Form API: Show/Hide/Select option based on a previous option Possible Duplicate:
  Easiest method for conditional fields in a form  




Essentially I have 2 select fields on a form:

EDIT: I should mention, this is an entirely custom form built with Form API, I'm not modifying a core/contrib form.


Type


Global
Regional

City


Any
London
New York
Paris
LA



If Type:Global is selected, then the City field is hidden and City:Any is selected.

If Type:Regional is selected, then City is revealed, City:London selected and City:Any hidden.

Is there a Drupal or Form API compliant method to achieve this or is it simply a case of loading a JS file with drupal_add_js and writing entirely custom JS? 

If it's simply custom JS/JQuery no need to write it out for me, I can handle that, but I would like to try and stick to the standard and best practices if there are any for my problem.

Thanks!
You can solve this problem using AHAH on your Form API. The module AHAH Helper may be very useful for you... take a look there...