readOnly('foos.@each.bars')

controllers.application.js

import Ember from 'ember';
import { readOnly } from '@ember/object/computed';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  
  foos: [{
    bars: ['a', 'b']
  }, {
    bars: ['c', 'd']
  }],
  
  fooBars: readOnly('foos.@each.bars')
});

templates.application.hbs

<p><code>fooBars:</code></p>

<ul>
  {{#each this.fooBars as |bar|}}
    <li>{{bar}}</li>
  {{else}}
  	<li>No fooBars?</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": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}