How to use Mock functionality in Jest? How to create Mock functions and mock modules? - 面试题

Jest's Mock functionality is an important tool for isolating dependencies and verifying behavior in tests:

1. Creating Mock Functions:

javascript
const mockFn = jest.fn();
mockFn('arg1', 'arg2');
expect(mockFn).toHaveBeenCalledWith('arg1', 'arg2');
expect(mockFn).toHaveBeenCalledTimes(1);

2. Mock Return Values:

javascript
const mockFn = jest.fn();
mockFn.mockReturnValue(42);
mockFn.mockReturnValueOnce(10).mockReturnValueOnce(20);

// Or use mockResolvedValue for Promises
mockFn.mockResolvedValue('data');

3. Mock Implementation:

javascript
const mockFn = jest.fn((a, b) => a + b);
// Or use mockImplementation
mockFn.mockImplementation((a, b) => a * b);

4. Mock Modules:

javascript
jest.mock('./api', () => ({
  fetchData: jest.fn(() => Promise.resolve('data'))
}));

import { fetchData } from './api';

5. Spy Functions:

javascript
const spy = jest.spyOn(object, 'method');
spy.mockRestore(); // Restore original function

Common Assertions:

toHaveBeenCalled(): Was called
toHaveBeenCalledWith(...args): Called with specific arguments
toHaveBeenCalledTimes(n): Number of calls
toHaveReturnedWith(value): Returned specific value
toHaveLastReturnedWith(value): Last returned value

Best Practices:

Mock external dependencies, not the code being tested
Clean up mocks after testing
Use jest.clearAllMocks() to clear call records
Use jest.restoreAllMocks() to restore original implementations