Ivo Wetzel's answer doesn't seem to be valid in Socket.io 0.9 anymore.

In short you must now save the socket.id and use io.sockets.socket(savedSocketId).emit(...) to send messages to it.

This is how I got this working in clustered Node.js server:

First you need to set Redis store as the store so that messages can go cross processes:

var express = require("express");
var redis = require("redis");
var sio = require("socket.io");

var client = redis.createClient()
var app = express.createServer();
var io = sio.listen(app);

io.set("store", new sio.RedisStore);


// In this example we have one master client socket 
// that receives messages from others.

io.sockets.on('connection', function(socket) {

  // Promote this socket as master
  socket.on("I'm the master", function() {

    // Save the socket id to Redis so that all processes can access it.
    client.set("mastersocket", socket.id, function(err) {
      if (err) throw err;
      console.log("Master socket is now" + socket.id);
    });
  });

  socket.on("message to master", function(msg) {

    // Fetch the socket id from Redis
    client.get("mastersocket", function(err, socketId) {
      if (err) throw err;
      io.sockets.socket(socketId).emit(msg);
    });
  });

});

I omitted the clustering code here, because it makes this more cluttered, but it's trivial to add. Just add everything to the worker code. More docs here http://nodejs.org/api/cluster.html

For your convenience, each socket automatically joins a room identified by its id (see Socket#id). This makes it easy to broadcast messages to other sockets:

io.on("connection", (socket) => {
  socket.on("say to someone", (id, msg) => {
    // send a private message to the socket with the given id
    socket.to(id).emit("my message", msg);
  });
});

docs: https://socket.io/docs/v4/server-api/#socketjoinroom

The simplest, most elegant way

verified working with socket.io v3.1.1

It's as easy as:

client.emit("your message");

And that's it. Ok, but how does it work?

Minimal working example

Here's an example of a simple client-server interaction where each client regularly receives a message containing a sequence number. There is a unique sequence for each client and that's where the "I need to send a message to a particular client" comes into play.

Server

server.js

const
    {Server} = require("socket.io"),
    server = new Server(8000);

let
    sequenceNumberByClient = new Map();

// event fired every time a new client connects:
server.on("connection", (socket) => {
    console.info(`Client connected [id=${socket.id}]`);
    // initialize this client's sequence number
    sequenceNumberByClient.set(socket, 1);

    // when socket disconnects, remove it from the list:
    socket.on("disconnect", () => {
        sequenceNumberByClient.delete(socket);
        console.info(`Client gone [id=${socket.id}]`);
    });
});

// sends each client its current sequence number
setInterval(() => {
    for (const [client, sequenceNumber] of sequenceNumberByClient.entries()) {
        client.emit("seq-num", sequenceNumber);
        sequenceNumberByClient.set(client, sequenceNumber + 1);
    }
}, 1000);

The server starts listening on port 8000 for incoming connections. As soon as a new connection is established, that client is added to a map that keeps track of its sequence number. The server also listens for the disconnect event to remove the client from the map when it leaves.

Each and every second, a timer is fired. When it does, the server walks through the map and sends a message to every client with their current sequence number, incrementing it right after. That's all that is to it. Easy peasy.

Client

The client part is even simpler. It just connects to the server and listens for the seq-num message, printing it to the console every time it arrives.

client.js

const
    io = require("socket.io-client"),
    ioClient = io.connect("http://localhost:8000");

ioClient.on("seq-num", (msg) => console.info(msg));

Running the example

Install the required libraries:

npm install [email protected] [email protected]

Run the server:

node server

Open other terminal windows and spawn as many clients as you want by running:

node client

I have also prepared a gist with the full code here.

