socket.io v.4 How to get list of sockets already in room when joining

Question

I am using socket.io (v4.0.1) to build a chat app with react and node. I would like to get a list of sockets (users) who have already joined a room so that when a users joins a room I can send a notification to them to let them know if there are other people online already. There are a lot of examples how to do this with earlier versions of socket.io but none seem to work with version 4. Here is my server code for this event:

  // join a room (chat) with the chat id
  socket.on('join chat', ({ chat, userId }) => {
    socket.join(chat.id);

    // this line is not working
    const clients = Object.keys(io.sockets.adapter.rooms[chat.id].sockets);
    // does not work
    // var roster = io.sockets.clients('chatroom1');


    const { users } = chat;

    if (!chat.users) return console.log('users not defined');

    // for each user in the conversation, send a notification to their own room
    // telling them someone joined the chat
    // want to let the user who joined (userId) know who is already there
    chat.users.forEach(user => {
      if (user._id === userId) return;
      socket.in(user._id).emit('user joined', userId);
    });

    console.log(`User with id ${userId} joined chat with id ${chat.id}`);
  });

Answer 1

Got an answer to this on Github. Hope this helps someone:

// return all Socket instances
const sockets = await io.fetchSockets();
    
// return all Socket instances in the "room1" room of the main namespace
const sockets = await io.in("room1").fetchSockets();
    
// return all Socket instances in the "room1" room of the "admin" namespace
const sockets = await io.of("/admin").in("room1").fetchSockets();
    
// this also works with a single socket ID
const sockets = await io.in(theSocketId).fetchSockets();

Documentation: https://socket.io/docs/v4/server-instance/#Utility-methods