Error-handling middleware in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does error-handling middleware behave in Express?

Consider an Express app with a route that throws an error. What happens when an error-handling middleware is defined with four parameters?

Which statement best describes the behavior?

A The error-handling middleware runs for every request, regardless of errors.

B The error-handling middleware only handles errors if they are thrown asynchronously inside setTimeout.

C The error-handling middleware catches errors passed with next(err) or thrown synchronously and handles them.

D The error-handling middleware must be the first middleware to catch errors.

💡 Hint

Error-handling middleware in Express has a special signature with four parameters.

In Express, error-handling middleware is defined with four parameters: (err, req, res, next). It catches errors passed to next(err) or thrown synchronously in routes or middleware. It does not run for every request, only when an error occurs.

📝 Syntax

intermediate

Identify the correct error-handling middleware syntax in Express

Which of the following middleware functions is correctly defined as an error-handling middleware in Express?

A function (req, res) { res.status(500).send('Error occurred'); }

B function (err, req, res, next) { res.status(500).send('Error occurred'); }

C function (req, res, next) { res.status(500).send('Error occurred'); }

D function (err, req, res) { res.status(500).send('Error occurred'); }

💡 Hint

Error-handling middleware must have four parameters.

Error-handling middleware in Express must have exactly four parameters: (err, req, res, next). This signature tells Express to treat it as error-handling middleware.

🔧 Debug

advanced

Why does this error-handling middleware not catch errors?

Given this Express app code:

app.get('/test', (req, res) => { throw new Error('fail'); });
app.use((req, res, next) => { res.status(404).send('Not found'); });
app.use((err, req, res, next) => { res.status(500).send('Server error'); });

Why does the error-handling middleware not catch the thrown error?

A Because the 404 middleware is placed before the error-handling middleware and does not call next(err).

B Because the error-handling middleware must be defined before any routes.

C Because the thrown error is asynchronous and not caught by Express.

D Because the error-handling middleware is missing the next parameter.

💡 Hint

Middleware order matters in Express.

The 404 middleware catches all requests not handled by previous routes but does not pass errors to next(err). Since it is placed before the error-handling middleware and does not forward errors, the error-handling middleware never runs.

❓ state_output

advanced

What is the response status code when an error is passed to next()?

In this Express app snippet:

app.get('/fail', (req, res, next) => { next(new Error('fail')); });
app.use((err, req, res, next) => { res.status(400).send('Bad request'); });

What status code will the client receive when requesting /fail?

A 404

B 500

C 200

D 400

💡 Hint

The error-handling middleware sets the status code explicitly.

The error-handling middleware sets the response status to 400 before sending the response. So the client receives status 400.

🧠 Conceptual

expert

Which statement about error-handling middleware chaining is true?

In Express, if multiple error-handling middleware are defined, how does Express decide which one to run?

A Express runs error-handling middleware in the order they are defined until one sends a response or calls next().

B Express runs only the first error-handling middleware and ignores the rest.

C Express runs all error-handling middleware in parallel.

D Express randomly picks one error-handling middleware to run.

💡 Hint

Think about how middleware chaining works in Express.

Express runs error-handling middleware in the order they are defined. Each can handle the error or pass it on by calling next(err). This allows chaining multiple error handlers.