New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  prop1: Ember.computed('appName', function() {
    return this.get('appName') + '!';
  }),
  
  prop2: Ember.computed('appName', {
    get(key) {
       return this.get('appName') + '!';
    },
    set(value) {
      return value;
    }
  }),
  
  actions: {
    doAction() {
      this.set('prop1', 'new prop1');
      this.set('prop2', 'new prop2');
    }
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
Prop1: {{prop1}}
<br>
Prop2: {{prop2}}
<b>
<button {{action "doAction"}}>Do action</button>
<br>
<br>

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"
  }
}