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.