New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
 
  things: Ember.A([1,2,3]),
  firstA: Ember.computed.oneWay('things.firstObject'),
  firstB: Ember.computed.oneWay('things.0'),
  firstC: Ember.computed(function() {
    return this.get('things').objectAt(0);
  }),
 
  actions: {
    modify() {
      let value = this.get('things').objectAt(0);
      this.set('things.0', value + 1);
    }
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{firstA}}
{{firstB}}
{{firstC}}
<br>
<br>
<button onclick={{action 'modify'}}>Change</button>

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"
  }
}