Does Modified Java Script Value support all of JavaScript?

I am relatively new to JavaScript but trying to use it to do a few things and cannot figure out if I am doing something wrong or trying to do something Spoon will not support. My specific case is trying to use the JavaScript prompt() method but I am also interested in the general answer to whether this step should be able to run JavaScript in general or only a selected subset of it.

I suppose related, is there a more detailed discussion of this step somewhere than http://wiki.pentaho.com/display/EAI/...a+Script+Value?

Thanks.