New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  noteError: false,
  actions: {
    toggle() { this.toggleProperty('noteError'); }
  }
});

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

.error {
  background: red;
}

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
{{textarea
  classNames="ham--sandwich"
  classNameBindings="noteError:error"
	value=noteError
}}
  {{textarea
   id="add-note__textarea"
   classNames="add-note__textarea"
   classNameBindings="noteError:error"
   value=noteError
   rows="4"
   placeholder="i18n_leave_a_note"
   required=true
 }}
<button {{action 'toggle'}}>toggle</button>
<br>
<br>

twiddle.json

{
  "version": "0.10.6",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.5.1",
    "ember-data": "2.9.0",
    "ember-template-compiler": "2.5.1",
    "ember-testing": "2.5.1"
  },
  "addons": {}
}