How to know which user is online?

Question

I want to know that if 2 users in a room are online or not. if both are online then I want to tell it to the room that the user is online but I can't find a way to know how to tell they are online.

I have a global array which stores the acc_id of all the socket.id and gets updated everytime the socketid changes, by this I can know the user has not open another tab as a new tab gives a new client.

io.of('/').in(room).clients(function(error, clients) {
    console.log(clients.length);
    clients.forEach(function(iiid){
    ds.some(function(el) {
       if (el.socket_id === iiid && el.acc_id !== decrypted && clients.length >= 2) {
       io.to(room).emit('isasdjfg__-dfko', {
             dfhdf: "Oui", //Yes in French 
             pfg: room
        });
      }
 });
});
});

This checks if the clients in the room have a acc_id that is not equal to the current user id means if there are 2 people in the room, but this doesn't works.

Answer 1

The below code contains a users array in the server . When a user connects it sends a connect event with an id . That id is stored in the users array . When the user disconnects the id is removed from the array. So you can make a condition when the user id is in the array user is online else offline .

if(users.includes(id))
console.log('online')

For checking if 2 users with id1 and id2 respectively are online

if(users.includes(id1) && users.includes(id2))
console.log("both are online")

Server

var users = [];

io.on('connection', function(socket){
  socket.on('connect', function(data){
 users[socket.id] = data.id;
  });
  socket.on('disconnect', function(){
    users.splice(socket.id,1);
  });
});

Client

var socket = io();
 socket.emit('connect',{id:'id'});