New Twiddle

controllers.application\.js

import Controller from '@ember/controller';

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

helpers.my-helper\.js

import { helper } from '@ember/component/helper';
import { htmlSafe } from '@ember/string';

export default helper(function myHelper(params/*, hash*/) {
  return htmlSafe('<script>alert(1)<' + '/script>');
});

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>
<br>
{{my-helper}}
<br>
<br>

twiddle\.json

{
  "version": "0.17.0",
  "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/3.3.1/jquery.js",
    "ember": "3.15.0",
    "ember-template-compiler": "3.15.0",
    "ember-testing": "3.15.0"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}