expression session how does it work - node.js

Question

I am reading an article about nodejs express module and sessions here
https://www.codementor.io/emjay/how-to-build-a-simple-session-based-authentication-system-with-nodejs-from-scratch-6vn67mcy3
I am confused on this portion of the code
app.use((req, res, next) => {
if (req.cookies.user_sid && !req.session.user) {
res.clearCookie('user_sid');
}
next();
});
from where did the req object get the user property ?
If we look at the article the session object is created like
app.use(session({
key: 'user_sid',
secret: 'somerandonstuffs',
resave: false,
saveUninitialized: false,
cookie: {
expires: 600000
}
So how does this even work
if (req.cookies.user_sid && !req.session.user)
how does the req.cookies.user_id shouldnt it be req.cookies.key ?
How does req.session.user work ?
user is not even a property of the session object no ?

Answer 1

Look further down in the article. There are several instances where req.user is an lvalue. In fact it's entirely up to you to assign to it:
The article posits this code block:
Signup:
// route for user signup
app.route('/signup')
.get(sessionChecker, (req, res) => {
res.sendFile(__dirname + '/public/signup.html');
})
.post((req, res) => {
User.create({
username: req.body.username,
email: req.body.email,
password: req.body.password
})
.then(user => {
/*
* The user just signed up, so let's sign him in
*/
req.session.user = user.dataValues;
res.redirect('/dashboard');
})
.catch(error => {
res.redirect('/signup');
});
});
Logging In:
// route for user Login
app.route('/login')
.get(sessionChecker, (req, res) => {
res.sendFile(__dirname + '/public/login.html');
})
.post((req, res) => {
var username = req.body.username,
password = req.body.password;
User.findOne({ where: { username: username } }).then(function (user) {
if (!user) {
res.redirect('/login');
} else if (!user.validPassword(password)) {
res.redirect('/login');
} else {
/*
* The user just logged in so lets sessionize them
*/
req.session.user = user.dataValues;
res.redirect('/dashboard');
}
});
});