We've seen two kinds of middleware functions so far. In one, the first argument is the handler function. In the other, the first argument is a string containing a URL snippet, and the second argument is the handler function.
What's actually going on is app.use has an optional first argument: the path the middleware is mounted on. The path is a pattern match against the request URL, and the given function is triggered if the URL matches the pattern. There's even a method to supply named parameters in the URL:
app.use('/user/profile/:id', function(req, res, next) { userProfiles.lookup(req.params.id, (err, profile) => { if (err) return next(err); // do something with the profile // Such as display it to the user ...
