👀 Views: 263 💬 Answers: 1 📅 Created: 2025-06-05

Node.js Socket.IO CORS Express JavaScript

I'm reviewing some code and I'm working with issues with CORS configuration in my Node.js application that uses Socket.IO. I'm trying to set up a basic chat application where the client connects to the server using Socket.IO, but I'm running into issues when the client is hosted on a different domain. The behavior I consistently see in the browser's console is: `Access to XMLHttpRequest at 'http://localhost:3000/socket.io/?EIO=4&transport=polling&t=NS7I...` has been blocked by CORS policy: Response to preflight request doesn't pass access control check: It does not have HTTP ok status.` To resolve this, I have set up CORS middleware using the `cors` package. Here's what my server code looks like: ```javascript const express = require('express'); const http = require('http'); const cors = require('cors'); const { Server } = require('socket.io'); const app = express(); const server = http.createServer(app); const io = new Server(server); const corsOptions = { origin: 'http://localhost:4000', // client-side URL methods: ['GET', 'POST'], allowedHeaders: ['Content-Type'], }; app.use(cors(corsOptions)); io.on('connection', (socket) => { console.log('A user connected'); socket.on('disconnect', () => { console.log('User disconnected'); }); }); server.listen(3000, () => { console.log('Server is running on port 3000'); }); ``` On the client side, I’m simply connecting to the Socket.IO server like so: ```javascript const socket = io('http://localhost:3000'); ``` Despite configuring CORS, I'm still getting the CORS behavior. I've double-checked that the client is indeed running on `http://localhost:4000`, and I've also tried adding `credentials: true` to the CORS options, but I still face this scenario. I've confirmed that the server logs don't show any incoming requests from the client prior to the behavior. Is there something I might be overlooking in the CORS setup for Socket.IO specifically? Any insights would be greatly appreciated!