Jest

Assertions, async tests and focused mock behavior.

Assertions

test('adds', () => { expect(1 + 2).toBe(3); });Define an isolated test

expect(result).toEqual({ id: 1 });Compare object structure

expect(items).toContain('Ada');Assert collection membership

expect(() => fail()).toThrow();Assert a synchronous error

await expect(load()).resolves.toEqual({ ok: true });Assert a fulfilled promise

await expect(load()).rejects.toThrow('missing');Assert promise rejection

beforeEach(() => resetState());Reset shared fixtures before each test

afterEach(() => jest.restoreAllMocks());Restore spies and replaced properties

const save = jest.fn().mockResolvedValue({ id: 1 });Stub an asynchronous dependency

expect(save).toHaveBeenCalledWith({ name: 'Ada' });Assert a meaningful call contract

npx jest --runInBandRun tests serially

npx jest --coverageReport exercised source coverage