Array Property Gotcha

components.ui-test.js

import Ember from 'ember';

export default Ember.Component.extend({
  testArray: [],
  
  didReceiveAttrs() {
    this._super(...arguments);
    const elementId = this.get('elementId')
    this.get('testArray').push(elementId)
  }
});

controllers.application.js

import Ember from 'ember';

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

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
{{ui-test}}

{{ui-test}}

templates.components.ui-test.hbs

<h2>Component Instance ID: {{elementId}}</h2>

<ul>
  {{#each testArray as |test|}}
    <li>{{test}}</li>
  {{/each}}
</ul>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.2"
  }
}