手机站顶部效果，渐变背景色

/**
* Created by zhousg on 2015/12/18.
*/
window.onload = function(){
search();
};
/*头部搜索*/
var search = function(){
/*搜索框对象*/
var search = document.getElementsByClassName('jd_header_box')[0];
/*banner对象*/
var banner = document.getElementsByClassName('jd_banner')[0];
/*高度*/
var height = banner.offsetHeight;

window.onscroll = function(){
var top = document.body.scrollTop;
/*当滚动高度大于banner的高度时候颜色不变*/
if(top > height){
search.style.background  = "rgba(201,21,35,0.85)";
}else{
var op = top/height * 0.85;
search.style.background  = "rgba(201,21,35,"+op+")";
}
};

};