useMutation hook in GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the useMutation hook in GraphQL?

The useMutation hook is used to send data-changing requests (mutations) to a GraphQL server, such as creating, updating, or deleting data.

beginner

How do you call a mutation function returned by useMutation?

You call the mutation function like a regular function, optionally passing variables as an object, for example: mutateFunction({ variables: { id: 1 } }).

intermediate

What does useMutation return?

useMutation returns an array with two elements: the mutation function to trigger the mutation, and an object containing the mutation's current status like loading, error, and data.

intermediate

Why is it important to handle the loading and error states when using useMutation?

Handling loading shows the user the mutation is in progress, and handling error helps display any problems that happened during the mutation, improving user experience.

advanced

Can you update the UI automatically after a mutation using useMutation? How?

Yes, you can update the UI by using the refetchQueries option to reload queries or by updating the Apollo cache manually in the update callback after the mutation completes.

What does the useMutation hook primarily do?

A Send data-changing requests to the server

B Fetch data from the server

C Subscribe to real-time data updates

D Manage local component state

The useMutation hook is designed to send mutations, which change data on the server.

What is the first element returned by useMutation?

A An object with loading and error states

B A function to trigger the mutation

C The mutation result data

D A boolean indicating success

The first element is the mutation function you call to execute the mutation.

How do you pass variables to a mutation using useMutation?

A Variables are not supported in mutations

B By setting a global variable before calling the mutation

C By including variables in the GraphQL query string

D Inside the mutation function call as an object with a <code>variables</code> key

Variables are passed as an object inside the mutation function call under the variables key.

Which of these is NOT part of the mutation result object returned by useMutation?

A <code>subscribe</code>

B <code>error</code>

C <code>data</code>

D <code>loading</code>

subscribe is not part of the mutation result; it relates to subscriptions.

What option can you use with useMutation to automatically refresh queries after a mutation?

A <code>variables</code>

B <code>onCompleted</code>

C <code>refetchQueries</code>

D <code>fetchPolicy</code>

refetchQueries lets you specify queries to reload after the mutation finishes.

Explain how to use the useMutation hook to send a mutation and handle its result.

Think about the steps from setup to calling and handling the mutation.

Describe ways to update the UI after a mutation using useMutation.

Consider how to keep the UI in sync with server changes.

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