next() function and flow control in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when next() is called in Express middleware?

Consider this Express middleware function:

app.use((req, res, next) => {
  console.log('Middleware 1');
  next();
  console.log('After next');
});

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

Express

app.use((req, res, next) => {
  console.log('Middleware 1');
  next();
  console.log('After next');
});

A No output

B Middleware 1\nAfter next

C After next\nMiddleware 1

D Middleware 1

💡 Hint

Think about what next() does and when the code after it runs.

Calling next() passes control to the next middleware but does not stop the current function. So, 'Middleware 1' is logged, then next() moves to the next middleware, but after next() call, 'After next' is also logged.

❓ lifecycle

intermediate

How does next() affect error handling middleware?

Given these middleware functions:

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

app.use((err, req, res, next) => {
  res.send('Error handled');
});

What will the client receive when a request is made?

Express

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

app.use((err, req, res, next) => {
  res.send('Error handled');
});

A Middleware skipped, next middleware runs

B No response, request hangs

C Error handled

D Syntax error

💡 Hint

What happens when next() is called with a string argument?

Calling next() with an argument triggers Express to skip normal middleware and jump to error-handling middleware. So the error handler sends 'Error handled' to the client.

📝 Syntax

advanced

Which middleware setup causes a runtime error?

Identify which option will cause an error when a request is made:

A app.use((req, res, next) => { throw new Error('Fail'); });

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

C app.use((err, req, res, next) => { res.send('Error'); });

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

💡 Hint

Check if calling next() after sending a response causes issues.

Calling next() after res.send() can cause Express to try to send another response, leading to a runtime error about headers already sent.

🔧 Debug

advanced

Why does this middleware never call next() or send a response?

Look at this middleware:

app.use((req, res, next) => {
  if (req.query.pass) {
    next();
  }
});

What happens if req.query.pass is false or missing?

Express

app.use((req, res, next) => {
  if (req.query.pass) {
    next();
  }
});

A Request hangs because neither next() nor res.send() is called

B Request proceeds normally

C Error thrown due to missing next()

D Response sent automatically

💡 Hint

What happens if middleware neither calls next() nor sends a response?

If next() is not called and no response is sent, Express waits indefinitely, causing the request to hang.

🧠 Conceptual

expert

What is the effect of calling next('route') in Express middleware?

Consider this route setup:

app.get('/test', (req, res, next) => {
  next('route');
}, (req, res) => {
  res.send('First handler');
});

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

What response will the client receive when requesting '/test'?

Express

app.get('/test', (req, res, next) => {
  next('route');
}, (req, res) => {
  res.send('First handler');
});

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

A Second handler

B First handler

C Error: route not found

D Request hangs

💡 Hint

What does next('route') do inside route middleware?

Calling next('route') skips the remaining middleware for the current route and moves to the next matching route handler. So the second handler responds.

Practice

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

easy

A. It logs the request details to the console.

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

C. It restarts the current middleware from the beginning.

D. It ends the request and sends a response to the client.

Solution

1. Step 1: Understand middleware flow

   Express middleware functions run in sequence. Calling next() moves control to the next middleware or route.

2. Step 2: Identify next() role

   Calling next() does not send a response but passes control forward, allowing the app to continue processing.

3. Final Answer:

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

4. Quick Check:

   next() moves control forward [OK]

Hint: Remember: next() moves to next step, doesn't send response [OK]

Common Mistakes:

• Thinking next() sends a response
• Confusing next() with ending the request
• Assuming next() restarts middleware

2. Which of the following is the correct syntax to use next() in an Express middleware function?

easy

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

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

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

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

Solution

1. Step 1: Check middleware parameters

   Express middleware must have three parameters: req, res, and next to use next().

2. Step 2: Verify calling next()

   The correct way to call next() is as a function with parentheses: next();

3. Final Answer:

   function(req, res, next) { next(); } -> Option C

4. Quick Check:

   Correct syntax includes next as parameter and calls next() [OK]

Hint: next() must be called as a function with parentheses [OK]

Common Mistakes:

• Omitting next parameter
• Calling next without parentheses
• Using res.next() instead of next()

3. Consider this Express middleware chain:

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

B. First\nSecond\nThird

C. Second\nThird

D. First

Solution

1. Step 1: Trace middleware execution order

   The first middleware logs 'First' and calls next(), so the second middleware runs next.

2. Step 2: Check response and flow stop

   The second middleware logs 'Second' and sends a response with res.send('Done'). Sending a response ends the request, so the third middleware does not run.

3. Final Answer:

   First\nSecond -> Option A

4. Quick Check:

   Response sent stops flow, so third middleware skipped [OK]

Hint: Sending response stops middleware chain immediately [OK]

Common Mistakes:

• Assuming all middleware run even after response
• Thinking next() runs after res.send()
• Ignoring that res.send ends request

4. What is wrong with this Express middleware?

app.use((req, res, next) => {
  console.log('Logging request');
  // forgot to call next or send response
});

medium

A. It will cause the request to hang and never finish.

B. It will automatically call next() after logging.

C. It will send an empty response to the client.

D. It will throw a syntax error.

Solution

1. Step 1: Identify missing flow control

   The middleware logs a message but neither calls next() nor sends a response.

2. Step 2: Understand Express behavior

   Without calling next() or sending a response, Express waits indefinitely, causing the request to hang.

3. Final Answer:

   It will cause the request to hang and never finish. -> Option A

4. Quick Check:

   Missing next() or response causes hanging [OK]

Hint: Always call next() or send response to finish middleware [OK]

Common Mistakes:

• Assuming Express auto-calls next()
• Thinking console.log ends request
• Expecting default response without sending

5. You want to create middleware that checks if a user is logged in. If not logged in, it should send a 401 response. If logged in, it should continue to the next middleware. Which code correctly uses next() to achieve this?

hard

A. app.use((req, res, next) => {
if (!req.user) {
next();
} else {
res.status(401).send('Unauthorized');
}
});

B. app.use((req, res, next) => {
next();
if (!req.user) {
res.status(401).send('Unauthorized');
}
});

C. app.use((req, res, next) => {
if (req.user) {
res.status(401).send('Unauthorized');
} else {
next();
}
});

D. app.use((req, res, next) => {
if (!req.user) {
res.status(401).send('Unauthorized');
} else {
next();
}
});

Solution

1. Step 1: Check user login condition

   If req.user is missing, the middleware should send a 401 Unauthorized response immediately.

2. Step 2: Use next() correctly

   If the user is logged in, call next() to pass control to the next middleware or route handler.

3. Final Answer:

   Middleware sends 401 if no user, else calls next() -> Option D

4. Quick Check:

   Send response before next(), or call next() to continue [OK]

Hint: Send response first if unauthorized; call next() only if authorized [OK]

Common Mistakes:

• Calling next() before sending response
• Swapping conditions for sending response
• Calling next() after sending response