rendering.rb

module React
  module Server
    module Rendering
      extend ActiveSupport::Concern
      include ::BeforeRender

      included do
        before_render :set_rendered

        attr_reader :rendered
        helper_method :rendered
      end

      def set_rendered
        @rendered = self.class.server_js.eval(
          "this.render(#{data.to_json})" # `this.render` is the executable name of the bundle, `data` are our props (ex. {foo: :bar}) 
        )
      end

      module ClassMethods
        @@server_js = "" # cache the server js
        @@server_js_last_modified = Time.at 0

        def server_js
          if @@server_js_last_modified < mtime = File.mtime(filepath)
            @@server_js = File.open(filepath).read
            @@server_js = ExecJS.compile @@server_js # this will be WAY faster if you minify your js
            @@server_js_last_modified = mtime
          end
          @@server_js
        end

        private

        def filepath
          Rails.configuration.react.server_js # allows me to make this configurable in a config file. You could just return a string here.
        end
      end
    end
  end
end