socket.io determine if a user is online or offline

Question

We can trace if a connection is established or disconnected by this code

console.log('a user connected');     socket.on('disconnect', function () {         console.log('user disconnected');     }); 

Well, its fine. But how can we determine what user connected or gone offline. My client is written in PHP/HTML so they have a user ID.

Answer 1

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.

Answer 2

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 parameters const 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 port const 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