Angular2 CLI Socket.io Cannot read property 'on' undefined - node.js

Question

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">
{{msg}}
</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);
Solution:
constructor(private _zone: NgZone, public http: Http) {
this.socket = sio(this.url);
}
in your on callback use arrow function to preserve this keyword :
sendMessage(message){
this.socket.on('connect', (data) => {
this.socket.emit('add-message', message);
});
}

Answer 1

you have to instantiate your socket :
constructor(private _zone: NgZone, public http: Http) {
this.socket = sio(this.url);
}
in your on callback use arrow function to preserve this keyword :
sendMessage(message){
this.socket.on('connect', (data) => {
this.socket.emit('add-message', message);
});
}