Include JS/CSS assets from anywhere in your Rails application - even outside /app/assets

assets_anywhere.rb

module AssetsAnywhere
  class Middleware
    def initialize(app)
      @app = app
      @builder = Builder.new
    end

    def call(env)
      @builder.attempt(env) || @app.call(env)
    end
  end

  class Builder
    ASSET_TYPES = /\.(js|css)$/i

    def initialize
      @file_server = Rack::Files.new(Rails.root)
    end

    def attempt(env)
      request = Rack::Request.new(env)

      (request.get? || request.head?) && serve(request)
    end

    private

    def serve(request)
      return unless ASSET_TYPES.match?(request.path_info)

      file_readable?(request.path_info) && @file_server.call(request.env)
    end

    def clean_path(path_info)
      path = Rack::Utils.unescape_path path_info.chomp('/').delete_prefix('/')
      Rack::Utils.clean_path_info path if Rack::Utils.valid_path? path
    end

    def file_readable?(path)
      return unless (path = clean_path(path))

      file_stat = File.stat(Rails.root.join(path.delete_prefix('/').b).to_s)
    rescue SystemCallError
      false
    else
      file_stat.file? && file_stat.readable?
    end
  end
end

Rails.application.config.middleware.insert_after ActionDispatch::Static, AssetsAnywhere::Middleware

view.html.erb

<%# In your views you can do... %>
<%= javascript_include_tag '/lib/some_javascript.js' %>
<%= javascript_include_tag '/app/models/user.js' %>
<%= stylesheet_link_tag '/app/views/layouts/application.css' %>