computed-observer

controllers.application.js

import Ember from 'ember';
const {observer, computed} = Ember;

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  count: 0,
  
  countObserver: observer("count", function() {
    console.log("observer is working");
    this.set("message", `The count is ${this.get("count")}`);
  }),
  
  countSquare: computed("count", function() {
    console.log("computed is working");
    let count = this.get('count');
    return count * count;
  }),
  
  actions: {
    incrementCount() {
      this.incrementProperty("count");
    }
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
Message is {{message}}
<br>
<button onclick={{action 'incrementCount'}}>Increment Count</button>
<br>

Count's square is {{countSquare}}

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"
  }
}