What is middleware in Node.js HTTP servers?

Direct answer

Middleware is a chain of functions (req, res, next) that inspect or transform requests before the final handler — auth, logging, body parsing, CORS; each calls next() to continue or ends the response to stop the chain.

Raw http.createServer has no middleware — frameworks compose ordered functions. Fastify uses hooks and plugins with similar pipeline semantics and encapsulation.

Why interviewers ask

Order matters — body parser before route that reads req.body; auth before protected routes.
next(err) — skip to error-handling middleware in Express-style stacks.
Short-circuit — send 401 and return without next() for failed auth.
Cross-cutting concerns — logging, rate limits, request IDs — not duplicated per route.

middleware-chain.mjs

function logger(req, res, next) {
  console.log(req.method, req.url);
  next();
}

function requireJson(req, res, next) {
  if (!req.headers["content-type"]?.includes("application/json")) {
    res.writeHead(415).end("JSON only");
    return;
  }
  next();
}

function compose(...fns) {
  return (req, res) => {
    let i = 0;
    const next = () => {
      const fn = fns[i++];
      if (fn) fn(req, res, next);
    };
    next();
  };
}