Maintain a Map of online users (userId → socketId). When sending a message, check if the receiver is online. If not, send an email notification with the sender's name, message preview, and a link to the chat. This keeps offline users informed.
On message:send event: validate input, save to MongoDB, update conversation, broadcast to room with io.to(roomId).emit('message:received', message), check if receiver is online (if not, send email), and acknowledge with callback. Client emits message:send with a callback, listens for message:received, and updates the UI.
The client passes a callback as the third argument: socket.emit('message:send', data, callback). The server receives the callback as the second parameter and calls it with the result: callback({ success: true, message }). This confirms the server received and processed the event.
When a user opens a chat or receives a message, emit messages:read with the targetUserId. The server updates all unread messages from that sender to read: true and resets the unread count in the conversation. Broadcast messages:read to the room so the sender can update read receipts.
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