Express Custom 404

app.js

var express = require('express'),
	mongoose = require('mongoose'),
	app;

app = module.exports = express.createServer();
app.configure(function() {
	app.set('views', __dirname + '/views');
	app.set('view engine', 'jade');
	app.use(express.bodyParser());
	app.use(express.methodOverride());
	app.use(express.cookieParser());
	app.use(express.session({ secret: 'meeper' }));
	app.use(require('stylus').middleware({ src: __dirname + '/public' }));
	app.use(app.router);
	app.use(express.static(__dirname + '/public'));
});
app.configure('development', function() { app.use(express.errorHandler({ dumpExceptions: true, showStack: true })); });
app.configure('production', function() { app.use(express.errorHandler()); });

app.error(function(err, req, res, next) {
	if (err instanceof NotFound) {
		res.render('404', { status: 404 });
	} else {
		res.render('500', { status: 500 });
	}
});
function NotFound(msg) {
	this.name = 'NotFound';
	Error.call(this, msg);
	Error.captureStackTrace(this, arguments.callee);
}
NotFound.prototype.__proto__ = Error.prototype;

// Routes
app.get('/', function(req, res) {
	res.render('index', {
		title: 'Express'
	});
});
app.get('/*', function(req, res) {
	throw new NotFound('Page not found.');
});

// Listen
// Only listen on $ node app.js
if (!module.parent) {
	app.listen(3000);
	console.log("Express server listening on port %d", app.address().port);
}

Hi, thanks for example. Why you are use prototypes ?