How to keep track when session expires in express session

// Access the session as req.session
app.get('/', (req,res) => {
    console.log(req.session);
    if (req.session.viewCount) {
        req.session.viewCount++;
        viewCount++;
    }
    else {
        req.session.viewCount = 1;
        viewCount = 1;
    }
    console.log(`${req.session.id}`);
    res.send(`<h1> Hello world...view count(s): ${req.session.viewCount}`);
});