Handling CORS

Configuration

Since Socket.IO v3, you need to explicitly enable Cross-Origin Resource Sharing (CORS).

const io = require("socket.io")(httpServer, {
  cors: {
    origin: "https://example.com",
    methods: ["GET", "POST"]
  }
});

All options will be forwarded to the cors package. The complete list of options can be found here.

Example with cookies (withCredentials) and additional headers:

// server-side
const io = require("socket.io")(httpServer, {
  cors: {
    origin: "https://example.com",
    methods: ["GET", "POST"],
    allowedHeaders: ["my-custom-header"],
    credentials: true
  }
});

// client-side
const io = require("socket.io-client");
const socket = io("https://api.example.com", {
  withCredentials: true,
  extraHeaders: {
    "my-custom-header": "abcd"
  }
});

Note: this also applies to localhost if your web application and your server are not served from the same port

const io = require("socket.io")(httpServer, {
  cors: {
    origin: "http://localhost:8080",
    methods: ["GET", "POST"]
  }
});

httpServer.listen(3000);

You can disallow all cross-origin requests with the allowRequest option:

const io = require("socket.io")(httpServer, {
  allowRequest: (req, callback) => {
    const noOriginHeader = req.headers.origin === undefined;
    callback(null, noOriginHeader);
  }
});

Troubleshooting

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at xxx/socket.io/?EIO=4&transport=polling&t=NMnp2WI. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing).

If you have properly configured your server (see above), this could mean that your browser wasn't able to reach the Socket.IO server.

The following command:

curl "https://api.example.com/socket.io/?EIO=4&transport=polling"

should return something like:

0{"sid":"Lbo5JLzTotvW3g2LAAAA","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":5000}

If that's not the case, please check that your server is listening and is actually reachable on the given port.