Node.js variable getting reset - undefined

I have a global variable (openTokSessionID) that is set by a function. When the function is run again, it should check to see if openTokSessionID is set. If it is, it should use the current value. Instead, however, it complains that openTokSessionID is undefined. My server code is below. Thanks for your help.

var http = require('http').createServer(handler), 
io   = require('socket.io').listen(http).set('log level', 1),
opentok = require('opentok'),
key = '',    // Replace with your API key  
secret = '';  // Replace with your API secret  
var ot = new opentok.OpenTokSDK(key,secret);
ot.setEnvironment("staging.tokbox.com");
//ot.setEnvironment("api.opentok.com"); //only for production
http.listen(8080);
console.log('Chatserver listening on port 8080');

var nicknames = {};
var log = {};
var connectedUsersObject={};
var privateSessionObject={};
var data;
var openTokSessionID='';


function handler(req, res) {
  res.writeHead(200);
  res.end();
}

////////////////////////////////////////////SOCKET.IO FUNCTIONS///////////////////////////////////////////////////////
io.sockets.on('connection', function (socket) {
    socket.on('private message', function(data) {
        console.log('OpenTok Session ID is: ....' + openTokSessionID);
        if(data.messageType=='openTokDemoRequest'){ //if new session, create unique session ID, add to current chat object
            console.log(data.messageType + ' sender: ' + data.from);

            var location = '127.0.0.1'; // use an IP or 'localhost' 
            console.log('message type is: "openTokDemoRequest". openTokSessionID is: ' + openTokSessionID);
            var messageRecipient=data.from;
            if(openTokSessionID==''){
                console.log('The session ID is: ' + openTokSessionID + '++++++++');openTokSessionID= ot.create_session(location, function(openTokSessionID){

                    var data= {'to':messageRecipient, 'message':{'token': ot.generate_token({'session_id':openTokSessionID, 'role': "publisher"}), 'sessionID': openTokSessionID, 'apikey':key}, 'from': 'openTok', 'messageType':'demoTokenInfo', 'privateSessionID':''};
                    console.log('NEW session id is: ' + openTokSessionID + '. token is: ' + data.message.token);
                //  privateMessageSend(data);
                //  sendToUser(data);
                    connectedUsersObject[messageRecipient].emit('private message', data);
                    console.log ('message recipient is: ' + messageRecipient);
                }); 
            }
            else{
                console.log('OpenTok Session ID is: ////' + openTokSessionID);
                var data= {'to':messageRecipient, 'message':{'token': ot.generate_token({'session_id':openTokSessionID, 'role': "publisher"}), 'sessionID': openTokSessionID, 'apikey':key}, 'from': 'openTok', 'messageType':'demoTokenInfo', 'privateSessionID':''};
                console.log('session id is: ' + openTokSessionID + '. token is: ' + data.message.token);
                connectedUsersObject[messageRecipient].emit('private message', data);
                console.log ('message recipient is: ' + messageRecipient);
            }   

        }
    });