How can I display either a login or logout button in navbar?

My Problem

I’m using Express/Passport to create a login system.

My problem is very simple but every time I try something I get undefined errors. I’m trying to figure out how I can pass my req.body.username to my navbar so that it:

1. Displays the username
2. Display either the logout or the register/login buttons, depending if the user is logged in or not.

I know that I will need to use an if statement inside my navbar ejs only like I said, every time I try something like:

  <%if (req.body.username) { %>
  <!--Logout-->
  <li><a href="/users/logout">Logout</a></li>
  <% }else{ %>
  <!--Sign In/Register-->
  <li><a href="/users/register">Register</a></li>
  <li><a href="/users/login">Login</a></li>
  <%}%>

… I get undefined errors.

Please help me understand what I’m doing wrong and how I can fix my code.

My Code

app.js

// setup session
app.use(
  session({
    secret: process.env.SECRET,
    resave: false,
    saveUninitialized: false,
  })
);

// initialize passport
app.use(passport.initialize());

// use passport to deal with session
app.use(passport.session());

app.use(function (req, res, next) {
  res.locals.isAuthenticated = req.isAuthenticated();
  next();
});

userController.js

exports.register = function (req, res, next) {
  if (req.isAuthenticated()) {
    res.redirect("/");
  } else {
    res.render("register");
  }
};

exports.login = function (req, res, next) {
  if (req.isAuthenticated()) {
    res.redirect("/");
  } else {
    res.render("login");
  }
};

exports.register_account = async (req, res) => {
  try {
    const registerUser = await User.register(
      { username: req.body.username },
      req.body.password
    );
    if (registerUser) {
      passport.authenticate("local")(req, res, function () {
        res.redirect("/");
      });
    } else {
      res.redirect("/register");
    }
  } catch (err) {
    res.send(err);
  }
};

exports.login_account = (req, res) => {
  // Create new user object
  const user = new User({
    username: req.body.username,
    password: req.body.password,
  });

  // Check if credentials are correct
  req.login(user, (err) => {
    if (err) {
      console.log(err);
    } else {
      passport.authenticate("local")(req, res, function () {
        res.redirect("/");
      });
    }
  });
};

exports.logout = function (req, res, next) {
  req.logout(function (err) {
    if (err) {
      return next(err);
    }
    res.redirect("/");
  });
};

users.js

/// USER ROUTE ///
router.get("/register", user_controller.register);

router.get("/login", user_controller.login);

// register user in DB
router.post("/register", user_controller.register_account);

// login user
router.post("/login", user_controller.login_account);

// Logout user
router.get("/logout", user_controller.logout);

module.exports = router;

navbar.js

<ul class="navbar">
  <p>USER PANEL</p>

  <!--Logout-->
  <li><a href="/users/logout">Logout</a></li>
  <!--Sign In/Register-->

  <li><a href="/users/login">Login</a></li>
  <li><a href="/users/register">Register</a></li>
</ul>