怪异行为的Javascript实例

Question

Why does it returns false?

function f(){ return f; }
new f() instanceof f; // Prints false instead of true 

As I understand, in this particular case instanceof should check something like:

newObj.__proto__ === f.prototype .

And newObj.__proto__ should be set automatically on new f() call.

Answer 1

Constructor functions implicitly return this . this is an instance of the constructor function and so, if you return this the function will evaluate to true.

You do not return this . You return f which is not an instance of the constructor, thus the expression returns false .