jest mock resolved value code example

Example 1: jest mock reset

test("mockFn.mockReset", () => {
  const mockFn = jest.fn().mockImplementation(() => 43);
  const MockClass = jest.fn();

  new MockClass();
  expect(mockFn()).toBe(43);

  expect(mockFn.mock.calls).toHaveLength(1);
  expect(MockClass.mock.instances).toHaveLength(1);

  mockFn.mockReset();
  MockClass.mockReset();

  new MockClass();
  expect(mockFn()).toBeUndefined();

  expect(mockFn.mock.calls).toHaveLength(1);
  expect(MockClass.mock.instances).toHaveLength(1);
});

Example 2: jest mock implementation

describe("mockImplementation", () => {
  test("function", () => {
    const mockFn1 = jest.fn().mockImplementation(() => 42);
    const mockFn2 = jest.fn(() => 42);

    expect(mockFn1()).toBe(42);
    expect(mockFn2()).toBe(42);
  });

Example 3: jest mock mockname

test("mockName", () => {
  const mockFn = jest.fn().mockName("mockedFunction");
  mockFn(); // comment me
  expect(mockFn).toHaveBeenCalled();
});