Angular2 CLI Socket.io Cannot read property 'on' undefined

I want to use socket.io module for sending message and I am newbie in using it. I am trying to run socket.io within my Angular2 CLI + Node.js application and I am getting following error:

TypeError: Cannot read property 'on' of undefined at MessagesComponent.webpackJsonp.363.MessagesComponent.sendMessage (messages.component.ts:34)

What is wrong with my code and how can I connect and send message to the socket.io server?

messages.component.html

<div class="stick" style="background-color:#F5F5F5;">
  <h5>Messages:</h5>

<ul>
<li *ngFor="let msg of msgs">

</li>
</ul>

<input #mm/>
<button (click)="sendMessage(mm.value); mm.value=''">Send</button>
</div>

messages.component.ts

import { Component,Input,OnInit,Output,EventEmitter,HostListener,ElementRef, NgZone} from "@angular/core";

import * as sio from 'socket.io-client';
import { Observable } from 'rxjs/Observable';
@Component({

    selector: "messages",
    templateUrl: './messages.component.html'

})

export class MessagesComponent implements  OnInit{

             socket: SocketIOClient.Socket;


             private url = 'http://localhost:4200';


        constructor(private _zone: NgZone, public http: Http) {}




        ngOnInit() {


        }



              sendMessage(message){
                      this.socket.on('connect', function(data) {
                this.socket.emit('add-message', message);
                });
              }

              getMessages() {
                let observable = new Observable(observer => {
                  this.socket = io(this.url);
                  this.socket.on('message', (data) => {
                    observer.next(data);
                  });
                  return () => {
                    this.socket.disconnect();
                  };
                })
                return observable;
              }


}

www.ts

import { app } from '../app';
import * as http from 'http';




/**
 * Get port from environment and store in Express.
 */
const port = normalizePort(process.env.PORT || 3000);
app.set('port', port);

/**
 * Create HTTP server.
 */
const server = http.createServer(app);

let io = require('socket.io').listen(server);



io.on('connection', (socket) => {

  socket.on('disconnect', function(){
    console.log('user disconnected');
  });

  socket.on('add-message', (message) => {
    io.emit('message', {type:'new-message', text: message});
  });
});
/**
 * Listen on provided port, on all network interfaces.
 */

server.listen(port);
server.on('error', onError);
server.on('listening', onListening);

via Tony