Error-first callback convention in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the error-first callback convention in Node.js?

It is a pattern where the first argument of a callback function is reserved for an error object. If there is no error, this argument is null or undefined. The following arguments carry the successful result.

beginner

Why do Node.js callbacks use the error-first pattern?

To clearly separate error handling from success logic, making it easy to check if an error occurred before processing results.

beginner

Example: In the callback function function(err, data) { ... }, what does err represent?

err represents the error object if an error happened during the asynchronous operation. If no error occurred, err is null or undefined.

beginner

How should you handle the error argument in an error-first callback?

Always check if the error argument is truthy first. If it is, handle or report the error. Only proceed with the result if the error is null or undefined.

intermediate

What happens if you ignore the error argument in an error-first callback?

Ignoring the error can cause your program to behave unexpectedly or crash later because you miss handling problems early.

In an error-first callback, what is the first argument usually used for?

A To pass configuration options

B To pass the successful result

C To pass an error object if an error occurred

D To pass a callback function

The first argument is reserved for an error object if an error happened; otherwise, it is null or undefined.

What should you do first inside an error-first callback function?

A Return without any checks

B Check if the error argument is truthy

C Ignore the error argument

D Process the data immediately

You should always check if the error argument is truthy to handle errors before processing data.

If the error argument is null in an error-first callback, what does it mean?

A The function is synchronous

B An error occurred, stop processing

C The callback was not called

D No error occurred, proceed with the result

A null error means no error happened, so you can safely use the result data.

Which of these is a correct error-first callback signature?

A function(err, result) { ... }

B function(result, err) { ... }

C function(data) { ... }

D function(callback, error) { ... }

The error-first callback always has the error as the first argument, followed by the result.

What is a common problem if you forget to check the error argument in a callback?

A Your program might crash or behave unexpectedly

B The callback will never be called

C The error argument will be automatically handled

D The program will run faster

Ignoring errors can cause crashes or bugs because problems are not handled early.

Explain the error-first callback convention and why it is useful in Node.js.

Think about how callbacks tell you if something went wrong first.

Describe how you should write a callback function following the error-first convention.

Focus on the order and handling of error and result.