Mocking & Spying in Jest

Jest Mock Functions

const mockFn = jest.fn((x: number) => x * 2);
mockFn(2);
expect(mockFn).toHaveBeenCalledWith(2);
expect(mockFn).toHaveReturnedWith(4);

Manual Mock Implementation

jest.spyOn(Math, 'random').mockReturnValue(0.5);
expect(Math.random()).toBe(0.5);

Auto‑mocking a module

jest.mock('../src/api'); // creates a mock based on the module’s shape
import { fetchUser } from '../src/api';
(fetchUser as jest.Mock).mockResolvedValue({ id: 1, name: 'Bob' });

Resetting mocks

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