Middleware on notFound / onError

[na-trium-144]

What is the feature you are proposing?

Currently, onError and notFound only accept a single handler. It would be great if they could accept middlewares, similar to route definitions.

Use Case: running specific middleware (e.g., languageDetector) when rendering 404 page or error page, without registering it globally.
(For example you might not want to use languageDetector globally because it has the side effect of disabling the edge cache due to the Set-Cookie.)

const app = new Hono()

app.notFound(
  languageDetector(),
  (c) => {
    return c.html(render404Page(c.get("language")))
  }
)

app.onError(
  languageDetector(),
  (err, c) => {
    return c.html(renderErrorPage(c.get("language")))
  }
)

[MonisMS]

This would be useful for my case too (X). @na-trium-144 are you planning to implement this? If not, I'd be happy to take it

[na-trium-144]

I think this won't be a simple patch but requires consideration on changes to the internal structure, so I would wait for the maintainer.
Personally I don't mind who take this though.

[yusukebe]

To implement this, we have to add code to hono-base.ts. This is not as easy as some think.

[SnowingFox]

Opened a PR to address this: #5211

[usualoma]

While I think it would be overengineering if we viewed this as a solution intended solely for this issue, it might be worth considering something like the following.
#5229