Ajax JavaScript

Question

Hi

  I am working in ajax and face a problem in javascript and xml. how can we use java script and xml file in AJAX.

 switch (eventId)     {         ...             case 'imageUrl' :             ajaxion.SetEventMonior('imageUrl', '');             parameters = window.document.getElementById('dropDown').value;             break;         ...     }     return parameters; }
in this code what is event id and what is the meaning of ajaxion?