重写自己如何减少判断？其实我也是学习别人的代码，才明白的。

具体代码：

 //兼容的绑定事件方法
        function addHandler(target, eventType, handler) {
            //重写自己
            if (target.addEventListener) { //DOM2 Events 
                addHandler = function (target, eventType, handler) {
                    target.addEventListener(eventType, handler, false);
                };
            } else { //IE 
                addHandler = function (target, eventType, handler) {
                    target.attachEvent("on" + eventType, handler);
                };
            }
            //关键，调用新方法，重写自己，减少判断，第二次调用时，使用新方法，没有判断
            addHandler(target, eventType, handler);
        }
        function removeHandler(target, eventType, handler) {
            //o//重写自己 
            if (target.removeEventListener) { //DOM2 Events 
                removeHandler = function (target, eventType, handler) {
                    target.addEventListener(eventType, handler, false);
                };
            } else { //IE 
                removeHandler = function (target, eventType, handler) {
                    target.detachEvent("on" + eventType, handler);
                };
            }
            //关键，调用新方法，重写自己，减少判断，第二次调用时，使用新方法，没有判断
            removeHandler(target, eventType, handler);
        }

 function addHandler(target, eventType, handler) {          
            if (target.addEventListener) { //DOM2 Events 
                target.addEventListener(eventType, handler, false);
            } else { //IE 
                target.attachEvent("on" + eventType, handler);
            }           
        }