Empty brace expansion

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  a: 1,
  aa: 2,
  ab: 3,
  sum: Ember.computed('a{,a,b}', function() {
    const { a, aa, ab } = this.getProperties('a', 'aa', 'ab');
    return  a + aa + ab;
  }),
  actions : {
    incA() {
      this.set('a', this.get('a') + 1);
    }
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
{{sum}}
<button {{action 'incA'}}>Click me</button>
<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": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}