Access Ajax POSTed data in a cfm page – Jquery ajax

I am trying to use Ajax/JQuery to submit a form, and on the target page (response.cfm) I am returning a Json which will return either a success or a failure message (based on the checkbox) on the form page (form.cfm). But I can’t seem to access the data sent by the ajax request.

form.cfm

<form id="frm_test">
  <input type="text" name="firstname" id="firstname" placeholder="First name" required /><br>
  <input type="text" name="surname" id="surname" placeholder="Surname" required /><br>
  <input type="email" name="email" id="email" placeholder="Email" required /><br>
  <input type="checkbox" name="error" id="error" />Return error (check to see the error message. leave empty to see a success message)<br>
  <input type="submit" value="Save changes" id="submitdata" />
</form>

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>

<script>

  $("#frm_test").submit(function (eventData) {
    event.preventDefault();
    $("#message").empty();
    var postData = {
      first_name: $('#firstname').val(),
      family_name: $('#surname').val(),
      email: $('#email').val(),
      error: $('#error').is(':checked')
    };

    $.ajax({
      url: "response.cfm",
      method: "POST",
      data: postData
    }).done(
      function(response) {
        if (response["SUCCESSMESSAGE"] !== null) {
          $("#message").append(response["SUCCESSMESSAGE"]);
        } else if (response["EXCEPTIONMESSAGE"] !== null) {
          $("#message").append(response["EXCEPTIONMESSAGE"]);
        }
      });

    $("#frm_test").trigger("reset");
  });

</script>


<div id="message"></div>

response.cfm

<cfprocessingdirective suppresswhitespace="Yes">

  <cfset response = {
    SuccessMessage: 'checkbox value' ? JavaCast("null", "") : "Success",
    ExceptionMessage: 'checkbox value' ? "An error occurred" : JavaCast("null", "")
  }>
  
  <cfheader name="Content-Type" value="application/json">
    
  <cfoutput> #serializeJSON(response)# </cfoutput>  
  
</cfprocessingdirective>