How do I send a private message to another user through sockets?

I have seen the tutorials of sockets in all types of messages, like emit, broadcast and direct message. But I haven’t seen a private message feature. I want to build a whatsapp like clone to send message to a selected particular user. How can I build this using sockets ?

And one bug is also there. While performing an action using action flows in front-end nodejs I cannot see the socket events variables.