window.onload = function () {
    var oButton = document.getElementById('button');
    addEvent(oButton, 'click', fn1); 
};

function fn1(e) {
    alert('1');
};

function addEvent(obj, type, fn) {
    if (typeof obj.addEventListener != 'undefined') {
        obj.addEventListener(type, fn, false);
    } else{
        if(!obj.events) obj.events={};
        if(!obj.events[type]){
            obj.events[type] = [];
            if(obj['on'+type]) obj.events[type][0] =fn; // ？？问题在这里
        }
        obj.events[type][addEvent.ID++] = fn; 
        obj['on'+type]=function(){
            for(var i in obj.events[type]){
                obj.events[type][i]();
            }
        }
    }
};

addEvent.ID=1; 

为什么要做判断事件存不存在'这句话是不是多余。