Middleware reference

Middleware sits between the HTTP listener and the endpoint router. Each entry has an optional pre(req) (before routing) and post(req, res) (after the handler runs).

Built-ins

import { mockr, logger, delay, auth, errorInjection } from '@yoyo-org/mockr';

await mockr({
  middleware: [
    logger(),                          // ascii log line per request
    delay({ ms: 250 }),                // fixed or per-route delay
    auth({ token: 'secret' }),         // 401 unless Authorization matches
    errorInjection({ rate: 0.05 }),    // 5% chance of 500
  ],
  endpoints: [/* ... */],
});

Helper	Behavior
logger()	Logs method url → status once per request.
delay({ ms })	Adds latency; supports (req) => number for per-route timing.
auth({ token })	Rejects with 401 unless Authorization matches.
errorInjection({ rate })	Random 500 for chaos testing.

Custom middleware

import type { Middleware } from '@yoyo-org/mockr';

const cors: Middleware = {
  name: 'cors',
  pre(req) {
    if (req.method === 'OPTIONS') {
      return {
        status: 204,
        headers: {
          'Access-Control-Allow-Origin': '*',
          'Access-Control-Allow-Methods': 'GET,POST,PUT,PATCH,DELETE,OPTIONS',
          'Access-Control-Allow-Headers': '*',
        },
        body: '',
      };
    }
  },
  post(req, res) {
    return {
      ...res,
      headers: { ...(res.headers ?? {}), 'Access-Control-Allow-Origin': '*' },
    };
  },
};

pre returning a response short-circuits the chain — handler never runs. post rewrites the response after the handler. Either may be async.

Runtime registration

const server = await mockr({ /* ... */ });
server.use(cors);

Works after startup — useful for tests that conditionally inject behavior.