classes

components.my-component\.js

import Component from '@ember/component';

export default class extends Component {
  classNames = ['my-component'];
}

controllers.application\.js

import Controller from '@ember/controller';

export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';
}

styles.app\.css

.my-component::after {
  display: block;
  font-family: monospace;
  content: "class: " attr(class);
}

templates.application\.hbs

<MyComponent />

<MyComponent @classNames="hi" />
<MyComponent @class="hi" />
<MyComponent class="hi" />

templates.components.my-component\.hbs

{{yield}}

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}