app.get() being called multiple times express

Question

I'm fairly new to node.js and trying to make a simple website which first asks the authentication and then redirects the user to a page.

so, what i do is that i create a middleware which listenes to every request made to my website.

what this middleware does that it checks if the the user is logged in with my website or not is yes then redirect to the requested page if not, then redirect to the login page, here is my code for that.

var express = require('express');
var app = express();

// middleware for using static files 
app.use('/public', express.static(__dirname + '/public')); // all the js files for check_before.html
app.use('/templates', express.static(__dirname + '/templates')); // here are css/js files for login.html 

// setting up views folder
app.set('views', __dirname + '/views'); // check_before.html is sitting here
app.engine('html', require('ejs').renderFile);
app.set('view engine', 'html');

app.use((req, res, next) => {
    res.render('check_before.html'); 
// here in the html I implement the logic using js files which are located in public folder.

    next();

});

// if not logged in , the user gets here
app.get('/login', (req, res, next) => {

    res.render('login.html')

});

// if logged in redirect to some page 
app.get('/welcome_page', (req, res) => {
    return 'welcome'

});

everything goes well untill the user hits the http://localhost:8000/login page (after the check if they are signed in or not) the page keeps on loading multiple times and it won't stop reloading. I have defined all the css, js files of login.html page in the templates folder which is loaded above the middleware by reffereing to this question Express middleware getting called many times. could that be a problem?

what could be the reason for this?

here is the error i'm getting in the console.

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

any guesses?

Edit1
I went through this question Error: Can't set headers after they are sent to the client , and i guess it concludes that setting headers explicitly could be problematic.

Could this be a reason? because in my logic if the user is not signed In, I'm just using window.location.replace('http://localhost:8000/login') to redirect the user to login page. should I use any another method for redirection?

Edit2

There are suggestions that i must write a middleware to check is the user is authenticated or not, and get a sort of flag for that, but as i've stated above that i'm implementing the logic in check_before.html(client side). so it won't be possible to use that.

Answer 1

I have two guesses:

1. You shouldn't call send (or any other function )after res.render.

2. Middleware to verify user is logged in should be something like this (applied only to routes you want to verify user)

Middleware should be something like this

const isAuthenticated = (req, res, next) => {
    if(req.isAuthenticated()) {
        next();
    } else {
        res.redirect('/');
    }
}    

app.get('/welcome_page', isAuthenticated, (req, res) => {
  return 'welcome'

});