array.length != array.[]

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  array: ['a', 'b', 'c'],
  
  lengthBased: Ember.computed('array.length', function() {
    return this.get('array').join(" ");
  }),
  
  brackedBased: Ember.computed('array.[]', function() {
    return this.get('array').join(" ");
  }),
  
  actions: {
    swap() {
      this.get('array').replace(0,1,['d']);
    }
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

{{#each array as |item|}}
  {{item}} <br>
{{/each}}

lengthBased: {{lengthBased}}
brackedBased: {{brackedBased}}

<button {{action 'swap'}}>Swap</button>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}