testland/jest-tests
Configures and runs Jest - Meta-built batteries-included JS/TS unit framework with built-in `expect`, snapshot testing, mocking (`jest.mock`, `jest.fn`, `jest.spyOn`, manual `__mocks__/`), test environment selection (`jsdom` / `node`), parallel workers, coverage via Istanbul, watch mode, and CI integration via `--ci` flag. Use when the user works with React (CRA / older Next.js) or Node services and needs the most ecosystem-supported JS test framework.
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mocking.mdreferences/
Jest mocking and fake timers
Deep reference for jest-tests SKILL.md. Consult for the three mock forms,
manual __mocks__/, and fake-timer control. This is per-framework mocking
lifecycle, not mocking hygiene (for anti-patterns see test-code-conventions).
Three forms:
// jest.fn() - standalone mock function
const myMock = jest.fn();
myMock.mockReturnValue(42);
expect(myMock(5)).toBe(42);
expect(myMock).toHaveBeenCalledWith(5);
// jest.mock('./module') - automatic module mock
jest.mock('./api-client');
import { fetchUser } from './api-client';
fetchUser.mockResolvedValue({ id: 1, name: 'Alice' });
// jest.spyOn(obj, 'method') - wrap existing method
const spy = jest.spyOn(myObject, 'someMethod')
.mockImplementation(() => 'mocked');
expect(myObject.someMethod()).toBe('mocked');
spy.mockRestore();Manual mocks live in __mocks__/ adjacent to the module:
src/
api-client.js
__mocks__/
api-client.js # automatically used when jest.mock('./api-client') runsTimer mocks:
jest.useFakeTimers();
setTimeout(callback, 1000);
jest.advanceTimersByTime(1000);
expect(callback).toHaveBeenCalled();
jest.useRealTimers();