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

Recall & Review

beginner

What is error-handling middleware in Express?

Error-handling middleware is a special type of middleware in Express that catches and processes errors occurring in the app. It has four parameters: (err, req, res, next).

beginner

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

You define it with four parameters: (err, req, res, next). For example: function (err, req, res, next) { /* handle error */ }.

intermediate

Why must error-handling middleware have four parameters?

Express uses the number of parameters to identify error-handling middleware. Having four parameters tells Express this middleware handles errors.

intermediate

Where should error-handling middleware be placed in the middleware stack?

It should be placed after all other middleware and routes, so it can catch errors from them.

beginner

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

Calling next(err) passes the error to the next error-handling middleware, skipping normal middleware.

Which of the following is the correct signature for an Express error-handling middleware?

A (err, req, res, next)

B (req, res, next)

C (req, res)

D (err, req, res)

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

Where should you place error-handling middleware in your Express app?

A Before all routes

B In the middle of route definitions

C After all routes and other middleware

D It does not matter

Error-handling middleware should be last to catch errors from all previous middleware and routes.

What does calling next(err) inside a route handler do?

A Skips to the next error-handling middleware

B Ends the request without response

C Skips to the next normal middleware

D Restarts the request

Calling next with an error passes control to the next error-handling middleware.

If an error occurs in a route and you don't have error-handling middleware, what happens?

A The request hangs and never responds

B Express automatically handles the error and sends a response

C The error crashes the server

D The error is ignored

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

Which parameter in error-handling middleware holds the error object?

A next

B res

C req

D err

The first parameter, err, contains the error passed to the middleware.

Explain how error-handling middleware works in Express and why it needs four parameters.

Think about how Express knows which middleware handles errors.

Describe the steps to add custom error handling to an Express app.

Consider where and how you write the error middleware function.