Ember Test

controllers.application.js

import Ember from 'ember';
import { computed, get } from '@ember/object';

export default Ember.Controller.extend({
  isButtonOn: false,
  // This toggle will run when the button is pressed, it should toggle the isButtonOn property
  // which should trigger the value of text to recompute
  // -- begin edit area
  toggle () {
  },
  text: computed(function () {
  // -- end edit area
  	return get(this, 'isButtonOn') ?
      'Congrats you\'ve fixed it and turned on the button' :
    	'You must fix controllers/application.js and press the button';
  }),
});

templates.application.hbs

<h1>Test</h1>
<button {{action toggle}}>
	Button ({{if isButtonOn "On" "Off"}})
</button>
<br>
<br>
The text below should change when the button has been toggled
<br>
<br>
{{text}}
<br>
<br>

twiddle.json

{
  "version": "0.15.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.2.2",
    "ember-template-compiler": "3.2.2",
    "ember-testing": "3.2.2"
  },
  "addons": {
    "ember-data": "3.2.0"
  }
}