原生js一个简单的拖动

style如下

#div1{
　　width: 200px;
　　height: 20px;
　　background: red;
　　position: relative;
　　margin-top:200px;
　　margin-left:auto;
　　margin-right:auto;
}
#div2{
　　width: 20px;
　　height: 20px;
　　background: blue;
　　position: absolute;
　　left: 0px;
　　top:0px;
}

html如下

<div id="div1">
    <div id="div2"></div>
</div>

<p id="con"></p>

js如下

    window.onload = function(){
        div2.onmousedown = function(ev){
            var e = ev || event;
            var disX = e.clientX - this.offsetLeft;

            document.onmousemove = function(ev){
                var e = ev || event;
                var t = e.clientX - disX;

                if( t <= 0  ) t = 0;

                if( t >= div1.clientWidth - div2.offsetWidth ){
                    t = div1.clientWidth - div2.offsetWidth;
                };

                var Scale = t / (div1.clientWidth - div2.offsetWidth);

                div2.style.left = t + "px";  

                con.innerHTML = t;
            };

            document.onmouseup = function (){
                document.onmousemove = null;
                document.onmouseup = null;  
            };
        }
    }