isLoggedIn.js

module.exports = function isLoggedIn(req, res, next) {
  if (req.user) {
    next();
  } else {
    // return unauthorized
    res.send(401, "Unauthorized");
  }
};

server.js

const express = require("express");

const setCurrentUser = require("./middleware/setCurrentUser.js");
const isLoggedIn = require("./middleware/isLoggedIn.js");

const app = express();

app.use(setCurrentUser);

app.get("/users", isLoggedIn, function(req, res) {
  // ...
});