custom helper

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  selectedMenuIndex: 0,
  menuItems: [
    { text: 'A' },
    { text: 'B' },
    { text: 'C' },
  ],
  actions: {
    selectMenuItem: function(itemIndex) {
      this.set('selectedMenuIndex', itemIndex);
    }
  }
});

application.template.hbs

{{#each menuItems as |item index|}}
  <li class="item {{active-class index selectedMenuIndex}}" {{action "selectMenuItem" index}}>{{item.text}}</li>
{{/each}}

helpers.active-class.js

import Ember from 'ember';

export function activeClass(params/*, hash*/) {
  const index = params[0];
  const active = params[1];
  
  return (index === active) ? 'active-class': '';
}

export default Ember.Helper.helper(activeClass);

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

.active-class {
  color: blue; 
}

.item {
  list-style: none;
  cursor: pointer
}

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"
  }
}