In the FormServer Documentation, there is a pdf (transformation_reference.pdf) which outlines the objects, properties and methods which you can use from either Server-side scripting or Javascript client in HTML with Form Server.
You can also look it the xfasubset.js file which is included in every HTML page rendered by form server. This basically maps Browser properties to the XFA object model. You can get it by viewing the source of the HTML form, scroll to the bottom and grab the url for the SCRIPT tag and enter it into the browser. This will download the JS file. If you know javascripting and HTML it should not be much of a problem for anyone to understand what is available.