Middleware execution flow (req, res, next) in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a middleware function that logs the request method.

Express

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

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

Asend

Bnext

Cend

Dhandle

Attempts:

3 left

💡 Hint

Common Mistakes

Using send or end instead of next causes the request to stop.

The next function is used to pass control to the next middleware.

2fill in blank

medium

Complete the code to call the next middleware after logging the URL.

Express

app.use((req, res, [1]) => {
  console.log(req.url);
  [1]();
});

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

Ahandle

Bsend

Cend

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to call next() causes the request to hang.

Calling next() passes control to the next middleware in the stack.

3fill in blank

hard

Fix the error in the middleware to properly pass control to the next function.

Express

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

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

Anext

Bsend

Cend

Dhandle

Attempts:

3 left

💡 Hint

Common Mistakes

Writing next; instead of next(); does not call the function.

You must call next() as a function, not just reference it.

4fill in blank

hard

Fill both blanks to create middleware that checks if user is authenticated and calls next or sends 401.

Express

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

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

Anext

Bsend

Cjson

Dend

Attempts:

3 left

💡 Hint

Common Mistakes

Using end or json instead of send for the response.

The middleware uses next to continue if authenticated, otherwise sends a 401 response with send.

5fill in blank

hard

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

Express

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

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

Anext

Bmethod

Curl

Dbody

Attempts:

3 left

💡 Hint

Common Mistakes

Using body instead of url for the request path.

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

Practice

1. What does the next() function do in Express middleware?

easy

A. It passes control to the next middleware or route handler.

B. It sends the response back to the client immediately.

C. It stops the middleware chain and ends the request.

D. It modifies the request object permanently.

Solution

1. Step 1: Understand middleware flow

   Middleware functions receive req, res, and next. Calling next() tells Express to continue to the next middleware or route.

2. Step 2: Identify the role of next()

   Calling next() does not send a response or stop the chain; it moves control forward.

3. Final Answer:

   It passes control to the next middleware or route handler. -> Option A

4. Quick Check:

   next() = pass control [OK]

Hint: Remember: next() moves to the next step in middleware [OK]

Common Mistakes:

• Thinking next() sends a response
• Assuming next() stops middleware chain
• Confusing next() with modifying req or res

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

easy

A. app.use((req, res, next) => { next(); });

B. app.use((req, res) => { next(); });

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

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

Solution

1. Step 1: Check middleware function signature

   Express middleware must have three parameters: req, res, and next.

2. Step 2: Verify correct usage of next()

   Calling next() as a function is required to pass control. app.use((req, res, next) => { next(); }); correctly uses next() with all parameters.

3. Final Answer:

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

4. Quick Check:

   Middleware signature + next() call = app.use((req, res, next) => { next(); }); [OK]

Hint: Middleware needs (req, res, next) and call next() as a function [OK]

Common Mistakes:

• Omitting the next parameter
• Calling next without parentheses
• Using res.next() which does not exist

3. Consider this Express middleware chain:

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

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

medium

A. A

B. A B

C. B A

D. No output

Solution

1. Step 1: Trace middleware execution order

   The first middleware logs 'A' and calls next(), so control moves to the second middleware.

2. Step 2: Continue to second middleware

   The second middleware logs 'B' and calls next(), moving to the third middleware which sends the response.

3. Final Answer:

   A B -> Option B

4. Quick Check:

   Middleware logs in order = A B [OK]

Hint: Middleware runs in order; next() moves forward [OK]

Common Mistakes:

• Assuming logs appear after response
• Thinking next() skips middleware
• Confusing order of middleware calls

4. What is the problem with this middleware code?

app.use((req, res, next) => {
  console.log('Start');
  // missing next() call
});
app.use((req, res) => {
  res.send('Hello');
});

medium

A. The second middleware is missing next parameter.

B. console.log() causes an error in middleware.

C. res.send() cannot be called in middleware.

D. The first middleware never calls next(), so request hangs.

Solution

1. Step 1: Check middleware flow control

   The first middleware logs 'Start' but does not call next() or send a response, so Express waits indefinitely.

2. Step 2: Understand consequence of missing next()

   Without calling next(), the second middleware never runs, and the client gets no response.

3. Final Answer:

   The first middleware never calls next(), so request hangs. -> Option D

4. Quick Check:

   Missing next() causes hang [OK]

Hint: Always call next() or send response to avoid hanging [OK]

Common Mistakes:

• Ignoring missing next() causes no response
• Thinking console.log() breaks middleware
• Assuming res.send() is invalid in middleware

5. You want to create middleware that logs request method and URL, then passes control only if the method is GET. Otherwise, it should send a 405 status. Which code correctly implements this?

hard

A. app.use((req, res, next) => {
console.log(req.method, req.url);
next();
res.status(405).send('Method Not Allowed');
});

B. app.use((req, res, next) => {
console.log(req.method, req.url);
if (req.method === 'GET') res.send('OK');
else next();
});

C. app.use((req, res, next) => {
console.log(req.method, req.url);
if (req.method === 'GET') next();
else res.status(405).send('Method Not Allowed');
});

D. app.use((req, res) => {
console.log(req.method, req.url);
if (req.method !== 'GET') next();
else res.status(405).send('Method Not Allowed');
});

Solution

1. Step 1: Log method and URL

   The middleware logs req.method and req.url correctly in all options.

2. Step 2: Control flow based on method

   app.use((req, res, next) => { console.log(req.method, req.url); if (req.method === 'GET') next(); else res.status(405).send('Method Not Allowed'); }); calls next() only if method is GET, else sends 405 status. This matches the requirement.

3. Step 3: Identify incorrect options

   app.use((req, res, next) => { console.log(req.method, req.url); if (req.method === 'GET') res.send('OK'); else next(); }); sends 'OK' on GET instead of passing control. app.use((req, res, next) => { console.log(req.method, req.url); next(); res.status(405).send('Method Not Allowed'); }); calls next() then tries to send response (invalid). app.use((req, res) => { console.log(req.method, req.url); if (req.method !== 'GET') next(); else res.status(405).send('Method Not Allowed'); }); lacks next parameter and reverses logic.

4. Final Answer:

   app.use((req, res, next) => { console.log(req.method, req.url); if (req.method === 'GET') next(); else res.status(405).send('Method Not Allowed'); }); -> Option C

5. Quick Check:

   next() only on GET, else 405 = app.use((req, res, next) => { console.log(req.method, req.url); if (req.method === 'GET') next(); else res.status(405).send('Method Not Allowed'); }); [OK]

Hint: Call next() only for GET; send 405 otherwise [OK]

Common Mistakes:

• Calling next() then sending response
• Swapping condition logic
• Missing next parameter in middleware