The "instanceof" operator in JavaScript

In this blog you will learn about the "instanceof" operator in JavaScript.

The "instanceof" operator in JavaScript

 

The instanceof allows to check if the object has been created by the given constructor or not:

 

For example:

function Abhi() { }
var abhijeet = new Abhi
alert(abhijeet instanceof Abhi) // true

It follows the __proto__ chain instead.

 

The logic behind obj instanceof F:

 

1. Get obj.__proto__ 

2. Compare obj.__proto__ against F.prototype

3. If no match then set temporarily obj = obj.__proto__ and repeat step 2 until either match is found or the chain ends.

 

In the above example, the match is found at the first step,

 

because: abhijeet.__proto__ == Abhi.prototype

 

Another example:

function Abhi() { }
var abhijeet = new Abhi
alert(abhijeet instanceof Object) // true

Here we can see that the match is found at abhijeet.__proto__.__proto__ == Object.prototype.

 

Note that the comparison only uses __proto__ and prototype, the function object itself isn’t involved.