req.app

This property holds a reference to the instance of the Express application that is using the middleware.

If you follow the pattern in which you create a module that just exports a middleware function and require() it in your main file, then the middleware can access the Express instance via req.app

For example:

app.get('/viewdirectory', require('./mymiddleware.js'));

module.exports = (req, res) => {
  res.send(`The views directory is ${req.app.get('views')}`);
};