我发现这个例子很有帮助：
https://github.com/visionmedia/express/blob/master/examples/error-
pages/index.js
所以实际上是这部分：
// "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');});
