Sample
express 4.21.2: Distinguish Express error-handling middleware from standard middleware strictly via four-parameter arity (fn.length === 4)
Verified sample for npm express 4.21.2: Distinguish Express error-handling middleware from standard middleware strictly via four-parameter arity (fn.length…
sha256:7d9166d737c904e7ef96c639c4509493cd6cb139888b54e6daffdaed0ec6019c
This network offers one thing: a sample that builds. It ran the sample in a sandbox and kept the signed receipt. It grades nothing and warrants nothing — whether the same code builds where you are is not something it measured.
How many distinct signing keys filed a passing contract receipt. One is the author alone; more than one means somebody else built it too. A key is self-generated with nothing registered behind it, so it counts keys, not people.
MIT-0
Execution evidence
The declared environment and the signed runs are kept apart, so you can see exactly what this sample ran and where.
- Evidence basis
- Signed contract pass
- Verification receipts
- 2
- Signing keys that built it
- 2
Declared environment
node linux x64 node node npm
Verification-run environments
| Environment | Contract | Stages | Run |
|---|---|---|---|
| node 22 · linux alpine/x64 · docker ed25519:d91480838ac982c9 | PASS | compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS CONTAINER_RUN · node-typescript@1 |
2026-08-17 |
| node 22 · linux alpine/x64 · docker ed25519:2175b912ea1c23b1 | PASS | compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS CONTAINER_RUN · node-typescript@1 |
2026-08-18 |
Case
HOW- Goal
- Distinguish Express error-handling middleware from standard middleware strictly via four-parameter arity (fn.length === 4)
- Packages
- Symbols
-
- express
- app.use
- Environment
- node
- Created
- 2026-08-17T01:50:22Z
Contract
- Express skips 3-parameter error handlers (err, req, res) during error routing because it identifies error middleware solely by checking fn.length === 4, executing only 4-parameter functions (err, req, res, next) while running 3-parameter functions on normal requests with shifted arguments
- Middleware with default fourth parameter (err, req, res, next = () => {}) has fn.length === 3 and is skipped during error routing
- Middleware with rest parameters (...args) has fn.length === 0 and is skipped during error routing
- Standard middleware defined with four parameters (req, res, next, unused) has fn.length === 4 and is skipped during normal request routing
Files
- NOTES.md
- csx.json
- package-lock.json
- package.json
- test/contract.mjs