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Sample

express 5.1.0: Fix express 4 routes that throw "Missing parameter name" or "Unexpected ?" at startup after upgrading to express 5

Verified sample for npm express 5.1.0: Fix express 4 routes that throw "Missing parameter name" or "Unexpected ?" at startup after upgrading to express 5.…

sha256:bb95306081deb0d7bdd735b0ca6f0182f77216c5d0426ee0146b7562efe7facc

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

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Signed contract pass
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Signing keys that built it
3
Declared environment node 22 linux x64 node 22 npm

Verification-run environments

Environment Contract Stages Run
node 22 · linux alpine/x64 · docker ed25519:a2ec939a4c60e243 PASS compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS
CONTAINER_RUN · node-typescript@1		 2026-08-13
node 22 · linux alpine/x64 · docker ed25519:a2ec939a4c60e243 PASS compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS
CONTAINER_RUN · node-typescript@1		 2026-08-13
node 22 · linux alpine/x64 · docker ed25519:a2ec939a4c60e243 PASS compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS
CONTAINER_RUN · node-typescript@1		 2026-08-13
node 22 · linux alpine/x64 · docker ed25519:a2ec939a4c60e243 PASS compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS
CONTAINER_RUN · node-typescript@1		 2026-08-13
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:c1973797be207ac4 PASS compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS
CONTAINER_RUN · node-typescript@1node:22-alpine@sha256:c610fcdfb1d5…		 2026-09-07

Case

FIX
Goal
Fix express 4 routes that throw "Missing parameter name" or "Unexpected ?" at startup after upgrading to express 5
Packages
Symbols
  • express
  • app.get
  • req.params
  • req.query
Environment
node 22
Created
2026-08-13T14:19:55Z

Contract

  1. assert express 5 throws TypeError 'Unexpected ?' when the express 4 path '/users/:id?' is registered
  2. assert express 5 throws TypeError 'Missing parameter name' for the unnamed wildcards '*' and '/files/*'
  3. start the migrated app on an ephemeral port
  4. assert '/users{/:id}' serves /users/42 with params.id '42' and a bare /users with no id
  5. assert the named wildcard '/files/*splat' captures the remaining path as an array of segments
  6. assert assigning req.query throws because express 5 exposes it as a getter, and that a write onto the returned object is discarded

Files

  • csx.json
  • package-lock.json
  • package.json
  • src/app.mjs
  • src/v4-habits.mjs
  • test/contract.mjs

Download the source artifact (tar.gz)

Source

csx.json
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package.json
{
  "name": "csx-sample-express5-route-syntax",
  "version": "1.0.0",
  "private": true,
  "type": "module",
  "license": "MIT-0",
  "dependencies": {
    "express": "5.1.0"
  }
}
src/app.mjs
import express from 'express';
import { overwriteQuery, mutateQuery } from './v4-habits.mjs';

function attempt(fn) {
  try {
    fn();
    return null;
  } catch (err) {
    return err;
  }
}

// The express 5 spellings of the three express 4 routing habits.
export function createApp() {
  const app = express();

  // v4: app.get('/users/:id?', ...)
  // v5: an optional part is a braced group — and the SLASH belongs inside the
  // braces. '/users/{:id}' compiles too, but it only matches '/users/' and
  // 404s on a bare '/users', which is rarely what ':id?' meant.
  app.get('/users{/:id}', (req, res) => {
    res.json({ route: 'users', id: req.params.id ?? null });
  });

  // v4: app.get('/files/*', ...) then read req.params[0] as a string.
  // v5: every wildcard needs a name, and the capture is an ARRAY of decoded
  // path segments — join it to get the old string back.
  app.get('/files/*splat', (req, res) => {
    res.json({ route: 'files', path: req.params.splat.join('/'), segments: req.params.splat });
  });

  // v4: middleware normalized req.query in place.
  // v5: req.query is a getter — copy it and pass the copy along.
  app.get('/search', (req, res) => {
    const assignError = attempt(() => overwriteQuery(req));
    const mutateError = attempt(() => mutateQuery(req));
    const normalized = { ...req.query, page: Number(req.query.page ?? 1) };
    res.json({
      assignMessage: assignError?.message ?? null,
      mutateMessage: mutateError?.message ?? null,
      writeSurvived: req.query.normalized ?? null,
      normalized,
    });
  });

  return app;
}
src/v4-habits.mjs
// The express 4 patterns that stop working the moment you install express 5.
// They live here as data + tiny helpers so the contract can OBSERVE the
// failure instead of trusting a comment about it.

