New Twiddle

controllers.application\.js

import Controller from '@ember/controller';
import { set, action, computed } from '@ember/object';

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

	@action setAppName(arg) {
    this.set('appName', arg);
  }

	@computed('appName')
	get appNameComputed() {
    return this.appName;
  }

  get appNameComputedThenGetted() {
    return this.appNameComputed;
  }

	get appNameGetted() {
    return this.appName;
  }
}

templates.application\.hbs

appName: {{this.appName}}<br>
appNameComputed: {{this.appNameComputed}}<br>
appNameGetted: {{this.appNameGetted}}<br>
appNameComputedThenGetted: {{this.appNameComputedThenGetted}}
<br>

<button {{on "click" (fn this.setAppName "newApp")}}>
change with .set()
</button>

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"
  }
}