谁能告诉我为什么在成功注册新用户后我的成功消息没有显示在HTML表单上？我可以从我的数据库中看到用户已经注册，但是没有得到"success message"页面重新加载而且没有显示任何消息 . 以下是我正在使用的HTML和JavaScript代码 . 它也抛出这个错误，我不知道如何解决它 "Uncaught ReferenceError: $ is not defined" . 谢谢 .

$(document).ready(function() {

});

function newUser() {

  // object
  var user = new Object();
  var error;

  // user inputs
  user.fname = $('#fname').val();
  user.lname = $('#lname').val();
  user.username = $('#username').val();
  user.usertype = $('#usertype').val();
  user.email = $('#email').val();
  user.phoneno = $('#phoneno').val();
  user.pswd1 = $('#pswd1').val();
  user.pswd2 = $('#pswd2').val();

  error = true;
  if (error == true) {

    $
      .ajax({
        url: 'AddUser',
        type: 'POST',
        dataType: 'json',
        data: JSON.stringify(user),
        contentType: 'application/json',
        mimeType: 'application/json',

        success: function(data) {

          if (data.message == "Registered") {
            alert("Registered!");

            $("#message").empty();
            $("#message").append(
              $('<div class="alert alert-success alert-dismissible" role="alert">' +
                '<button type="button" class="close" data-dismiss="alert" aria-label="Close">' +
                '<span aria-hidden="true">&times;</span></button>' +
                '<i class="fa fa-check-circle" style="text-align: center">' +
                data.message +
                '</i></div>'));

            $('html,body').scrollTop(0);

          } else {
            $("#message").empty();
            $("#message").append(
              $('<div class="alert alert-danger alert-dismissible" role="alert">' +
                '<button type="button" class="close" data-dismiss="alert" aria-label="Close">' +
                '<span aria-hidden="true">&times;</span></button>' +
                '<i class="fa fa-times-circle" style="text-align: center">' +
                data.message +
                '</i></div>'));

            $('html,body').scrollTop(0);
          }
        },

        error: function(data, status, err) {
          alert("error: " + data + " status: " + status + " er:" + err);
        }

      });
  }
}

<!DOCTYPE HTML>
<html>

<head>
  <title>Registration Page</title>

  <meta name="viewport" content="width=device-width, initial-scale=1">
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
  <meta name="keywords" content="Pooled Responsive web template, Bootstrap Web Templates, Flat Web Templates, Android Compatible web template, 
Smartphone Compatible web template, free webdesigns for Nokia, Samsung, LG, SonyEricsson, Motorola web design" />

  <script type="application/x-javascript">
    addEventListener("load", function() {
      setTimeout(hideURLbar, 0);
    }, false);

    function hideURLbar() {
      window.scrollTo(0, 1);
    }
  </script>

  <!-- Bootstrap Core CSS -->
  <link href="web/css/bootstrap.min.css" rel='stylesheet' type='text/css' />
  <!-- Custom CSS -->
  <link href="web/css/style.css" rel='stylesheet' type='text/css' />
  <link rel="stylesheet" href="css/morris.css" type="text/css" />
  <!-- Graph CSS -->
  <link href="web/css/font-awesome.css" rel="stylesheet">
  <link rel="stylesheet" href="web/css/jquery-ui.css">
  <!-- jQuery -->
  <script src="js/jquery-2.1.4.min.js"></script>
  <!-- //jQuery -->

  <link href='//fonts.googleapis.com/css?family=Roboto:700,500,300,100italic,100,400' rel='stylesheet' type='text/css' />
  <link href='//fonts.googleapis.com/css?family=Montserrat:400,700' rel='stylesheet' type='text/css'>
  <!-- lined-icons -->
  <link rel="stylesheet" href="web/css/icon-font.min.css" type='text/css' />
  <!-- //lined-icons -->


  <!-- These are my own scripts -->
  <script src="js/registration.js"></script>

</head>

<body>
  <div id="message" style="width: 20vw;margin-left: 24vw;"></div>
  <div class="main-wthree">
    <div class="container">
      <div class="sin-w3-agile">
        <h2>New User Registration</h2>

        <form action="#" method="post">

          <div class="username">
            <span class="username">First name:</span> <input type="text" name="fname" class="name" id="fname" placeholder="" required="">
            <div class="clearfix"></div>
          </div>

          <div class="username">
            <span class="username">Last name:</span> <input type="text" name="lname" class="name" id="lname" placeholder="" required="">
            <div class="clearfix"></div>
          </div>

          <div class="username">
            <span class="username">Username:</span> <input type="text" name="name" class="name" id="username" placeholder="" required="">
            <div class="clearfix"></div>
          </div>

          <div class="username">
            <span class="username">User type:</span>
            <select name="usertype" id="usertype" required="">
              <option></option>
              <option value=1>Admin</option>
              <option value=2>Normal</option>
            </select>
            <div class="clearfix"></div>
            <br>
          </div>

          <div class="username">
            <span class="username">Email:</span> <input type="text" name="email" class="name" id="email" placeholder="" required="">
            <div class="clearfix"></div>
          </div>

          <div class="username">
            <span class="username">Phone number:</span> <input type="tel" name="phoneno" class="name" id="phoneno" placeholder="" required="">
            <div class="clearfix"></div>
          </div>

          <div class="password-agileits">
            <span class="username">Password:</span> <input type="password" name="password" class="password" id="pswd1" placeholder="" required="">
            <div class="clearfix"></div>
          </div>
          <div class="password-agileits">
            <span class="username">Confirm Password:</span> <input type="password" name="password" id="pswd2" class="password" placeholder="" required="">
            <div class="clearfix"></div>
          </div>

          <div class="login-w3">
            <input type="submit" class="login" id="registerUser" onclick="newUser()" value="Register">
          </div>
          <div class="clearfix"></div>
        </form>

      </div>
    </div>
  </div>
</body>

</html>
javascript html