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.

Practice

1. What is the main purpose of router-level middleware in Express?

easy

A. To replace the main app middleware completely

B. To run middleware only for routes defined in a specific router

C. To handle errors globally across all routers

D. To serve static files from a directory

Solution

1. Step 1: Understand router-level middleware scope

   Router-level middleware runs only for routes defined in that router, not the entire app.

2. Step 2: Compare with other middleware types

   Main app middleware runs for all routes, error middleware handles errors, static middleware serves files. Router middleware is specific to router routes.

3. Final Answer:

   To run middleware only for routes defined in a specific router -> Option B

4. Quick Check:

   Router middleware scope = specific router routes [OK]

Hint: Router middleware affects only its router's routes [OK]

Common Mistakes:

• Confusing router middleware with app-level middleware
• Thinking router middleware handles errors globally
• Assuming router middleware serves static files

2. Which of the following is the correct way to add middleware to an Express router?

easy

A. router.middleware(function(req, res, next) { next(); });

B. router.add(function(req, res, next) { next(); });

C. app.useRouter(function(req, res, next) { next(); });

D. router.use(function(req, res, next) { next(); });

Solution

1. Step 1: Recall Express router middleware syntax

   The correct method to add middleware to a router is router.use().

2. Step 2: Check other options for validity

   router.middleware, app.useRouter, and router.add are not valid Express methods.

3. Final Answer:

   router.use(function(req, res, next) { next(); }); -> Option D

4. Quick Check:

   Use router.use() to add middleware [OK]

Hint: Use router.use() to add middleware to routers [OK]

Common Mistakes:

• Using non-existent methods like router.middleware or router.add
• Confusing app.useRouter with router.use
• Forgetting to call next() inside middleware

3. Consider this Express router code:

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

router.use((req, res, next) => {
  console.log('Middleware A');
  next();
});

router.get('/test', (req, res) => {
  res.send('Hello from test');
});

module.exports = router;

What will be printed in the console when a client requests /test on this router?

medium

A. Middleware A

B. Hello from test

C. Middleware A and then Hello from test

D. Nothing will be printed

Solution

1. Step 1: Identify middleware behavior on route request

   The router middleware logs 'Middleware A' and calls next(), so it runs before the route handler.

2. Step 2: Understand console output vs response

   The console.log prints 'Middleware A' to the server console. The response sends 'Hello from test' to the client but does not print to console.

3. Final Answer:

   Middleware A -> Option A

4. Quick Check:

   Middleware console.log prints 'Middleware A' [OK]

Hint: Middleware logs to console, route sends response [OK]

Common Mistakes:

• Confusing console output with response content
• Expecting both console and response text in console
• Ignoring the next() call in middleware

4. Given this router code:

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

router.use((req, res, next) => {
  console.log('First middleware');
  next();
});

router.get('/hello', (req, res) => {
  res.send('Hello World');
});

router.use((req, res) => {
});

What is the problem with this code?

medium

A. The last middleware does not call next(), causing requests to hang

B. The last middleware is missing next() and will cause 404 for all routes

C. The last middleware will never run because it is after a route handler

D. The last middleware should have 4 parameters to handle errors

Solution

1. Step 1: Analyze middleware execution order

   Middleware and routes execute in the order defined. First middleware runs for all requests and calls next().

2. Step 2: Trace unmatched route behavior

   For unmatched routes like /foo: first middleware calls next(), /hello route doesn't match so next(), last middleware runs but does nothing--no response and no next() call.

3. Step 3: Identify the consequence

   The request pipeline stops without a response, causing the client request to hang indefinitely.

4. Final Answer:

   The last middleware does not call next(), causing requests to hang -> Option A

5. Quick Check:

   No response + no next() = request hangs [OK]

Hint: Every middleware must either respond or call next() [OK]

Common Mistakes:

• Thinking empty catch-all middleware is harmless
• Forgetting unmatched routes reach the last middleware
• Assuming the router automatically handles 404s

5. You want to apply a logging middleware only to routes under /api in your Express app. You create a router and add middleware like this:

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

apiRouter.use((req, res, next) => {
  console.log('API request:', req.method, req.url);
  next();
});

apiRouter.get('/users', (req, res) => {
  res.send('User list');
});

app.use('/api', apiRouter);

app.get('/', (req, res) => {
  res.send('Home');
});

What will happen when a client requests /api/users and /?

hard

A. Logging middleware does not run for any route

B. Logging middleware runs for both /api/users and /

C. Logging middleware runs only for /api/users, not for /

D. Logging middleware runs only for / route

Solution

1. Step 1: Understand router mounting path

   The router is mounted at '/api', so its middleware runs only for routes starting with '/api'.

2. Step 2: Analyze requests

   Request to '/api/users' triggers the router middleware and route handler. Request to '/' goes directly to app.get('/') and skips router middleware.

3. Final Answer:

   Logging middleware runs only for /api/users, not for / -> Option C

4. Quick Check:

   Router middleware runs only on mounted path routes [OK]

Hint: Router middleware runs only on its mounted path routes [OK]

Common Mistakes:

• Assuming router middleware runs for all app routes
• Confusing app-level and router-level middleware scopes
• Expecting middleware to run for '/' when mounted on '/api'