Slimmer provides Rack middleware for applying a standard header and footer around pages returned by a Ruby (Rack) application.

It does this by taking the page rendered by the application, extracting the contents of a div with id 'wrapper' and inserting that into a div with the same id in one of its templates. It also transfers various other details, such as meta, script, and style tags.

Use in a Rails app

Slimmer provides a Railtie so no configuration is necessary.

Asset tag helpers

To get asset tag helpers to point to your external asset server, add

config.action_controller.asset_host = ""

to application.rb.


Slimmer can be configured with a logger by passing in a logger instance (anything that quacks like an instance of Logger). For example, to log to the Rails log, put the following in an initializer:

YourApp::Application.configure do
  config.slimmer.logger = Rails.logger

Note: This can't be in application.rb because the Rails logger hasn't been initialized by then.

Debug logging

By default if you pass in a logger with its log level set to debug, slimmer will dup this logger and reduce the level to info. (Slimmer's debug logging is very noisy). To prevent this, set the enable_debugging option to true. e.g. for Rails:

YourApp::Application.configure do
  config.slimmer.enable_debugging = true

GOV.UK Components

To use shared template components you need to include the GOV.UK component module:

class ApplicationController < ActionController::Base
  include Slimmer::GovukComponents

This will make calls out to static when you try and render a partial prefixed with govuk_component:

<%= render partial: 'govuk_component/example_component' %>

You will need a copy of static running for the templates to be loaded from.

Testing components

In test mode (when Rails.env.test? returns true), shared components are not fetched from Static. Instead they are rendered as a dummy tag which contains a JSON dump of the locals - the arguments passed to the component.

A test helper is included which returns a CSS selector for finding a given component to assert that it was used. You can make it available in your tests with:

require 'slimmer/test_helpers/govuk_components'
include Slimmer::TestHelpers::GovukComponents

And then assert that the component has been used:

page.should have_css(shared_component_selector('metadata'))

Or look for one of the arguments to the component which will have been JSON.dumped inside the tag:

within(shared_component_selector('title')) do
  expect(page).to have_content(expected_title_text)


Add the following code to features/support:

require 'slimmer/cucumber'


Add the following code to spec/spec_helper:

require 'slimmer/rspec'