Custom error extensions in GraphQL

Introduction

Custom error extensions help you add extra details to errors in GraphQL. This makes it easier to understand and fix problems.

When you want to give users more information about why a request failed.

When you need to send error codes that your app can use to handle errors better.

When you want to include extra data like which field caused the error.

When debugging and you want to log detailed error info.

When building APIs that need clear error messages for clients.

Syntax

GraphQL

throw new GraphQLError('Error message', {
  extensions: {
    code: 'ERROR_CODE',
    detail: 'Extra info about the error'
  }
});

The extensions object can hold any extra info you want to send with the error.

The code field inside extensions is often used to identify error types.

Examples

This example sends a simple error with a code to identify the problem.

GraphQL

throw new GraphQLError('User not found', {
  extensions: { code: 'USER_NOT_FOUND' }
});

This example adds a list of invalid arguments to help the client fix the input.

GraphQL

throw new GraphQLError('Invalid input', {
  extensions: {
    code: 'BAD_USER_INPUT',
    invalidArgs: ['email']
  }
});

This example explains why the user is forbidden from accessing a resource.

GraphQL

throw new GraphQLError('Permission denied', {
  extensions: {
    code: 'FORBIDDEN',
    reason: 'User does not have admin rights'
  }
});

Sample Program

This code checks if a user exists. If not, it throws a GraphQL error with a custom code. The catch block prints the error extensions as JSON.

GraphQL

const { GraphQLError } = require('graphql');

function checkUser(user) {
  if (!user) {
    throw new GraphQLError('User not found', {
      extensions: { code: 'USER_NOT_FOUND' }
    });
  }
  return 'User exists';
}

try {
  console.log(checkUser(null));
} catch (error) {
  console.log(JSON.stringify(error.extensions));
}

Important Notes

Always use meaningful codes in extensions.code to help clients handle errors.

You can add any extra fields inside extensions to provide more context.

Custom error extensions improve API clarity and debugging.

Summary

Custom error extensions add extra info to GraphQL errors.

Use extensions to send codes and details.

This helps clients understand and handle errors better.

Practice

1. What is the main purpose of using extensions in GraphQL error responses?

easy

A. To change the data returned by the query

B. To hide errors from the client

C. To speed up the query execution

D. To provide extra information like error codes and details

Solution

1. Step 1: Understand the role of error extensions

   Extensions add extra info to errors, such as codes or metadata.

2. Step 2: Compare with other options

   Extensions do not change data, speed, or hide errors; they enrich error info.

3. Final Answer:

   To provide extra information like error codes and details -> Option D

4. Quick Check:

   Extensions add info = B [OK]

Hint: Extensions add details to errors, not data or speed [OK]

Common Mistakes:

• Thinking extensions change query data
• Assuming extensions hide errors
• Confusing extensions with performance tools

2. Which of the following is the correct way to add a custom error extension in a GraphQL error object?

easy

A. { message: 'Error', extensions: ['BAD_REQUEST'] }

B. { message: 'Error', code: 'BAD_REQUEST' }

C. { message: 'Error', extensions: { code: 'BAD_REQUEST' } }

D. { error: 'Error', extensions: 'BAD_REQUEST' }

Solution

1. Step 1: Identify correct error object structure

   The error object must have a message and an extensions object with code inside.

2. Step 2: Check each option's syntax

   { message: 'Error', extensions: { code: 'BAD_REQUEST' } } correctly nests code inside extensions; others misuse or misplace fields.

3. Final Answer:

   { message: 'Error', extensions: { code: 'BAD_REQUEST' } } -> Option C

4. Quick Check:

   Extensions is an object with code = D [OK]

Hint: Extensions must be an object with keys like code [OK]

Common Mistakes:

• Putting code outside extensions
• Using extensions as a string or array
• Misnaming fields like error instead of message

3. Given this GraphQL error response:

{"errors":[{"message":"Not found","extensions":{"code":"NOT_FOUND","timestamp":"2024-06-01T12:00:00Z"}}]}

What will the client see in the extensions field?

medium

A. {"code":"NOT_FOUND","timestamp":"2024-06-01T12:00:00Z"}

B. "Not found"

C. {"message":"Not found"}

D. An empty object {}

Solution

1. Step 1: Locate the extensions field in the error

   The extensions field contains code and timestamp as key-value pairs.

2. Step 2: Understand what client receives

   The client sees the extensions object exactly as given, with code and timestamp.

3. Final Answer:

   {"code":"NOT_FOUND","timestamp":"2024-06-01T12:00:00Z"} -> Option A

4. Quick Check:

   Extensions content = A [OK]

Hint: Extensions field shows all custom error details as object [OK]

Common Mistakes:

• Confusing message with extensions
• Expecting extensions to be a string
• Ignoring extra fields like timestamp

4. You wrote this error response in your GraphQL server:

{ message: 'Unauthorized', extensions: 'AUTH_ERROR' }

Why does this cause a problem?

medium

A. Extensions must be an object, not a string

B. Message cannot be 'Unauthorized'

C. Extensions field is missing

D. The error object must not have extensions

Solution

1. Step 1: Check the type of extensions field

   Extensions must be an object with key-value pairs, not a string.

2. Step 2: Identify the error cause

   Using a string for extensions causes syntax or runtime errors in GraphQL.

3. Final Answer:

   Extensions must be an object, not a string -> Option A

4. Quick Check:

   Extensions type error = A [OK]

Hint: Extensions must always be an object, never a string [OK]

Common Mistakes:

• Using string instead of object for extensions
• Ignoring GraphQL error format rules
• Assuming message content causes error

5. You want to send a GraphQL error with a custom extension that includes an error code and a retry flag set to false. Which of these error objects correctly implements this?

hard

A. { message: 'Timeout', extensions: { code: 'TIMEOUT', retry: 0 } }

B. { message: 'Timeout', extensions: { code: 'TIMEOUT', retry: false } }

C. { message: 'Timeout', extensions: { code: 'TIMEOUT', retry: 'no' } }

D. { message: 'Timeout', extensions: { code: 'TIMEOUT', retry: null } }

Solution

1. Step 1: Check the data types for extensions fields

   The retry flag should be a boolean false, not string or null or number.

2. Step 2: Validate each option's retry value

   { message: 'Timeout', extensions: { code: 'TIMEOUT', retry: false } } uses boolean false correctly; others use wrong types.

3. Final Answer:

   { message: 'Timeout', extensions: { code: 'TIMEOUT', retry: false } } -> Option B

4. Quick Check:

   Boolean retry flag = C [OK]

Hint: Use boolean false for retry, not string or null [OK]

Common Mistakes:

• Using string 'no' instead of boolean false
• Using null or 0 for boolean flags
• Ignoring correct data types in extensions