Express: how do I let the template view know that I'm logged in?

Question

I need to render a logout button and for that it needs a condition based on the login state. The problem is: I'm using a template engine, which has the navigation bar in the layout.pug, so I can't access just the bar, also I don't want to repeat the same code to check the session on all pages. After search a little I decided to use res.locals

At first I simply tried using the login route to load a boolean into the res.locals.isloggedIn variable, but it didn't work at all. So after thinking a lot I came to this conclusion:`

app.use(function (
  req: express.Request,
  res: express.Response,
  next: express.NextFunction
): void {
  res.locals.isLoggedIn = Boolean(req.session.token);
  next();
});

I'm going to make this look better later, I promise

So that works, great, but then why would I ask about it? Because it left me with more questions than answers.

First, why does it work here and not within a route? I read that it should be loaded before any route, but why? Another thing, wouldn't this sort of thing make all requests share the same state? I know it doesn't work that way, but it's something else that I don't really know why it happens.

And finally, and most importantly, is that really the right way to solve this problem? You see, I made a middleware to allow access to some pages, but why waste time on this if this solution was apparently simpler?

Here is the middleware:

export function ensureAuthenticated(
  req: Request,
  res: Response,
  next: NextFunction
) {
  const token = req.session.token;
  const loginPath = "/auth/login";

  if (!token) {
    req.flash("error", "Please log in");
    res.redirect(loginPath);
    return;
  }

  try {
    const decoded = jwt.verify(token, process.env.JWT_SECRET as string) as any;

    models.user
      .findById(decoded.id)
      .then((user) => {
        if (!user) {
          req.flash("error", "User not found");
          res.redirect(loginPath);
          return;
        }

        req.user = user;
        next();
      })
      .catch((err) => {
        console.log(err);
        req.flash("error", "Something went wrong");
        res.redirect(loginPath);
      });
  } 
  
  catch (err) {
    req.flash("error", "Please log in");
    res.redirect(loginPath);
  }
}

small note: normally when I ask this kind of question people think I haven't studied. I took a ten hour freecodecamp course yesterday so I decided to do something myself, unfortunately it didn't teach me how to log out. please don't judge me for asking too much, i can assure you that if i didn't have other things to do i would spend all the time in the world looking for an answer on my own. thank you for understanding and peace