New Twiddle

components.my-component\.js

import Component from '@glimmer/component';
import {action} from '@ember/object';
import {tracked} from '@glimmer/tracking';
export default class extends Component {
  @tracked selected = [];
  @action select(event) {
    this.selected = [...this.selected, event.target.value];
    event.target.value = "";
  }
}

controllers.application\.js

import Controller from '@ember/controller';

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

templates.application\.hbs

<MyComponent />

templates.components.my-component\.hbs

<select {{on "change" this.select}}>
  <option value=""></option>
  <option value="one">One</option>
  <option value="two">Two</option>
  <option value="three">Three</option>
</select>

Ausgewählt:
<div>
{{this.selected}}
</div>

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"
  }
}