express-emits

Typed EventEmitter instrumentation for Express. Every middleware and request emits start/end events with real durations, so you can wire up logging, tracing, or metrics without the library deciding how you output them.

import express from 'express';
import { expressEmits } from 'express-emits';

const app = express();
const { on, log, namedMiddleware } = expressEmits(app);

on('request.start', (r) => console.log(`-> ${r.method} ${r.url} [${r.requestID}]`));
on('request.end', (r) => console.log(`<- ${r.statusCode} in ${r.duration.toFixed(1)}ms`));

app.use(namedMiddleware('auth', (req, res, next) => {
  log(res, 'checking token');
  next();
}));

app.get('/', (_req, res) => res.send('Hello World'));
app.listen(3000);

Install

npm install express-emits

Express 5 is a peer dependency. Node 18+.

API

expressEmits(app, options?)

Instruments app and returns { on, log, namedMiddleware }. Each call gets its own emitter, so two apps in one process stay isolated.

Option	Default
unguessableRequestIds	false	Use randomUUID() instead of a per-process counter. Turn on if IDs ever leave the process or need to be unpredictable. The counter is faster and fine for correlating logs.

on(event, listener)

Typed against the event map. Wrong event name or wrong payload shape is a compile error.

Event	Payload
request.start	{ requestID, method, url, path, headers }
request.end	{ requestID, statusCode, duration }
middleware.start	{ requestID, middlewareName }
middleware.end	{ requestID, middlewareName, duration }
log	(requestID, middlewareName, ...args)

namedMiddleware(name, middleware)

Labels a middleware so its events carry name instead of an anonymous N placeholder.

log(res, ...args)

Emits a log event tagged with the current request ID and middleware name. Takes res because that is where per-request state lives.

Anonymous middleware

Middleware registered without namedMiddleware is numbered by execution order within each request, restarting at 1 every request:

request.start  id=1
  middleware.start  anonymous 1
  middleware.start  auth

A middleware that a given request skips does not consume a number for that request.

Performance

npm run bench runs bench/rps-bench.ts: five scenarios at 30s / 50 connections via autocannon. Two independent runs on the same machine:

Scenario	Run 1	Run 2
express (base)	18330 req/s	17201 req/s
zero listeners, counter IDs	16551 req/s (-9.7%)	16885 req/s (-1.8%)
silent listeners, counter IDs	16405 req/s (-10.5%)	16260 req/s (-5.5%)
zero listeners, unguessableRequestIds: true	16342 req/s (-10.8%)	16455 req/s (-4.3%)
silent listeners, unguessableRequestIds: true	16063 req/s (-12.4%)	15791 req/s (-8.2%)

Benchmark was run on my local computer, that is why the benchmark results are not stable.

Development

npm run build    # compile to dist/
npm test         # build, then run the test suite
npm run bench    # 30s x 5 RPS benchmark against plain Express, see Performance
npm run dev      # watch mode, runs the demo in main.ts

License

MIT