You can use

//send message only to sender-client

socket.emit('message', 'check this');

//or you can send to all listeners including the sender

io.emit('message', 'check this');

//send to all listeners except the sender

socket.broadcast.emit('message', 'this is a message');

//or you can send it to a room

socket.broadcast.to('chatroom').emit('message', 'this is the message to all');

In 1.0 you should use:

io.sockets.connected[socketid].emit();

Whatever version we are using if we just console.log() the "io" object that we use in our server side nodejs code, [e.g. io.on('connection', function(socket) {...});], we can see that "io" is just an json object and there are many child objects where the socket id and socket objects are stored.

I am using socket.io version 1.3.5, btw.

If we look in the io object, it contains,

 sockets:
  { name: '/',
    server: [Circular],
    sockets: [ [Object], [Object] ],
    connected:
     { B5AC9w0sYmOGWe4fAAAA: [Object],
       'hWzf97fmU-TIwwzWAAAB': [Object] },

here we can see the socketids "B5AC9w0sYmOGWe4fAAAA" etc. So, we can do,

io.sockets.connected[socketid].emit();

Again, on further inspection we can see segments like,

 eio:
  { clients:
     { B5AC9w0sYmOGWe4fAAAA: [Object],
       'hWzf97fmU-TIwwzWAAAB': [Object] },

So, we can retrieve a socket from here by doing

io.eio.clients[socketid].emit();

Also, under engine we have,

engine:
 { clients:
    { B5AC9w0sYmOGWe4fAAAA: [Object],
      'hWzf97fmU-TIwwzWAAAB': [Object] },

So, we can also write,

io.engine.clients[socketid].emit();

So, I guess we can achieve our goal in any of the 3 ways I listed above,

1. io.sockets.connected[socketid].emit(); OR
2. io.eio.clients[socketid].emit(); OR
3. io.engine.clients[socketid].emit();

You can do this

On server.

global.io=require("socket.io")(server);

io.on("connection",function(client){
    console.log("client is ",client.id);
    //This is handle by current connected client 
    client.emit('messages',{hello:'world'})
    //This is handle by every client
    io.sockets.emit("data",{data:"This is handle by every client"})
    app1.saveSession(client.id)

    client.on("disconnect",function(){
        app1.deleteSession(client.id)
        console.log("client disconnected",client.id);
    })

})

    //And this is handle by particular client 
    var socketId=req.query.id
    if(io.sockets.connected[socketId]!=null) {
        io.sockets.connected[socketId].emit('particular User', {data: "Event response by particular user "});
    }

And on client, it is very easy to handle.

var socket=io.connect("http://localhost:8080/")
    socket.on("messages",function(data){
        console.log("message is ",data);
        //alert(data)
    })
    socket.on("data",function(data){
        console.log("data is ",data);
        //alert(data)
    })

    socket.on("particular User",function(data){
        console.log("data from server ",data);
        //alert(data)
    })

As of version 1.4.5, be sure you provide a properly prefixed socketId in io.to(). I was taking the socketId the Client logged to debug and it was without prefix so I ended up searching forever till I found out! So you might have to do it like this if the Id you have is not prefixed:

io.to('/#' + socketId).emit('myevent', {foo: 'bar'});

io.sockets.sockets[socket.id].emit(...) worked for me in v0.9

Also you can keep clients refferences. But this makes your memmory busy.

Create an empty object and set your clients into it.

const myClientList = {};

  server.on("connection", (socket) => {
    console.info(`Client connected [id=${socket.id}]`);
     myClientList[socket.id] = socket;   
  });
  socket.on("disconnect", (socket) => {
    delete myClientList[socket.id];
  });

then call your specific client by id from the object

myClientList[specificId].emit("blabla","somedata");

Socket.IO allows you to “namespace” your sockets, which essentially means assigning different endpoints or paths.

This might help: http://socket.io/docs/rooms-and-namespaces/

The solution that worked for me is the following:

var room = "room_"+socket.id;
socket.join(room);
io.to(room).emit("eventName", data);
io.to(socket.id).emit("eventName",data);

I am not sure why the solution worked for me. The steps that I did were: I created a unique room, joined the room to the socket, emitted the event to the room, and emitted the event to the socket ID.

Would someone explain to me, please?