Why middleware is fundamental in Node.js - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a middleware function that logs every request.

Node.js

app.use(function(req, res, next) { console.log(req.[1]); next(); });

Drag options to blanks, or click blank then click option'

Aurl

Bmethod

Cbody

Dheaders

Attempts:

3 left

💡 Hint

Common Mistakes

Using req.method logs the HTTP method, not the URL.

The middleware logs the URL of each request using req.url.

2fill in blank

medium

Complete the code to create middleware that ends the response with 'Hello World'.

Node.js

app.use(function(req, res, [1]) { res.send('Hello World'); });

Drag options to blanks, or click blank then click option'

Aclose

Bend

Cfinish

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'end' or 'finish' instead of 'next' causes errors.

The middleware function uses next as the third parameter to pass control if needed.

3fill in blank

hard

Fix the error in the middleware that should call the next function.

Node.js

app.use(function(req, res, next) { console.log('Request received'); [1](); });

Drag options to blanks, or click blank then click option'

Ares.send

Bnext

Creq.next

Dres.end

Attempts:

3 left

💡 Hint

Common Mistakes

Calling res.end() or res.send() instead of next() stops middleware chain.

The middleware must call next() to pass control to the next middleware.

4fill in blank

hard

Fill both blanks to create middleware that checks if user is authenticated and calls next if true.

Node.js

app.use(function(req, res, next) { if (req.[1]) { [2](); } else { res.status(401).send('Unauthorized'); } });

Drag options to blanks, or click blank then click option'

AisAuthenticated

Bnext

Csend

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Calling res.send() instead of next() stops the chain.

The middleware checks req.isAuthenticated and calls next() if true.

5fill in blank

hard

Fill all three blanks to create middleware that logs method, URL, and then calls next.

Node.js

app.use(function(req, res, next) { console.log(req.[1] + ' ' + req.[2]); [3](); });

Drag options to blanks, or click blank then click option'

Amethod

Burl

Cnext

Dbody

Attempts:

3 left

💡 Hint

Common Mistakes

Logging req.body instead of req.url or forgetting to call next().

The middleware logs the HTTP method and URL, then calls next() to continue.

Practice

1. What is the main role of middleware in a Node.js application?

easy

A. To run code between receiving a request and sending a response

B. To directly send HTML pages to the client

C. To store data permanently in a database

D. To compile JavaScript code into machine code

Solution

1. Step 1: Understand middleware position in request flow

   Middleware functions run after a request is received but before the response is sent back.

2. Step 2: Identify middleware purpose

   Middleware is used to process or modify requests and responses, like logging or authentication, not to send pages or store data directly.

3. Final Answer:

   To run code between receiving a request and sending a response -> Option A

4. Quick Check:

   Middleware = code between request and response [OK]

Hint: Middleware runs between request and response [OK]

Common Mistakes:

• Thinking middleware sends the final response directly
• Confusing middleware with database operations
• Assuming middleware compiles code

2. Which of the following is the correct way to define a middleware function in Express.js?

easy

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

B. app.get((req, res) => { console.log('Middleware'); });

C. app.listen((req, res, next) => { next(); });

D. app.middleware((req, res, next) => { next(); });

Solution

1. Step 1: Recall Express middleware syntax

   Middleware is added using app.use() with a function that takes req, res, and next parameters.

2. Step 2: Check each option

   app.use((req, res, next) => { console.log('Middleware'); next(); }); uses app.use() correctly and calls next() to pass control. Others use wrong methods or miss next().

3. Final Answer:

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

4. Quick Check:

   Middleware uses app.use() and next() [OK]

Hint: Middleware uses app.use() with next() call [OK]

Common Mistakes:

• Using app.get() instead of app.use() for middleware
• Forgetting to call next() inside middleware
• Using non-existent app.middleware() method

3. Consider this Express middleware code snippet:

app.use((req, res, next) => { console.log('First'); next(); });
app.use((req, res, next) => { console.log('Second'); res.send('Done'); });
app.use((req, res, next) => { console.log('Third'); next(); });

What will be printed in the console when a request is made?

medium

A. First\nSecond\nThird

B. First\nSecond

C. Second\nThird

D. First only

Solution

1. Step 1: Trace middleware execution order

   Middleware runs in order. First logs 'First' and calls next(), so second runs next.

2. Step 2: Check response behavior in second middleware

   Second logs 'Second' and sends response with res.send('Done'), ending the request cycle. Third middleware is not called.

3. Final Answer:

   First\nSecond -> Option B

4. Quick Check:

   Middleware stops after res.send() [OK]

Hint: Middleware stops after res.send() without next() [OK]

Common Mistakes:

• Assuming all middleware always run
• Ignoring that res.send() ends response
• Thinking next() is called automatically after res.send()

4. You wrote this middleware but your app crashes with an error:

app.use((req, res) => { console.log('Logging request'); });

What is the problem?

medium

A. Middleware must send a response directly

B. Middleware cannot use console.log

C. Middleware must have three parameters: req, res, and next

D. Middleware should be defined after app.listen()

Solution

1. Step 1: Check middleware function parameters

   Express middleware needs three parameters: req, res, and next to pass control to next middleware.

2. Step 2: Identify missing next parameter

   The given middleware only has req and res, so it cannot call next(), causing the app to hang or crash.

3. Final Answer:

   Middleware must have three parameters: req, res, and next -> Option C

4. Quick Check:

   Middleware needs next parameter [OK]

Hint: Middleware needs (req, res, next) parameters [OK]

Common Mistakes:

• Omitting the next parameter
• Thinking console.log causes errors
• Believing middleware must always send response

5. You want to add middleware that logs request method and URL, then checks if user is authenticated before continuing. Which code correctly combines these tasks?

hard

A. app.use((req, res, next) => { console.log(req.method, req.url); next(); res.send('Unauthorized'); });

B. app.use((req, res) => { console.log(req.method, req.url); next(); if (!req.user) res.send('Unauthorized'); });

C. app.use((req, res, next) => { if (!req.user) next(); console.log(req.method, req.url); next(); });

D. app.use((req, res, next) => { console.log(req.method, req.url); if (!req.user) return res.status(401).send('Unauthorized'); next(); });

Solution

1. Step 1: Log request method and URL first

   app.use((req, res, next) => { console.log(req.method, req.url); if (!req.user) return res.status(401).send('Unauthorized'); next(); }); logs method and URL immediately, which is correct for logging middleware.

2. Step 2: Check authentication before calling next()

   app.use((req, res, next) => { console.log(req.method, req.url); if (!req.user) return res.status(401).send('Unauthorized'); next(); }); checks if req.user exists; if not, it sends 401 response and stops. Otherwise, it calls next() to continue.

3. Step 3: Verify other options

   The other options misuse next() or res.send(), causing incorrect flow or multiple calls.

4. Final Answer:

   app.use((req, res, next) => { console.log(req.method, req.url); if (!req.user) return res.status(401).send('Unauthorized'); next(); }); -> Option D

5. Quick Check:

   Log then check auth, call next() only if authorized [OK]

Hint: Log first, then check auth, call next() only if authorized [OK]

Common Mistakes:

• Calling next() after sending response
• Not stopping middleware on unauthorized access
• Logging after next() causing missed logs