Custom error extensions in GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are custom error extensions in GraphQL?

Custom error extensions are additional fields added to GraphQL error responses to provide more details about the error, such as error codes or extra context.

intermediate

How do you add custom error extensions in a GraphQL resolver?

You throw an error with an 'extensions' property, for example:
throw new ApolloError('Not found', 'NOT_FOUND', { resource: 'User' }) where the third argument adds custom extensions.

beginner

Why use custom error extensions instead of just error messages?

Custom extensions allow clients to programmatically identify error types and handle them differently, improving user experience and debugging.

beginner

What is the standard field name for custom error details in GraphQL errors?

The standard field name is 'extensions'. It is an object that can hold any extra information about the error.

intermediate

Give an example of a custom error extension field you might add.

You might add a field like { code: 'UNAUTHENTICATED', timestamp: '2024-06-01T12:00:00Z' } to indicate the error type and when it happened.

What is the purpose of the 'extensions' field in a GraphQL error?

A To store the query string

B To hold the user's authentication token

C To provide extra structured information about the error

D To define the GraphQL schema

The 'extensions' field holds extra details about the error to help clients understand and handle it.

How do you include custom error extensions when throwing an error in Apollo Server?

A By changing the HTTP status code

B By passing an object as the third argument to ApolloError

C By adding a new query

D By modifying the GraphQL schema

ApolloError accepts a third argument for extensions, allowing you to add custom error details.

Which of these is NOT a good reason to use custom error extensions?

A To store sensitive user passwords

B To provide user-friendly error codes

C To help clients handle errors programmatically

D To add timestamps or context to errors

Sensitive data like passwords should never be included in error extensions.

What type of data can the 'extensions' field contain?

A Any JSON-serializable data

B Only numbers

C Only strings

D Only booleans

The 'extensions' field can hold any data that can be converted to JSON.

If you want to indicate an error is due to authentication failure, which extension code might you use?

A SUCCESS

B NOT_FOUND

C INTERNAL_ERROR

D UNAUTHENTICATED

'UNAUTHENTICATED' is commonly used to indicate authentication errors.

Explain how custom error extensions improve error handling in GraphQL.

Think about why just an error message might not be enough.

Describe how to add custom error extensions in a GraphQL resolver using Apollo Server.

Focus on the ApolloError constructor parameters.

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