How can I use CORS with a socket.io server not built on top of a HTTP server

I'm trying to write a server running under nodejs that is just supposed to be a socket server; not supposed to accept http requests at all. Here's how I am setting it up:

import { Server as ServerSocket } from 'socket.io';
import { ClientToServerEvents, ServerToClientEvents } from './types';

const server = new ServerSocket<
  ClientToServerEvents,
  ServerToClientEvents>({cors:{
    origin: "*"
  }});

server.on('connection', (client) => {
  console.log('got connection');
});

server.listen(55555);

server.engine.on("initial_headers", (headers:unknown, req:unknown) => {
    if (typeof headers === 'object' && headers) {
        Object.assign(headers, {"Access-Control-Allow-Origin": "*"});
     }
  });
  
  server.engine.on("headers", (headers:unknown, req:unknown) => {
    if (typeof headers === 'object' && headers) {
        Object.assign(headers, {"Access-Control-Allow-Origin": "*"});
    }
  });

As you can see, I have tried three ways to set up CORS (from random web pages, hacked to allow T开发者_开发问答ypeScript to be happy), but none of them work. The client (a React-based web page on firefox) is unable to connect. It uses:

import { Socket, io } from 'socket.io-client';
...
this.socket = io(':55555');

I always get:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://localhost:55555/socket.io/?EIO=4&transport=polling&t=OJgOc80. (Reason: CORS request did not succeed). Status code: (null).

Indeed this happens even if the server isn't running. It seems to be blocked on the client. Is it because socket.io-client is still using http?