What is middleware in Express, and how does the execution order work?

Answer: Middleware is the core abstraction in Express — a function that runs during the request/response cycle.

Signature and flow:

function logger(req, res, next) {
  console.log(`${req.method} ${req.url}`);
  next(); // pass control to the next middleware/route
}
app.use(logger);

Each middleware can:

• run code / modify req or res (e.g., attach req.user),
• end the cycle by sending a response, or
• call next() to continue, or next(err) to jump to error handling.

Execution order = registration order:

app.use(express.json());        // 1. parse JSON body
app.use(requestLogger);         // 2. log
app.use('/api', authenticate);  // 3. auth for /api routes
app.get('/api/me', handler);    // 4. route handler
app.use(errorHandler);          // 5. error middleware (last)

If express.json() were registered after the route, req.body would be undefined in the handler — a classic ordering bug.

Types of middleware:

• Application-level — app.use(fn).
• Router-level — mounted on an express.Router().
• Path-scoped — app.use('/admin', fn) runs only for matching paths.
• Built-in — express.json(), express.static().
• Third-party — helmet, cors, morgan.
• Error-handling — four args (err, req, res, next).

Key idea: it's a pipeline — control flows top to bottom until something sends a response or calls next(err).