Unit Tests For UseFetchStaticFilters Hook: 80% Coverage

In this comprehensive guide, we'll walk through the process of adding unit tests for the useFetchStaticFilters hook, aiming for a coverage of at least 80%. This is crucial for ensuring the reliability and stability of your application, particularly within the Liatoshynsky-Foundation (lf-client) project. Unit tests help catch bugs early, make refactoring safer, and provide confidence in the correctness of your code. Let's dive into the specifics of testing this hook.

Understanding the useFetchStaticFilters Hook

The useFetchStaticFilters hook, located in app/shared/hooks/use-search/useFetchStaticFilters.ts, is designed to fetch static filter data for tables. Before we can effectively test it, we need to understand its core responsibilities:

• Fetching Data: The primary function of this hook is to retrieve static data used for filtering tables within the application. This data might include predefined categories, options, or other static datasets that users can use to refine their searches.
• Handling Endpoints: The hook needs to gracefully handle scenarios where the endpoint is null. This is a common situation in web applications where certain data might not always be available or applicable.
• Query Key Management: The hook utilizes a query key to manage the caching and invalidation of fetched data. This is important for performance and ensuring that the UI reflects the most up-to-date information.
• Integration with tableClientService: The hook interacts with the tableClientService to fetch the actual data. This service likely encapsulates the logic for making API requests and handling responses.

With these responsibilities in mind, we can formulate a strategy for testing the hook thoroughly.

Setting Up the Testing Environment

Before writing the tests, we need to set up our testing environment. This involves installing the necessary testing libraries and configuring our test runner. For this guide, we'll assume you're using Jest as your test runner and next-intl for internationalization.

1. Install Dependencies: Ensure you have Jest and any other required testing libraries installed. Typically, this involves running npm install --save-dev jest @testing-library/react @testing-library/jest-dom or the equivalent yarn command.
2. Configure Jest: Set up your jest.config.js file with the necessary configurations. This might include specifying test file patterns, setting up module aliases, and configuring any necessary transformations.
3. Mocks: We will be using mocks extensively to isolate the hook and control its dependencies. This involves mocking next-intl, the useQuery hook, and the tableClientService. Mocks allow us to simulate the behavior of these dependencies and assert that the hook interacts with them correctly.

Mocking next-intl

Since the hook likely uses next-intl for handling locales, we need to mock it to return a stable locale during testing. This ensures that our tests are consistent and not affected by changes in the user's locale.

jest.mock('next-intl', () => ({
  useLocale: () => 'en',
}));

This mock replaces the actual next-intl module with a mock implementation that always returns 'en' as the locale.

Mocking useQuery

The useQuery hook is a crucial part of fetching data in modern React applications. To test useFetchStaticFilters in isolation, we need to mock useQuery to record the arguments it's called with and optionally execute the provided queryFn. This allows us to verify that the hook is calling useQuery with the correct parameters and that the query function behaves as expected.

jest.mock('~/shared/hooks/query/useQuery', () => {
  const mockUseQuery = jest.fn((queryKey, queryFn) => {
    mockUseQuery.mock.calls.push({ queryKey, queryFn });
    return {
      data: null,
      isLoading: false,
      error: null,
    };
  });
  return mockUseQuery;
});

This mock provides a function that records the queryKey and queryFn arguments passed to useQuery. It also returns a default object with data, isLoading, and error properties, which can be customized in individual tests.

Mocking tableClientService

Finally, we need to mock the tableClientService and its getTableStaticData method. This allows us to control the data returned by the service and verify that the hook calls it with the correct locale.

jest.mock('~/services/client/tableService', () => ({
  getTableStaticData: jest.fn(),
}));

This mock replaces the actual tableClientService with a mock object that has a getTableStaticData method. The getTableStaticData method is a Jest mock function, which allows us to track its calls and return custom values.

Writing Unit Tests

With our testing environment set up, we can now write unit tests for the useFetchStaticFilters hook. We'll focus on validating the following:

• Query Key: The hook should generate the correct query key based on the locale.
• Endpoint Handling: The hook should handle cases where the endpoint is null gracefully.
• queryFn Behavior: The queryFn should call tableClientService.getTableStaticData with the correct locale.
• Data Fetching: The hook should return the data fetched by tableClientService.
• Loading and Error States: The hook should correctly handle loading and error states.

Here's an example of a test suite for useFetchStaticFilters:

import { renderHook } from '@testing-library/react-hooks';
import { useFetchStaticFilters } from '~/shared/hooks/use-search/useFetchStaticFilters';
import { useQuery } from '~/shared/hooks/query/useQuery';
import { getTableStaticData } from '~/services/client/tableService';
import { useLocale } from 'next-intl';

jest.mock('next-intl', () => ({
  useLocale: () => 'en',
}));

jest.mock('~/shared/hooks/query/useQuery', () => {
  const mockUseQuery = jest.fn((queryKey, queryFn) => {
    mockUseQuery.mock.calls.push({ queryKey, queryFn });
    return {
      data: null,
      isLoading: false,
      error: null,
    };
  });
  return mockUseQuery;
});

jest.mock('~/services/client/tableService', () => ({
  getTableStaticData: jest.fn(),
}));

describe('useFetchStaticFilters', () => {
  const mockUseQuery = useQuery as jest.Mock;
  const mockGetTableStaticData = getTableStaticData as jest.Mock;

