Centralized error handler in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a centralized error handler in Express?

It is a single middleware function that catches and processes all errors in an Express app, so you don't have to handle errors in every route separately.

beginner

How do you define a centralized error handler middleware in Express?

You create a middleware function with four parameters: (err, req, res, next). Express recognizes it as an error handler and calls it when an error occurs.

intermediate

Why should you place the centralized error handler after all other middleware and routes?

Because Express calls middleware in order, placing the error handler last ensures it catches errors from all previous middleware and routes.

intermediate

What is the role of the 'next' function in an Express error handler?

It can be used to pass the error to the next error handler if you have multiple, or to continue the middleware chain if needed.

beginner

How can you send a user-friendly error message from a centralized error handler?

You can set the HTTP status code and send a JSON or text response with a simple message explaining the error without exposing sensitive details.

Which of these is the correct signature for an Express centralized error handler?

A (err, req, res, next)

B (req, res, next)

C (req, res)

D (err, req, res)

Express recognizes error handlers by having four parameters: err, req, res, and next.

Where should you place the centralized error handler middleware in your Express app?

A It doesn't matter

B Before all routes

C In the middle of routes

D After all routes and other middleware

The error handler should be last to catch errors from all previous middleware and routes.

What happens if you forget to call next() inside an error handler?

A The request will hang and not continue

B Nothing, it works fine

C Express will crash

D The error will be passed to the next middleware automatically

If next() is not called or response is not sent, the request will hang waiting for a response.

How can you send a 404 error using the centralized error handler?

A Use res.sendStatus(404) in routes only

B Call res.status(404).send() inside the error handler

C Throw an error with status 404 and pass it to next()

D You cannot handle 404 errors in centralized error handler

You create an error with status 404 and pass it to next(), so the centralized handler can respond.

What is a benefit of using a centralized error handler?

A It makes the app slower

B It reduces repeated error handling code in routes

C It hides all errors from developers

D It only works for syntax errors

Centralized error handling keeps your code clean by handling errors in one place.

Explain how to create and use a centralized error handler in an Express app.

Think about the special middleware signature and order.

Describe why centralized error handling improves Express app code quality.

Consider how handling errors in one place helps.