Note: When I say "open source", I am talking about GPL. Substitute with "free" if you prefer. When I say "closed source", I mean "not GPL or MIT, etc".
We have an open source (GPLv3) webapp that we develop and sell support and hosting for (similar model to redhat). Note, this isn't a website that we host that happens to be open source. It's a website that our clients host (or pay us to host). Each client makes their own instance of the webapp. The entire webapp is under GPL.
A particular script that we use (FullCalendar) is licensed under MIT. But the author has added a new add-on feature that will not be open sourced (maybe, it's undecided). The exact details of this license are not available yet.
Can I purchase this new add-on script and use it in my webapp (distribute it with the website's gpl code)? I have a feeling, that distributing the script that I bought would be illegal since my clients haven't bought it. Could I work around this? Could I host the script on our servers and leave it out of the distributed source? Only distribute a minified version?
What are the rules for using closed source JS in a GPL based project?
And I'm not looking for legal advice. We're not paranoid about legal issues, we just want to make sure we aren't doing something that is obviously illegal/unethical and to understand exactly how you apply GPL to JS files. Also, I'd like to give the author of the script some feedback.