JS实现轮播图效果

<div id="swiper-container" class="swiper-container" onmouseenter="swiperImg()" onmouseleave="stopSwiper()">
  <div id="img-list" style="left:0px;">
   <img src="img/swiper1.png" alt="1">
   <img src="img/swiper2.png" alt="2">
   <img src="img/swiper1.png" alt="1">
   <img src="img/swiper2.png" alt="2">
  </div>
  <div id="swiper-btn">
   <span index="1" class="on"></span>
   <span index="2"></span>
  </div>
</div>

 * {
  margin: 0;
  padding: 0;
 }

 a {
  text-decoration: none;
 }

 .swiper-container {
  position: relative;
  width: 300px;
  height: 300px;
  margin: 0 auto;
  border: 1px solid;
  overflow: hidden;
 }

 #img-list {
  position: absolute;
  width: 1200px;
  height: 300px;
 }

 #img-list img {
  float: left;
 }

 #swiper-btn {
  position: absolute;
  bottom: 5%;
  left: 45%;
 }

 #swiper-btn span {
  display: inline-block;
  width: 10px;
  height: 10px;
  border-radius: 5px;

 }

 .on {
  background-color: goldenrod;
 }

 span {
  background-color: #d7d7d7;
 }

var timer;
  var div = document.getElementById('img-list');
  var span = document.getElementById('swiper-btn').getElementsByTagName('span');
  var offset = -300;
  var index = 1;
  function swiperImg() {
   timer = setInterval(() => {
    var left = parseInt(div.style.left);
    var newLeft = left + offset;
    if (newLeft <= -1200) {
     div.style.left = '0px';
    }
    else {
     div.style.left = newLeft + 'px';
    }
    showBtn(parseInt(div.style.left));
   }, 3000);
  }

  function showBtn(left) {
   if (left == 0 || left == -600) {
    span[0].className = "on";
    span[1].className = "";
   }
   else {
    span[0].className = "";
    span[1].className = "on";
   }
  }

  function stopSwiper() {
   clearInterval(timer);
  }

  for (var i = 0; i < span.length; i++) {
   span[i].onclick = function () {
    if (this.className == "on") {
     return false;
    }
    var myIndex = parseInt(this.getAttribute("index"));
    if (myIndex == 1)
     div.style.left = 0 + 'px';
    if (myIndex == 2)
     div.style.left = -300 + 'px';
    index = myIndex;
    showButton();
   }
  }

  function showButton() {
   for (var i = 0; i < span.length; i++) { //全部取消掉on样式
    if (span[i].className == "on") {
     span[i].className = "";
     break;
    }
   }
   span[index - 1].className = "on";
  }