JavaScript实现水平进度条拖拽交互

<html>
<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    *{
      margin: 0;
      padding: 0;
    }
    .scroll{
      margin:100px;
      width: 500px;
      height: 5px;
      background: #ccc;
      position: relative;
    }
    .bar{
      width: 10px;
      height: 20px;
      background: #369;
      position: absolute;
      top: -7px;
      left: 0;
      cursor: pointer;
    }
    .mask{
      position: absolute;
      left: 0;
      top: 0;
      background: #369;
      width: 0;
      height: 5px;
    }
  </style>  
</head>
<body>
  <div class="scroll" id="scroll">
    <div class="bar" id="bar">

    </div>
    <div class="mask" id="mask"></div>
  </div>
  <p></p>
  <script>  
    var scroll = document.getElementById('scroll');
    var bar = document.getElementById('bar');
    var mask = document.getElementById('mask');
    var ptxt = document.getElementsByTagName('p')[0];
    var barleft = 0;
    bar.onmousedown = function(event){
      var event = event || window.event;
      var leftVal = event.clientX - this.offsetLeft;
      var that = this;
       // 拖动一定写到 down 里面才可以
      document.onmousemove = function(event){
        var event = event || window.event;
        barleft = event.clientX - leftVal;     
        if(barleft < 0)
          barleft = 0;
        else if(barleft > scroll.offsetWidth - bar.offsetWidth)
          barleft = scroll.offsetWidth - bar.offsetWidth;
        mask.style.width = barleft +'px' ;
        that.style.left = barleft + "px";
        ptxt.innerHTML = "已经走了" + parseInt(barleft/(scroll.offsetWidth-bar.offsetWidth) * 100) + "%";

        //防止选择内容--当拖动鼠标过快时候，弹起鼠标，bar也会移动，修复bug
        window.getSelection ? window.getSelection().removeAllRanges() : document.selection.empty();
      }

    }
    document.onmouseup = function(){
      document.onmousemove = null; //弹起鼠标不做任何操作
    }
  </script>
</body>
</html>