set return

application.controller.js

import Ember from 'ember';

const { computed } = Ember;

export default Ember.Controller.extend({
  appName:'Ember Twiddle',
  thing: computed(function () {
    let thing = this.set('appName', 'Ember');
    
    return thing;
  })
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>

{{thing}}

<br>
<br>
{{outlet}}
<br>
<br>

twiddle.json

{
  "version": "0.5.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "1.13.11",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "1.13.11"
  }
}