gistfile1.js

var request = new XMLHttpRequest();

request.onload = function () {
  // request.status is the HTTP status code of the response
  // request.responseText is whatever data the server sent back
  // easiest for this would be something like
  if (response.status === 200) {
    // form posted ok
  } else {
    // form failed to post
  }
};

request.open('POST', '/submit', true);
request.setRequestHeader("Content-Type", "application/json");
request.send(formData);