//如果需要头部导航栏固定，把下面代码全部删除即可。

var new_scroll_position = 0;

var last_scroll_position;

var header = document.getElementById("header");

 

window.addEventListener('scroll', function(e) {

  last_scroll_position = window.scrollY;

 

  // 向下滚动

  if (new_scroll_position < last_scroll_position && last_scroll_position > 80) {

    header.classList.remove("slideDown");

    header.classList.add("slideUp");

 

  // 向上滚动

  } else if (new_scroll_position > last_scroll_position) {

    header.classList.remove("slideUp");

    header.classList.add("slideDown");

  }

 

  new_scroll_position = last_scroll_position;

});