Client cannot recognize failed connect

[wmorgansoftware]

I'm doing some development using flask-socketio and socket.io-client, most recent versions as of the time this was written; Python 3.10. With a successful connection, I'm not having any problems with communicating between server and client. However, I'm running into some issues when I intentionally fail the 'connect' event on the server side (happens with both a "return False" and raise of the ConnectionRefusedError from flask-socketio, obviously changing the name to not conflict with Python's built-in).

On Firefox, I see the following:

Firefox can’t establish a connection to the server at ws://127.0.0.1:5000/socket.io/?EIO=4&transport=websocket&sid=knBqXuIQnfdUhrAKAAAC. websocket.js:54:26
The connection to ws://127.0.0.1:5000/socket.io/?EIO=4&transport=websocket&sid=knBqXuIQnfdUhrAKAAAC was interrupted while the page was loading. websocket.js:54:26

In addition, server-side errors of:

simple_websocket.ws.ConnectionClosed: Connection closed: 1005
OSError
(not sure if the full stack-trace is needed)

On the Chromium side of things, there's no server error, but on the client I am seeing:

websocket.js:54 WebSocket connection to 'ws://127.0.0.1:5000/socket.io/?EIO=4&transport=websocket&sid=fQgITd3EwkgSil7pAAAE' failed: Invalid frame header

Any thoughts as to why I would be seeing this, as well as what I could do to resolve? I haven't had issues with previous versions that are now outdated; they just went right to the disconnect handler so I could redirect to another path. Obviously I'm trying to intentionally fail the connection if someone isn't logged in, making them do so first. Here's how I'm calling things client side:

var socket;
var interval;
$(document).ready(function() {
  socket = io.connect('http://' + document.domain + ':' + location.port + '/judge');
  // RECV sockets
  ...
});
// SEND sockets

[wmorgansoftware]

OBE; I was not using on('connect_error'). Still having issues with the server throwing tracebacks, but that's not socket.io's problem, so this can be closed.