[gayanMatch]

Errors thrown by Express now carry a stable code property so error handlers can branch on the code instead of matching message text. Implements the proposal in #6951. Original implementation by @gayanMatch.

What's included

• lib/errors.js with one constant per error and factory helpers used at every throw site
• express.errorCodes, a frozen object with all 15 codes
• express.isExpressError(err), true only for errors carrying one of these codes
• Tests for every code and for the helper
• History.md entry listing each code and what throws it

Decisions

• Codes are prefixed EXPRESS_ERR_ so they do not collide with Node.js's ERR_* namespace
• Error types and messages are unchanged: instanceof TypeError and RangeError checks keep working, so this is semver-minor
• No custom error class. isExpressError() covers the "did Express throw this" question without changing instanceof behaviour

Follow-ups

• [6.x] res.send() no longer serializes non-string bodies as JSON #7553 (res.send throws for non-string bodies) and feat(res.redirect): add validation for url and status arguments #6404 (res.redirect validation) add new throw sites and will need codes once they land
• The two file-serving errors from the original table, request aborted and EISDIR, already carry Node-style codes and are intentionally untouched
• Documenting the codes on expressjs.com

Refs #6951

[UlisesGascon]

If we want to wait for express@6 also is an option

[UlisesGascon]

@gayanMatch awesome work with this PR!

I just pushed a72565c to include a prefix and 685d941 to include the express.isExpressError() helper and solved conflicts as the PR was a bit old.

[krzysdz]

I have two questions after a quick look at the code:

1. There are 3 functions that are identical, except for the specific Error subclass. Are there any advantages to this solution over passing the type as an argument¹?

   function createError(type, code, message) {
       const error = new type(message);
       // ...

2. Error.captureStackTrace is not standard and originates from V8, but according to browser-compat-data it should be supported by all platforms that can run Express. Does it make sense to guard it behind if?

By the way #6951 is tagged as 6.x and this PR as 5.x .

Footnotes

1. in languages with better type systems I'd probably do this as a template parameter, but this is JS... ↩