Is it possible to make custom events on the client??

Question

Socket.io

Hello. I'm doing a chat on a project using socket.io.
On the back, I have all the necessary endpoints (to get users, their messages, etc.)
Reading about the socket, I understood that the back must register an event using socket.io.emit.
Can I do the same on the client without the help of a back?
That is an example:
I want to receive a notification when a message arrives:

socket.on('receiveMessage', () => {
 notifAllUsers();
 renderUnreadMessage();
})

Answer 1

What does it mean to receive without the help of a bek?
Who will send the events then if not the server?