Javascript for regular Formstack Forms?

Is it possible to add Javascript to regular Formstack forms? Some of the "recipes" used in the examples for how to use Javascript with Salesforce forms would be very useful for my regular formstack forms as well (see https://help.formstack.com/s/article/Form-Settings-Overview).

Also, on a similar note, is it possible for users to create plugins? Or, are plugins only available if the Formstack devs release them?

The reason I'm asking these two questions is because I would like to add more complex validation to my Formstack forms. For example, it'd be nice to have a way to enforce a policy that only one word be entered into a name field. Or, perhaps I want to have multiple questions with the same set of answers, but only allow users to use an answer only once per submission. Etc...etc...