//一般写法

//缺点：当他每次被调用的时候都都会执行里面的if条件，

var addEvent = function(elem, type, handler){

    if(window.addEventLister){

        return elem.addEventListener(type, handler, false);

    }

    if(window.attachEvent){

        return elem.attachEvent('on'+type, handler);

    }

}

//改进

var addEvent = (function(){

    if(window.addEventListener){

        return function(elem, type, handler){

            elem.addEventListener(type, handler, false);

        }

    }

    if(window.attachEvent){

        return function(elem, type, handler){

            elem.atttachEvent('on'+handler);

        }

    }

})();

//惰性载入方案

var addEvent = function(elem, type, handler){

    if(window.addEventListener){

        addEvent = function(elem, type, handler){

            elem.addEventListener(type, handler, false);

        }

    } else if(window.atttachEvent){

        addEvent = function(elem, type, handler){

            elem.atttachEvent('on'+type, handler);

        }

    }

    addEvent(elem, type, handler);

};