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Explain the concept of 'middleware' in Express.js.

Q: Explain the concept of 'middleware' in Express.js.

**TL;DR.** **Middleware** are functions with the signature (req, res, next) that run **in order** for each request. Each can inspect/modify req / res , end the response, or call next() to pass control onward. They form a **pipeline** for cross-cutting concerns: logging, auth, parsing, validation, and error handling. request logger next() <rect cl

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A fundamental concept for web development with Node.js and Express.

express.jsmiddlewareweb developmenthttp

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Logger + auth + error middleware

// runs for every request, then passes on
app.use((req, res, next) => { console.log(req.method, req.url); next(); });

// guard: respond OR continue
function auth(req, res, next) {
  if (!req.headers.authorization) return res.status(401).end();
  next();
}
app.get('/secret', auth, (req, res) => res.send('ok'));

// error middleware — note the 4 args
app.use((err, req, res, next) => res.status(500).json({ error: err.message }));

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