I'm trying to understand the difference beetween functions arrow and functions in javascript, with the help of some website, and from what I seemed to understand, it's the this binding that change in the case So I understood that in function arrow, this is binded to the closest non-arrow function parent this and in regular function, this is binded from the closest this

So with that in mind, I was doing some test in the browser console and came up with this:

test = {
    test_name: "OK",
    test_func: function() {
        console.log(this.test_name);
        (() => {
            console.log(this.test_name);
            function b() {
                console.log(this.test_name);
            }
            b();
        })();
    }
}
test.test_func();

I was excepting to get

OK
OK
OK

but what I get is

OK
OK
undefined

And I can't understand why since I get 'OK' in the arrow function, why don't I get 'OK' in the b() function?