Writing Unit Tests with Jest

What testing best practice is demonstrated here?

describe('User service', () => {
  const mockDb = {
    query: jest.fn()
  };
  
  const userService = new UserService(mockDb);
  
  beforeEach(() => {
    mockDb.query.mockClear();
  });
});

Global mocking
Mock persistence
Mock cleanup
This shows mock cleanup best practices: 1) Clears mock state between tests, 2) Prevents test interference, 3) Ensures mock call history is fresh, 4) Maintains test isolation, 5) Improves test reliability, 6) Common practice for mock-heavy tests, 7) Essential for stateful mock testing, 8) Prevents false positives from previous tests.
Mock initialization

Next Question (12/20)