flatten nested arrays

controllers.application.js

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

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  user: computed(function() {
    return {
      accounts: [
        {
          sites: [
        		{ name: 'site-a-1' },
        		{ name: 'site-a-2' }
          ],
        },
        {
          sites: [
            { name: 'sites-b-1' },
          ]
        }
      ],
    };
  }),
});

templates.application.hbs

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

{{#let (flatten (map-by "sites" user.accounts)) as |sites|}}
	{{log sites}}
  
	<ul>
    {{#each sites as |site|}}
      <li>{{site.name}}</li>
    {{/each}}
  </ul>
{{/let}}

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-composable-helpers": "2.1.0"
  }
}