Error handling on client in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output when a GraphQL query returns an error?

Consider a GraphQL query that requests a user's name but the server returns an error due to a missing user ID. What will the client receive?

GraphQL

query GetUserName {
  user(id: null) {
    name
  }
}

A {"data": {"user": null}, "errors": [{"message": "User ID is required"}]}

B {"data": null}

C {"error": "User ID is required"}

D {"data": {"user": {"name": ""}}, "errors": []}

💡 Hint

GraphQL responses include both data and errors fields when errors occur.

When a GraphQL query has an error, the response contains a data field with null or partial data and an errors array describing the problem.

🧠 Conceptual

intermediate

How should a client handle partial data with errors in GraphQL?

A GraphQL query returns some data but also includes errors. What is the best practice for the client to handle this?

A Discard all data and show only the error messages.

B Retry the query automatically without informing the user.

C Display the data and show error messages to the user.

D Ignore the errors and use the data as is.

💡 Hint

GraphQL allows partial data with errors; clients should inform users appropriately.

Clients should use the available data but also inform users about errors to avoid confusion and improve user experience.

📝 Syntax

advanced

Which GraphQL client code snippet correctly handles errors in a query response?

Given the following client code snippets, which one properly checks for errors in the GraphQL response?

GraphQL

const response = await fetchGraphQL(query);
// Which snippet correctly handles errors?

A if (response.errors) { console.error(response.errors); } else { console.log(response.data); }

B if (response.data.errors) { console.error(response.data.errors); } else { console.log(response.data); }

C if (response.error) { console.error(response.error); } else { console.log(response.data); }

D if (response.data) { console.error(response.data); } else { console.log(response.errors); }

💡 Hint

GraphQL responses have an errors field at the top level, not inside data.

The errors field is a top-level array in the response object. Checking response.errors is correct.

❓ optimization

advanced

How can a client optimize error handling to reduce UI disruptions?

When a GraphQL query returns errors, how can the client minimize negative impact on user experience?

A Ignore all errors to avoid confusing the user.

B Batch multiple queries to reduce the number of error messages shown.

C Show detailed error stack traces to the user immediately.

D Reload the entire page on any error.

💡 Hint

Batching queries can reduce the frequency of errors shown to users.

Batching queries reduces the number of separate error messages, making error handling smoother and less disruptive.

🔧 Debug

expert

Why does this GraphQL client code fail to catch errors properly?

Examine the client code below. It does not detect errors returned by the GraphQL server. What is the cause?

GraphQL

const response = await fetchGraphQL(query);
if (!response.data) {
  console.error('No data received');
} else {
  console.log('Data:', response.data);
}

A The code should check 'response.data.errors' to find errors.

B The code incorrectly checks 'response.data' instead of 'response.error'.

C The code should use try-catch around the fetchGraphQL call to catch errors.

D The code does not check the 'errors' field in the response, so errors are ignored.

💡 Hint

GraphQL errors are in a separate 'errors' field, not inside 'data'.

The code only checks if 'data' exists but does not check the 'errors' field, so errors are missed.

Practice

1. What is the main purpose of error handling on the client side in GraphQL applications?

easy

A. To speed up the server response time

B. To keep the app stable and show clear messages to users

C. To change the GraphQL schema dynamically

D. To store data permanently on the client

Solution

1. Step 1: Understand client-side error handling

   Error handling on the client is about managing problems that happen when fetching or processing data, so the app doesn't crash.

2. Step 2: Identify the main goal

   The goal is to keep the app stable and inform users clearly about what went wrong.

3. Final Answer:

   To keep the app stable and show clear messages to users -> Option B

4. Quick Check:

   Error handling = stability + clear messages [OK]

Hint: Error handling means stability and clear user messages [OK]

Common Mistakes:

• Thinking error handling speeds up server
• Confusing error handling with schema changes
• Assuming error handling stores data

2. Which of the following is the correct way to catch errors from a GraphQL query using promises on the client?

easy

A. client.query(...).then(...).catch(error => handleError(error))

B. client.query(...).catch(...).then(error => handleError(error))

C. client.query(...).try(error => handleError(error))

D. client.query(...).error(error => handleError(error))

Solution

1. Step 1: Recall promise error handling syntax

   Promises use .then() for success and .catch() for errors.

2. Step 2: Match correct chaining

   The correct order is .then(...).catch(...). client.query(...).then(...).catch(error => handleError(error)) matches this.

3. Final Answer:

   client.query(...).then(...).catch(error => handleError(error)) -> Option A

4. Quick Check:

   Promise catch method = .catch() [OK]

Hint: Use .then() before .catch() to handle errors [OK]

Common Mistakes:

• Placing .catch() before .then()
• Using non-existent .try() or .error() methods
• Ignoring promise chaining order

3. Given the following code snippet, what will be logged if the GraphQL query fails?

client.query({ query: GET_USER })
  .then(response => console.log('User:', response.data.user))
  .catch(error => console.log('Error:', error.message))

medium

A. User: undefined

B. No output

C. User: null

D. Error: [error message from server]

Solution

1. Step 1: Understand promise flow on failure

   If the query fails, the .catch() block runs, logging the error message.

2. Step 2: Identify logged output

   The console logs 'Error:' followed by the error message from the server.

3. Final Answer:

   Error: [error message from server] -> Option D

4. Quick Check:

   Failed query triggers .catch() logging error [OK]

Hint: Failed queries run .catch() logging error message [OK]

Common Mistakes:

• Assuming .then() runs on failure
• Expecting 'User: undefined' instead of error
• Thinking no output appears on error

4. Identify the error in this client-side GraphQL error handling code:

try {
  const response = await client.query({ query: GET_POSTS });
  console.log(response.data.posts);
} catch {
  console.log('Failed to fetch posts');
}

medium

A. client.query does not return a promise

B. Using await inside try block is invalid

C. Missing error parameter in catch block

D. console.log cannot be used inside catch

Solution

1. Step 1: Review try-catch syntax

   The catch block should include an error parameter to access error details.

2. Step 2: Identify missing error parameter

   The code uses catch { ... } without (error), preventing access to the error details.

3. Final Answer:

   Missing error parameter in catch block -> Option C

4. Quick Check:

   catch needs (error) parameter [OK]

Hint: Always include error parameter in catch block [OK]

Common Mistakes:

• Omitting error parameter in catch
• Thinking await is invalid in try
• Believing client.query is not a promise

5. You want to show a friendly message to users when a GraphQL mutation fails due to network issues. Which approach correctly handles this on the client?

async function submitData() {
  try {
    const result = await client.mutate({ mutation: ADD_ITEM, variables: { name: 'Book' } });
    console.log('Item added:', result.data.addItem.id);
  } catch (error) {
    if (error.networkError) {
      alert('Network problem, please try again later.');
    } else {
      alert('An error occurred.');
    }
  }
}

hard

A. This code correctly distinguishes network errors and shows messages

B. The catch block should not check error properties

C. Using alert is not allowed in error handling

D. await cannot be used with client.mutate

Solution

1. Step 1: Analyze error handling logic

   The catch block checks if the error is a network error and shows a specific message.

2. Step 2: Confirm correct usage of await and alerts

   Using await with client.mutate is valid, and alerts are acceptable for user messages.

3. Final Answer:

   This code correctly distinguishes network errors and shows messages -> Option A

4. Quick Check:

   Check error.networkError to show friendly messages [OK]

Hint: Check error.networkError to show specific messages [OK]

Common Mistakes:

• Ignoring networkError property in catch
• Thinking alerts are disallowed
• Misunderstanding await usage with mutate