在最后一张幻灯片中停止自动播放

Question

我希望滑块到达最后一张幻灯片时自动停止播放。

我已经尝试过这个：

if (currentSlide == 3) { 
  clearInterval(isPlaying); 
}

但是这个没有用，知道怎么做吗？

这是我的代码：

var currentSlide = 0;

function showSlide() {
  var slides = document.getElementsByClassName("slider__slide");

  if (currentSlide >= slides.length) {
    currentSlide = 0;
  }

  if (currentSlide < 0) {
    currentSlide = 3;
  }

  for (i = 0; i < slides.length; i++) {
    slides[i].classList.remove("slide--active");
  }

  slides[currentSlide].classList.add("slide--active");
}

var isPlaying = setInterval(function() { showSlide(currentSlide += 1); }, 8000);

function nextSlide() {
  showSlide(currentSlide += 1);
  clearInterval(isPlaying);  
  isPlaying = setInterval(function() { showSlide(currentSlide += 1); }, 8000);
}

function prevSlide() {
  showSlide(currentSlide -= 1);
  clearInterval(isPlaying);  
  isPlaying = setInterval(function() { showSlide(currentSlide += 1); }, 8000);
}