What is middleware in Express and how does it work?

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In Express.js, middleware is a function that runs during the request-response cycle. It can access the request (req), the response (res), and the next() function, which passes control to the next middleware in the stack. Middleware is used to perform tasks like logging, authentication, parsing JSON, error handling, etc.

🔧 How Middleware Works:

Middleware functions are executed in the order they are defined. Each function can:

Modify req or res
End the response
Call next() to move to the next middleware

🧱 Syntax:

app.use((req, res, next) => {
  console.log('This is a middleware!');
  next(); // pass to the next middleware or route
});

🔍 Example Use Cases:

Logging Requests:

app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next();
});

Parsing JSON:

app.use(express.json());

Custom Authentication:

const auth = (req, res, next) => {
  if (req.headers.token === '123') next();
  else res.status(401).send('Unauthorized');
};
app.use(auth);

Error Handling Middleware:

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});

In short, middleware is a powerful way to modularize and structure your Express applications.

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