mocking function in object jest 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 spy on class method

//class.js
class MyClass {
  methodOne() {
    return 1;
  }
  methodTwo() {
    return 2;
  }
}
module.exports = MyClass;

// class.test.js
test('spy using class method', () => {
  const result = new MyClass()
  const spy = jest.spyOn(result, 'methodOne')
  result.methodOne()

  // check class method is call or not
  expect(spy).toHaveBeenCalled()

  // expect old value
  expect(result.methodOne()).toBe(1)

  // expect new value
  spy.mockReturnValueOnce(12)
  expect(result.methodOne()).toBe(12)
})