window.onscroll=function(){console.log('第一个');}

var oldMethod = window.onscroll;

if(typeof oldMethod == 'function'){

    window.onscroll = function(){

        oldMethod.call(this);

        console.log('第二个');

    }

}

function addEvent(obj,type,fn){

    if(obj.attachEvent){ //ie

        obj.attachEvent('on'+type,function(){

            fn.call(obj);

        })

    }else{

        obj.addEventListener(type,fn,false);

    }

}

addEvent(window,'scroll',function(){console.log('第n个')});

addEvent(window,'scroll',function(){console.log('第n+1个')});