How to track that there is a new unread message?

Question

Node.js

For the site, you need to make a messaging system based on a chat, like in VK. I already did all this through socket.io using rooms, but I stopped at the implementation of receiving a notification to users that they have a new unread message if they are not in a chat (in a specific room). What is the best way to implement the definition of what is a new unread message?