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Mocking Apollo Client with Jest for Unit Testing

This challenge focuses on testing components that interact with Apollo Client. We'll learn how to mock Apollo Client requests in Jest to isolate your components and ensure they behave correctly under various data scenarios, without needing a live GraphQL server. This is crucial for robust and fast unit testing of your frontend applications.

Problem Description

You are tasked with writing unit tests for a React component that fetches data using Apollo Client. Your goal is to mock the Apollo Client's query and mutate operations using Jest to simulate different GraphQL responses. This will allow you to test your component's rendering, loading states, error handling, and data display in isolation.

What needs to be achieved:

  1. Create a mock Apollo Client instance.
  2. Configure the mock client to respond to specific GraphQL queries and mutations with predefined data or errors.
  3. Write Jest tests for a React component that utilizes this mocked Apollo Client.

Key Requirements:

  • Use jest.mock to mock the apollo-client package.
  • Implement a mock ApolloClient class or object that has query and mutate methods.
  • The query and mutate mock methods should return a Promise that resolves with an object matching Apollo's ExecutionResult structure (e.g., { data: {...} } or { errors: [...] }).
  • Test scenarios should cover:
    • Successful data fetching.
    • Handling GraphQL errors.
    • Component rendering based on loading states (optional, but good practice).

Expected Behavior:

Your tests should verify that the component correctly renders data when the mock client returns successful responses, displays appropriate error messages when errors are returned, and potentially shows loading indicators.

Edge Cases to Consider:

  • What happens if a query is called with variables that were not mocked?
  • How to handle scenarios where a mutation returns no data but is considered successful.

Examples

Let's assume you have a simple React component UserProfile that fetches user data.

Example Component Snippet (Conceptual):

// UserProfile.tsx (for context, not part of the solution to implement)
import React from 'react';
import { gql, useQuery, useMutation } from '@apollo/client';

const GET_USER = gql`
  query GetUser($id: ID!) {
    user(id: $id) {
      id
      name
      email
    }
  }
`;

const UPDATE_USER_EMAIL = gql`
  mutation UpdateUserEmail($id: ID!, $email: String!) {
    updateUserEmail(id: $id, email: $email) {
      id
      email
    }
  }
`;

interface UserData {
  user: {
    id: string;
    name: string;
    email: string;
  };
}

interface UpdateUserData {
  updateUserEmail: {
    id: string;
    email: string;
  };
}

export const UserProfile: React.FC<{ userId: string }> = ({ userId }) => {
  const { loading, error, data } = useQuery<UserData>(GET_USER, {
    variables: { id: userId },
  });

  const [updateEmail] = useMutation<UpdateUserData>(UPDATE_USER_EMAIL);

  if (loading) return <p>Loading...</p>;
  if (error) return <p>Error: {error.message}</p>;

  const user = data?.user;

  const handleUpdateEmail = async () => {
    try {
      await updateEmail({
        variables: { id: userId, email: 'new.email@example.com' },
      });
      // Potentially refetch or update UI
    } catch (e) {
      console.error("Mutation failed", e);
    }
  };

  return (
    <div>
      <h1>User Profile</h1>
      {user ? (
        <>
          <p>Name: {user.name}</p>
          <p>Email: {user.email}</p>
          <button onClick={handleUpdateEmail}>Update Email</button>
        </>
      ) : (
        <p>No user data found.</p>
      )}
    </div>
  );
};

Example 1: Mocking a Successful Query

Input:
A test file (`UserProfile.test.tsx`) that imports and renders the `UserProfile` component. The test should mock Apollo Client to return successful user data for the `GET_USER` query.

Mocked Apollo Client Configuration:
The mock client's `query` method should be set up to intercept the `GET_USER` query with `variables: { id: '123' }` and return:
{
  data: {
    user: {
      id: '123',
      name: 'Alice Smith',
      email: 'alice.smith@example.com',
    },
  },
}

Expected Output (in the rendered component):
The component should display:
<h1>User Profile</h1>
<p>Name: Alice Smith</p>
<p>Email: alice.smith@example.com</p>

Example 2: Mocking a Query Error

Input:
A test file that renders the `UserProfile` component, but this time, the mock Apollo Client should simulate a network error or a GraphQL error for the `GET_USER` query.

Mocked Apollo Client Configuration:
The mock client's `query` method should be set up to intercept the `GET_USER` query with `variables: { id: '123' }` and return an error:
{
  errors: [new Error('Failed to fetch user data')],
}

Expected Output (in the rendered component):
The component should display:
<h1>User Profile</h1>
<p>Error: Failed to fetch user data</p>

Example 3: Mocking a Successful Mutation

Input:
A test file that renders the `UserProfile` component. The test should first mock the `GET_USER` query for initial data. Then, it should simulate a successful `UPDATE_USER_EMAIL` mutation.

Mocked Apollo Client Configuration:
- For `GET_USER` query with `variables: { id: '123' }`:
  {
    data: {
      user: {
        id: '123',
        name: 'Alice Smith',
        email: 'alice.smith@example.com',
      },
    },
  }
- For `UPDATE_USER_EMAIL` mutation with `variables: { id: '123', email: 'new.email@example.com' }`:
  {
    data: {
      updateUserEmail: {
        id: '123',
        email: 'new.email@example.com',
      },
    },
  }

Expected Behavior:
1.  The component initially renders with Alice's original email.
2.  When the "Update Email" button is clicked, the mock mutation is triggered.
3.  The test should assert that the mutation was called with the correct variables.
4.  (Optional for this challenge, but good to consider) If the component refetches or updates its state based on the mutation, that behavior should also be tested. For simplicity, we'll focus on verifying the mutation call.

Constraints

  • You must use Jest for testing.
  • Your solution must be in TypeScript.
  • Do not use any actual GraphQL server or network requests during testing.
  • The mock client should be able to handle multiple query and mutate calls, potentially with different configurations for each.
  • Aim for clear and readable test code.

Notes

  • Consider using a mocking library like jest-mock-extended for creating more sophisticated mocks, but it's not strictly required for this challenge. The core is understanding how to mock the query and mutate methods.
  • The useQuery and useMutation hooks from @apollo/client internally call the ApolloClient instance. Your mock needs to intercept these calls.
  • Think about how to match the incoming GraphQL query document and variables to the correct mock response. You might need to stringify queries or compare ASTs for more robust matching, but for this challenge, direct comparison of query objects or string representations might suffice.
  • Remember to provide mock data in a structure that mirrors what your actual Apollo client would return (e.g., { data: { ... } } or { errors: [ ... ] }).
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