<script>
    window.onload = function (){
      //1.获取需要的标签
      var box = document.getElementById("box");
      var box_top = box.children[0];
      var box_bottom = box.children[1];
      var mask = box_bottom.children[0];
      //2.获取滚动条长度
      // 滚动条长度 = ( 盒子的宽度 / 内容的宽度) * 盒子的宽度
      var mask_length = (box.offsetWidth /box_top.offsetWidth) * box.offsetWidth;
      mask.style.width = mask_length + "px";
      //3.监听鼠标按下的事件
      mask.onmousedown = function (event){
        var event = event || window.event;
        //3.1设置起始位置
        var beginX = event.clientX - mask.offsetLeft;
        //3.2 监听鼠标的移动
        document.onmousemove =function (event){
          var event = event || window.event;
          //3.2.1求移动的位置
          var endX = event.clientX - beginX;

          //3.2.2处理边界值
          if(endX < 0){
            endX = 0
          }else if(endX >= box.offsetWidth - mask.offsetWidth){
            endX = box.offsetWidth - mask.offsetWidth;
          }
          //3.2.3让滚动条滚动
          mask.style.left = endX + "px";

          //3.2.4 让商品随着滚动
          //公式：内容走的距离 = （内容的长度 - 盒子的长度） / (盒子长度 - 滚动条的长度) * 滚动条走的距离
          var content_len = (box_top.offsetWidth - box.offsetWidth) / (box.offsetWidth - mask.offsetWidth) * endX;
          box_top.style.left = -content_len + "px";

          return false;
        };
        document.onmouseup = function (){
          document.onmousemove = null;
        }
      }
    }
</script>