useMutation hook in GraphQL

Introduction

The useMutation hook helps you change data on the server, like adding or updating information.

When you want to add a new item to a list, like a new user or post.

When you need to update existing information, such as changing a user's email.

When you want to delete an item from the database.

When you want to send a form and save the data on the server.

Syntax

GraphQL

const [mutateFunction, { data, loading, error }] = useMutation(MUTATION_QUERY);

mutateFunction is called to run the mutation.

The object contains data (result), loading (in progress), and error (if any).

Examples

This sets up a function addUser to add a user when called.

GraphQL

const [addUser] = useMutation(ADD_USER);

This sets up updatePost and tracks the mutation status.

GraphQL

const [updatePost, { data, loading, error }] = useMutation(UPDATE_POST);

Sample Program

This example shows how to add a new todo item using useMutation. When the button is clicked, it sends the new todo text to the server.

GraphQL

import { gql, useMutation } from '@apollo/client';

const ADD_TODO = gql`
  mutation AddTodo($text: String!) {
    addTodo(text: $text) {
      id
      text
      completed
    }
  }
`;

function AddTodo() {
  const [addTodo, { data, loading, error }] = useMutation(ADD_TODO);

  const handleAdd = () => {
    addTodo({ variables: { text: 'Learn GraphQL' } });
  };

  if (loading) return 'Saving...';
  if (error) return `Error! ${error.message}`;

  return (
    <>
      <button onClick={handleAdd}>Add Todo</button>
      {data && <p>Added: {data.addTodo.text}</p>}
    </>
  );
}

Important Notes

Always handle loading and error states to improve user experience.

Pass variables as an object inside the mutateFunction call.

Summary

useMutation lets you send changes to the server.

Call the mutation function with needed data to update the database.

Watch for loading and error to know what is happening.

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