js实现简单轮播图

<div class="slideshow-container">
  <div class="mySlides fade">
    <img src="img1.jpg">
  </div>
  <div class="mySlides fade">
    <img src="img2.jpg">
  </div>
  <div class="mySlides fade">
    <img src="img3.jpg">
  </div>
</div>
<br>
<div style="text-align:center">
  <span class="dot"></span>
  <span class="dot"></span>
  <span class="dot"></span>
</div>

.slideshow-container {
  max-width: 500px;
  position: relative;
  margin: auto;
}
.mySlides {
  display: none;
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
}
.mySlides img {
  width: 100%;
  height: auto;
}
.dot {
  cursor: pointer;
  height: 15px;
  width: 15px;
  margin: 0 2px;
  background-color: #bbb;
  border-radius: 50%;
  display: inline-block;
  transition: background-color 0.6s ease;
}
.active,
.dot:hover {
  background-color: #717171;
}

var slideIndex = 0;
showSlides();
function showSlides() {
  var i;
  var slides = document.getElementsByClassName("mySlides");
  var dots = document.getElementsByClassName("dot");
  for (i = 0; i < slides.length; i++) {
    slides[i].style.display = "none";
  }
  slideIndex++;
  if (slideIndex > slides.length) {
    slideIndex = 1;
  }
  for (i = 0; i < dots.length; i++) {
    dots[i].className = dots[i].className.replace(" active", "");
  }
  slides[slideIndex - 1].style.display = "block";
  dots[slideIndex - 1].className += " active";
  setTimeout(showSlides, 3000); // 每3秒切换一张图片
}
// 当点击小圆点时，切换到对应的图片
var dots = document.getElementsByClassName("dot");
for (var i = 0; i < dots.length; i++) {
  dots[i].onclick = function () {
    for (var j = 0; j < dots.length; j++) {
      dots[j].className = dots[j].className.replace(" active", "");
    }
    this.className += " active";
    var slides = document.getElementsByClassName("mySlides");
    for (var k = 0; k < slides.length; k++) {
      slides[k].style.display = "none";
    }
    slides[Array.prototype.indexOf.call(dots, this)].style.display = "block";
    slideIndex = Array.prototype.indexOf.call(dots, this) + 1;
  };
}