title: How to setup javascript files as :defaults in rails 4?
body: <p>My current RoR application using rails 3.2.10 and I want to upgrade it on rails 4.0.0.</p>
  
  <p>I solve the gem dependence but when I run the rails server it give the error :-</p>
  
  <pre><code>undefined method `[]=' for nil:NilClass
  config/application.rb:39:in `&lt;class:Application&gt;'
  </code></pre>
  
  <p>My config/application.rb file has this code at line 39.</p>
  
  <pre><code>#JavaScript files you want as :defaults (application.js is always included).
  config.action_view.javascript_expansions[:defaults] = %w(jquery rails)
  </code></pre>
  
  <p>for that what should do- change the code or remove, any dependence or any other option.</p>
  
  <p>Rails 4 also support assets precompile so I comment the this line:-</p>
  
  <pre><code>#config.assets.enabled = true   In rails 4 assets pipline enable by default
  </code></pre>