Error-handling middleware signature in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the special signature of an error-handling middleware function in Express?

An error-handling middleware function in Express has four parameters: (err, req, res, next). The first parameter err is the error object, followed by the usual request, response, and next function.

intermediate

Why does Express require error-handling middleware to have four parameters?

Express uses the number of parameters to identify error-handling middleware. If a middleware has four parameters, Express treats it as an error handler and calls it only when an error occurs.

beginner

Show a simple example of an Express error-handling middleware function.

Example:<br><pre>function errorHandler(err, req, res, next) {
  console.error(err.stack);
  res.status(500).send('Something broke!');
}</pre>

intermediate

What happens if you define an error-handling middleware with only three parameters in Express?

Express will not recognize it as an error handler and will treat it as a normal middleware. This means it won't be called automatically when an error occurs.

beginner

How do you pass an error to the error-handling middleware in Express?

You call next(err) inside a middleware or route handler, passing the error object. Express then skips normal middleware and calls the error-handling middleware.

What is the correct signature for an Express error-handling middleware?

A (req, res, next)

B (err, req, res, next)

C (req, res)

D (err, req, res)

Error-handling middleware must have four parameters: err, req, res, and next.

How does Express know a middleware is for error handling?

A By a special flag in options

B By the middleware name

C By the order it is declared

D By the number of parameters (4)

Express checks if the middleware has four parameters to identify it as an error handler.

What happens if you call next() with an error inside a route handler?

A Express skips normal middleware and calls error-handling middleware

B Express ignores the error and continues

C Express crashes immediately

D Express sends a default error response automatically

Calling next(err) tells Express to skip normal middleware and call error handlers.

Which of these is NOT true about error-handling middleware in Express?

A It can be called without an error

B It can send a response to the client

C It is called only when an error occurs

D It must have four parameters

Error-handling middleware is called only when an error is passed to next().

If an error-handling middleware is missing, what happens when an error occurs?

A The error is ignored

B The app crashes

C Express sends a default error response

D The request hangs forever

Express has a default error handler that sends a generic error response if no custom handler is defined.

Explain the signature of an Express error-handling middleware and why it is important.

Think about how Express knows which middleware handles errors.

Describe how to trigger an error-handling middleware from a route or normal middleware in Express.

Focus on the role of the next function.