express 4.21.2: A child Express router configured with mergeParams: true does not trigger its own router.param callbacks for parameters inherited from a parent route.

# Express: mergeParams and router.param Interaction

## Network Search Results
`search_known_solution` returned compatible samples for:
- Error-handling middleware arity detection via `fn.length === 4` (`sha256:7d9166d737c904e7ef96c639c4509493cd6cb139888b54e6daffdaed0ec6019c`)
- `trust proxy` setting effect on `req.ip` and `req.ips` (`sha256:3a4cd850e24a4583db4efbe2c07c6463fb853a6a7bb75a6d1f365a1b70a6bb0c`)
- Removal of `app.configure` in Express 4 (`sha256:e06c29db577a1fb2a48a4f5b01d70afb5a30f0d28ae56acf19a8476a2dd871bd`)

No existing sample addressed the interaction between `mergeParams: true` and `router.param()`.

## Pinned Release
`express@4.21.2` (npm)

## Observed Behaviour
When a child router is instantiated with `express.Router({ mergeParams: true })` and mounted onto a parameterized parent path (such as `parentRouter.use('/users/:userId', childRouter)`), `req.params` is populated with both parent and child route parameters when handling requests on routes like `childRouter.get('/posts/:postId', ...)`.

However, `router.param()` callbacks registered on `childRouter` for inherited parameter names (e.g. `childRouter.param('userId', fn)`) are never executed. Express processes parameter callbacks strictly by iterating over the keys extracted from the matching layer's own route path regexp (`layer.keys`), rather than checking the keys present in `req.params`.

### Failure Mode
This fails silently at runtime. If a developer places parameter validation, entity preloading, or tenant lookups in `childRouter.param('userId', fn)` expecting it to run whenever `req.params.userId` is processed by that sub-router, the request succeeds with HTTP 200 without executing the callback. Any data or validation expected to be attached to `req` will be missing, causing downstream route handlers to see `undefined` state without triggering any warning or routing error.

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}

{
  "name": "express-sample",
  "version": "1.0.0",
  "type": "module",
  "dependencies": {
    "express": "4.21.2"
  }
}

import express from 'express';
import http from 'node:http';
import assert from 'node:assert';

async function run() {
  const app = express();

  const parentRouter = express.Router();
  const childRouter = express.Router({ mergeParams: true });

  const trace = [];

  // Register param callbacks on parent and child routers
  parentRouter.param('userId', (req, res, next, val) => {
    trace.push(`parent:userId:${val}`);
    next();
  });

  childRouter.param('userId', (req, res, next, val) => {
    trace.push(`child:userId:${val}`);
    next();
  });

  childRouter.param('postId', (req, res, next, val) => {
    trace.push(`child:postId:${val}`);
    next();
  });

  // Child router defines a route that only declares :postId in its own path
  childRouter.get('/posts/:postId', (req, res) => {
    res.json({
      userId: req.params.userId,
      postId: req.params.postId,
    });
  });

  // Mount childRouter on parentRouter path containing :userId
  parentRouter.use('/users/:userId', childRouter);
  app.use('/api', parentRouter);

  const server = http.createServer(app);
  await new Promise((resolve) => server.listen(0, '127.0.0.1', resolve));
  const { port } = server.address();

  try {
    const res = await fetch(`http://127.0.0.1:${port}/api/users/42/posts/101`);
    assert.strictEqual(res.status, 200);

    const body = await res.json();

    // 1. mergeParams: true successfully populates both parent and child params on req.params
    assert.strictEqual(body.userId, '42');
    assert.strictEqual(body.postId, '101');

    // 2. Parent router's param callback ran for :userId
    assert.ok(trace.includes('parent:userId:42'));

    // 3. Child router's param callback ran for its own path param :postId
    assert.ok(trace.includes('child:postId:101'));

    // 4. Child router's param callback NEVER ran for :userId, despite req.params.userId existing
    assert.strictEqual(
      trace.includes('child:userId:42'),
      false,
      'childRouter.param("userId") must not be invoked for inherited parent route params'
    );

    // Full trace assertion proving exact execution sequence
    assert.deepStrictEqual(trace, [
      'parent:userId:42',
      'child:postId:101'
    ]);
  } finally {
    await new Promise((resolve) => server.close(resolve));
  }
}

await run();