Glimmer

components.test-component\.js

import Component from '@glimmer/component';
import { action } from '@ember/object';
export default class extends Component {
  
  @action
  addElement() {
    this.args.testArray.pushObject("two");
  }
  
}

controllers.application\.js

import Controller from '@ember/controller';
export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';
	testArray = ["one"];
}

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>

<h3>Application Template</h3>
{{#each this.testArray as |element|}}
	{{element}}
{{/each}}

<TestComponent
	@testArray={{this.testArray}}
/>

<br>
<br>
{{outlet}}
<br>
<br>

templates.components.test-component\.hbs

<br>

<h3>Inside Component</h3>
<button {{on "click" this.addElement}}>add element</button>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0",
    "ember-data": "3.17.1"
  }
}