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05 — Middleware ​

{ pre, post } wrappers run around every request. pre can short-circuit; post can rewrite the response. Built-ins ship in the package.

Run this chapter in 30 seconds

  1. Open in StackBlitz → — full Node sandbox in your browser, no install.
  2. Wait for npm install to finish, then in the Terminal tab run:
    npx tsx examples/05-middleware/server.ts
  3. Paste any request from the Try it section below into the Terminal (use curl — the StackBlitz preview port is forwarded).

Concept ​

Middleware order matters — top of the array runs first. A pre that returns a HandlerResult skips downstream handlers entirely. server.use(...) adds middleware after boot (e.g., admin guards toggled at runtime).

Code ​

ts
import { mockr, mockGroup, auth, delay, logger } from '@yoyo-org/mockr';
import { z } from 'zod';

type Endpoints = {
  '/api/health': { status: string };
  '/api/login': { token: string };
  '/api/secret': { flag: number };
};

const api = mockGroup<Endpoints>()
  .data('/api/health', { status: 'ok' })
  .post('/api/login', {
    body: z.object({ email: z.string() }),
    fn: (req, ctx) => {
      if (req.body.email === 'admin@example.com') return { token: 'admin-token-123' };
      return ctx.error(401, 'Invalid credentials');
    },
  })
  .data('/api/secret', { flag: 42 })
  .done();

mockr({
  port: 3005,
  middleware: [
    logger(),
    delay({ min: 50, max: 150 }),
    auth({
      type: 'bearer',
      validate: (token) => token === 'admin-token-123',
      exclude: ['/api/health', '/api/login'],
    }),
    {
      name: 'request-id',
      post: (_req, res) => ({
        ...res,
        headers: { ...res.headers, 'x-request-id': `req-${Date.now()}` },
      }),
    },
  ],
  groups: [api],
});

Built-in middleware ​

HelperPurpose
logger()Logs each request to stdout.
delay({ min, max })Random latency between bounds.
auth({ type, validate, exclude })Bearer / Basic guard.
errorInjection({ rate, status })Random N% failure for retry-logic testing.

Try it ​

Open in StackBlitz → — paste each curl into the StackBlitz Terminal once npx tsx examples/05-middleware/server.ts is running.

bash
# excluded from auth — 200
curl -s http://localhost:3005/api/health

# get a token
curl -s -X POST http://localhost:3005/api/login \
  -H 'Content-Type: application/json' \
  -d '{"email":"admin@example.com"}'

# authed — 200, plus x-request-id header
curl -s http://localhost:3005/api/secret \
  -H 'Authorization: Bearer admin-token-123' -i

# no token — 401
curl -s http://localhost:3005/api/secret -i

What's next ​

Switch the server between named states → 06 — Scenarios.

MIT License

mockr