A MERN + Socket.IO chat app where the Express API and the Socket.IO server share one HTTP server instance. On connect, the client passes its userId as a handshake query param; the server keeps an in-memory userSocketMap (userId → socketId) and broadcasts getOnlineUsers to every connected client whenever someone joins or leaves. REST handles persistence (auth, message history) while sending a message also emits a newMessage event straight to the recipient's socket for instant delivery — the JWT lives in localStorage and rides along as a token header on every request.