Web Socket: cannot detect client connection on internet disconnect

I want to detect the client connection if client turn off the internet using web socket. My code is:

 //Include util library
    var util = require('util');
    // Include underscore library
    var _ = require('underscore')._;
    //For websocket
var webSocketServer = new (require('ws')).Server({port: (process.env.PORT || 5000)}),
webSockets = {} // userID: webSocket

// CONNECT /:userID
// wscat -c ws://localhost:5000/1
webSocketServer.on('connection', function (webSocket) 
{
  var userID = webSocket.upgradeReq.url.substr(1);
  //console.log('User_id is ',userID);
  webSockets[userID] = webSocket
                   util.log('User_id: [' + userID + '] enter in connected users list of [ ' + Object.getOwnPropertyNames(webSockets)+' ]')
                   // Call function which check id exist in letswalkee DB table
                   check_userid(userID);


                   // Send msg like: [fromUserID, text]    [1, "Hello, World!"]
webSocket.on('message', function(message) {
util.log('Received from [' + userID + ']: ' + message)
var messageArray = JSON.parse(message)
                                    var toUserWebSocket = webSockets[messageArray[0]]
                                    if (toUserWebSocket) {
                                    util.log('Sent to [' + messageArray[0] + ']: ' + JSON.stringify(messageArray))
                                    messageArray[0] = userID
                                    toUserWebSocket.send(JSON.stringify(messageArray))
                                    }
                                    })

                   webSocket.on('close', function () 
                   {
                      delete webSockets[userID]
                      util.log('User_id Deleted from connected users: [ ' + userID+' ]');

                    })
webSocket.on('disconnect',function()
  {
   console.log('hello i am disconnected');  
  });
})

I used that code (webSocket.on('disconnect',function()) but did not worked? Help asap?