var login                   = document.getElementById(‘box‘);login.onmousedown 	= function(e) {			var e 	= getEvent(e);//获取event对象			var _this 	= this;			var diffX 	= e.clientX - _this.offsetLeft;//鼠标点击的坐标点x减去box元素距离左边的距离，得到的差为鼠标垫距离box元素左边的距离			var diffY 	= e.clientY - _this.offsetTop;			document.onmousemove 	= function(e) { //第一种方式，每次判断，然后给box元素的left赋值   				if(e.clientX - diffX <= 0) {					_this.style.left 	= ‘0px‘;				}else if(e.clientX - diffX >= document.body.clientWidth - _this.offsetWidth) {					_this.style.left 	= (document.body.clientWidth - _this.offsetWidth) + ‘px‘;				}else {					_this.style.left 	= (e.clientX - diffX) + ‘px‘;				}//这种方法更简洁，好看				var top 				= e.clientY - diffY;				if(top <= 0) 					top 				= 0;				else if(top >= document.body.clientHeight - _this.offsetHeight)					top 				= document.body.clientHeight - _this.offsetHeight				_this.style.top 	= top + ‘px‘;							}			document.onmouseup 		= function() {//鼠标放开后，注销移动跟放开的事件				this.onmousemove 		= null;				this.onmouseup 		= null;			}		}        function getEvent(event) {   return event || window.event;      }