useMutation hook in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this mutation call?

Given the following GraphQL mutation using useMutation, what will be the value of data after the mutation completes successfully?

GraphQL

const [addUser, { data, loading, error }] = useMutation(ADD_USER);

addUser({ variables: { name: "Alice", age: 30 } });

A null

B {"loading": true}

C {"addUser": {"id": "1", "name": "Alice", "age": 30}}

D {"error": "User already exists"}

💡 Hint

The data object contains the server response after the mutation completes.

When the mutation completes successfully, data holds the returned object from the server, which includes the new user's details.

🧠 Conceptual

intermediate

What does the useMutation hook return?

Which of the following correctly describes the values returned by the useMutation hook in Apollo Client?

A A promise that resolves when the mutation completes.

B A single function that executes the mutation and returns the result directly.

C An object with only the mutation function and no state information.

D An array with a mutation function and an object containing mutation state like loading, error, and data.

💡 Hint

Think about how React hooks usually return multiple values.

The useMutation hook returns a tuple: the first element is the mutation function, and the second is an object with the mutation's current state.

📝 Syntax

advanced

Identify the syntax error in this mutation usage

Which option contains the correct syntax to call a mutation with variables using useMutation?

GraphQL

const [updateUser] = useMutation(UPDATE_USER);

updateUser(???);

A updateUser({ variables: { id: 1, name: "Bob" } })

B updateUser(variables: { id: 1, name: "Bob" })

C updateUser({ id: 1, name: "Bob" })

D updateUser({ vars: { id: 1, name: "Bob" } })

💡 Hint

The variables must be passed inside an object with a key named variables.

The mutation function expects an object with a variables property containing the variables object.

❓ optimization

advanced

How to update the cache after a mutation?

After running a mutation with useMutation, which option correctly updates the Apollo Client cache to include the new item?

A Rely on automatic cache updates; no action needed.

B Use the <code>update</code> function in <code>useMutation</code> options to modify the cache manually.

C Call <code>refetchQueries</code> to reload all queries after mutation.

D Use <code>setState</code> to update local React state instead of cache.

💡 Hint

Manual cache updates allow precise control without extra network requests.

The update function lets you write directly to the Apollo cache after a mutation, avoiding unnecessary refetches.

🔧 Debug

expert

Why does this mutation cause an error?

Consider this mutation call:

const [deleteUser] = useMutation(DELETE_USER);
deleteUser({ variables: { id: undefined } });

What error will this cause and why?

A GraphQL error: Variable '$id' of required type 'ID!' was not provided.

B TypeError: Cannot read property 'id' of undefined.

C SyntaxError: Unexpected token in mutation variables.

D No error; mutation runs successfully with undefined id.

💡 Hint

Required variables must be provided with valid values.

The mutation expects a non-null ID, but undefined was passed, causing a GraphQL validation error.

Practice

1. What is the primary purpose of the useMutation hook in GraphQL?

easy

A. To subscribe to real-time updates

B. To fetch data from the server

C. To cache data locally

D. To send changes or updates to the server

Solution

1. Step 1: Understand the role of useMutation

   The useMutation hook is designed to send changes or updates to the server, such as creating, updating, or deleting data.

2. Step 2: Differentiate from other hooks

   Unlike useQuery which fetches data, useMutation is for sending data changes.

3. Final Answer:

   To send changes or updates to the server -> Option D

4. Quick Check:

   useMutation = send changes [OK]

Hint: useMutation always sends updates, not fetches [OK]

Common Mistakes:

• Confusing useMutation with useQuery
• Thinking useMutation fetches data
• Assuming useMutation caches data

2. Which of the following is the correct way to call a mutation function returned by useMutation?

easy

A. const [addUser] = useMutation(ADD_USER); addUser({ variables: { name: 'Alice' } });

B. const addUser = useMutation(ADD_USER); addUser({ name: 'Alice' });

C. const addUser = useMutation(ADD_USER); addUser();

D. const [addUser] = useMutation(ADD_USER); addUser('Alice');

Solution

1. Step 1: Understand useMutation return value

   useMutation returns an array where the first element is the mutation function.

2. Step 2: Correctly call the mutation function

   The mutation function is called with an object containing a variables key holding the data to send.

3. Final Answer:

   const [addUser] = useMutation(ADD_USER); addUser({ variables: { name: 'Alice' } }); -> Option A

4. Quick Check:

   Call mutation with variables object [OK]

Hint: Call mutation with { variables: {...} } object [OK]

Common Mistakes:

• Calling mutation without variables object
• Not destructuring the mutation function
• Passing variables directly without wrapping

3. Given the code below, what will be the value of loading immediately after calling addPost({ variables: { title: 'Hello' } })?

const [addPost, { loading, error }] = useMutation(ADD_POST);
addPost({ variables: { title: 'Hello' } });

medium

A. undefined

B. false

C. true

D. null

Solution

1. Step 1: Understand loading state in useMutation

   When the mutation function is called, loading becomes true until the server responds.

2. Step 2: Check immediate state after calling mutation

   Immediately after calling addPost, the mutation is in progress, so loading is true.

3. Final Answer:

   true -> Option C

4. Quick Check:

   Mutation called = loading true [OK]

Hint: loading is true while mutation runs [OK]

Common Mistakes:

• Assuming loading is false immediately
• Confusing loading with error
• Expecting loading to be undefined

4. Identify the error in the following code snippet using useMutation:

const [updateUser, { loading, error }] = useMutation(UPDATE_USER);

updateUser({ name: 'Bob' });

medium

A. Mutation function called without wrapping variables in an object

B. Mutation function not destructured from useMutation

C. Missing import of useMutation

D. Using wrong hook for mutation

Solution

1. Step 1: Check how mutation function is called

   The mutation function expects an object with a variables key, but here it is called with { name: 'Bob' } directly.

2. Step 2: Identify correct call format

   The correct call should be updateUser({ variables: { name: 'Bob' } }).

3. Final Answer:

   Mutation function called without wrapping variables in an object -> Option A

4. Quick Check:

   Variables must be inside variables object [OK]

Hint: Always wrap variables inside { variables: {...} } [OK]

Common Mistakes:

• Passing variables directly without wrapping
• Forgetting to destructure mutation function
• Calling mutation without arguments

5. You want to update a user's email and then immediately fetch the updated user data. Which approach using useMutation is best to ensure the UI shows fresh data?

hard

A. Call the mutation and manually update the cache without refetching

B. Call the mutation, then use refetchQueries option to reload the user query

C. Call the mutation without any options and rely on cache update

D. Call the mutation and ignore loading and error states

Solution

1. Step 1: Understand data freshness after mutation

   After a mutation, the UI may show stale data unless the cache is updated or queries are refetched.

2. Step 2: Use refetchQueries to reload fresh data

   Using the refetchQueries option with useMutation triggers a fresh fetch of specified queries, ensuring updated UI.

3. Final Answer:

   Call the mutation, then use refetchQueries option to reload the user query -> Option B

4. Quick Check:

   Use refetchQueries for fresh data after mutation [OK]

Hint: Use refetchQueries to refresh data after mutation [OK]

Common Mistakes:

• Ignoring cache updates causing stale UI
• Not handling loading or error states
• Assuming mutation auto-refreshes queries