Why I like TypeScript — 1 Reliable Mocking with Interface

test_without_interface.ts

describe('sendSMS()', () => {
  it('returns status code of smsClient.singleSend method', async () => {
    // given
    const verification = new Verification();
    const mockSMSClient = jest.fn();
    // which mocks real response from thir party request
    const fakeSMSResponse = {
      code: 0,
      msg: 'dummy message',
    };
    mockSMSClient.mockReturnValue(Promise.resolve(fakeSMSResponse));
    verification.SMSClient.singleSend = mockSMSClient;
    // when
    const result = await verification.sendSMS();
    // then
    expect(result).toBe(fakeSMSResponse.code);
  });
});