Express Application: How do I handle 404 responses?

Source: https://expressjs.com/en/starter/faq.html

In Express, 404 responses are not the result of an error, so the error-handler middleware will not capture them. This behavior is because a 404 response simply indicates the absence of additional work to do; in other words, Express has executed all middleware functions and routes, and found that none of them responded. All you need to do is add a middleware function at the very bottom of the stack (below all other functions) to handle a 404 response:

app.use(function (req, res, next) {
  res.status(404).send("Sorry can't find that!")
})

Add routes dynamically at runtime on an instance of express.Router() so the routes are not superseded by a middleware function.