New Twiddle

controllers.application.js

import Ember from 'ember';
import Foo from '../utils/foo';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  myFoo: new Foo()
});

templates.application.hbs

Native class example

{{this.myFoo.countText}}

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": "3.10.0",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
  }
}

utils.foo.js

import { computed, set, action } from '@ember/object';

export default class Foo {
  
  count = 0;
  
  @computed('count')
  get countText() {
    return `the count is ${this.count}`
  }
  
  @action
  increment() {
    set(this, 'count', this.count+1);
  }
}