How are these two Java Script codes different?

Question

In second code "Bird.prototype and Dog.prototype is not instance of Animal"

Why is so? No such problem in first code.

    //First
    function Animal() { }
    function Bird() { }
    function Dog() { }
    
    Bird.prototype = Object.create(Animal.prototype);
    Dog.prototype = Object.create(Animal.prototype);
    
    // Only changes  in code below this line
    Bird.prototype.constructor=Bird;
    Dog.prototype.constructor=Dog;
    
    let duck = new Bird();
    let beagle = new Dog();

    //Second
    function Animal() { }
    function Bird() { }
    function Dog() { }
    
    Bird.prototype = Object.create(Animal.prototype);
    Dog.prototype = Object.create(Animal.prototype);
    
    // Only change code below this line
    Bird.prototype={constructor:Bird};
    Dog.prototype={constructor:Dog};
    
    let duck = new Bird();
    let beagle = new Dog();

instanceof does not depend on constructor, but rather "prototype chain", which is apparently broken by the second approach. See this post for more details — Jan Stránský
– Jan Stránský, Commented Aug 31, 2020 at 19:58

Quentin · Accepted Answer · 2020-08-31 19:53:40Z

3

In the first example you modify the object assigned to prototype.

In the second example you replace it.

const thing = { a: "value" };

const a = {};
const b = {};

a.example = Object.create(thing);
b.example = Object.create(thing);

a.example.b = "other";
b.example = { different: "object" };

console.log( { a, b } );

answered Aug 31, 2020 at 19:53

Quentin

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