Example of using the flash hash in Rails, with AJAX and a global jQuery listener.

ajax_flashable.rb

# Add Flash messages to response
# 
# Reference: http://stackoverflow.com/a/10167659/224139
# Location: app/controllers/concerns
module Concerns::AjaxFlashable
  extend ActiveSupport::Concern
  
  included do
    after_filter :flash_to_headers
  end

  private

  def flash_to_headers
    return unless request.xhr?
    response.headers['X-Message'] = flash_message
    response.headers["X-Message-Type"] = flash_type.to_s

    flash.discard # don't want the flash to appear when you reload page
  end

  def flash_message
    [:error, :alert, :warning, :notice].each do |type|
      return flash[type] unless flash[type].blank?
    end
  end

  def flash_type
    [:error, :alert, :warning, :notice].each do |type|
      return type unless flash[type].blank?
    end
  end

end

jquery.global-ajax-flash.js

jQuery(document).ajaxComplete(function (event, xhr, opts) {

  var type = xhr.getResponseHeader('X-Message-Type') || 'notice';
  var message = xhr.getResponseHeader('X-Message');

  if (!!message) { 
    // *** Here is where the HTML would be written, or a call to a 
    //     notification plugin.
    console.log('FLASH: [' + type + '] ' + message);
  }

});

posts_controller.rb

class PostsController < ApplicationController

  include Concerns::AjaxFlashable

  def create
    @post = Post.new(params[:post])
    if @post.save
      flash[:notice] = "Your post has been created!"
      respond_to do |format|
        format.html { redirect_to :action => :index }
        format.js { head :ok }
      end
    else
      flash[:error] = "Sorry, there was an error saving your post."
      # Could append error messages here as well...
      respond_to do |format|
        format.html { render :new }
        format.js { head :bad_request }
      end
    end
  end

end