at helper

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  someArray: [{ num: 1},{ num: 2},{num: 3},{num:4},{num:5}],
  actions: {
   increm() {
     let obj = this.get('someArray').objectAt(3);
    Ember.set(obj, 'num', Ember.get(obj, 'num') + 1);
  }
  }
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
{{get (at someArray 3) 'num'}}
<br>
<button type="button" {{action "increm"}}>Incrememt</button>
<br>
{{#each someArray as |arr index|}}
{{arr.num}}
{{/each}}

helpers.at.js

import Ember from 'ember';

export function at([array, index]/*, hash*/) {
  return array.objectAt(index);
}

export default Ember.Helper.helper(at);

twiddle.json

{
  "version": "0.6.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.3.1/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.3.3/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js"
  }
}