jest testing function calls 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 spyon

// jest.spyOn(object, methodName)
const spy = jest.spyOn(video, 'play');

// jest.spyOn(object, methodName, accessType?)
const spy = jest.spyOn(video, 'play', 'get'); // we pass 'get'