getWithDefault

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',

  defaultValue: 'this is the default value',
  
  // displayValue: this.getWithDefault(this, 'maybeValue', this.get('defaultValue')),
  displayValue: this.getWithDefault('maybeValue', 'default'),
  
  actions: {
    rollTheDice() {
      const value = Math.random() > 0.5 ? 'a value' : null;
      this.set('maybeValue', value);
    }
  }

});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
<p>The default value is: {{defaultValue}}</p>
<p>The maybe value is: {{maybeValue}}</p>

<p>The value is: {{displayValue}}</p>

<button onClick={{action 'rollTheDice'}}>Roll the dice</button>
<br>
<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"
  }
}