<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>js实现拖拽的效果</title>
    <style>
        body{margin:0;padding:0;font-size:12px;}
        .scale{background: #ddd; width: 200px; height: 3px; position: relative;margin: 20px;}
        .scale span{background:#aaa;width:8px;height:16px;position:absolute;left:-2px;top:-6px;cursor:pointer; border-radius: 3px;}
        .scale div{background: #66D9EF; position: absolute; height: 3px; width: 0; left: 0; bottom: 0; }
    </style>
</head>
<body>

<div class="scale" id="bar">
    <div></div>
    <span id="btn"></span>
</div>
音量：<span id="title">0</span>

<script>
    var scale = function (btn,bar,title){
        this.btn = document.getElementById(btn);
        this.bar = document.getElementById(bar);
        this.title = document.getElementById(title);
        this.step = this.bar.getElementsByTagName("div")[0];

        this.init = function (){
            var f=this,g=document,b=window,m=Math;
            f.btn.onmousedown=function (e){
                var x=(e||b.event).clientX;
                var l=this.offsetLeft;
                var max=f.bar.offsetWidth-this.offsetWidth;
                g.onmousemove=function (e){
                    var thisX=(e||b.event).clientX;
                    var to=m.min(max,m.max(-2,l+(thisX-x)));
                    f.btn.style.left=to+'px';
                    f.ondrag(m.round(m.max(0,to/max)*100),to);
                    b.getSelection ? b.getSelection().removeAllRanges() : g.selection.empty();
                };
                g.onmouseup=new Function('this.onmousemove=null');
            };
        };
        this.ondrag = function (pos,x){
            this.step.style.width=Math.max(0,x)+'px';
            this.title.innerHTML=pos+'%';
        };
        this.init();
    }

    new scale('btn','bar','title'); //实例化一个拖拽

</script>
</body>
</html>