Custom Radio Option

controllers.application.js

import Ember from 'ember';
import { computed } from '@ember/object';

const radioOptions = [
	'Option 1',
  'Option 2',
  'Option 3',
]

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  radioOptions,
  
  selectedOption: 'Something',
  
  isCustomOption: computed('selectedOption', function() {
  	let options = this.radioOptions;
    let selected = this.selectedOption;
    
    return !options.includes(selected);
  })
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>

<p>{{if selectedOption selectedOption "Nothing selected"}}</p>

{{#each radioOptions as |opt|}}
	<p>
    <label>
      <input
        type="radio"
        name="select-the-thing"
        value={{opt}}
        checked={{eq opt selectedOption}}
        onchange={{action (mut selectedOption) value="target.value"}}
       >
      {{opt}}
    </label>
  </p>
{{/each}}

<p>
  <label>
    <input
      type="radio"
      name="select-the-thing"
      value=""
      checked={{isCustomOption}}
      onchange={{action (mut selectedOption) value="target.value"}}
    >
    Custom
  </label>
</p>

{{#if isCustomOption}}
	<textarea
  	value={{selectedOption}}
  	oninput={{action (mut selectedOption) value="target.value"}}
  />
{{/if}}

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": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2",
    "ember-truth-helpers": "2.0.0"
  }
}