if

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName:'Ember Twiddle',
  selected: 0,
  options: [1, 2],
  
  actions: {
    select(index) {
      this.set('selected', index);
    }
  }
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
{{#each options as |label index|}}
<button {{action "select" index}} class="{{if index selected 'selected'}}" {{action 'toggle'}}>
  {{label}} {{if index selected '*'}}
</button>
{{/each}}
<br>
<br>

twiddle.json

{
  "version": "0.4.17",
  "EmberENV": {
    "FEATURES": {}
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js"
  }
}