mock constructor parameters jest code example

Example 1: jest mock class

test("es6 class", () => {
    const SomeClass = jest.fn();
    const mMock = jest.fn();

    SomeClass.mockImplementation(() => {
      return {
        m: mMock
      };
    });

    const some = new SomeClass();
    some.m("a", "b");
    expect(mMock.mock.calls).toEqual([["a", "b"]]);
  });

Example 2: 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;
});