Express Middleware

🔹 What is Middleware in Express?

Middleware functions in Express is a functions that runs between the incoming request(req) and an outgoing response(res).
They can:
- Access the request(req) and response(res) objects.
- Modify them.
- End the request-response cycle.
- Or pass control to the next middleware using next().
Think of them as layers of functions that request passess through before reaching the final handler.

🔹 Syntax

    app.use((req, res, next) => {
        console.log('Middleware function executed.');
        next();             // Pass control to the next middleware
    })

🔹 Types of Middleware

Application-level Middleware ->

Attach directly to the Express app object.

Eg:

  app.use((req, res, next) => {
      console.log("Application Middleware");
      next();
  });

Router-level Middleware ->

Works the same but bounds to an Express router.

Eg:

    const router = Router()
    router.use((req, res, next)=>{
        console.log('Router Middleware');
        next();
    })

Built-in Middleware ->
- Examples:
  - express.json() → parse JSON bodies.
  - express.urlencoded({ extended: true }) → parse URL-encoded bodies.
  - express.static('public') → serve static files.
Third Party Middleware ->
- Examples:
  - morgan → logging
  - cors → handling cross-origin requests.
  - cookie-parser → parsing cookies.
```
    const cors = require('cors')
    app.use(cors())
```

Error Handling Middleware ->

Special middleware with 4 parameters (err, req, res, next).

Eg:

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

👉 Request Flow: Client → Middleware 1 → Middleware 2 → Route Handler → Response

🔹 When to use Middleware?

Logging requests (morgan, custom loggers).
Authentication/Authorization (check tokens, user roles).
Parsing data (JSON, forms, cookies).
Error handling (catch errors in routes).
Serving static files (images, CSS, JS).
Rate limiting / Security checks.