New Twiddle

controllers.application\.js

import Controller from '@ember/controller';
import EmberObject, {computed} from '@ember/object';


export default class ApplicationController extends Controller {
  persons = [
    EmberObject.create({name: 'foo'}),
    EmberObject.create({name: 'bar'}),
  ];

  length = computed('[email protected]', function () {
    console.log('length', this.persons.length);
    return this.persons.length;
  });
}

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>
<br>
Length: {{this.length}}

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"
  }
}