Socket IO error "Access to XMLHttpRequest has been blocked by CORS policy"

Question

my app has recently received an error

"Access to XMLHttpRequest at 'https://my-service-domain/socket.io/?EIO=3&transport=polling&t=M-eWtUQ' from origin 'https://my-app-domain' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource."

when i connecting to socket.io. I have not yet found a solution to this problem, I will describe in detail as the image below, has anyone ever encountered this error.

Server config:

var express = require('express');
var app = express();

app.use(function(req, res, next) {
  res.header("Access-Control-Allow-Origin", "*");
  res.header("Access-Control-Allow-Headers", "X-Requested-With");
  res.header("Access-Control-Allow-Headers", "Content-Type");
  res.header("Access-Control-Allow-Methods", "PUT, GET, POST, DELETE, OPTIONS");
  next();
});
var http = require('http');
var server = http.createServer(app);
var io = require('socket.io').listen(server, {log:false, origins:'*:*'});

Client config:

var socket = io('https://my-service-domain', {transports: ['polling']});
socket.on(channel_id, function(data){
     // some code
});

I tried to switch the websocket connection option var socket = io('https://my-service-domain', {transports: ['websocket']}), I got the error

"WebSocket connection to 'wss://my-service-domain/socket.io/?EIO=3& transport = websocket' failed: Error during WebSocket handshake: Unexpected response code: 400 "

Answer 1

If you are using Socket.IO v3, you need to explicitly enable Cross-Origin Resource Sharing (CORS).
const io = require("socket.io")(httpServer, {
  cors: {
    origin: "http://localhost:8080",
    methods: ["GET", "POST"]
  }
});

httpServer.listen(3000);

Answer 2

It is 100% working.. I have spend 2 hours in it and at last find the answer . Just replace code with this...

const express = require("express")
var app = express();
var server = app.listen(4000);
var io = require('socket.io')(server, {
    cors: {
      origin: '*',
    }
});