Sample
A child Express router configured with mergeParams: true does not trigger its own router.param callbacks for parameters inherited from a parent route.
sha256:b6befb92f54a3e5eedbcb73414c24d4307ca49a2714eb462879efeb235f9c28a
PUBLISHED
L3_CONTRACT_PASS
MIT-0
Execution evidence
Declared environment and signed verification runs are separated so you can see exactly what this sample proves.
Evidence basisSigned contract pass
Verification receipts1
Verification levelL3_CONTRACT_PASS
Declared environment
- Execution context
- node
- Operating system
- linux
- Architecture
- x64
- Runtime
- node
- Language
- node
- Package manager
- npm
Verification-run environments
- Execution context
- node 22
- Operating system
- linux alpine · musl
- Architecture
- x64
- Runtime
- node 22
- Language
- javascript
- Package manager
- npm
- Execution
- container · docker
CONTAINER_RUN · compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS · node-typescript@1 · 2026-08-17
Case
- Goal
- A child Express router configured with mergeParams: true does not trigger its own router.param callbacks for parameters inherited from a parent route. HOW
- Packages
-
express 4.21.2
- Environment
- node
- Created
- 2026-08-17T06:38:46Z
Commonly assumed
router.param callbacks defined on a child router with mergeParams: true execute for all named parameters present in req.params during route handling.
The sample's author recorded this as what a developer or model would expect here. The contract below is what actually ran.
Contract
- Express router.param callbacks only execute when the parameter placeholder is declared in the matching route's own path pattern, ignoring inherited parameters populated on req.params by mergeParams: true.
- req.params retains all parent route parameters merged alongside child route parameters when mergeParams: true is enabled on the child router.
- The parent router's router.param callback executes during parent layer dispatch, but childRouter.param for that same parameter name is never invoked.
Files
- NOTES.md
- csx.json
- package-lock.json
- package.json
- test/contract.mjs
Download the source artifact (tar.gz)
Origin Seeder
csx-seed
Verification receipts
- node 22 · linux alpine/x64 · docker · CONTAINER_RUN · compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS · node-typescript@1 · 2026-08-17 · ed25519:d91480838ac982c9