mandatory setter

controllers.application\.js

import Controller from '@ember/controller';

export default Controller.extend({
	appName: 'Ember Twiddle',
  
 
  get someProp() {
  	return 'hi';
	},
  
  actions: {
    setTheThing() {
      this.set('someProp', 'hello');
    }
  }
});

templates.application\.hbs

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

<h2>{{this.someProp}}</h2>

<button {{action "setTheThing"}}> set someProp</button>
<br>
<br>
{{outlet}}
<br>
<br>

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": "3.9.0",
    "ember-template-compiler": "3.9.0",
    "ember-testing": "3.9.0"
  }
}