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Standardize on error codes for errors express throws #6951
Description
Capturing an idea we've talked about offline.
It would be great to move to using (and documenting) explicit error codes for errors that express itself generates, to allow folks to use them as a contract if they wish. That would free us up from having to worry that someone is matching on error messages themselves, and therefore making error messages part of the semver contract with users.
to jog memories, this came up recently when discussing #6756
We don't have all that many tbh, but here's a table I had claude make w/ sites where we generate an error:
| File | Line | Error | Proposed Code |
|---|---|---|---|
| response.js | 67 | Invalid status code: ... Status code must be an integer. |
ERR_INVALID_STATUS_CODE |
| response.js | 71 | Invalid status code: ... must be greater than 99 and less than 1000. |
ERR_STATUS_CODE_OUT_OF_RANGE |
| response.js | 386 | path argument is required to res.sendFile |
ERR_MISSING_ARG |
| response.js | 390 | path must be a string to res.sendFile |
ERR_INVALID_ARG_TYPE |
| response.js | 400 | path must be absolute or specify root to res.sendFile |
ERR_INVALID_PATH |
| response.js | 681 | Content-Type cannot be set to an Array |
ERR_INVALID_ARG_TYPE |
| response.js | 755 | cookieParser("secret") required for signed cookies |
ERR_COOKIE_SECRET_REQUIRED |
| response.js | 936 | Request aborted |
ERR_REQUEST_ABORTED |
| response.js | 946 | EISDIR, read |
ERR_EISDIR |
| request.js | 66 | name argument is required to req.get |
ERR_MISSING_ARG |
| request.js | 70 | name must be a string to req.get |
ERR_INVALID_ARG_TYPE |
| utils.js | 148 | unknown value for etag function |
ERR_INVALID_OPTION |
| utils.js | 180 | unknown value for query parser function |
ERR_INVALID_OPTION |
| view.js | 61 | No default engine was specified... |
ERR_NO_VIEW_ENGINE |
| view.js | 84 | Module "..." does not provide a view engine. |
ERR_INVALID_VIEW_ENGINE |
| application.js | 213 | app.use() requires a middleware function |
ERR_INVALID_MIDDLEWARE |
| application.js | 296 | callback function required |
ERR_MISSING_CALLBACK |
| application.js | 562 | Failed to lookup view "..." |
ERR_VIEW_NOT_FOUND |
Fastify does this quite well already: https://fastify.dev/docs/latest/Reference/Errors/
Activity
efekrskl commentedon Dec 17, 2025
We should definitely do this. I was recently going through the codebase, and our errors messages are really inconsistent 😅
Should we also brand our errors like Fastify does? (e.g FST_xxx)
sanwalsulehri commentedon Dec 18, 2025
byteforge38 commentedon Jan 9, 2026
Hi @jonchurch , can you assign this issue to me?
feat: add error codes for Express-thrown errors (expressjs#6951)
jonchurch commentedon Jan 10, 2026
This is in the discussion phase currently so Im not planning on assigning it to anyone outside the core team currently 🙇
Delapouite commentedon Feb 9, 2026
Related: a new TC39 proposal has recently been created to officialize the
error.codeproperty:https://github.com/jasnell/proposal-error-code-property
feat: brand error codes with EXPRESS_ERR_ prefix and document them
feat: add express.isExpressError() helper