How to mock react-query useQuery in jest

When using the useQuery hook from React Query, unit and integration tests often require mocking requests and responses to ensure that components behave correctly across various data states. Jest provides multiple ways to mock these asynchronous requests, ensuring our tests are both accurate and reliable. Here is a common approach to mock useQuery:

1. Install the necessary libraries

First, ensure that your project includes jest and @testing-library/react-hooks, which is specifically designed for testing React hooks.

bash
npm install --save-dev jest @testing-library/react-hooks

2. Set up the mock

For mocking useQuery, we typically need to mock the entire react-query library since useQuery is imported from it. You can achieve this using Jest's jest.mock() method.

javascript
import { renderHook } from '@testing-library/react-hooks';
import { useQuery } from 'react-query';

jest.mock('react-query', () => ({
  useQuery: jest.fn()
}));

3. Configure the mock behavior

After setting up the mock, we can define how useQuery should respond. This typically depends on the specific scenario we want to test. For example, you might want to test loading states, success states, and error states.

javascript
// Set useQuery mock to return data
useQuery.mockImplementation((queryKey, queryFn) => ({
  isLoading: false,
  isError: false,
  data: { items: ['item1', 'item2'] },
  error: null
}));

// Or mock a loading state
useQuery.mockImplementation(() => ({
  isLoading: true,
  isError: false,
  data: undefined,
  error: null
}));

// Or mock an error state
useQuery.mockImplementation(() => ({
  isLoading: false,
  isError: true,
  data: undefined,
  error: new Error('Fetching error')
}));

4. Test your component or hook

Use the renderHook function from @testing-library/react-hooks to render your hook and perform tests.

javascript
const { result, waitForNextUpdate } = renderHook(() => useQuery('fetchData', fetchDataFn));

expect(result.current.isLoading).toBe(true);  // Check if in loading state

await waitForNextUpdate();  // Wait for asynchronous operation to complete

expect(result.current.data).toEqual({ items: ['item1', 'item2'] });  // Verify data correctness

5. Clean up after tests

After each test, reset all mocks to ensure test independence.

javascript
afterEach(() => {
  jest.resetAllMocks();
});

By following these steps, you can effectively mock the useQuery hook to validate the behavior of your components or custom hooks across different test scenarios. This approach is very helpful for maintaining test control and predictability.