Difference between application level and router level middleware in Express?

If you’ve used Express even a little, you know how important middleware is. Those little functions run on every request and let you do stuff like logging, authentication, file uploads, and more. In this article, we will see what is Application-level middleware and Router-level middleware and what are the key differences between them.

Application-Level Middleware:

Application-level middleware is like a personal assistant that helps out with every incoming request to your Express app. No matter which route a client hits, this middleware jumps in to work its magic. For example, you can use app-level middleware to log every request that comes in for debugging. Or to check authentication for your entire application. Or even set global headers that you want on every response.

Syntax:

app.use(params...);

Key Features:

• Global Scope: It operates on all routes within the application.
• Initialization Order: It’s defined before any specific route handlers.
• Common Use Cases: Logging, setting headers, authentication checks, error handling, and serving static files.

Example:

Javascript

//app.js

const express = require('express');
const app = express();
const PORT = 3000;

// Application-level middleware for logging
app.use((req, res, next) => {
    console.log(`Request received at: ${new Date()}`);
    next();
});

// Application-level middleware for parsing JSON
app.use(express.json());

// Route
app.get('/', (req, res) => {
    res.send('Hello from the root route!');
});

app.listen(PORT, () => {
    console.log(`Server listening on port ${PORT}`);
});

Output:

Console Output:

Router-Level Middleware:

Router-level middleware is more specialized – it only applies to particular routing paths you choose. Think of it like a special helper that comes in to handle requests for certain routes. For example, you could set up some router-level middleware that checks for authentication on your /account routes or validation middlewares that check data before your /purchase or /signup routes.

Syntax:

router.use(params...);

Key Features:

• Specific Scope: It operates on specific routes or groups of routes.
• Initialization Order: It’s defined closer to route handlers, affecting only those routes.
• Common Use Cases: Route-specific authentication, input validation, handling specific routes’ behaviors.

Example:

Javascript

//app.js

const express = require('express');
const app = express();
const PORT = 3000;

// Router-level middleware
const router = express.Router();

router.use((req, res, next) => {
    console.log(`Middleware specific to this router`);
    next();
});

// Route within the router
router.get('/route', (req, res) => {
    res.send('Response from the router-level route');
});

// Mounting the router
app.use('/api', router);

app.listen(PORT, () => {
    console.log(`Server listening on port ${PORT}`);
});

Output:

Console Output:

Difference between Application and Router level Middleware:

Application middleware is better for global concerns like logging, while router middleware is better for things you only want to apply to certain routes, like authentication.

Feature	Application-level Middleware	Router-Level Middleware
Where it’s applied ?	Applies globally to the whole app.	Applied to a router.
How it’s applied ?	App.use() or app.METHOD().	Router.use() or router.METHOD()
Paths affected	Affects all routes and paths.	Only affects routes defined in router.
Common uses	Logging, authentication, error handling	Authentication for parts of app validation.