next()

I think you are referring to this section of the code in app.js

/* ERROR HANDLERS */
/* 404 handler to catch undefined or non-existent route requests */ 
app.use((req, res, next) => {
  console.log('404 error handler called');

  /* TODO 1: Send a response to the client
    - Set the response status to 404
    - Render the 'not-found' view
  */ 
});

Which post-instructions converts to this -

/* ERROR HANDLERS */
/* 404 handler to catch undefined or non-existent route requests */ 
app.use((req, res, next) => {
  console.log('404 error handler called');

  res.status = 404;
  res.render('not-found');

});

For some reason the app.use( ( [the one with 3 params, not 4] ) ... seemed to ALWAYS be run (after the quotes/router part if applicable), regardless whether the user actually triggered an invalid url. I don't know what was wrong, but now it's not doing that anymore (thus I had to edit and update my response hehe).

Anyways, I think we can use next(err) instead of rendering. But you will also get the stack trace message in the error page. For instance, localhost:3000/asdkahdkljah would normally get the not-found.pug page without the stack trace message, but with the next(err) it gets caught in the Global Error handler and it will include the stack trace message.