jest.fn 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();
});

Example 4: jest mock instance

test("mock.instances", () => {
  const mockFn = jest.fn();

  const a = new mockFn();
  const b = new mockFn();

  mockFn.mock.instances[0] === a;
  mockFn.mock.instances[1] === b;
});

Example 5: jest mock restore

// equvalent to mockReset

test("mockFn.mockRestore", () => {
  const StringUtils = {
    toUpperCase(arg) {
      return arg && arg.toUpperCase();
    }
  };

  const spy = jest.spyOn(StringUtils, "toUpperCase").mockImplementation(() => "MOCK");

  expect(StringUtils.toUpperCase("arg")).toBe("MOCK");
  expect(spy).toHaveBeenCalledTimes(1);
  expect(jest.isMockFunction(StringUtils.toUpperCase)).toBeTruthy();

  spy.mockRestore();

  expect(spy("arg")).toBeUndefined();
  expect(jest.isMockFunction(StringUtils.toUpperCase)).not.toBeTruthy();
  expect(StringUtils.toUpperCase("arg")).toBe("ARG");
  expect(spy).toHaveBeenCalledTimes(1);
 
});