Router-level Middleware in an Express Js

• Let's create a complete example of a router-level middleware in an Express.js application. In this example, we'll create a router-level middleware that checks for authentication before allowing access to certain routes.

• Project Structure: Create a project structure with the following directories and files:

    - my-express-app/

        - middlewares/

            - authenticationMiddleware.js

        - routes/

            - index.js

            - admin.js

        - app.js

        - package.json

• Middleware File (`authenticationMiddleware.js`): Create a router-level middleware that simulates authentication by checking for a token in the request header:

    // middlewares/authenticationMiddleware.js

    const authenticationMiddleware = (req, res, next) => {

        const authToken = req.header('Authorization');

        if (!authToken || authToken !== 'mySecretToken') {

            return res.status(401).send('Unauthorized. Please provide a valid token.');

        }

        next(); // Call the next middleware in the stack

    };

    module.exports = authenticationMiddleware;

• Route Files (`index.js` and `admin.js`): Create two route files in the `routes` directory:

    // routes/index.js

    const express = require('express');

    const router = express.Router();

    router.get('/', (req, res) => {

        res.send('Hello from the index route!');

    });

    module.exports = router;

    // routes/admin.js

    const express = require('express');

    const router = express.Router();

    const authenticationMiddleware = require('../middlewares/authenticationMiddleware');

    // Apply the router-level middleware only to the '/admin' route

    router.use(authenticationMiddleware);

    router.get('/dashboard', (req, res) => {

        res.send('Welcome to the Admin Dashboard!');

    });

    module.exports = router;

• Express App (`app.js`): Set up the Express.js application and use the router-level middleware:

    // app.js

    const express = require('express');

    const authenticationMiddleware = require('./middlewares/authenticationMiddleware');

    const indexRoute = require('./routes/index');

    const adminRoute = require('./routes/admin');

    const app = express();

    const PORT = 3000;

    // Use the router-level middleware only for the '/admin' route

    app.use('/admin', authenticationMiddleware);

    // Use the index and admin routes

    app.use('/', indexRoute);

    app.use('/admin', adminRoute);

    app.listen(PORT, () => {

        console.log(`Server is running on http://localhost:${PORT}`);

    });

• Run the Application: Run your application using the command:

    node app.js

• Visit `http://localhost:3000` in your browser to access the index route, and you should see the message "Hello from the index route!".

• Visit `http://localhost:3000/admin/dashboard` to access the admin route. Since the router-level middleware (`authenticationMiddleware`) is applied only to the `/admin` route, it will check for the authorization token. If the token is missing or incorrect, it will return a 401 Unauthorized response. If the token is valid, it will allow access to the admin dashboard route.

• This example demonstrates the use of a router-level middleware (`authenticationMiddleware`) in an Express.js application. The middleware is applied only to specific routes, allowing for targeted authentication checks.