helpers should be pure

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  items: [2, 5, 3, 1, 4]
});

helpers.sort.js

import Ember from 'ember';

const { K } = Ember;
const sortMap = {
  ascending(a, b) { return a - b; },
  descending(a, b) { return b - a; } 
};

export function sort([order, items]) {
  let sortFn = sortMap[order] || K;
  items.sort(sortFn);
  return items;
}

export default Ember.Helper.helper(sort);

templates.application.hbs

<h1>Helpers should be pure</h1>

Items: {{items}}
<hr>
Sorted: {{sort "ascending" items}}
<hr>
Items: {{items}}

twiddle.json

{
  "version": "0.10.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.6.0",
    "ember-data": "2.6.1",
    "ember-template-compiler": "2.6.0"
  },
  "addons": {}
}