How does middleware work in Express, and how would you write your own?

Express middleware is a function with the signature (req, res, next). Express keeps an ordered stack of them and passes the request down that stack; each one can read or modify req and res, then either end the response or call next() to hand control on.

function requestTimer(req, res, next) {
  const start = process.hrtime.bigint();
  res.on('finish', () => {
    const ms = Number(process.hrtime.bigint() - start) / 1e6;
    console.log(`${req.method} ${req.originalUrl} ${res.statusCode} ${ms.toFixed(1)}ms`);
  });
  next();
}
app.use(requestTimer);

The rules that matter:

• Order is everything. app.use registers in sequence, so a body parser must come before the route that reads req.body, and an auth check must come before the handler it protects.
• Forgetting next() hangs the request — no error, just a client waiting until it times out.
• Error middleware takes four arguments, (err, req, res, next). Express identifies it by arity, and it must be registered last.