Authentication errors in context in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Authentication Error Propagation

In a GraphQL API, when a user provides invalid credentials, how should the authentication error be handled in the resolver context to ensure the client receives a clear message?

A Throw an error inside the resolver without catching it, so the GraphQL server sends a generic error message.

B Return null from the resolver and log the error only on the server side without informing the client.

C Attach an authentication error to the context and throw a specific AuthenticationError that the client can catch and display.

D Ignore the error and return default data to avoid breaking the query.

💡 Hint

Think about how to communicate errors clearly to the client while maintaining security.

Throwing a specific AuthenticationError attached to the context allows the GraphQL server to send a clear, secure error message to the client. This helps the client understand the issue and respond appropriately.

❓ query_result

intermediate

Result of Query with Missing Authentication Token

Given a GraphQL query that requires authentication, what will be the result if the request is sent without an authentication token and the server uses context to check authentication?

GraphQL

query {
  userProfile {
    id
    name
  }
}

A {"errors":[{"message":"Internal server error"}],"data":null}

B {"data":{"userProfile":{"id":"123","name":"Alice"}}}

C {"data":{"userProfile":null}}

D {"errors":[{"message":"Authentication token missing","locations":[{"line":2,"column":3}],"path":["userProfile"]}],"data":{"userProfile":null}}

💡 Hint

Consider how GraphQL reports errors when authentication fails.

When authentication fails due to a missing token, the server returns an error message in the errors array and null for the data field requested.

📝 Syntax

advanced

Identify the Syntax Error in Authentication Middleware

Which option contains a syntax error in this GraphQL authentication middleware snippet that adds user info to context?

GraphQL

function context({ req }) {
  const token = req.headers.authorization || ''
  if (!token) {
    throw new AuthenticationError('No token')
  }
  const user = verifyToken(token)
  return { user }
}

A

function context({ req }) {
  const token = req.headers.authorization || ''
  if (!token) {
    throw new AuthenticationError('No token')
  }
  const user = verifyToken(token)
  return { user }
}

B

function context(req) {
  const token = req.headers.authorization || ''
  if (!token) {
    throw new AuthenticationError('No token')
  }
  const user = verifyToken(token)
  return { user }
}

C

function context({ req }) {
  const token = req.headers.authorization || ''
  if (!token) {
    throw new AuthenticationError('No token')
  }
  const user = verifyToken(token)
  return user
}

D

function context({ req }) {
  const token = req.headers.authorization || ''
  if (!token) {
    throw new AuthenticationError('No token')
  }
  const user = verifyToken(token)
  return { user: user }
}

💡 Hint

Check the function parameter syntax carefully.

Option B incorrectly uses req directly as a parameter without destructuring, but the function expects an object with a req property. This causes a runtime error when accessing req.headers.

❓ optimization

advanced

Optimizing Authentication Checks in GraphQL Context

Which option best optimizes authentication checks in the GraphQL context function to avoid redundant token verification on every resolver call?

A Verify the token once in the context function and attach the user info to context for all resolvers.

B Verify the token inside each resolver separately to keep context simple.

C Skip token verification and trust client-provided user info in context.

D Verify the token in a middleware before GraphQL execution and pass user info via context.

💡 Hint

Think about where to do the token verification for best performance and code clarity.

Verifying the token once in the context function and attaching user info avoids repeating verification in every resolver, improving performance and keeping code clean.

🔧 Debug

expert

Debugging Unexpected Authentication Error in Context

A GraphQL server returns an 'AuthenticationError: Invalid token' even when a valid token is sent. The context function is:

function context({ req }) {
  const token = req.headers.authorization.split(' ')[1]
  const user = verifyToken(token)
  return { user }
}

What is the most likely cause of the error?

A The token is being split incorrectly because the authorization header is undefined or empty.

B The verifyToken function is not imported correctly, causing it to always throw an error.

C The authorization header is missing the 'Bearer' prefix, causing split to fail.

D The context function does not return the user object properly.

💡 Hint

Check how the token is extracted from the authorization header.

If the authorization header is missing or empty, calling split(' ')[1] causes an error or undefined token, leading verifyToken to throw an 'Invalid token' error.

