Sending a message to a user that is not connected? - node.js

Question

I'm trying to create a user to user chat application - no group chat or anything.
I'm using NodeJS and Socket.io on the backend and React Native on the frontend.
I ended up having a Map that stores a user id and it's corresponding socket id, my problem is that only when a user connects to the server, he will get a socket id.
But what if User A is connect and is trying to send a message to User B, and User B is not connected, so it does not have a socket id, I don't really know what to do then.
This is what I got so far:
io.on("connection", (socket: Socket) => {
//Whenever a user will connect, the user will emit 'initiate' to register itself and it's socket id to the server.
//We will be using this userSocketMap to send messages between users.
socket.on(SocketEvents.INITIATE, (data) => {
const uid = data.uid;
const socketID = socket.id;
userSocketMap.set(uid, socketID);
});
//This function will get called when a user is sending message to another user.
socket.on(SocketEvents.SEND, (data) => {
const to = data.to;
const from = data.from;
const content = data.content;
const payload: MessagePayload = {
to: to,
from: from,
content: content,
};
const dstUser = userSocketMap.get(to); // getting the socket id of the receiver.
// '/#' is the prefix for the socketID, if the socketID already has this prefix, this should be deleted - need to check this.
//MessageEvent.RECEIVE is used to signal the dstUser to receive a message.
io.to("/#" + dstUser).emit(SocketEvents.RECEIVE, { payload: payload });
});
socket.on(SocketEvents.DISCONNECT, (socket: Socket) => {
const userID = getByValue(socket.id);
if (!userID) return;
userSocketMap.delete(userID);
});
});

Answer 1

You should do two things when working with react-native and socket.io in case user lost internet connection. Use socket.io heartbeat mechanism inorder to get the users that lost connection and are not responding and user NetInfo package to inform the mobile user that he has lost internet connection.
Socket.io
var server = app.listen(80);
var io = socketio(server,{'pingInterval': 2000});
io.on("connection", (socket: Socket) => {
socket.on(SocketEvents.INITIATE, (data) => {
const uid = data.uid;
const socketID = socket.id;
userSocketMap.set(uid, socketID);
})
socket.on('heartbeat', (socket: Socket) => {
const userID = getByValue(socket.id)
userSocketMap.MARK_USER_AS_INACTIVE(userID)
})
});
React-Native - use NetInfo - it used to be part of the core but got separated to a community module
import NetInfo from "#react-native-community/netinfo";
NetInfo.fetch().then(state => {
console.log("Connection type", state.type);
console.log("Is connected?", state.isConnected);
});
const unsubscribe = NetInfo.addEventListener(state => {
console.log("Connection type", state.type);
console.log("Is connected?", state.isConnected);
});
// Unsubscribe
unsubscribe();