ajax.js

$('#dat-form').onsubmit = function (ev) {
  ev.preventDefault();
  dat_value = ev.target.elements.dat_field_name.value

  var xhr = new XMLHttpRequest();

  xhr.onreadystatechange = function () {
    if (xhr.readyState === 4) {
      if (xhr.status === 200) {
        dat_action(xhr.responseText);
      } else {
        alert('Error: ' + xhr.status);
      }
    }
  }
  
  xhr.open('GET', '/dat-api?dat_field_name=' + dat_value, true);
  xhr.send();
}