我照着打的，为什么scrollTop总是0？

//获取元素样
var getElem=function getElem(selector){
return document.querySelector(selector)
}
var getAllElem=function getAllElem(selector){
return document.querySelectorAll(selector)
}
//获取元素样式
var getCls=function(element){
return element.getAttribute('class')

}
var setCls=function(element,cls){
return element.setAttribute('class',cls)
}

//W为元素添加样式
var addCls=function(element,cls){
var baseCls=getCls(element);
if(baseCls.indexOf(cls)===-1){
setCls(element,baseCls+' '+cls);
}
}
//W为元素删除样式 outline AAA
var delCls=function(element,cls){
var baseCls=getCls(element);
if(baseCls.indexOf(cls)!=-1){
setCls(element,baseCls.split(cls).join(' ').replace(/\s+/g,' '));
}
}

//第一步：初始化样式 init
var screenAniamteElements = {
'.screen-1': [
'.screen-1__heading',
'.screen-1__phone',
'.screen-1__shadow',
],
'.screen-2': [
'.screen-2__heading',
'.screen-2__phone',
'.screen-2__subheading',
'.screen-2__point_i_1',
'.screen-2__point_i_2',
'.screen-2__point_i_3',
],
'.screen-3': [
'.screen-3__heading',
'.screen-3__subheading',
'.screen-3__phone',
'.screen-3__features',
],
'.screen-4': [
'.screen-4__heading',
'.screen-4__subheading',
'.screen-4__type__item_i_1',
'.screen-4__type__item_i_2',
'.screen-4__type__item_i_3',
'.screen-4__type__item_i_4',
],
'.screen-5': [
'.screen-5__heading',
'.screen-5__subheading',
'.screen-5__bg',
]
};
var setScreenAnimateInit=function(screenCls){
var screen = document.querySelector(screenCls); //获取当前屏
var animateElements = screenAniamteElements[screenCls];
for (var i = 0; i < animateElements.length; i++) {
var element = document.querySelector(animateElements[i]);
var baseCls = element.getAttribute('class');
element.setAttribute('class', baseCls + ' ' + animateElements[i].substr(1) + '_animate_init');
}
}
window.onload=function(){
for(var k in screenAniamteElements){
setScreenAnimateInit(k);
}
}

//第二步，滚动到哪就播放到哪
window.onscroll=function(){
var top=document.body.scrollTop;
console.log(top);
}