Cannot read property 'push' of undefined Typescript / Angular 6

Question

I'm Implementing small chat application using socket.io gladly it's working fine. But my concern is when I got a new chat I need to assign it to string array to display in listview.

I have simply define array as "messages: string[] = [];" and pushed sample string on page load and it's working fine.But when I got new message from a socket this.socket.on('newmessage', function (data) method will fire and can read new message.those all things working correctly.

But When I push new string into my "messages: string[] = [];" array. I'm getting 'Cannot read property 'push' of undefined' error.

 import { Component, OnInit} from '@angular/core'; import * as io from 'socket.io-client'; @Component({ selector: 'app-chatbox', templateUrl: './chatbox.component.html', styleUrls: ['./chatbox.component.css'], }) export class ChatboxComponent implements OnInit { socket; messages: string[] = []; constructor() { this.socket = io.connect('http://localhost:8000'); } ngOnInit() { this.initializeChatServer(); } initializeChatServer() { this.messages.push( 'test 55');//This line works this.socket.on('newmessage', function (data) { console.log('message -> ' + data.nick + '>' + data.msg); this.messages.push(data.msg); //Cannot read property 'push' of undefined }); } } 

Answer 1

this.messages.push(data.msg); //Cannot read property 'push' of undefined

Because you have the wrong this . Arrow functions will fix that eg Change this.socket.on('newmessage', function (data) { to this.socket.on('newmessage', (data) => {

Answer 2

import { Component, OnInit} from '@angular/core';
import * as io from 'socket.io-client';

@Component({
  selector: 'app-chatbox',
  templateUrl: './chatbox.component.html',
  styleUrls: ['./chatbox.component.css'],
})

export class ChatboxComponent implements OnInit {
  socket;
  messages: string[] = [];

  constructor() { this.socket = io.connect('http://localhost:8000'); }

  ngOnInit() {
    this.initializeChatServer();
  }

  initializeChatServer() {

    this.messages.push( 'test 55');//This line works

    this.socket.on('newmessage', data => {
      console.log('message -> ' + data.nick + '>' + data.msg);     
      this.messages.push(data.msg); //Cannot read property 'push' of undefined
    });

  }

}

Answer 3

I think it's because of synchronize call.I think it's not the standard way but it worked after I change like this..

  initializeChatServer() { this.messages.push( 'test 55'); var self = this;//assgin this to var variable this.socket.on('newmessage', data => { console.log('message -> ' + data.nick + '>' + data.msg); self.messages.push(data.msg); }); }