How do I unit test TanStack's queryClient

In unit testing, we typically focus on whether the code functions as expected while testing in an isolated environment to avoid interference from dependencies or external factors. For TanStack Query (previously known as React Query)'s queryClient, we can achieve unit testing through the following steps:

1. Setting Up the Test Environment

First, ensure your project has the necessary testing libraries installed, such as Jest and React Testing Library, which help render components and execute unit tests.

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

2. Creating a Test QueryClient Instance

In testing, we typically create an independent QueryClient instance to ensure isolation between tests, preventing state sharing from causing test results to interfere with each other.

javascript
import { QueryClient } from 'react-query';

const createTestQueryClient = () => new QueryClient({
  defaultOptions: {
    queries: {
      retry: false, // Disable retries to reduce uncertainty
    },
  },
});

3. Writing Test Cases

Write test cases based on the functionality you want to test. For example, if we want to verify whether QueryClient correctly fetches and caches data, we can use mocked API responses to simulate the entire data fetching process.

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

jest.mock('axios');
const mockedAxios = axios as jest.Mocked<typeof axios>;

test('Using queryClient to fetch data', async () => {
  const queryClient = createTestQueryClient();
  const wrapper = ({ children }) => (
    <QueryClientProvider client={queryClient}>
      {children}
    </QueryClientProvider>
  );

  // Mock API response
  mockedAxios.get.mockResolvedValue({ data: 'test data' });

  const { result, waitFor } = renderHook(() => useQuery('testQuery', () => axios.get('fakeurl')), { wrapper });

  await waitFor(() => result.current.isSuccess);

  expect(result.current.data).toEqual('test data');
  expect(queryClient.getQueryData('testQuery')).toEqual('test data');
});

4. Cleanup and Reset

After each test run, clean up and reset the mock or the created QueryClient instance state to ensure test independence.

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

Through these steps, we can effectively unit test TanStack's queryClient, ensuring its functionality is correct and stable.