ReferenceError: $ is not defined

F12查看问题，发现jquery加载速度太慢

<script src="https://cdn.bootcss.com/jquery/3.3.1/jquery.min.js"></script>

 

换了源秒加载

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>