[Jest][備忘録] mockFn.mockImplementation(fn)に引数を渡さなかったら?

TL;DR

undefinedが返る

試してみたコード

環境は

    "jest": "^28.1.3",
    "ts-jest": "^28.0.8",
    "typescript": "^4.9.3",

1~3はオマケ

sample.spec.ts

const obj = {
  sampleAdd: function (num: number) {
    return num + 10;
  },
};

beforeEach(() => {
  jest.clearAllMocks();
  jest.resetAllMocks();
  jest.restoreAllMocks();
});

it('1', () => {
  expect(obj.sampleAdd(10)).toBe(20);
});

it('2', () => {
  const spy = jest.spyOn(obj, 'sampleAdd');
  expect(obj.sampleAdd(2)).toBe(12);
  expect(spy).toHaveBeenCalledTimes(1);
});

it('3', () => {
  const spy = jest.spyOn(obj, 'sampleAdd').mockImplementation((num: number) => {
    return num + 100;
  });
  expect(obj.sampleAdd(2)).toBe(102);
  expect(spy).toHaveBeenCalledTimes(1);
});

it('4', () => {
  const spy = jest.spyOn(obj, 'sampleAdd').mockImplementation();
  expect(obj.sampleAdd(2)).toBeUndefined(); // undefinedを期待してPASSする
  expect(spy).toHaveBeenCalledTimes(1);
});

参照