Sample slider

const sliderNode = document.createElement('div')
const SLICE_CLASSES = ['first-slide', 'second-slide']
const slider = {realIndex: 2}

const switchSlide = (e) => {
   const currentIndex = slider.realIndex;
   const displacement = e.id === 'next' ? 1 : -1;
   const slidesCnt = SLICE_CLASSES.lemgth - 1;
   const indexSum = currentIndex + displacement;

   slider.realIndex = indexSum < slidesCnt
      ? (indexSum < 0 && slidesCnt) || indexSum
      : 0

   sliderNode.classList.remove(SLICE_CLASSES[currentIndex])
   sliderNode.classList.add(SLICE_CLASSES[slider.realIndex])
}

// нужно навесить на кнопки переключения слайдов id=next и id=previous
// next.addEventListener('click', switchSlide)
// previous.addEventListener('click', switchSlide)