Socket.IO version - 4.4.1

Server

const express = require("express");
const app = express();
const http = require("http");
const server = http.createServer(app);
const { Server } = require("socket.io");
const cors = require("cors");

const io = new Server(server, {
  cors,
});

const PORT = 3000;

app.use(cors());

io.on("connection", (socket) => {
  console.log("A user connected :)");
  socket.on("msg", console.log);

  socket.on("connect_error", (err) => {
    console.log(`connect_error due to ${err.message}`);
  });
});

server.listen(PORT, () => {
  console.log("listening on *:"   PORT);
});

HTML

Inside body

 <script
      src="https://cdn.socket.io/4.4.1/socket.io.min.js"
    ></script>
    <script>
      // socket.io
      console.log("Connecting..")
      const socket = io("http://localhost:3000")
      socket.on("connect", () => {
        console.log(socket.connected); // true
      });
    </script>

There is no error on either client or server, but still i am not able to connect to the server.

CodePudding user response：

If you change this:

const io = new Server(server, {
   cors,
});

to this:

const io = new Server(server);

Then, your code starts working for me. So, something is wrong with how you're trying to use cors with socket.io. FYI, if you change the client to always use a webSocket as the transport, then you won't need CORS for your socket.io connection because a websocket connection is not subject to same origin restrictions.

const socket = io("http://localhost:3000", {transports: ["websocket"]});

By default a socket.io connection starts with http polling (which is subject to same origin restrictions) and then switching to a websocket after a few http requests.