Q51- Explain middleware in Express.js with examples.

Middleware in Express.js is a function that has access to the request (req) and response (res) objects, and the next function (next) in the request-response cycle.

• Middleware can execute code, modify req/res, end the request, or pass control to the next middleware.
• Middleware is used for logging, authentication, parsing request bodies, error handling, etc.

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🔹 Middleware Signature

function middleware(req, res, next) {
  // Do something
  next(); // Pass to next middleware/route
}

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🔹 Types of Middleware

1. Application-level middleware – applied to all routes or specific routes.
2. Router-level middleware – applied to routes using an Express router.
3. Built-in middleware – like express.json(), express.static().
4. Error-handling middleware – handles errors with 4 parameters (err, req, res, next).

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🔹 Example 1: Application-level Middleware

const express = require("express");
const app = express();
const PORT = 3000;

// Built-in middleware to parse JSON
app.use(express.json());

// Custom middleware for logging
app.use((req, res, next) => {
  console.log(`${req.method} ${req.url} - ${new Date().toISOString()}`);
  next();
});

app.get("/hello", (req, res) => {
  res.send("Hello World");
});

app.listen(PORT, () => console.log(`Server running on port ${PORT}`));

Output when accessing /hello:

GET /hello - 2025-10-22T10:00:00.000Z

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🔹 Example 2: Route-specific Middleware

function authMiddleware(req, res, next) {
  const token = req.headers["authorization"];
  if (token === "12345") next();
  else res.status(401).json({ message: "Unauthorized" });
}

app.get("/protected", authMiddleware, (req, res) => {
  res.send("This is protected content");
});

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🔹 Example 3: Error-handling Middleware

app.get("/error", (req, res) => {
  throw new Error("Something went wrong!");
});

// Error-handling middleware
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).json({ message: err.message });
});

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✅ Key Points

• Middleware is executed in order they are added.
• Must call next() to pass control unless you end the response.
• Common use cases: logging, authentication, request parsing, error handling, compression, CORS.