XMLHttpRequest返回状态0

Question

When this function is run, the request.status = 0. This usually means that the network is down but since the webpage loaded, this cannot be possible. I think it may be because I am using XAMPP and running on a localhost but I am not sure how to fix the problem.

Here is the javascript function:

   <script type="text/javascript">
    function submitForm() {
      var userKey;
      var username = login_form.elements["username"].value;

      function getKey() {
        var request = new XMLHttpRequest();

        request.onreadystatechange = function() {
          if(request.readyState == 4 && request.status == 200){
            userKey = request.responseText;
          }
        }

        request.open("POST", "key.php", true);
        request.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
        request.send("username="+username);
      }

      getKey();
   }
  </script>

Here is the php function called "key.php". It is in the same folder as the html file:

<?php
if (array_key_exists('username', $_POST)) {

    $username = $_POST['username'];

    echo '<script language="javascript">';
    echo 'alert("message successfully sent")';
    echo '</script>';

} else {
    echo 'Invalid parameters!';
}
?>