Middleware in Express (body-parser, auth)

Use Middleware (like `body-parser`, Auth)

📌 What: Middleware functions in Express.js are functions that execute between the time a request is received and a response is sent. They can modify the req or res objects or end the request-response cycle.

🎯 Why: Middleware is useful for handling:

Parsing: incoming JSON/form data (e.g. body-parser)
Authentication: verifying user tokens (e.g. JWT)
Logging: request info (e.g. morgan)
Validation: check incoming inputs

⚙️ How: Use app.use() to register middleware globally, or pass it to a specific route.

// Install middleware
npm install express body-parser jsonwebtoken

// server.js
const express = require('express');
const bodyParser = require('body-parser');
const jwt = require('jsonwebtoken');

const app = express();
app.use(bodyParser.json()); // Middleware to parse JSON body

// 🔐 Auth Middleware Example
function authMiddleware(req, res, next) {
  const token = req.headers.authorization?.split(" ")[1];
  if (!token) return res.status(401).json({ error: "No token provided" });

  try {
    const decoded = jwt.verify(token, "SECRET_KEY");
    req.user = decoded;
    next(); // go to next middleware or route
  } catch (err) {
    res.status(401).json({ error: "Invalid token" });
  }
}

// 🌐 Route with auth middleware
app.get("/profile", authMiddleware, (req, res) => {
  res.json({ message: "Welcome, user!", user: req.user });
});

app.listen(3000, () => {
  console.log("🚀 Server running on http://localhost:3000");
});
      

🕐 When: Use middleware:

app.use(express.json()) at the start to parse request bodies.
For protected routes with authentication.
For logging, input validation, error handling.

💡 Tip: You can create your own custom middleware and stack them in any order. Use next() to pass control to the next function in line.

Use Middleware (like body-parser, Auth)

Use Middleware (like `body-parser`, Auth)