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## 1. What `express.Router()` does

> `express.Router()` creates a **"mini application"** (a mini Express instance)
> that works the same way `app` does, but is meant **for grouping routes and middleware**.

The idea:
instead of describing every route in `app.js`,
you can split them across files and import them into the main application.

### An example without Router (everything in one file):

```javascript
const express = require('express');
const app = express();

app.get('/users', (req, res) => res.send('List of users'));
app.get('/products', (req, res) => res.send('List of products'));

app.listen(3000);
```

The downside: the code quickly gets unwieldy.

## 2. An example using `express.Router()`

Let's create two separate files for routes:

### routes/users.js

```javascript
const express = require('express');
const router = express.Router();

router.get('/', (req, res) => {
  res.send('List of users');
});

router.get('/:id', (req, res) => {
  res.send(`User with ID: ${req.params.id}`);
});

module.exports = router;
```

### routes/products.js

```javascript
const express = require('express');
const router = express.Router();

router.get('/', (req, res) => {
  res.send('List of products');
});

router.post('/', (req, res) => {
  res.send('Product created');
});

module.exports = router;
```

### app.js

```javascript
const express = require('express');
const app = express();

const usersRouter = require('./routes/users');
const productsRouter = require('./routes/products');

// Mount the routers with a prefix
app.use('/users', usersRouter);
app.use('/products', productsRouter);

app.listen(3000, () => console.log('Server running'));
```

Now:

- `GET /users` → "List of users"
- `GET /users/42` → "User with ID: 42"
- `GET /products` → "List of products"
- `POST /products` → "Product created"

## 3. How `Router` works internally

Every `Router` is a **middleware instance**
that holds its own set of routes and in-between handlers.
When Express receives a request:

1. It checks whether the path matches (`/users`, `/products`, etc.).
2. If it matches, the request is handed to the **inner Router**,
which runs its own little "mini Event Loop" of routes and middleware.

## 4. Router supports everything `app` does

| Feature | Works in Router? | Example |
|---|---|---|
| Middleware | Yes | `router.use(authMiddleware)` |
| Routes | Yes | `router.get('/', handler)` |
| Parameters | Yes | `router.param('id', callback)` |
| Sub-routers | Yes | `router.use('/admin', adminRouter)` |

## 5. Why use Router, the advantages

| Advantage | Explanation |
|---|---|
| Modularity | Lets you split the app into logical parts (users, products, orders, etc.) |
| Reusability | One Router can be mounted into different applications |
| Readability and structure | The code stays clean and easy to navigate |
| Local middleware | Middleware can apply only to a specific group of routes |
| Testability | Makes it easier to write unit tests for individual API modules |

## 6. Example: middleware for a single Router only

```javascript
const router = express.Router();

function authMiddleware(req, res, next) {
  if (req.headers.token === '123') next();
  else res.status(401).send('Access denied');
}

router.use(authMiddleware); // applies only to this Router's routes

router.get('/secret', (req, res) => {
  res.send('The secret zone');
});

module.exports = router;
```

Now the authorization check applies **only** to `/secret`,
and doesn't affect the rest of the application.

## 7. Summary

> `express.Router()` is a mini Express application
> that lets you group routes and middleware for a specific part of the API.
>
> It's used to:
>
> - organize code (splitting it into modules);
> - mount it with a prefix (`app.use('/api/users', usersRouter)`);
> - add local middleware;
> - make maintenance and testing simpler.
>
> Almost every sizable Express application is built on `Router()`.

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