从输入jquery获取值

我试图从输入中获取值,并将其传递给脚本以注册用户,但没有任何运气存储它。我设置的测试警报显示没有值。以下是我必须获取用户名值并将其传递到脚本中的步骤:

                    <form id ="form-signin" class="form-signin" action="" method="">
                      <input type="text" id="username" class="form-control" placeholder="Username" required autofocus>
                      <input type="email" class="form-control" placeholder="Email" required autofocus>
                      <input type="password" class="form-control" placeholder="Password" required>
                      <input type="submit" id="createUser" class="btn btn-lg btn-default btn-block" value="Sign Up" />
                    </form>



<script type="text/javascript">

    Parse.initialize("u3BTp3Efoko8hbNhl5MCeli8Kd2iiEk8mE4vYgn4", "tQEGymAWeB8Tr2LA3YDGoq2Lt2xpGMW9ikeFSTtD");

    //get the input data
    var username = $('#username').val();

    //set the user
    var user = new Parse.User();

    $( "form" ).submit(function( event ) {

      alert( "Error: " + username + " " );

        user.set("username", " " + username + " ");
        user.set("password", "my pass");
        user.set("email", "email@example.com");

        // other fields can be set just like with Parse.Object
        user.set("phone", "415-392-0202");

        user.signUp(null, {
          success: function(user) {
            // Hooray! Let them use the app now.
          },
          error: function(user, error) {
            // Show the error message somewhere and let the user try again.
            alert("Error: " + error.code + " " + error.message);
          }
        });

    });

</script>