In your ensureAuthenticated
method save the return url in the session like this:
...
req.session.returnTo = req.originalUrl;
res.redirect('/login');
...
Then you can update your passport.authenticate route to something like:
app.get('/auth/google/return', passport.authenticate('google'), function(req, res) {
res.redirect(req.session.returnTo || "https://stackoverflow.com/");
delete req.session.returnTo;
});