Can not get data of current logged in user

I am making some test changes directly into plugin. I use jQuery ajax to get response from one of the methods inside plugin. I am logged in as subscriber in application.
I can’t get data of logged user in any method (get_current_user_id returns 0, is_user_logged_in returns false). Logged in or not logged in, I always get 0 or false as result.

Then I output result of those same methods inside getUserList (where you even check if user is logged in), and I get same results; can’t get logged in user data, it’s always 0 or false.

How to get logged in user data? How to fix this issue?