New Twiddle

controllers.application.js

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

const stateA = 'A'; 
const stateB = 'B'; 
const computedSome = (propName, ...criterias) => computed(propName, function() {
  const prop = this.get(propName);
  return criterias.some(c => c === prop);
});

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  currentState: 'A',
  IS_ACTIVE: computedSome('currentState', stateA, stateB),
	actions: {
  	setState(s){
     this.set('currentState', s);
    }
  }
}); 

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
currentState: {{currentState}}
{{#if IS_ACTIVE}} <br>
You should see it if is action
{{/if}}<br>
<button {{action 'setState' 'A'}}>A</button>
<button {{action 'setState' 'B'}}>B</button>
<button {{action 'setState' 'C'}}>C</button>
<br>
<br> 
{{outlet}}
<br>
<br>

twiddle.json

{
  "version": "0.14.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "2.18.2",
    "ember-template-compiler": "2.18.2",
    "ember-testing": "2.18.2"
  },
  "addons": {
    "ember-data": "2.18.2"
  }
}