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.