socket.io determine if a user is online or offline

If your clients have specific user IDs they need to send them to socket.io server. E.g. on client side you can do

<script> const socket = io(); socket.emit('login',{userId:'YourUserID'});</script>

And on server you will put something like

const users = {};io.on('connection', function(socket){  console.log('a user connected');  socket.on('login', function(data){    console.log('a user ' + data.userId + ' connected');    // saving userId to object with socket ID    users[socket.id] = data.userId;  });  socket.on('disconnect', function(){    console.log('user ' + users[socket.id] + ' disconnected');    // remove saved socket from users object    delete users[socket.id];  });});

Now you can pair socket ID to your user ID and work with it.

In addition of @galethil's answer, What if user opens more than one tab (socket connection), each tab (socket connection) has unique socket id for single user, so we need to manage array of socket ids for particular user,

Client Side Connection:Support Socket IO Client v3.x,

<!-- SOCKET LIBRARY IN HTML --><script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/3.0.5/socket.io.js"></script>

const host = "http://yourdomain.com";// PASS your query parametersconst queryParams = { userId: 123 };const socket = io(host, {    path: "/pathToConnection",    transports: ['websocket'],  // https://stackoverflow.com/a/52180905/8987128    upgrade: false,    query: queryParams,    reconnection: false,    rejectUnauthorized: false});socket.once("connect", () => {        // USER IS ONLINE    socket.on("online", (userId) => {        console.log(userId, "Is Online!"); // update online status    });    // USER IS OFFLINE    socket.on("offline", (userId) => {        console.log(userId, "Is Offline!"); // update offline status    });});

Server Side Connection: Support Socket IO Server v3.x,

• Dependencies:

const _ = require("lodash");const express = require('express');const app = express();const port = 3000; // define your portconst server = app.listen(port, () => {  console.log(`We are Listening on port ${port}...`);});

• Connection:

const io = require('socket.io')(server, {    path: "/pathToConnection"});let users = {};io.on('connection', (socket) => {  let userId = socket.handshake.query.userId; // GET USER ID    // CHECK IS USER EXHIST   if (!users[userId]) users[userId] = [];    // PUSH SOCKET ID FOR PARTICULAR USER ID  users[userId].push(socket.id);     // USER IS ONLINE BROAD CAST TO ALL CONNECTED USERS  io.sockets.emit("online", userId);  // DISCONNECT EVENT  socket.on('disconnect', (reason) => {    // REMOVE FROM SOCKET USERS    _.remove(users[userId], (u) => u === socket.id);    if (users[userId].length === 0) {      // ISER IS OFFLINE BROAD CAST TO ALL CONNECTED USERS      io.sockets.emit("offline", userId);      // REMOVE OBJECT      delete users[userId];    }       socket.disconnect(); // DISCONNECT SOCKET  });});

GitHub Demo

We can identify the socket id in server who is connected and who is disconnected. So you can do something like this. You can use this setup if you have an identifier in client side

CLIENT

socket.emit('login', userId);

SERVER SIDE

const users = {};io.on("connection", (socket) => {   socket.on("login", (data) => {      users[socket.id] = data;    });  socket.on("disconnecting", (reason) => {        delete users[socket.id]; // remove the user. -- maybe not the exact code  });});

Hope you get the idea.