calulator.spec.tsx

it('should calculate on click', () => {
    // given:
    render(<Calculator/>);

    // when
    typeIn('expression', '6 * 7');
    clickOn('calculate');

    // then
    expect(textOf('result')).toBe('42');
  }
);

const typeIn = (selector: string, value: string) => 
    userEvent.type(screen.getByTestId(selector), value);
const clickOn = (selector: string) => userEvent.click(screen.getByTestId(selector));
const textOf = (selector: string) => screen.getByTestId(selector).textContent;