javascript - Express.js: send results after next() is called

Question

I have an app that enables users to define their own routes on the fly. And I still want to display an custom 404 message, so in the last middleware I do this:

app.use((req, res, next) => {
  // ...normal logic

  // check for user defined routes 
  next()
  // if res is not written, i.e. request not processed by user defined routes
  if (!res.writableEnded) {
    res.status(404).send(`not found!`)
  }
})

However, I got

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

. How can I implement this?

Answer 1

There is no way for middleware to know if a subsequent route handler is going to pick up the path.

You need to define it last, really last, after any user defined routes.

Then you need to not call next().