype);
Try it

[Edit]

function Mammal() {}
function Dog() {}
Mammal.prototype.walk = function () {
    console.log( 'walking..walking....' );
};
Dog.prototype.bark = function () {
    console.log( 'wof wof!' );
};

[/Edit]

let describe = Object.getOwnPropertyDescriptor;
let descriptor = Object.keys( Mammal.prototype ).reduce( function ( desc, key ) {
    desc[key] = describe( Mammal.prototype, key );
    return desc;
}, {} );
Dog.prototype = Object.create( Dog.prototype, descriptor );

[Edit]

Dog.prototype.constructor = Dog;
let dogInstance = new Dog();
dogInstance.walk();
dogInstance.bark();

Or use Object Inheritance follow this

function Mammal( key ) { this.key = key; }
function Dog() {
    Mammal.call( this, "Dog" );
}
Dog.prototype = Object.create( Mammal.prototype );
Dog.prototype.bark = function () {
    console.log( 'wof wof!' );
};
Mammal.prototype.walk = function () {
    console.log( 'walking..walking....' );
    return `${this.key} from Mammal.`;
};
Dog.prototype.constructor = Dog;
let dogInstance = new Dog();
dogInstance.walk();
dogInstance.bark();

Compare dogInstance

 Mammal.prototype.isPrototypeOf( dogInstance );
 Dog.prototype.isPrototypeOf( dogInstance );

It's work for me :(

[/Edit]

Answer 2

I think there is something wrong when resetting the constructor.

No, there's not.

Your problem is that you do

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

after having created the bark method on the old Dog.prototype object - which you are overwriting with a new, empty object. Just do it before any assignments:

function Mammal() {
}
Mammal.prototype.walk = function(){
    console.log('walking..walking....');
};

function Dog() {
}
Dog.prototype = Object.create(Mammal.prototype); // do this before creating properties
Dog.prototype.constructor = Dog;
Dog.prototype.bark = function() {
    console.log('wof wof!');
};

Answer 3

I'm not completely sure about this, but this line is the problem:

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

---

Apparently you can't give Mammal prototype directly to Dog prototype

function Mammal() {
}
function Dog() {

}
Mammal.prototype.walk = function () {
 console.log( 'walking..walking....' );
}
Dog.prototype.bark = function () {
 console.log( 'wof wof!' );
}

Dog.prototype.mammal = Object.create( Mammal.prototype );
Dog.prototype.constructor = Dog;
let dogInstance = new Dog();
dogInstance.mammal.walk();
dogInstance.bark();

this seems to work