I’m trying to create a simple game on javascript where people can play together, I had issues on the communication between the client and the server using a JSON file.

Without doing any changing on my code and starting the code again I saw this error on the chrome console:

SyntaxError: Unexpected token 'I', "Invalid JS"... is not valid JSON

and this error:

Failed to load resource: the server responded with a status of 500 (Internal Server Error)

Going in the server code, in the python console this has been printed:

Uncaught exception GET /game (::1)
HTTPServerRequest(protocol='http', host='localhost:1234', method='GET', uri='/game', version='HTTP/1.1', remote_ip='::1')
Traceback (most recent call last):
  File "C:UsersLucaAppDataLocalProgramsPythonPython312Libsite-packagestornadowebsocket.py", line 631, in _run_callback
    result = callback(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "c:UsersLucaDocumentsScriptsGiocoOnlineserver_5.py", line 36, in on_message
    contenutoFile = json.load(f)
                    ^^^^^^^^^^^^
  File "C:UsersLucaAppDataLocalProgramsPythonPython312Libjson__init__.py", line 293, in load
    return loads(fp.read(),
           ^^^^^^^^^^^^^^^^
  File "C:UsersLucaAppDataLocalProgramsPythonPython312Libjson__init__.py", line 346, in loads
    return _default_decoder.decode(s)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:UsersLucaAppDataLocalProgramsPythonPython312Libjsondecoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:UsersLucaAppDataLocalProgramsPythonPython312Libjsondecoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

Followed by a ton of errors like these:

500 GET /json?ts=2024-04-25T18:51:24.390Z (::1) 5.00ms
500 GET /json?ts=2024-04-25T18:51:24.401Z (::1) 1.00ms
500 GET /json?ts=2024-04-25T18:51:24.422Z (::1) 2.00ms
500 GET /json?ts=2024-04-25T18:51:24.434Z (::1) 2.00ms
500 GET /json?ts=2024-04-25T18:51:24.448Z (::1) 2.00ms
500 GET /json?ts=2024-04-25T18:51:24.464Z (::1) 2.00ms
500 GET /json?ts=2024-04-25T18:51:24.481Z (::1) 3.00ms
500 GET /json?ts=2024-04-25T18:51:24.496Z (::1) 1.00ms
500 GET /json?ts=2024-04-25T18:51:24.513Z (::1) 2.00ms

This is the server code

import tornado.ioloop
import tornado.web
import tornado.websocket
import json

clients = []
players = []

corrispondenza = ["P1", "P2"]

class JsonHandler(tornado.web.RequestHandler):
    def get(self):
        self.set_header('Cache-Control', 'no-store, no-cache, must-revalidate, max-age=0')
        try:
            with open("C:/Users/Luca/Documents/Scripts/GiocoOnline/base.json", "r") as file:
                data = json.load(file)
                self.write(data)
        except FileNotFoundError:
            # Handle file not found error
            self.set_status(404)
            self.write("File not found")
        except json.JSONDecodeError:
            # Handle invalid JSON format
            self.set_status(500)
            self.write("Invalid JSON format in file")
        
class GameHandler(tornado.websocket.WebSocketHandler):
    def open(self):
        clients.append(self)
        self.id = clients.index(self)
        self.write_message(json.dumps({"clientId" : self.id}))

    def on_message(self, player):
        player = json.loads(player)
        f = open("C:/Users/Luca/Documents/Scripts/GiocoOnline/base.json", "r+")
        contenutoFile = json.load(f)
        
        contenutoFile[corrispondenza[self.id]] = player
        f.close()
        with open ("C:/Users/Luca/Documents/Scripts/GiocoOnline/base.json", "w") as file:
            json.dump(contenutoFile, file)
        

    def on_close(self):
        x = self.id
        clients.remove(self)
        for i in range(x, len(clients) - x):
            clients[i].id -= 1

class MainHandler(tornado.web.RequestHandler):
    def get(self):
        self.render("client_5.html")


# Create a Tornado application with routing
application = tornado.web.Application([
    (r"/", MainHandler),
    (r"/game", GameHandler),
    (r"/json", JsonHandler),
])

if __name__ == "__main__":
    application.listen(1234)
    tornado.ioloop.IOLoop.current().start()

This is the part of the client code, where i get the error

function sendData() {
    console.log(JSON.stringify(player))
    ws.send(JSON.stringify(player))
}

let getData = async () => {
    let ts = (new Date()).toISOString();
    let response = await fetch("http://localhost:1234/json?ts=" + ts)
    data = await response.json()
}

All the server appears when i connect a client to the server.

I already tried to check if the json exists and i tried to create another one form zero thinking that the file could be corrupted.

I have never used tornado or javascript before.

Some words are in italian so if you need a translation please ask.

Thank you 🙂