Super Computed

components.my-component.js

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

export default Ember.Component.extend({
  items: computed(function() {
    return ['a', 'b'];
  })
});

components.my-sub-component.js

import MyComponent from './my-component';
import Ember from 'ember';

const { computed } = Ember;

export default MyComponent.extend({
  items: computed(function() {
    return this._super('items').concat(['c', 'd']);
  })
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{my-component}}
<br>
<br>
{{my-sub-component}}
<br>
<br>

templates.components.my-component.hbs

{{items}}

templates.components.my-sub-component.hbs

{{items}}

twiddle.json

{
  "version": "0.13.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "3.14.0",
    "ember-template-compiler": "3.14.0",
    "ember-testing": "3.14.0"
  }
}