@each not working

controllers.application.js

import Ember from 'ember';

let i=0;

export default Ember.Controller.extend({
  arr: [],
  
  counter: (function(){
    return this.get('arr.firstObject');
	}).property('arr.@each.x'),
  
  lala: (function(){
    let self = this;
    setInterval(function(){
      self.set('arr', [i++]);
    }, 1000);
  }).on('init')
                                       
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
counter: {{counter}}
<br>
<br>

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": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}