Express server middleware execute twice? - node.js

Question

After trying to log some data on index file. I found my express server execute twice. Why do i get this error/bug?
Running Node 12.13.0 LTS, Express 4.17.1 and latest packages versions by the date of this post. I’ve tried on commenting some parts of code and always seem to end up running twice.
My app.js code:
const express = require('express');
const expressLayouts = require('express-ejs-layouts');
const path = require('path');
const bodyParser = require('body-parser');
const favicon = require('serve-favicon');
const app = express();
// ENV Variables
require('dotenv').config();
const PORT = process.env.PORT;
// Authentication Packages
const session = require('express-session');
const passport = require('passport');
// Middlewares
app.use(favicon(path.join(__dirname,'public','images','favicon.ico')));
app.use(bodyParser.urlencoded({extended: false}));
app.use(express.json());
app.use(session({
secret: 'GBR6N7^?5Xx-Ldqxf&*-Hv$',
resave: false,
saveUninitialized: false,
//cookie: { secure: true }
}));
app.use(passport.initialize());
app.use(passport.session());
// View Engine
app.use(expressLayouts);
app.use(express.static(path.join(__dirname, 'public')));
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'ejs');
// Routes
app.use('/', require('./routes/index'));
// Controllers
app.use('/profile', require('./routes/profile'));
app.use('/products', require('./routes/products'));
app.use('/bookmarks', require('./routes/bookmarks'));
// Catch 404
app.use((req, res) => {
res.render('pages/404');
});
app.listen(PORT, () => console.log(`Server started on port ${PORT}`));
and my index.js code:
const express = require('express');
const router = express.Router();
// Official pages
router.get('/', (req, res) => {
// THIS IS THE CODE I GET TWICE ON CONSOLE
console.log(req.user);
console.log(req.isAuthenticated());
// THIS IS THE CODE I GET TWICE ON CONSOLE
res.render('pages/index');
});
router.get('/about', (req, res) => {
res.render('pages/about');
});
router.get('/features', (req, res) => {
res.render('pages/features');
});
// Footer pages
router.get('/terms', (req, res) => {
res.render('pages/terms');
});
router.get('/refunds', (req, res) => {
res.render('pages/refunds');
});
module.exports = router;
Also i have those two functions on my profile.js (for passport.js):
passport.serializeUser((userId, done) => {
done(null, userId);
});
passport.deserializeUser((userId, done) => {
done(null, userId);
});
I get those results twice:
console.log(req.user);
console.log(req.isAuthenticated());
Output (Executed twice!):
undefined
false
undefined
false
and I expect one:
undefined
false

Answer 1

Due to how Express routing works, the path / will match / and /about and /favicon.ico etc. This is because Express supports not just endpoint routing but also path mounting. In other words, express supports things like this:
const app = express();
const route = express.Router();
route.get('/world', (req, res) => { res.send('hello') });
app.get('/hello', route); // mounts world to hello
// so we can access /hello/world
In order to support the feature above, express needs to interpret paths such as /hello to mean both /hello and /hello/anything/else. It needs to treat it as both the endpoint and potentially just a path leading to an endpoint.
This means that if you have a path:
app.get('/', () => {});
It will also trigger if the browser requests /favicon.ico. And browsers request favicon.ico to draw the tiny icon in the browser tab. This is why your route is triggered twice.
A few things to keep in mind when writing Express routes/controllers:
Make sure that the / path is last because otherwise it will also respond to requests to all your other paths.
If you are using express.static() make sure it is set up before the / path. Yes, the first rule above should also cover this but I see this issue often enough that it merits its own point.
In your case you can possibly fix the issue by simply creating a favicon.ico icon and saving it in the static (public) folder.