New Twiddle

controllers.application.js

import Ember from 'ember';
import aliasDefault from '../file'
export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  s: undefined,
  foo: aliasDefault('s', 'foo'),
  actions: {
   	 changeAlias () {
       this.set('s', 'bar')
       // this.set('foo', 'bar')
     }
  }
});

file.js

import Ember from 'ember'
export default function aliasDefault (altKey, defaultRes) {
  var prev = Ember.computed.alias(altKey)
  var prevGet = prev.get
  prev.get = function (obj, keyName) {

		var res = prevGet.apply(this,[obj, keyName])
    return Ember.isPresent(res) ? res : defaultRes 
  }
  return prev 
}

templates.application.hbs

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

<h3>Alias Default: {{foo}}</h3>
<h3>Original: {{s}}</h3>
<br>
<button {{action "changeAlias"}}>Clicker</button>
<br>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}