JavaScript简单拖拽事件（鼠标跟随事件）

本例演示简单来说分为两步：

第一步：鼠标按下时的事件对象操作。

第二步：鼠标移动时对事件对象的操作。

 <style>
            *{margin: 0;
                padding: 0;
            }
            #box{ 100px;height: 100px;background: #CCC;border: 1px solid #999;position: absolute;
                    top: 100px;left: 500px;text-align: center;line-height: 100px;cursor: move;
            }
        
</style>
<div id="box">想要去哪？</div>
<script type="text/javascript">
            var oBox =document.getElementById("box");
            oBox.onmousedown = function(ev){
                ev =ev ||event;
                var oLeft = oBox.offsetLeft;
                var oTop = oBox.offsetTop;
                var a =ev.clientX-oLeft;
                var b =ev.clientY-oTop;
                oBox.onmousemove =function (ev){
                    ev = ev || event;
                    oBox.style.left =ev.clientX-a+'px';
                    oBox.style.top =ev.clientY-b+'px';
                }
            }    
            
            document.onmouseup =function(){
                oBox.onmousemove =function(){
                    null;
                }
            }
        
</script>