how to check constructor of instance

Question

i created new instance('instance1' and 'instance2') using 'new' keyword. just like this.

1.with 'Child.prototype.constructor = Child'

function Parent() {

}

function Child() {
  Parent.call(this);
}

Child.prototype = new Parent();
Child.prototype.constructor = Child;

var instance1 = new Child();

2.without 'Child.prototype.constructor = Child'

function Parent() {

}

function Child() {
  Parent.call(this);
}

Child.prototype = new Parent();

var instance2 = new Child();

And i can check the constructor of instance using 'instanceof' keyword.

instance1 instanceof Child  // true
instance1 instanceof Parent // true

this result is make sense, because i clearly wrote 'Child.prototype.constructor = Child;'. so instanceof keyword can find both constructor. BUT

instance2 instanceof Child  // true
instance2 instanceof Parent // true

. but this result is not make sense for me. i expected

instance2 instanceof Child  // false

because i did not write 'Child.prototype.constructor = Child;'.

Why???

Answer 1

instanceof operator looks up if the Constructor.prototype object is present in the Prototype chain ( __proto__ ) of the object being tested.

So in your example:

function Parent() {}

function Child() {
  Parent.call(this);
}

Child.prototype = new Parent();

var instance2 = new Child();    

Since instance2 is constructed from Child() constructor, the __proto__ of instance2 points to the prototype object of Child() constructor ie Child.prototype .

When you test for:

instance2 instanceof Child

instanceof operator will look if Child.prototype object is present in the prototype chain of instance2 , which results in true since instance2 was constructed from Child() constructor.
In other words:

instance2.__proto__ === Child.prototype

Taking your example of second case:

instance2 instanceof Parent

Here also instance2 's prototype chain ie ( __proto__ ) has Parent.prototype object it will evaluate to true. ie

instance2.__proto__.__proto__ === Parent.prototype

Final Note:

instanceof operator works very much like the above conditional checks to test whether an object is an instance of a constructor. The constructor property present on Constructor function's prototype object is never used by instanceof operator while testing.

Hope this helps.