IANAL (yet), but here's what I think the answer is.
1. Yes, it's copyrightable, just like anything else that is written.
2. But if they took any of that JS from something else that was GPL'd, then technically their JS is GPL'd too. (Assuming the GPL is enforcible.)
3. Nothing stops you from reading their code and rewriting it yourself, so long as you don't just copy. Copyright extends to "expression," not "ideas." The way an algorithm works, the way a program is organized, the interfaces it uses: these are all ideas, not expression. So if you rewrote the algorithms in your own idiom, you could take pretty much as much inspiration from the original as you'd like.
1. Yes, it's copyrightable, just like anything else that is written.
2. But if they took any of that JS from something else that was GPL'd, then technically their JS is GPL'd too. (Assuming the GPL is enforcible.)
3. Nothing stops you from reading their code and rewriting it yourself, so long as you don't just copy. Copyright extends to "expression," not "ideas." The way an algorithm works, the way a program is organized, the interfaces it uses: these are all ideas, not expression. So if you rewrote the algorithms in your own idiom, you could take pretty much as much inspiration from the original as you'd like.