Rails 3 javascript asset path method

_manifest.html.erb

<script>
  <% if File.exists? Rails.root.join('public/assets/manifest.yml') %>
  window.railsAssetManifest = <%= raw YAML.load_file(Rails.root.join('public/assets/manifest.yml')).to_json %>;
  <% else %>
  window.railsAssetManifest = {};
  <% end %>

  window.railsAssetPath = function(assetName){
    result = (!!window.railsAssetManifest[assetName] ? window.railsAssetManifest[assetName] : assetName);
    return ('/assets/' + result);
  };
</script>