var tab_height = document.getElementById('nav').offsetTop;
document.onscroll = function() {
var acroll_height = document.body.scrollTop || document.documentElement.scrollTop;
if (acroll_height > tab_height) {
document.getElementById('nav').className = 'tab tabfixed';
} else {
document.getElementById('nav').className = 'tab';
}
// 判断是否到达底部
// var scrollTop = (window)).scrollTop();
// var scrollHeight =
($(window)).height();
// if (scrollTop + windowHeight == scrollHeight) {
// self._getData();
// }
};
