/server> npm install

/server> node index.js

var express = require("express");
var app = express();
var http = require("http").createServer(app);
var socketIO = require("socket.io")(http);
var socketID = "";

socketIO.on("connection", function (socket) {
    console.log("User is conneected ", socket.id);
    socketID = socket.id;
});

http.listen(3000, function () {
    console.log("Server has started running!!");
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/3.0.4/socket.io.js"></script>
<script>
    var socketIO = io("http://localhost:3000", { transports: ['websocket'] });
</script>