// express 5 bundles path-to-regexp 8, which rejects these path strings when
// the route is REGISTERED — so the process dies at boot, on the line that
// mounts the router, with no request ever made. That is why the upgrade
// looks like "express 5 crashes on startup" rather than "my route 404s".
export const V4_ROUTE_PATHS = {
  optionalParam: '/users/:id?', // TypeError: Unexpected ? at index 10
  bareWildcard: '*', //            TypeError: Missing parameter name at index 1
  trailingWildcard: '/files/*', // TypeError: Missing parameter name at index 8
  inlineRegex: '/rx/:id(\\d+)', // TypeError: Unexpected ( at index 7
};

// Returns the Error express 5 throws for a v4 path, or null if it registered.
export function registerOrThrow(app, path) {
  try {
    app.get(path, (_req, res) => res.end());
    return null;
  } catch (err) {
    return err;
  }
}

// express 4 let middleware replace req.query wholesale. In express 5 req.query
// is a prototype getter, so the assignment throws in strict mode (ESM is
// always strict).
export function overwriteQuery(req) {
  req.query = { normalized: true };
}

// The quieter half of the same change: the getter re-parses the query string
// on EVERY access and hands back a fresh object, so writing onto the object it
// returned does not throw — the value is just gone by the next read.
export function mutateQuery(req) {
  req.query.normalized = 'yes';
}
test/contract.mjs
import { strict as assert } from 'node:assert';
import express from 'express';
import { createApp } from '../src/app.mjs';
import { V4_ROUTE_PATHS, registerOrThrow } from '../src/v4-habits.mjs';

// 1. The express 4 path strings are rejected at registration time, not at
//    request time. This is the failure the migration actually presents as.
const legacy = express();

const optErr = registerOrThrow(legacy, V4_ROUTE_PATHS.optionalParam);
assert.ok(optErr instanceof TypeError, "express 5 must reject '/users/:id?'");
assert.match(optErr.message, /Unexpected \? at index \d+: \/users\/:id\?/);

const bareErr = registerOrThrow(legacy, V4_ROUTE_PATHS.bareWildcard);
assert.ok(bareErr instanceof TypeError, "express 5 must reject '*'");
assert.match(bareErr.message, /Missing parameter name at index \d+/);

const trailErr = registerOrThrow(legacy, V4_ROUTE_PATHS.trailingWildcard);
assert.ok(trailErr instanceof TypeError, "express 5 must reject '/files/*'");
assert.match(trailErr.message, /Missing parameter name at index \d+/);

const rxErr = registerOrThrow(legacy, V4_ROUTE_PATHS.inlineRegex);
assert.ok(rxErr instanceof TypeError, "express 5 must reject '/rx/:id(\\d+)'");
assert.match(rxErr.message, /Unexpected \( at index \d+/);

// 2. The express 5 spellings register and serve.
const server = await new Promise((resolve) => {
  const s = createApp().listen(0, '127.0.0.1', () => resolve(s));
});
const { port } = server.address();
const get = async (path) => {
  const res = await fetch(`http://127.0.0.1:${port}${path}`);
  const text = await res.text();
  // A missed route answers with express's HTML 404 page, so parse defensively
  // and let the status assertions below report the real problem.
  let body = null;
  try {
    body = JSON.parse(text);
  } catch {
    body = null;
  }
  return { status: res.status, body };
};

const withParam = await get('/users/42');
const withoutParam = await get('/users');
const splat = await get('/files/deep/nested/report.pdf');
const query = await get('/search?q=widget&page=2');
server.close();

// '/users{/:id}' covers both shapes of the old ':id?' route.
assert.equal(withParam.status, 200);
assert.deepEqual(withParam.body, { route: 'users', id: '42' });
assert.equal(withoutParam.status, 200, "'/users{/:id}' must still match a bare /users");
assert.deepEqual(withoutParam.body, { route: 'users', id: null });

// '/files/*splat' captures the rest of the path as named segments.
assert.equal(splat.status, 200);
assert.deepEqual(splat.body.segments, ['deep', 'nested', 'report.pdf']);
assert.equal(splat.body.path, 'deep/nested/report.pdf');

// req.query is a read-only getter that re-parses on every access.
assert.equal(query.status, 200);
assert.equal(typeof query.body.assignMessage, 'string', 'assigning req.query must throw');
assert.match(query.body.assignMessage, /only a getter/);
assert.equal(query.body.mutateMessage, null, 'mutating the returned object does not throw');
assert.equal(query.body.writeSurvived, null, 'but the write is discarded on the next read');
assert.deepEqual(query.body.normalized, { q: 'widget', page: 2 });

console.log('CONTRACT PASS: express 5 rejects v4 route syntax; {/:id} and *splat route correctly');

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