1.scroll家族和offset家族的结合运用
2.当nav的offsetTop大于屏幕卷去高度的时候，进行吸顶
3.添加一个固定类，如果满足条件，为nav加类名
运行效果
滚动页面时，保证导航栏吸顶
代码
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title></title> <style> *{margin: 0;padding: 0;list-style: none;border:none} img{vertical-align: top;width: 100%;} section{width: 70%;margin: 0 auto;} .nav{position: fixed;left: 0;top: 0;width: 100%} </style> </head> <body> <header> <img src="https://www.jb51.net/images/top.png" alt=""> </header> <nav> <img src="https://www.jb51.net/images/nav.png" alt=""> </nav> <section> <img src="https://www.jb51.net/images/body01.png" alt=""> <img src="https://www.jb51.net/images/body02.png" alt=""> </section> <script src=""></script> <script> window.addEventListener('load',function (ev) { // 1. 求出头部高度 var navTopHeight = myTool.$('nav').offsetTop; // 2. 监听页面滚动 window.addEventListener('scroll',function (ev1) { var scrollTopHeight = myTool.scroll().top; // 2.1 判断 if(scrollTopHeight >= navTopHeight){ myTool.addClassName(myTool.$('nav'),'nav') }else{ myTool.removeClassName(myTool.$('nav'),'nav'); } }) }) </script> </body> </html>