socket.emit('message', "this is a test"); //sending to sender-client only
socket.broadcast.emit('message', "this is a test"); //sending to all clients except sender
socket.broadcast.to('game').emit('message', 'nice game'); //sending to all clients in 'game' room(channel) except sender
socket.to('game').emit('message', 'enjoy the game'); //sending to sender client, only if they are in 'game' room(channel)
socket.broadcast.to(socketid).emit('message', 'for your eyes only'); //sending to individual socketid
io.emit('message', "this is a test"); //sending to all clients, include sender
io.in('game').emit('message', 'cool game'); //sending to all clients in 'game' room(channel), include sender
io.of('myNamespace').emit('message', 'gg'); //sending to all clients in namespace 'myNamespace', include sender
socket.emit(); //send to all connected clients
socket.broadcast.emit(); //send to all connected clients except the one that sent the message
socket.on(); //event listener, can be called on client to execute on server
io.sockets.socket(); //for emiting to specific clients
io.sockets.emit(); //send to all connected clients (same as socket.emit)
io.sockets.on() ; //initial connection from a client.

Client side, HTML 
 id="messages" is to display broadcasted messages. 
 id ="message-form" is to hold input-field.   

<ol id="messages"></ol>
<form action="" id="message-form">
<input type="text" name="message" id="message" placeholder="enter message">
<button>Send</button>
</form>

Client side, JS
// First entered message is emitted after button click event.  This message is sent through "sendMessage" channel.  Server side will receive message through "sendMessage" channel, then, it emits to all clients connected to the server.

$('#message-form').on('submit', function(event) {
    event.preventDefault();
    socket.emit(
        'sendMessage',
    {
        from: 'User',
        text: $('#message').val()
    },
    () => {//callback function here }
  );

// Client side event listener.  It takes new messages from the server and appends it to HTML.  
socket.on('receiveMessage', message => {
    console.log('receiveMessage -- client side', message);
    const li = $('<li></li>');
    li.text(`${message.from}: ${message.text}`);
    $('#messages').append(li);
});

Server side, JS : 
// io.on from server side listens all incoming events.   Socket.emit inside io.on emits only "receiveMessage" message to all clients connected to the server.   Socket.emit only emit to sender-client while Socket.broadcast sends to every client except the sender itself.

io.on('connection', socket => {
    console.log('New user connected -- msg from server');
    /* socket.emit does Welcome message to new chatter */
    socket.emit('newMessage', chatMessage('Chatbot', 'Welcome'));
    /* socket.braodcast.emit from Admin to new user joined. */
    socket.broadcast.emit(
        'newMessage',
        generatedMessage('Chatbot', 'New User joined')
    );
     /* socket.on listens "sendMessage" from client and io.emit sends the message out to clients */
    socket.on('sendMessage', (message) => {
    console.log('Send message -- server side', message);
    io.emit('receiveMessage', chatMessage(message.from, message.text));
    });


const chatMessage = (from, text) => {
    return {
        from,
        text,
        time: new Date().getTime()
   };
};