I found this example quite helpful:
https://github.com/visionmedia/express/blob/master/examples/error-pages/index.js
So it is actually this part:
// "app.router" positions our routes
// above the middleware defined below,
// this means that Express will attempt
// to match & call routes _before_ continuing
// on, at which point we assume it's a 404 because
// no route has handled the request.
app.use(app.router);
// Since this is the last non-error-handling
// middleware use()d, we assume 404, as nothing else
// responded.
// $ curl http://localhost:3000/notfound
// $ curl http://localhost:3000/notfound -H "Accept: application/json"
// $ curl http://localhost:3000/notfound -H "Accept: text/plain"
app.use(function(req, res, next){
res.status(404);
// respond with html page
if (req.accepts('html')) {
res.render('404', { url: req.url });
return;
}
// respond with json
if (req.accepts('json')) {
res.send({ error: 'Not found' });
return;
}
// default to plain-text. send()
res.type('txt').send('Not found');
});