send previous messages in group chat

Question

I work at a chat application. I want to store messages in an object like this :

{
  'room1': ['msg1', 'msg2', ...]
  'room2': ['msg3', 'msg4', ...]
  ...
}

I defined a variable in my socket.io server roomMessages the problem is when I want to add a message in a specific room using roomMessages[room].push(data); it shows me this error: TypeError: Cannot read properties of undefined (reading 'push') this is my socket.io code:

const roomMessages = {};

roomsNamespace.use((socket, next) => {
  const token = socket.handshake.query.token;
  console.log(token);
  if (!token) return next(new Error("not auth"));
  next();
});

roomsNamespace.on("connection", (socket) => {
  const token = socket.handshake.query.token;
  const apiService = new ApiService(process.env.BE_URL, token);
  const room = socket.handshake.query.room;
  socket.on("join-room", async () => {
    socket.join(room);
    await apiService.joinRoom(room);
    const usersInThisRoom = await apiService.getUsersInRoom(room);
    roomsNamespace.in(room).emit("all-users", usersInThisRoom);
    socket.emit("room-messages", roomMessages[room]);
  });
  socket.on("chat-message", (data) => {
    // console.log(data);
    if (roomMessages[room]) roomMessages[room].push(data);
    roomsNamespace.in(room).emit("chat-message", data);
  });

basically, I want to send previous messages when user joins the chat How to fix this error?

Answer 1

you can do something like this

const roomMessages = {};

const getMessages = (room) => roomMessages[room] || []
const addMessage = (room, message) => {
  roomMessages[room] = [...getMessages(room), message]
}



roomsNamespace.use((socket, next) => {
  const token = socket.handshake.query.token;
  console.log(token);
  if (!token) return next(new Error("not auth"));
  next();
});

roomsNamespace.on("connection", (socket) => {
  const token = socket.handshake.query.token;
  const apiService = new ApiService(process.env.BE_URL, token);
  const room = socket.handshake.query.room;
  socket.on("join-room", async () => {
    socket.join(room);
    await apiService.joinRoom(room);
    const usersInThisRoom = await apiService.getUsersInRoom(room);
    roomsNamespace.in(room).emit("all-users", usersInThisRoom);
    socket.emit("room-messages", getMessages(room));
  });
  socket.on("chat-message", (data) => {
    // console.log(data);
    addMessage(room, data)
    roomsNamespace.in(room).emit("chat-message", data);
  });

in this way if you want later you can change your implementation of messageRepository storing data in redis or wherever you want