computed-decorators-problem

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  @computed('appName')
  get screamingAppName() { return this.appName; }
});

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": "3.8.0",
    "ember-template-compiler": "3.8.0",
    "ember-testing": "3.8.0"
  },
  "addons": {
    "ember-data": "3.8.0",
    "ember-decorators": "5.1.0"
  }
}