setReadOnly

controllers.application.js

import Ember from 'ember'

const setReadOnly = (context, name, value) => {
  const writable = false
  const configurable = false
  const enumerable = true
  Ember.defineProperty(context, name, { value, writable, configurable, enumerable })
}

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

    init() {
        setReadOnly(this, 'foo', 'bar')
	}
})

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
foo: {{foo}}
<br><br>
Open your console
{{input value=foo}}
<br>
<br>

twiddle.json

{
  "version": "0.9.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.6.0",
    "ember-data": "2.6.0",
    "ember-template-compiler": "2.6.0"
  }
}