Reset an Inherited Constructor Property

[iuryCh]

In this code bellow:

function Animal() { }
function Bird() { }
function Dog() { }

Bird.prototype = Object.create(Animal.prototype);
Dog.prototype = Object.create(Animal.prototype);

// Only change code below this line



let duck = new Bird();
let beagle = new Dog();

console.log(Dog.prototype.isPrototypeOf(duck))

why the log is false if such as Dog and Bird were instanceOf Animal, why only in Bird.prototype the log returns true ?

[gabei]

There are two prototype chains stemming from the parent Animal prototype.

Animal —> Bird —> duck
Animal —> Dog —> beagle

When the isPrototypeof() method is called on Dog , it searches its prototype chain and duck is nowhere to be found. That is not true when Dog is replaced by Bird because Bird and duck are a part of the same chain.

Think of it as:
Same Parent —> Child —> Grandchild
Same Parent —> Different Child —> Different Grandchild

They share some blood, but there are differences between the cousins (grandchildren).

EDIT:
It is also worth mentioning that isPrototypeOf() works on objects, not prototypes themselves.
Animal.prototype.isPrototypeOf(Bird) will return false.
Animal.prototype.isPrototypeOf(duck) will return true.