Router level middleware in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is router-level middleware in Express?

Router-level middleware is a function that runs during the request-response cycle but is bound to an Express router instance. It helps organize middleware for specific routes.

beginner

How do you apply middleware only to routes in a specific router?

You attach middleware to the router instance using router.use() or directly in route handlers. This middleware runs only for routes handled by that router.

beginner

Example: What does this code do?<br><pre>const router = express.Router();<br>router.use((req, res, next) =&gt; {<br>  console.log('Router middleware');<br>  next();<br>});</pre>

This middleware logs 'Router middleware' for every request handled by this router, then passes control to the next middleware or route handler.

intermediate

Can router-level middleware modify the request or response objects?

Yes, router-level middleware can change req or res objects, like adding properties or headers, affecting downstream handlers.

intermediate

What is the difference between application-level and router-level middleware?

Application-level middleware applies to all routes in the app, while router-level middleware applies only to routes defined in a specific router instance.

Where do you attach router-level middleware in Express?

A On an Express router instance using router.use()

B Directly on the Express app using app.use()

C Inside route handler functions only

D In the package.json file

Router-level middleware is attached to a router instance using router.use(), so it only affects routes in that router.

What happens if router-level middleware does not call next()?

A The request will hang and not proceed to the next handler

B The request will automatically continue

C The server will restart

D The middleware will be skipped

Middleware must call next() to pass control; otherwise, the request stops and the client waits indefinitely.

Which of these is true about router-level middleware?

A It cannot modify the request object

B It only runs for routes defined in its router

C It runs for every route in the app

D It must be defined after all routes

Router-level middleware is scoped to the router it is attached to, so it only runs for routes in that router.

How do you create a router in Express to use router-level middleware?

A const router = Router();

B const router = new Router();

C const router = express.createRouter();

D const router = express.Router();

The correct way is to call express.Router() to create a new router instance.

What is a common use case for router-level middleware?

A To configure the Express app settings

B To serve static files globally

C To add authentication checks for routes in a specific router

D To connect to the database

Router-level middleware is often used to add checks like authentication only for routes in that router.

Explain how router-level middleware works in Express and how it differs from application-level middleware.

Think about where you attach the middleware and which routes it affects.

Describe a scenario where using router-level middleware is helpful and how you would implement it.

Consider grouping routes that share a common need.