possibly there is some miss understanding,
script from processing .js and p5.js might be
stored at some ( remote or local ) server
but they not run there ( as server side code )
your browser downloads them and they run on your computer
in the RAM and can create files there too.
and when that script ask you to store the file on your local computer file system
it feels for you like a download ( from server to client ) but it is not,
just from your local RAM to your local drive.
anyhow unless having
- server-side code and
- server-side file system declared and
- GET PUT communication
all will be local.
you can save that file local, upload it later
and use them in the next ( or changed ) script.
also
might give you a idea what could be possible.