How does the Express.js 4 Router let things reach the 404 page

Question

The express generator generates the following code in the app.js page:

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

// catch 404 and forward to error handler
app.use(function(req, res, next) {
    var err = new Error('Not Found');
    err.status = 404;
    next(err);
});

However, in the documentation, it says

Since path defaults to "/", middleware mounted without a path will be executed for every request to the app.

And gives this example:

// this middleware will not allow the request to go beyond it
app.use(function(req, res, next) {
  res.send('Hello World');
})

// requests will never reach this route
app.get('/', function (req, res) {
  res.send('Welcome');
})

So, how could the 404 page ever be reached (or the /users route for that matter) if nothing can get passed the app.use('/', routes) line?

Answer 1

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

// catch 404 and forward to error handler
app.use(function(req, res, next) {
    var err = new Error('Not Found');
    err.status = 404;
    next(err);
});

Let's say your app.js has the above code (straight from the generator) and your server receives a request to /foo. First, your app.use('/', routes); middleware gets to check if it can handle the request, because it is defined first in your middleware stack. If the routes object contains a handler for /foo, and that handler calls res.send(), then your server is done handling the request, and the rest of your middleware doesn't do anything. However, if the handler for /foo calls next() instead of res.send(), or if the routes object does not contain a handler for /foo at all, then we continue going down the list of middleware.

The app.use('/users', users); middleware does not execute, since the request is not to /users or to /users/*.

Finally, the 404 middleware executes last, since it was defined last. Since it was defined with no route, it executes for all requests that get past the first two middleware.