Setting default value for object-/array- type property

components.foo-bar.js

import Ember from 'ember';

export default Ember.Component.extend({
  items: Ember.computed(() => []),
  
  actions: {
    addItem() {
      const items = this.get('items');
      items.pushObject(`Item ${items.length}`);
    }
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
});

templates.application.hbs

<div>
  <p>Foo-bar 1</p>
  {{foo-bar}}
</div>
<div>
  <p>Foo-bar 2</p>
  {{foo-bar}}
</div>

templates.components.foo-bar.hbs

<button type="button" onclick={{action 'addItem'}}>Add Item</button>
<ul>
  {{#each items as |item|}}
    <li>{{item}}</li>
  {{/each}}
</ul>

twiddle.json

{
  "version": "0.10.6",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.9.0",
    "ember-data": "2.9.0",
    "ember-template-compiler": "2.9.0",
    "ember-testing": "2.9.0"
  },
  "addons": {}
}