generate a js asset helper

asset_helper.js.erb

<%
  manifest = {}
  app = Rails.application
  env = app.assets
  env.each_logical_path do |logical_path|
    if File.basename(logical_path)[/[^\.]+/, 0] == 'index'
      logical_path.sub!(/\/index\./, '.')
    end

    # grabbed from Sprockets::Environment#find_asset
    pathname = Pathname.new(logical_path)
    if pathname.absolute?
      return unless stat(pathname)
      logical_path = attributes_for(pathname).logical_path
    else
      begin
        pathname = resolve(logical_path)
      rescue Sprockets::FileNotFound
        return nil
      end
    end

    asset = Sprockets::Asset.new(env, logical_path, pathname)
    manifest[logical_path] = app.config.assets.digest ? asset.digest_path : asset.logical_path
  end
%>

!function(window, document, undefined){
  
  var manifest = <%= ActiveSupport::JSON.encode(manifest) %>;
  
  function asset_path(path){
    if(manifest.hasOwnProperty(path)){
      return '/assets/' + manifest[path];
    }
    else{
      throw Error('missing asset: ' + path);
    }
  };

  function asset_url(path){
    return window.location.protocol + '//' + window.location.host + asset_path(path);
  };

  function tag(tag, options){
    var tag = $(document.createElement(tag));
    tag.attr(options);
    return tag[0].outerHTML
  }

  function image_tag(path, options){
    var attr = $.extend({}, options, {src: asset_path(path)});
    return tag('img', attr);
  }

  function javascript_include_tag(path, options){
    var attr = $.extend({type: 'text/javascript'}, options, {src: asset_path(path)});
    return tag('script', attr);
  }

  function stylesheet_link_tag(path, options){
    var attr = $.extend({rel: 'stylesheet', media: 'screen'}, options, {href: asset_path(path)});
    return tag('link', attr);
  }

  window.asset_path = asset_path;
  window.asset_url = asset_url;
  window.image_tag = image_tag;
  window.javascript_include_tag = javascript_include_tag;
  window.stylesheet_link_tag = stylesheet_link_tag;

}(window, document);