socket.io determine if a user is online or offline

Solution:

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, each tab has unique socket id for single user, so we need to manage array of socket ids for particular user,

Client Side Connection:

<!-- SOCKET LIBRARY IN HTML -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/2.3.0/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
    allowUpgrades: false,
    query: queryParams,
    reconnect: false,
    secure: true,
    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:

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

  });

});