yes, this function is called when client disconnects but that is clientSide: and you cannot transmit it to the Server: Client got end-of-stream!!!
( i know the other links: some are irrelevant as for UDP or carnivore and as for websockets, which is the more interesting i dont remember why but i have decided that it was not ok for this project…I have also tried with node.js and even with Pusher…but they are rather ok for javaScript && P5JS, execept Pusher, which seems to have some unofficial library for httpRequests from Java)