How to use the middleware to check the authorization before entering each route in express?

As long as

app.use(authChecker);

is before

app.use(app.router);

it will get called for every request. However, you will get the “too many redirects” because it is being called for ALL ROUTES, including /auth. So in order to get around this, I would suggest modifying the function to something like:

function authChecker(req, res, next) {
    if (req.session.auth || req.path==='/auth') {
        next();
    } else {
       res.redirect("/auth");
    }
}

This way you won’t redirect for the auth url as well.