js实现拖拽（带阴影）

HTML代码:

<p>
    ksjdfkjlksjfdksjfskjfskljldffffffffffffffffffffffff
</p>
<div id="div"></div>

css代码：

<style>
    body{
        padding: 0;
        margin: 0;
    }
    #div{
        width: 100px;
        height: 100px;
        background: #cc0000;
        position: absolute;
        left: 0;
        top: 0;
    }
    .newbox{
        width: 100px;
        height: 100px;
        background: rgba(255,0,0,.3);
        position: absolute;
        left: 0;
        top: 0;
    }
</style>

js代码：

<script>
    window.onload = function () {
        var oBox = document.getElementById('div');
        oBox.onmousedown = function (ev) {
            //克隆原div ，拖拽时的阴影
            var newBox = oBox.cloneNode();
            newBox.removeAttribute('id');
            newBox.className = 'newbox';
            document.body.appendChild(newBox);
            //鼠标点击位置到移动元素所在的外框的距离
            var oEvent = ev||event;
            var disX = oEvent.clientX - newBox.offsetLeft;
            var disY = oEvent.clientY - newBox.offsetTop;
            document.onmousemove = function (ev) {
                var oEvent = ev||event;
                var left = oEvent.clientX - disX;
                var top = oEvent.clientY - disY;
                if (left<0){
                    left = 0;
                } else if (left>document.documentElement.clientWidth - oBox.offsetWidth){
                    left = document.documentElement.clientWidth - oBox.offsetWidth;
                }

                if (top < 0){
                    top = 0;
                } else if(top > document.documentElement.clientHeight - oBox.offsetHeight) {
                    top = document.documentElement.clientHeight - oBox.offsetHeight;
                }
                newBox.style.top = top +'px';
                newBox.style.left = left + 'px';
            }
            document.onmouseup = function () {
                oBox.style.top = newBox.style.top;
                oBox.style.left = newBox.style.left;
                document.body.removeChild(newBox);
                document.onmousemove = null;
                document.onmouseup = null;
                oBox.releaseCapture && oBox.releaseCapture();//IEA独有的释放捕获
            }
            /*为消除拖拽时会选中旁边的文字*/
            return false; //清除系统默认事件
            oBox.setCapture && oBox.setCapture();//IE独有的捕获鼠标
            /*
            在指定窗口里设置鼠标捕获。一旦窗口捕获了鼠标，所有鼠标输入都针对该窗口，无论光标是否在窗口的边界内。同一时刻只能有一个窗口捕获鼠标。如果鼠标光标在另一个线程创建的窗口上，只有当鼠标键按下时系统才将鼠标输入指向指定的窗口。
            * */
        }
    }
</script>