jest mockImplementationOnce code example

Example 1: 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 2: jest mockclear

test("mockFn.mockClear", () => {
  const mockFn = jest.fn().mockImplementation(() => 42);
  const MockClass = jest.fn();

  new MockClass();
  expect(mockFn()).toBe(42);

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

  new MockClass();
  expect(mockFn()).toBe(42);

  expect(mockFn.mock.calls).toHaveLength(2);
  expect(MockClass.mock.instances).toHaveLength(2);

  mockFn.mockClear();
  MockClass.mockClear();

   new MockClass();
  expect(mockFn()).toBe(42);

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