EmberArray autotracking

controllers.application\.js

import { A } from '@ember/array';
import Controller from '@ember/controller';
import { action } from '@ember/object';

export default class ApplicationController extends Controller {
  items = A([]);

  get even() {
    return this.items.length % 2 === 0;
  }

  get len() {
    return this.items.length;
  }

  get allNine() {
    return this.items.every((i) => i.includes('9'));
  }

  get someNine() {
    return this.items.some((i) => i.includes('9'));
  }

  @action
  add() {
    this.items.pushObject(`${Math.floor(Math.random() * 100000)}`);
  }
}

templates.application\.hbs

<button {{on 'click' this.add}}>add</button>

{{#each this.items as |item|}}
  <div>{{item}}</div>
{{/each}}

<div>from array.length: {{this.items.length}} items</div>
<div>from len: {{this.len}} items</div>
{{#if this.allNine}}<div>all items have 9s</div>{{/if}}
{{#if this.someNine}}<div>some items have 9s</div>{{/if}}

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"
  }
}