Express Login With Redirect Using Node.js

I'm trying to implement a basic login mechanism based on this answer: http://stackoverflow.com/a/8003291/5111904

In my backend I'm handling the login post request like this:

app.post('/login', function (req, res) {
    console.log(req.body);
    if (req.body.user === 'normal' && req.body.password === '12345') {
        req.session.user_id = 0; // This is failing (req.session is undefined)
        res.redirect('/index');
    } else {
        res.send('Bad user/pass');
    }
});

The server is using https:

server = https.createServer(https_options, app).listen(PORT, HOST);

When the client is clicking the login button this code is getting executed:

 function postLogin(){
    var url = "/login";
    var xhr = new XMLHttpRequest();
    var data = {
        user: userInput.value,
        password: passwordInput.value
    };

    xhr.open("POST", url, true);
    xhr.onreadystatechange = function (oEvent) {
        if(xhr.readyState === 4){
            // Checking status codes
            if(xhr.status === 200){
                onSuccess(xhr.responseText, xhr.responseType);
            }
            else{
                console.log(xhr.status);
                onError();
            }

        }
    }
    xhr.setRequestHeader("Content-type", "application/json; charset=UTF-8");
    console.log("Sending this data: " + JSON.stringify(data));
    xhr.send(JSON.stringify(data));
}

After the login the user should get redirected to the index page:

app.get('/index', (request, response) => {
    response.render('main', {});
})

So I got this issues:

• In the first code snippet req.session is undefined
• In the secod snippet the value of (xhr.responseText) evaluates to the html of the index page (where the user should be redirected)
• How do I redirect the user to the index page in a proper way?

There will be only one valid user so this code is not intended to be used by many people and it should only provide a basic type of security.

via Matthias Herrmann