New Twiddle

controllers.application\.js

import Controller from '@ember/controller';
import { tracked } from '@glimmer/tracking';
import { action } from '@ember/object';

export default class ApplicationController extends Controller {
  @tracked isOpen = false;

  @action
  toggle() {
    this.isOpen = !this.isOpen;
  }
}

templates.application\.hbs

{{#if this.isOpen}}
  {{set-body-class "is-open"}}
{{/if}}

<button type="button" {{on "click" this.toggle}}>Toggle</button>

{{outlet}}

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": true,
    "_APPLICATION_TEMPLATE_WRAPPER": false,
    "_JQUERY_INTEGRATION": false
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "ember": "3.22.0-beta.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0",
    "ember-set-body-class": "0.4.2"
  }
}