1. 对于posX和posY应该声明在drop函数中，应该声明在mousedown函数内

2. 当拖动时，应该去掉那个margin-left：-80px的属性

3. 当为left和right赋值时，不该再去用e.clientX - L

4. 应该把mouseup绑定事件和mousemove同级，否则会导致点击如果不动，而导致没有解绑mousemove

drap: function(obj) {
        obj.onmousedown = function(event) {
            var event = event || window.event;
            var posX = event.clientX - obj.offsetLeft;//在这儿声明pos
            var posY = event.clientY - obj.offsetTop;
            document.onmousemove = function(event) {
                var event = event || window.event;
                var L = event.clientX - posX;
                var T = event.clientY - posY;
                                obj.style.marginLeft = 0;
                                //console.log(L, T)
                if (L <= 0)
                    L = 0;
                else if (L > document.documentElement.clientWidth - obj.offsetWidth)
                    L = document.documentElement.clientWidth - obj.offsetWidth;
                if (T <= 0)
                    T = 0;
                else if (T > document.documentElement.clientHeight - obj.offsetHeight)
                    T = document.documentElement.clientHeight - obj.offsetHeight;

                obj.style.left = L + "px";//这儿
                obj.style.top = T + "px";
            }
                        document.onmouseup = function() {//这儿解绑
                                document.onmousemove = null;
                                document.onmouseup = null;
                        }
            return false;
        }
    }