Error-handling middleware in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is error-handling middleware in Node.js Express?

It is a special type of middleware designed to catch and handle errors that occur during request processing. It has four parameters: (err, req, res, next).

beginner

How do you define an error-handling middleware function in Express?

By creating a function with four arguments: (err, req, res, next). Express recognizes it as error-handling middleware because of the first 'err' parameter.

intermediate

Why should error-handling middleware be placed after all other middleware and routes?

Because it catches errors from previous middleware or routes. If placed earlier, it won't catch errors from later code.

intermediate

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

Express skips normal middleware and routes, then calls the next error-handling middleware to handle the error.

beginner

How can you send a user-friendly error message from error-handling middleware?

Use res.status() to set HTTP status code and res.json() or res.send() to send a clear message explaining the error.

Which of these is the correct signature for error-handling middleware in Express?

A (req, res)

B (err, req, res, next)

C (req, res, next)

D (err, req, res)

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

Where should error-handling middleware be placed in your Express app?

A Only inside route handlers

B Before all routes and middleware

C Anywhere in the code

D After all routes and middleware

It should be placed after all routes and middleware to catch errors from them.

What does calling next(err) inside a middleware do?

A Passes control to the next error-handling middleware

B Sends a response immediately

C Stops the request processing

D Passes control to the next normal middleware

Calling next with an error skips normal middleware and goes to error-handling middleware.

How can you send a 404 error using error-handling middleware?

A next(404)

B res.send(404)

C res.status(404).send('Not Found')

D throw 404

Use res.status(404) to set the status code and send a message.

What parameter identifies a middleware as error-handling middleware?

A The presence of 'err' as the first parameter

B The use of res.send()

C The use of next()

D The absence of next parameter

Error-handling middleware has four parameters, starting with 'err'.

Explain how error-handling middleware works in Express and why its signature is important.

Think about how Express knows which middleware handles errors.

Describe the steps to create and use error-handling middleware in a Node.js Express app.

Focus on function signature, placement, and usage.