Core language support for JavaScript is provided by the Mozilla Rhino engine. What HtmlUnit provides are all the browser specific host objects such as Document or Navigator.
SimpleScriptable
All of HtmlUnit's JavaScript objects subclass SimpleScripable either directly or indirectly. This class contains all the common methods needed by the HtmlUnit host objects. Additionally, it changes the way that JavaScript method lookup happens. The rhino engine doesn't walk up the class hierarchy when looking for methods starting with jsGet_, jsSet_ or jsFunction. SimpleScriptable changes this so that the superclass hierarchy is walked.
Debugging JavaScript Errors
Testing pages with JavaScript on them can produce a number of errors. These errors occur for several reasons. Some browsers accept shortcuts that are not part of the DOM model and currently not duplicated by HtmlUnit. Also, HtmlUnit still does not implement all extensions that have been added by the different browsers. HtmlUnit wraps the inline code into a wrapper function but when errors are reported from this code, it is hard to determine what code was really being called. A custom logger has been added to HtmlUnit to support this. If you set 'org.htmlunit.javascript' to debug level, the inline code will be logged with the wrapper function to help you determine what line of code is failing.
One of the problems with JavaScript is finding the objects that you expect to be defined but have not been defined within the code. To help find some of these errors, HtmlUnit will produce WARN level messages reporting these errors. Before submitting JavaScript bugs, please read this.