js拖拽阴影

css如下

#box{ width: 200px; height: 200px; position: absolute;background-color: red;}
.border_box{  position: absolute; width: 200px; height: 200px;  opacity: 0.5; background-color: red;}

html如下

<div id="box"></div>

javascript如下

var oBox=document.getElementById('box');

    oBox.onmousedown=function(ev){
        var oEvent=ev || event;

        var disX=oEvent.clientX-oBox.offsetLeft;
        var disY=oEvent.clientY-oBox.offsetTop;
        //鼠标位置减去box位置
        
        var left=0;
        var top=0;

        var oBorderBox=document.createElement('div');
        oBorderBox.className="border_box";

        oBorderBox.style.left=oBox.offsetLeft+'px';;
        oBorderBox.style.top=oBox.offsetTop+'px';
        document.body.appendChild(oBorderBox);

        document.onmousemove=function(ev){ //oBox改成document是怕速度拖拽速度快，鼠标偏离了box
            var oEvent=ev || event;
            
            left = oEvent.clientX - disX;
            top = oEvent.clientY - disY;

            oBorderBox.style.left=left+'px';
            oBorderBox.style.top=top+'px';

        }

        document.onmouseup=function(){ //oBox改成document是防止鼠标拖拽出可视区发生不了oBox.onmouseup事件
        
            if( !collision( oBorderBox,oBox ) ){
                oBox.style.left = left + "px";    
                oBox.style.top = top + "px";
            }
            
            document.onmousemove=null;
            document.onmouseup=null;

            document.body.removeChild(oBorderBox);
        }

        return false; //解决FF再次拖拽出现两个oBox的bug
    }
    
    
        /*
            obj1：拖拽的元素
            obj2：被碰撞的元素
        */
        function collision(obj1,obj2){

            var obj1L = obj1.offsetLeft;
            var obj1LW = obj1L + obj1.offsetWidth;
            var obj1T = obj1.offsetTop;
            var obj1TH = obj1T + obj1.offsetHeight;

            var obj2L = obj2.offsetLeft;
            var obj2LW = obj2L + obj2.offsetWidth;
            var obj2T = obj2.offsetTop;
            var obj2TH = obj2T + obj2.offsetHeight;

            if( obj1LW < obj2L || obj1L > obj2LW || obj1TH < obj2T || obj1T > obj2TH){
                return false;
            }

            return true;
        }