javascript中的instanceof运算符令人困惑的行为

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

1. Why is foo not an instance of Foo ?
2. Why is this an instance of Foo ?

Answer 1

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