Express.js Router Middleware

Router middleware in Express allows you to define middleware functions for specific routes or groups of routes. It acts as a function that is executed during the request-response cycle. Middleware can be used to perform tasks such as authentication, logging, validation, or transforming requests before they reach route handlers.

 

Key Features of Router Middleware

• Route-specific Middleware: Middleware can be applied to specific routes or groups of routes.
• Stacked Execution: Middleware functions are executed in the order they are defined.
• Modular and Reusable: Router middleware allows you to break down functionality into smaller, reusable modules.

 

Steps to Use Router Middleware in Express.js

Set up a Basic Express Application
If you haven’t already, initialize a Node.js project and install Express:

npm init -y
npm install express --save

Create an Express Application with Router Middleware
Create a file called app.js and set up route-specific middleware using express.Router():

const express = require('express');
const app = express();
const router = express.Router();

// Simple middleware function that runs for every request
const logRequest = (req, res, next) => {
    console.log(`Request made to: ${req.originalUrl}`);
    next();  // Pass control to the next middleware
};

// Apply middleware globally to all routes
app.use(logRequest);

// Define a route using middleware for a specific path
router.use((req, res, next) => {
    console.log('Middleware for /user routes');
    next();  // Pass control to the next middleware
});

// Route to handle user requests
router.get('/:id', (req, res) => {
    res.send(`User ID: ${req.params.id}`);
});

// Another route for updating user data
router.put('/:id', (req, res) => {
    res.send(`Updated user ID: ${req.params.id}`);
});

// Apply the router with its middleware to the '/user' path
app.use('/user', router);

// Start the server
app.listen(3000, () => {
    console.log('Server is running on port 3000');
});

Run the Application
Start the server by running:

node app.js

Test the Routes
Open your browser or use tools like Postman to test the following routes:

• http://localhost:3000/user/123 – Will print "User ID: 123"
• http://localhost:3000/user/123 (PUT request) – Will print "Updated user ID: 123"
• "Request made to: /user/123"
• "Middleware for /user routes"

Handling Error Middleware
You can also define error handling middleware in Express. For example, if you have an error in one of the routes, you can catch it with a middleware:

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

Using Middleware with Route Handlers
You can apply middleware only to specific routes or route groups:

// Route-specific middleware example
router.get('/profile', logRequest, (req, res) => {
    res.send('User Profile');
});

Router Middleware with Parameters
Middleware functions can be used with routes that contain parameters, such as user IDs or product IDs:

router.param('id', (req, res, next, id) => {
    console.log(`User ID: ${id}`);
    next();
});

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

 

Summary

Router middleware in Express is a powerful feature that enables you to modularize your middleware functions and apply them to specific routes or groups of routes. This helps in managing reusable functionality like authentication, logging, and request validation. You can stack middleware to process requests before they reach the route handlers and organize your application more efficiently.