How to implement a ban on the transition to url addresses if the user is not authorized?

Question

Node.js

I want to implement the backend part, but I don’t understand how to implement a ban on going to url addresses. That is, there is a url: localhost:3000/log_in, if the user is in the database, then there is a transition to localhost:3000/user, but you can also go to localhost:3000/user without authorization. How to fix it?