Why - setting value on pojo

controllers.application\.js

import Controller from '@ember/controller';


export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';

	constructor() {
    super(...arguments);
    
    console.log('initial', this.thePojo);
    
    let i = 0;
    setInterval(() => {
      console.log(this.thePojo);
      i++;
      
      this.thePojo.head.str = i;
    }, 1000);
  }

  thePojo = {
    head: {
      str: 'hi',
    } 
  }
}

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>
<br>
{{this.thePojo.head.str}}
<br>
<br>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}