Updating a plain object

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  // Fun fact: using an object as a property like this is kinda OK for controllers,
  // since they are singletons. But the instance of the object will leak across
  // tests! So please don't do what I do here:
  foo: {
    bar: 0
  },
  
  actions: {
    change() {
      this.set('foo.bar', Math.random());
      console.log('Note how getter and setter appear on the object:', this.foo);
    }
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

<p>
  foo.bar: {{foo.bar}}
</p>

<p>
  <button {{action 'change'}}>Change</button>
</p>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}