Question

Instead of string it is object

String.prototype.foo = function () { return this; };
typeof "hello".foo() // object ???
"hello".foo().toString(); //hello 

it should return string instead i guess.

Answer 1

No. A real string ("hello", 'booya') is a primitive value – it doesn’t have any functions or anything. It’s just a value.

When you do "string".foo, it turns into this:

Object("string").foo

Inside of foo, this points towards Object("string"), not the primitive value. Doing Object("string") turns it into an object, so typeof object === 'object'.

If you want the “underlying” primitive, call valueOf:

String.prototype.foo = function () {
    return typeof this.valueOf();
}
"meep".foo(); //string