CLASS-BINDING

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  isChecked: false
});

styles.app.css

.foo {
  background-color: red;
}
.bar {
  background-color: blue;
}

templates.application.hbs

{{input type="checkbox" checked=isChecked}}
<br>
{{textarea class=(if isChecked "foo" "bar")}}

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"
  }
}