纯JS实现图片匀速淡入淡出效果

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>淡入效果</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }
    div {
      border: 2px solid #aaa;
    }
    img {
      width: 300px;
      height: 300px;
      filter: alpha(opacity:30);
      opacity: .3;
      margin: 0 auto;
    }
  </style>
</head>
<body>
<div>
  <img src="img/timg.jpg" alt="" id="img">
</div>
<script>
  var alpha=30;
  var img = document.getElementById("img");
  img.onmouseover=function(){
    startMove(100)
  };
  img.onmouseout=function(){
    startMove(30)
  }
  var timer;
  function startMove(tar) {
    var img = document.getElementById("img");
    clearInterval(timer);
    timer = setInterval(function () {
      var ispeed=0;
      ispeed= alpha<tar?5:-5;
      if(alpha==tar){
        clearInterval(timer)
      }
      else{
        alpha+=ispeed;
        img.style.filter="alpha(opacity:"+alpha+")";
        img.style.opacity=alpha/100;
      }
    }, 30)
  }
</script>
</body>
</html>