js基础拖拽一

	var oDiv= document.getElementById("div");
	oDiv.onmousedown= function(e){
		var _this = this;
		var e= e || window.event;
		var diffX= e.clientX - _this.offsetLeft;
		var diffY= e.clientY - _this.offsetTop;
	     document.onmousemove= function(e){
			 var e= e || window.event;
			 _this.style.left= e.clientX -diffX + "px";
			 _this.style.top= e.clientY -diffY + "px";
		 
		 }
		 document.onmouseup= function(e){
			 this.onmousemove= null
			 this.onmouseup= null
		 }
	}