Computed Property Extensions

application.controller.js

import Ember from 'ember';

Ember.ComputedProperty.prototype.andNot = function(keyName) {
  let previousVal = this.get(this);
  console.log(this, previousVal);
  return Ember.computed(keyName, function () {
    console.log(previousVal, Ember.get(this, keyName));
  	return previousVal && !Ember.get(this, keyName);
  });
}

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  testx: [],
  test2: false,
  x: Ember.computed.empty('testx').andNot('test2'),
  z: Ember.computed.empty('test')
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>
x: {{x}}
<br>
z: {{z}}

twiddle.json

{
  "version": "0.6.5",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.4.3/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.4.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.4.3/ember-template-compiler.js"
  }
}