Let’s look at some best practices when it comes to using mocks, stubs, spies, and other kinds of test doubles.

Mock Only What You Need

• Avoid Over-Mocking: Mocking too much can make tests less valuable.
• Mock External Dependencies: Focus on mocking APIs, databases, or modules outside your control.

Reset Mocks Between Tests

Ensure Isolation: Use vi.clearAllMocks() or vi.resetAllMocks() in afterEach to reset mocks.

afterEach(() => {
	vi.clearAllMocks();
});

Use Spies to Verify Interactions

When Not to Mock: If you want to test the actual implementation but still verify interactions, use spies.

Keep Tests Readable

• Descriptive Names: Name your mocks and spies clearly to enhance readability.
• Arrange-Act-Assert Pattern: Organize tests into setup, execution, and verification phases.

Be Cautious with Global Mocks

• Restore Globals: When mocking global objects (like fetch or console), ensure they are restored after the test.

Common Pitfalls and How to Avoid Them

Overusing Mocks

Issue: Over-mocking can lead to tests that pass even when the code is broken.

Solution:

• Mock only the external dependencies.
• Allow the code under test to execute as much real logic as possible.

Mock Leakage Between Tests

Issue: Mocks retain state between tests, causing unexpected behavior.

Solution:

• Use vi.resetAllMocks() or vi.clearAllMocks() in a beforeEach or afterEach hook.

Forgetting to Restore Mocked Modules

Issue: Mocked modules persist across tests.

Solution:

• Use vi.resetModules() to reset the module registry if needed.

  beforeEach(() => {
  	vi.resetModules();
  });

Testing Implementation Details

Issue: Tests break when internal implementation changes, even if the external behavior is the same.

Solution:

• Focus on testing the outputs and side effects.
• Use mocks and spies to verify interactions, not internal logic.