experimental: error callback for middleware

[dai-shi]

(I'm still not sure how onError works. Isn't it documented?)

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Name	Status	Preview	Updated (UTC)
waku	✅ Ready (Inspect)	Visit Preview	Mar 6, 2025 11:52pm

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[rmarscher]

There is documentation for onError in the parameters section of renderToReadableStream

onError: A callback that fires whenever there is a server error, whether recoverable or not. By default, this only calls console.error. If you override it to log crash reports, make sure that you still call console.error. You can also use it to adjust the status code before the shell is emitted.

We should add console.error(err); to onError. I see that is missing from Waku's implementation.

Each of these usage sections discuss how onError can be used:

• Logging crashes on the server
• Recovering from errors inside the shell

  If there is an error while generating the shell, both onError and your catch block will fire. Use onError for error reporting and use the catch block to send the fallback HTML document

• Recovering from errors outside the shell
• Setting the status code
• Handling different errors in different ways

Logging crashes on the server is the primary use case that I have for onError. I have server-side console.error hooked up to my error tracking - so really I won't have to add anything else if we add that. But... for people that want to do their own instrumentation and maybe ignore certain errors, it would be really great to register a callback to do that.

In those React usage docs, the methods for setting status code or making the onError error accessible to the request handler are interesting... but as soon as there is a suspense boundary, they won't work as expected because the onError will happen after the response starts streaming.

If I want to add error reporting for everything collected by ctx.unstable_errs - how can I do that? Create Waku middleware and pipe the ctx.body through my own transform that reports new ctx.unstable_errs as they become available and then one final check when the stream is complete?

[rmarscher]

Isn't it documented?

There is documentation for onError in the parameters section of renderToReadableStream

Sorry... I realize I was confusing react-dom/server renderToReadableStream with react-server-dom-webpack renderToReadableStream. I wonder if they are intended to work in a similar way?

[dai-shi]

Yeah, I've read the react-dom docs once before.

We should add console.error(err); to onError. I see that is missing from Waku's implementation.

Nice catch.

If I want to add error reporting for everything collected by ctx.unstable_errs - how can I do that? Create Waku middleware and pipe the ctx.body through my own transform that reports new ctx.unstable_errs as they become available and then one final check when the stream is complete?

Yes, my expectation is write your own middleware. I haven't tried anything and not sure if it works as you expect. Feedback is welcome.

[rmarscher]

LGTM. I think it is ready to merge unless you aren't sure that you like how the callbacks are being passed through.

[dai-shi]

Thanks for the review. I'm not 100% confident, but as it's marked as unstable_, I'm fine to merge.