From the @LearnRPG answer but with 1.0:

 // send to current request socket client socket.emit('message', "this is a test"); // sending to all clients, include sender io.sockets.emit('message', "this is a test"); //still works //or io.emit('message', 'this is a test'); // sending to all clients except sender socket.broadcast.emit('message', "this is a test"); // sending to all clients in 'game' room(channel) except sender socket.broadcast.to('game').emit('message', 'nice game'); // sending to all clients in 'game' room(channel), include sender // docs says "simply use to or in when broadcasting or emitting" io.in('game').emit('message', 'cool game'); // sending to individual socketid, socketid is like a room socket.broadcast.to(socketid).emit('message', 'for your eyes only'); 

To answer @Crashalot comment, socketid comes from:

var io = require('socket.io')(server); io.on('connection', function(socket) { console.log(socket.id); }) 

Here is a more complete answer about what has changed from 0.9.x to 1.x.

 // send to current request socket client socket.emit('message', "this is a test");// Hasn't changed // sending to all clients, include sender io.sockets.emit('message', "this is a test"); // Old way, still compatible io.emit('message', 'this is a test');// New way, works only in 1.x // sending to all clients except sender socket.broadcast.emit('message', "this is a test");// Hasn't changed // sending to all clients in 'game' room(channel) except sender socket.broadcast.to('game').emit('message', 'nice game');// Hasn't changed // sending to all clients in 'game' room(channel), include sender io.sockets.in('game').emit('message', 'cool game');// Old way, DOES NOT WORK ANYMORE io.in('game').emit('message', 'cool game');// New way io.to('game').emit('message', 'cool game');// New way, "in" or "to" are the exact same: "And then simply use to or in (they are the same) when broadcasting or emitting:" from http://socket.io/docs/rooms-and-namespaces/ // sending to individual socketid, socketid is like a room io.sockets.socket(socketid).emit('message', 'for your eyes only');// Old way, DOES NOT WORK ANYMORE socket.broadcast.to(socketid).emit('message', 'for your eyes only');// New way 

I wanted to edit the post of @soyuka but my edit was rejected by peer-review.

Emit cheatsheet

io.on('connect', onConnect); function onConnect(socket){ // sending to the client socket.emit('hello', 'can you hear me?', 1, 2, 'abc'); // sending to all clients except sender socket.broadcast.emit('broadcast', 'hello friends!'); // sending to all clients in 'game' room except sender socket.to('game').emit('nice game', "let's play a game"); // sending to all clients in 'game1' and/or in 'game2' room, except sender socket.to('game1').to('game2').emit('nice game', "let's play a game (too)"); // sending to all clients in 'game' room, including sender io.in('game').emit('big-announcement', 'the game will start soon'); // sending to all clients in namespace 'myNamespace', including sender io.of('myNamespace').emit('bigger-announcement', 'the tournament will start soon'); // sending to individual socketid (private message) socket.to(<socketid>).emit('hey', 'I just met you'); // sending with acknowledgement socket.emit('question', 'do you think so?', function (answer) {}); // sending without compression socket.compress(false).emit('uncompressed', "that's rough"); // sending a message that might be dropped if the client is not ready to receive messages socket.volatile.emit('maybe', 'do you really need it?'); // sending to all clients on this node (when using multiple nodes) io.local.emit('hi', 'my lovely babies'); }; 

broadcast.emit sends the msg to all other clients (except for the sender)

socket.on('cursor', function(data) { socket.broadcast.emit('msg', data); }); 

For namespaces within rooms looping the list of clients in a room (similar to Nav's answer) is one of only two approaches I've found that will work. The other is to use exclude. E.G.

socket.on('message',function(data) { io.of( 'namespace' ).in( data.roomID ).except( socket.id ).emit('message',data); } 

Server-side all valid emit events list in V4.x

io.on("connection", (socket) => { // basic emit socket.emit(/* ... */); // to all clients in the current namespace except the sender socket.broadcast.emit(/* ... */); // to all clients in room1 except the sender socket.to("room1").emit(/* ... */); // to all clients in room1 and/or room2 except the sender socket.to(["room1", "room2"]).emit(/* ... */); // to all clients in room1 io.in("room1").emit(/* ... */); // to all clients in room1 and/or room2 except those in room3 io.to(["room1", "room2"]).except("room3").emit(/* ... */); // to all clients in namespace "myNamespace" io.of("myNamespace").emit(/* ... */); // to all clients in room1 in namespace "myNamespace" io.of("myNamespace").to("room1").emit(/* ... */); // to individual socketid (private message) io.to(socketId).emit(/* ... */); // to all clients on this node (when using multiple nodes) io.local.emit(/* ... */); // to all connected clients io.emit(/* ... */); // WARNING: `socket.to(socket.id).emit()` will NOT work, as it will send to everyone in the room // named `socket.id` but the sender. Please use the classic `socket.emit()` instead. // with acknowledgement socket.emit("question", (answer) => { // ... }); // without compression socket.compress(false).emit(/* ... */); // a message that might be dropped if the low-level transport is not writable socket.volatile.emit(/* ... */); }); 

Client-side

// basic emit socket.emit(/* ... */); // with acknowledgement socket.emit("question", (answer) => { // ... }); // without compression socket.compress(false).emit(/* ... */); // a message that might be dropped if the low-level transport is not writable socket.volatile.emit(/* ... */); 

archived link to source => http://archive.today/2021.06.22-023900/https://socket.io/docs/v4/emit-cheatsheet/index.html

Other cases

io.of('/chat').on('connection', function (socket) { //sending to all clients in 'room' and you io.of('/chat').in('room').emit('message', "data"); }; 

Updated the list for further documentation.

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. 

Hope this helps.

Updated for Socket.io v4.0.0+

To emit to all clients except sender in v4, you use:

socket.broadcast.emit(/* ... */); 

Example Usage

io.on("connection", (socket) => { socket.on("message", (message) => { //sends "newMessage" to all sockets except sender socket.broadcast.emit("newMessage", message) }) }) 

use this coding

io.sockets.on('connection', function (socket) { socket.on('mousemove', function (data) { socket.broadcast.emit('moving', data); }); 

this socket.broadcast.emit() will emit everthing in the function except to the server which is emitting

I am using namespaces and rooms - I found

socket.broadcast.to('room1').emit('event', 'hi'); 

to work where

namespace.broadcast.to('room1').emit('event', 'hi'); 

did not

(should anyone else face that problem)