完美拖拽

html代码：

<div id="div1"></div>

css代码：

#div1{width:100px;height:100px;background:red;position:absolute}

javascript代码：

window.onload=function(){
    var oDiv=document.getElementById("div1");
    var x=0;
    var y=0;
    oDiv.onmousedown=function(ev){
        var oEvent=ev||event;
        //鼠标的横坐标减去div的offsetLeft
        x=oEvent.clientX-oDiv.offsetLeft;
        //鼠标的纵坐标减去div的offsetTop
        y=oEvent.clientY-oDiv.offsetTop;  
        
        document.onmousemove=function(ev){
            var oEvent=ev||event;     
            var left=oEvent.clientX-x;
            var right=oEvent.clientY-y;
            //判断左边是否过界
            if(left<0){
                left=0;
            }
            //判断右边是否过界
            else if(left>document.documentElement.clientWidth-oDiv.offsetWidth){
                left=document.documentElement.clientWidth-oDiv.offsetWidth;
            }
            //判断上边是否过界
            if(right<0){
                right=0;
            }
            //判断下边是否过界
            else if(right>document.documentElenment.clientHeight){
                right=document.documentElenment.clientHeight-oDiv.offsetHeight;
            }
            oDiv.style.left=left+"px";
            oDiv.style.top=right+"px";
        }      
        document.onmouseup=function(){
            //清空document的事件
            document.onmousemove=null;
            document.onmouseup=null;
        }
        //解决低版本火狐bug，干掉系统默认
        return false;
    }

}