  beforeEach(() => {
    mockUseQuery.mockClear();
    mockGetTableStaticData.mockClear();
  });

  it('should generate the correct query key', () => {
    renderHook(() => useFetchStaticFilters('testEndpoint'));
    expect(mockUseQuery).toHaveBeenCalledWith(
      ['static-filters', 'en', 'testEndpoint'],
      expect.any(Function)
    );
  });

  it('should handle null endpoint gracefully', () => {
    renderHook(() => useFetchStaticFilters(null));
    expect(mockUseQuery).not.toHaveBeenCalled();
  });

  it('should call tableClientService.getTableStaticData with locale in queryFn', async () => {
    const endpoint = 'testEndpoint';
    const { result } = renderHook(() => useFetchStaticFilters(endpoint));
    const queryFnCall = mockUseQuery.mock.calls[0];
    if (!queryFnCall) {
      throw new Error('queryFn was not called');
    }
    const queryFn = queryFnCall.queryFn;
    await queryFn();
    expect(mockGetTableStaticData).toHaveBeenCalledWith('en', endpoint);
  });

  it('should return data from useQuery', () => {
    mockUseQuery.mockReturnValue({
      data: [{ id: 1, name: 'Test Filter' }],
      isLoading: false,
      error: null,
    });
    const { result } = renderHook(() => useFetchStaticFilters('testEndpoint'));
    expect(result.current).toEqual([{ id: 1, name: 'Test Filter' }]);
  });

  it('should return null when useQuery returns null data', () => {
    mockUseQuery.mockReturnValue({
      data: null,
      isLoading: false,
      error: null,
    });
    const { result } = renderHook(() => useFetchStaticFilters('testEndpoint'));
    expect(result.current).toBeNull();
  });

  it('should handle loading state from useQuery', () => {
    mockUseQuery.mockReturnValue({
      data: null,
      isLoading: true,
      error: null,
    });
    const { result } = renderHook(() => useFetchStaticFilters('testEndpoint'));
    expect(result.current).toBeNull();
  });

  it('should handle error state from useQuery', () => {
    mockUseQuery.mockReturnValue({
      data: null,
      isLoading: false,
      error: new Error('Test Error'),
    });
    const { result } = renderHook(() => useFetchStaticFilters('testEndpoint'));
    expect(result.current).toBeNull();
  });
});

Test Case Breakdown

Let's break down the test cases to understand what each one is verifying:

• should generate the correct query key: This test verifies that the hook calls useQuery with the correct query key. The query key should include the 'static-filters' prefix, the locale, and the endpoint.
• should handle null endpoint gracefully: This test ensures that the hook doesn't call useQuery when the endpoint is null. This prevents unnecessary API calls and potential errors.
• should call tableClientService.getTableStaticData with locale in queryFn: This test verifies that the queryFn passed to useQuery calls tableClientService.getTableStaticData with the correct locale and endpoint.
• should return data from useQuery: This test ensures that the hook returns the data fetched by useQuery. We mock useQuery to return some data and then assert that the hook returns the same data.
• should return null when useQuery returns null data: This test verifies that the hook returns null when useQuery returns null data. This is important for handling cases where no data is available.
• should handle loading state from useQuery: This test ensures that the hook handles the loading state correctly. When useQuery is in the loading state, the hook should return null.
• should handle error state from useQuery: This test verifies that the hook handles errors correctly. When useQuery returns an error, the hook should return null.

Achieving 80% Coverage

To achieve 80% coverage, you need to ensure that your tests cover all the critical paths and branches in your code. This includes:

• Statement Coverage: Ensure that every line of code is executed by at least one test.
• Branch Coverage: Ensure that every possible branch in your code (e.g., if/else statements, switch statements) is executed by at least one test.
• Function Coverage: Ensure that every function is called by at least one test.
• Line Coverage: Ensure that every line of code in a function is executed by at least one test.

Use a coverage tool (like Jest's built-in coverage reporter) to identify areas of your code that are not covered by tests. Write additional tests to cover these areas.

Tips for Improving Coverage

• Write Tests Early: Write tests as you develop your code. This makes it easier to identify gaps in your coverage and ensures that your tests are aligned with your code.
• Focus on Edge Cases: Pay attention to edge cases and boundary conditions. These are often the source of bugs, and they're important to test thoroughly.
• Use Mocking Effectively: Mocking allows you to isolate your code and test it in a controlled environment. Use mocks to simulate different scenarios and ensure that your code behaves correctly in each case.
• Review Your Tests: Regularly review your tests to ensure that they're still relevant and effective. As your code evolves, your tests may need to be updated to reflect the changes.

Conclusion

Adding unit tests for the useFetchStaticFilters hook is a crucial step in ensuring the reliability and stability of your application. By following the steps outlined in this guide, you can achieve a coverage of at least 80% and gain confidence in the correctness of your code. Remember to focus on testing the key responsibilities of the hook, including query key generation, endpoint handling, queryFn behavior, and data fetching.

By using mocks effectively and writing comprehensive test cases, you can ensure that your hook behaves as expected in various scenarios. This not only helps catch bugs early but also makes refactoring and maintaining your code easier in the long run.

For more information on best practices for unit testing React hooks, consider exploring resources like the React Testing Library documentation.