Madness Demo

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  root: 'aa',
  appName: Ember.computed('root', {
    get() {
      return this.get('root');
    },
    set(key, value) {
      let newValue = value;
      if (value === null || value === undefined) {
        newValue = this.defaultValue;
      }
      this.set('root', newValue);
      return newValue;
    }
  }),
  init() {
    this._super(...arguments);
    this.defaultValue = 'default';
    let appName = this.get('appName');
    if (appName === null || appName === undefined) {
      this.set('appName', this.defaultValue);
    }
    window.c = this;
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<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": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}