Practice

1. What is the main purpose of checking user information in the GraphQL context during a query?

easy

A. To prevent unauthorized users from accessing sensitive data

B. To speed up the query execution

C. To format the data before sending it

D. To log the query for analytics

Solution

1. Step 1: Understand the role of context in GraphQL

   The context carries user information that helps identify who is making the request.

2. Step 2: Recognize the purpose of authentication checks

   Checking user info in context ensures only authorized users get sensitive data.

3. Final Answer:

   To prevent unauthorized users from accessing sensitive data -> Option A

4. Quick Check:

   Authentication protects data = D [OK]

Hint: Always check user info in context to block unauthorized access [OK]

Common Mistakes:

• Thinking context is for speeding queries
• Assuming context formats data
• Believing context is only for logging

2. Which of the following is the correct way to throw an authentication error inside a GraphQL resolver using context?

easy

A. throw new Error('Not authenticated') without checking context

B. if (context.user) throw new Error('Not authenticated')

C. return 'Not authenticated' without throwing error

D. if (!context.user) throw new Error('Not authenticated')

Solution

1. Step 1: Check user presence in context

   We must verify if context.user exists to confirm authentication.

2. Step 2: Throw error when user is missing

   If no user info, throw an error to stop unauthorized access.

3. Final Answer:

   if (!context.user) throw new Error('Not authenticated') -> Option D

4. Quick Check:

   Throw error if no user = C [OK]

Hint: Throw error only when user info is missing in context [OK]

Common Mistakes:

• Throwing error when user exists
• Not checking context before throwing
• Returning error message instead of throwing

3. Given this resolver snippet:

function resolver(parent, args, context) {
  if (!context.user) {
    throw new Error('Not authenticated');
  }
  return 'Secret Data';
}

What will be the output if context.user is null?

medium

A. null

B. Error: Not authenticated

C. 'Secret Data'

D. undefined

Solution

1. Step 1: Check the condition with context.user null

   Since context.user is null, the condition !context.user is true.

2. Step 2: Understand error throwing behavior

   The resolver throws an error 'Not authenticated' and stops execution.

3. Final Answer:

   Error: Not authenticated -> Option B

4. Quick Check:

   Null user triggers error = A [OK]

Hint: Null or missing user causes error throw [OK]

Common Mistakes:

• Assuming it returns secret data anyway
• Expecting null instead of error
• Confusing error message with return value

4. Identify the error in this resolver code snippet:

function resolver(parent, args, context) {
  if (context.user === undefined) {
    throw new Error('Not authenticated');
  }
  return 'User data';
}

medium

A. Throwing error without message is wrong

B. Return statement is missing

C. It should check if context.user is falsy, not just undefined

D. No error, code is correct

Solution

1. Step 1: Analyze the user check condition

   The code only checks if context.user is exactly undefined, missing null or false cases.

2. Step 2: Explain why broader check is needed

   Users can be null or falsy; checking only undefined misses those cases, allowing unauthorized access.

3. Final Answer:

   It should check if context.user is falsy, not just undefined -> Option C

4. Quick Check:

   Check all falsy user values = B [OK]

Hint: Check all falsy user values, not just undefined [OK]

Common Mistakes:

• Checking only undefined misses null users
• Assuming error message is missing
• Thinking return is missing

5. You want to protect a GraphQL query that returns user profile data. Which approach correctly uses context to throw an authentication error only if the user is not logged in, and returns the profile otherwise?

function getUserProfile(parent, args, context) {
  // Your code here
}

hard

A. if (!context.user) throw new Error('Not authenticated'); return context.user.profile;

B. if (context.user) throw new Error('Not authenticated'); return context.user.profile;

C. return context.user.profile; if (!context.user) throw new Error('Not authenticated');

D. throw new Error('Not authenticated'); return context.user.profile;

Solution

1. Step 1: Check user presence before returning data

   The code must first verify if context.user exists to confirm authentication.

2. Step 2: Throw error if user is missing, else return profile

   If no user, throw error; otherwise, safely return user profile data.

3. Final Answer:

   if (!context.user) throw new Error('Not authenticated'); return context.user.profile; -> Option A

4. Quick Check:

   Check user first, then return profile = A [OK]

Hint: Check user before return; throw error if missing [OK]

Common Mistakes:

• Throwing error when user exists
• Returning before checking user
• Throwing error unconditionally