store clients example.js

var http = require('http')
  , io = require('socket.io') // for npm, otherwise use require('./path/to/socket.io') 
  , clients = {}
  , server = http.createServer(function(req, res){ 
 // your normal server code 
})
server.listen(80)
  

function sendToClient(id, message) {
  clients[id].send(message)
}

var socket = io.listen(server) 

socket.on('connection', function(client){ 
  clients[client.sessionId] = client

  client.on('disconnect', function(){ 
    delete clients[client.sessionId]
  }) 
  
  sendToClient("231489729578634289576", "hey dood!")
}) 

ty!