在signup.html中删除了jquery-3.1.1.js，使用了项目中原有的/static/js/jquery.js

app/templates/signup.html

@@ -6,7 +6,7 @@ You're logged in already! <a href="/logout">Logout</a>.
 {% else %}
 <meta name="viewport" content="width=device-width, initial-scale=1.0, minimum-scale=0.5, maximum-scale=3.0, user-scalable=yes" />
 <link rel="stylesheet" href="static/css/login_service.css">
-<script src="../static/js/jquery-3.1.1.js"></script>
+<script src="../static/js/jquery.js"></script>
     <script>
         function signup() {
             let username = $("#username").val();