Mock Generator

# Mock Generator > Generate mocks, stubs, and fakes for dependencies --- You are a test mock expert. The user wants to generate mocks, stubs, and fakes for external dependencies in unit tests. ## What to check first - Which mocking library is available (`jest.mock`, `unittest.mock`, `Mockito`, `Moq`, etc.) - What dependencies need mocking (HTTP clients, databases, file systems, APIs) - Whether to use automatic mocking or manual object creation - If you're mocking ES modules, CommonJS, or class-based dependencies ## Steps 1. Install the appropriate mocking library for your stack: `npm install --save-dev jest` (JavaScript), `pip install pytest-mock` (Python), or use built-in `unittest.mock` (Python standard library) 2. Identify the dependency to mock by reviewing the import statements and constructor parameters in the code under test 3. Create a mock object using the library's API: `jest.mock()` for auto-mocking or `jest.fn()` for manual function mocks in JavaScript 4. Define mock return values with `.mockReturnValue()` or `.mockResolvedValue()` for promises to control what the mock returns 5. Set up mock implementations for complex behavior using `.mockImplementation()` or side effects with `.mockImplementationOnce()` 6. Create spy assertions using `.toHaveBeenCalled()`, `.toHaveBeenCalledWith(args)`, or `.toHaveBeenCalledTimes(n)` to verify interactions 7. Reset mocks between tests with `jest.clearAllMocks()` or `jest.resetAllMocks()` to prevent test pollution 8. For advanced cases, use `.mockRejectedValue()` for error scenarios or `.spyOn()` to wrap real implementations while tracking calls ## Code ```javascript // Mock HTTP client dependency for a user service const axios = require('axios'); const UserService = require('./UserService'); jest.mock('axios'); describe('UserService', () => { let userService; beforeEach(() => { userService = new UserService(axios); jest.clearAllMocks(); }); test('should fetch user data successfully', async () => { // Arrange: Set mock return value const mockUser = { id: 1, name: 'Alice', email: '[email protected]' }; axios.get.mockResolvedValue({ data: mockUser }); // Act: Call the function under test const result = await userService.getUser(1); // Assert: Verify mock was called and result is correct expect(axios.get).toHaveBeenCalledWith('/users/1'); expect(axios.get).toHaveBeenCalledTimes(1); expect(result).toEqual(mockUser); }); test('should handle API errors gracefully', async () => { // Arrange: Mock rejection const error = new Error('Network error'); axios.get.mockRejectedValue(error); ``` *Note: this example was truncated in the source. See [the GitHub repo](https://github.com/Samarth0211/claude-skills-hub) for the latest full version.* ## Common Pitfalls - Treating this skill as a one-shot solution — most workflows need iteration and verification - Skipping the verification steps — you don't know it worked until you measure - Applying this skill without understanding the underlying problem — read the related docs first ## When NOT to Use This Skill - When a simpler manual approach would take less than 10 minutes - On critical production systems without testing in staging first - When you don't have permission or authorization to make these changes ## How to Verify It Worked - Run the verification steps documented above - Compare the output against your expected baseline - Check logs for any warnings or errors — silent failures are the worst kind ## Production Considerations - Test in staging before deploying to production - Have a rollback plan — every change should be reversible - Monitor the affected systems for at least 24 hours after the change --- *From [CLSkills.in](https://clskills.in/browse) — 2,300+ free Claude Code skills*