New Twiddle

controllers.application.js

import Ember from 'ember';

import { computed } from "@ember/object";
import { map } from "@ember/object/computed";

const Folder = Ember.Object.extend({
  subArrays: map('subfolders', 'allChildren'),
  allChildren: computed('subfolders.@each.allChildren', 'subArrays.@each.[]', function() {
    let children = [];
		this.get('subfolders').forEach(folder => {
      children.push(folder);
			folder.get('allChildren').forEach(child =>
        children.push(child)
     	)
    });
    return children;
  })
});

const deepFolder = Folder.create({
  name: "c",
  subfolders: []
})

const subfolders = [
    Folder.create({
      name: "a",
      subfolders: [
        Folder.create({
          name: "b",
          subfolders: [
            deepFolder
          ]
        }) 
      ]
    })
  ];

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  root: Folder.create({subfolders}),
  
  deepFolder,
  
  allNames: computed('root.allChildren.@each.name', function() {
    let names = this.get('root.allChildren').map(child => child.get('name'));
		return names.join(", ");
  }),
  
  actions: {
    addDeeperFolder() {
      this.get('deepFolder.subfolders').pushObject(Folder.create({name: 'Deeper Folder', subfolders: []}));
    }
  }
  
  
});

templates.application.hbs

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

{{allNames}}

name of deeply nested folder: {{input value=deepFolder.name}}

<button {{action 'addDeeperFolder'}}>Add deeper folder</button>

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"
  }
}