Express middleware error handler - node.js

Question

Following a generated template for express. In the app.js there is a following snippet
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);
});
// error handler
app.use(function(err, req, res, next) {
// set locals, only providing error in development
res.locals.message = err.message;
res.locals.error = req.app.get('env') === 'development' ? err : {};
// render the error page
res.status(err.status || 500);
res.render('error');
});
Per my understanding, the middleware will run in order from app.use('/users', users) to 404 handler to error handler. My question is how will it be possible to reach error handler and execute this line res.status(err.status || 500); ? Wont every failed request be passed through 404 handler first and therefor getting the status code of 404? Please let me know if I am missing something! Thank you

Answer 1

No, it won't. If you look at these event handlers declarations you'll see that error handler for unhandled error, has an additional err parameter:
app.use(function(req, res, next) {
app.use(function(err, req, res, next) {
Error-handling middleware always takes four arguments. You must provide four arguments to identify it as an error-handling middleware function. Even if you don’t need to use the next object, you must specify it to maintain the signature. Otherwise, the next object will be interpreted as regular middleware and will fail to handle errors. For details about error-handling middleware.
So, when the route is not found, the last declared middleware is calling, it's 404 error handler.
But when you call next with error: next(err) or your code throws an error, the last error handler is calling.

Answer 2

System error should be handled before 404
app.use('/users', users);
// error handler
app.use(function(err, req, res, next) {
// No routes handled the request and no system error, that means 404 issue.
// Forward to next middleware to handle it.
if (!err) return next();
// set locals, only providing error in development
res.locals.message = err.message;
res.locals.error = req.app.get('env') === 'development' ? err : {};
// render the error page
res.status(err.status || 500);
res.render('error');
});
// catch 404. 404 should be consider as a default behavior, not a system error.
app.use(function(req, res, next) {
res.status(404);
res.render('Not Found');
});

Answer 3

Wont every failed request be passed through 404 handler first and therefor getting the status code of 404?
No, the 404 route is just a standard middleware and usually wired up last, meaning if no other routes handle the request they will eventually hit the 404.
The 500 is a special type of middleware, you'll notice it has 4 parameters (first one being an error param). This middleware is invoked as soon as you call next with an error, or any sort of data for that matter.
See the docs

Answer 4

Something to note for anyone else that stumbles around this post.
Your last middleware to be invoked (if none gets invoked) - will be 404, it doesn't take an err as its middleware parameter. Hence, you can't/shouldn't try to catch the 404 before any other middleware. So your last middleware should be without the err parameter like the example below and of course, should be at the bottom of your application stack.
//at the botton of your app.js
// catch 404.
//no err parameter
app.use(function(req, res, next) {
res.render("404Page"{
url: req.url
});
});
If you want to catch your errors before it goes to this handler use a (err, req, res, next) before this middleware.