Compression middleware changes behavior of undefined statusText in writeHead #254

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1. Siretu

   added

   bug

   on Sep 5, 2025
2. Siretu

   added the

   Bug

   issue type on Sep 5, 2025
3. Siretu commented on Sep 5, 2025

   

   Siretu

   on Sep 5, 2025

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   Some more context on this. I figured this out after hours of chasing down a bug in our codebase. We're using the Vercel AI SDK (https://github.com/vercel/ai) and we noticed that the appropriate headers are not added unless we add a statusText to pipeUIMessageStreamToResponse.

   That method in return ends up calling into https://github.com/vercel/ai/blob/main/packages/ai/src/util/write-to-server-response.ts#L19 where it calls response.writeHead(status ?? 200, statusText, headers) which will fail if the statusText is undefined and you're using compression middleware.
4. abhii734

   added a commit that references this issue on Mar 10, 2026

   fix: resolve expressjs#254 - preserve headers set via writeHead

   ...

   This issue will close once commit a4e80f0 is merged into the 'master' branch.

   a4e80f0
5. Vansh1811

   added a commit that references this issue on Mar 24, 2026

   fix: preserve writeHead statusText and headers when compression is ac…

   ...

   This issue will close once commit 484e598 is merged into the 'master' branch.

   484e598
6. mahmoodhamdi

   added a commit that references this issue on Mar 29, 2026

   fix: handle undefined statusText in writeHead arguments

   ...

   2f36fd9
7. mahmoodhamdi commented on Mar 29, 2026

   

   mahmoodhamdi

   on Mar 29, 2026

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   I traced this bug to the on-headers package, which is used by compression to hook into writeHead. The issue is in on-headers's setWriteHeadHeaders function — it determines the header index based only on whether arguments[1] is a string, without accounting for the case where 3 arguments are passed with a non-string second argument.

   When writeHead(200, undefined, headers) is called:

   1. typeof undefined !== 'string', so headerIndex = 1
   2. headers = arguments[1] → undefined
   3. The actual headers at arguments[2] are never read

   Native Node.js writeHead handles this correctly because it checks the total argument count.

   I've opened a fix at jshttp/on-headers#49. Once that's released, this issue should be resolved without any changes needed in compression itself.