confusing behavior of instanceof operator in javascript

Question

Take a look at this code:

function Foo () {
    console.log(this instanceof Foo);
    return { name: "nitesh" }; 
}

foo = new Foo(); //true 
console.log(foo instanceof Foo) //false

Why is foo not an instance of Foo?
Why is this an instance of Foo?

You are basically doing { name: "nitesh" } instanceof Foo. — Felix Kling
– Felix Kling, Commented Aug 9, 2012 at 14:20

gen_Eric · Accepted Answer · 2012-08-09 14:16:54Z

11

In your Foo function, you are returning an object. This is what foo gets set to. That is not a Foo object, it's just a "normal" object.

Try it this way:

function Foo(){
    console.log(this instanceof Foo);
    this.name = "nitesh";
}

var foo = new Foo(); //true 
console.log(foo instanceof Foo) //true

answered Aug 9, 2012 at 14:16

gen_Eric

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2 Comments

nitesh sharma Over a year ago

thanks for explanation yet i am still confused how come is this an instance of Foo?

gen_Eric Over a year ago

@niteshsharma: That's how new works. From the docs: The constructor function is called with the specified arguments and 'this' bound to the newly created object.

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