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 
          res.send(profile.display()); 
    }); 
}); 

This path specification has a pattern, :id, and the value will land in req.params.id. In this example, we're suggesting a user profiles service, and that for this URL we want to display information about the named user.

Another way to use a middleware function is on a specific HTTP request method. With app.use, any request will be matched, but in truth, GET requests are supposed to behave differently to POST requests. You call app.METHOD where METHOD matches one of the HTTP request verbs. That is, app.get matches the GET method, app.post matches POST, and so on.

Finally, we get to the router object. This is a kind of middleware used explicitly for routing requests based on their URL. Take a look at routes/users.js:

var express = require('express'); 
var router = express.Router(); 
router.get('/', function(req, res, next) { 
  res.send('respond with a resource'); 
}); 
module.exports = router; 

We have a module whose exports object is a router. This router has only one route, but it can have any number of routes you think is appropriate.

Back in app.js, this is added as follows:

app.use('/users', users); 

All the functions we discussed for the app object apply to the router object. If the request matches, the router is given the request for its own chain of processing functions. An important detail is that the request URL prefix is stripped when the request is passed to the router instance.

You'll notice that the router.get in users.js matches '/' and that this router is mounted on '/users'. In effect, that router.get matches /users as well, but because the prefix was stripped, it specifies '/' instead. This means a router can be mounted on different path prefixes without having to change the router implementation.