为什么我在JS文件中用了window.onload以后，代码就不执行了。

window.onload = function (){

var index = 0,

nav = byId("nav").getElementsByTagName("span"),

img = byId("banner").getElementsByTagName("div"),

sec = document.getElementsByTagName("section")[0],

size = img.length,

time = null;

function byId(id){

return typeof(id) === "string" ? document.getElementById(id):id;

};

function addHandler(element,type,handler){

if(element.addEventListener){

element.addEventListener(type,handler);

}else if(element.attachEvent){

element.attachEvent("on"+type,handler);

}else{

element["on"+type] = handler;

}

};

//图片自动轮播

function startAutoplay(){

time = setInterval(function(){

           index++;

           if(index >= size){

               index = 0;

           };

           changeImg();

       },1000)

}

//停止图片自动轮播

function stopAutoplay(){

clearInterval(time);

}

//图片切换

function changeImg(){

for(var i = 0;i < size;i++){

nav[i].className = "";

img[i].style.display = "none";

}

nav[index].className = "active";

img[index].style.display = "block";

}

//导航切换图片

function navImg(){

for(var i = 0;i < size;i++){

nav[i].setAttribute("data-id",i)

addHandler(nav[i],"mousemove",function(){

               index = this.getAttribute("data-id");

               changeImg();

           })

};

}

function bannerImg(){

startAutoplay();

navImg();

addHandler(sec,"mousemove",stopAutoplay);

addHandler(sec,"mouseout",startAutoplay);

};

addHandler(window,"load",bannerImg);

}