get-programming-with-nodejs | Code samples for Get Programming | Runtime Evironment library

When an exception occurs, express will call the next() function behind the scenes. Next calls the subsequent middleware function.

When one of your HTTP methods returns successfully, the error middleware functions are never reached.

You can read here for more details.

Edit:

So this is a bit tricky. The pageNotFoundError does not actually occur after an exception is thrown. Rather, it is encountered after none of the get/post routes match. So if a request was made to '/nonexistent-path/*', next would be called until pageNotFoundError is hit.

However, if an actual exception is thrown, pageNotFoundError would not be called because it does not accept error as an argument in its method signature. Only internalServerError will be called, because it's the only middleware that accepts error as an argument.