Assets management for Ruby web projects


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Hanami::Assets supports Ruby (MRI) 2.3+ and JRuby


Add this line to your application's Gemfile:

gem 'hanami-assets'

And then execute:

$ bundle

Or install it yourself as:

$ gem install hanami-assets



Hanami::Assets provides asset-specific helpers to be used in templates. They resolve one or multiple sources into corresponding HTML tags. Those sources can be either a name of a local asset or an absolute URL.

Given the following template:

<!doctype HTML>
    <title>Assets example</title>
    <%= stylesheet 'reset', 'grid', 'main' %>

  <!-- ... -->
  <%= javascript 'https://code.jquery.com/jquery-2.1.1.min.js', 'application' %>
  <%= javascript 'modals' %>

It will output this markup:

<!doctype HTML>
    <title>Assets example</title>
    <link href="/assets/reset.css" type="text/css" rel="stylesheet">
    <link href="/assets/grid.css" type="text/css" rel="stylesheet">
    <link href="/assets/main.css" type="text/css" rel="stylesheet">

  <!-- ... -->
  <script src="https://code.jquery.com/jquery-2.1.1.min.js" type="text/javascript"></script>
  <script src="/assets/application.js" type="text/javascript"></script>
  <script src="/assets/modals.js" type="text/javascript"></script>

Let's have a look at the corresponding Ruby code. In this example we use ERb, but remember that Hanami::Assets is compatible with all the rendering engines such as HAML, Slim, Mustache, etc..

require 'erb'
require 'hanami/assets'
require 'hanami/assets/helpers'

class View
  include Hanami::Assets::Helpers

  def initialize
    @template = File.read('template.erb')
    @engine   = ERB.new(@template)

  def render

View.new.render # => HTML markup

For advanced configurations, please have a look at Hanami::Assets::Configuration.

Available Helpers

This gem ships with the following helpers:

  • javascript
  • stylesheet
  • favicon
  • image
  • video
  • audio
  • asset_path
  • asset_url

Development mode

Hanami::Assets can help you during the development process of your application. It can manage multiple source directories for each asset type or run a preprocessor for you.


Imagine to have your application's javascripts under app/assets/javascripts and that those assets depends on a vendored version of jQuery.

require 'hanami/assets'

Hanami::Assets.configure do
  compile true

  sources << [

When from a template you do:

<%= javascript 'jquery', 'jquery-ui', 'login' %>

Hanami::Assets looks at the defined sources and lazily copies those files under public/assets (by default), before the markup is generated.

Your public directory will have the following structure.

% tree public
└── assets
    ├── jquery.js
    ├── jquery-ui.js
    └── login.js

Please remember that sources are recursively looked up in order of declaration.

If in the example above we had a jquery.js under app/assets/javascripts/**/*.js that file would be copied into the public directory instead of the one under vendor/jquery. The reason is because we declared app/assets/javascripts first.


Hanami::Assets is able to run assets preprocessors and lazily compile them under public/assets (by default), before the markup is generated.

Imagine you have main.css.scss under app/assets/stylesheets and reset.css under vendor/stylesheets.

The two extensions are important. The first one is mandatory and it's used to understand which asset type we are handling: .css for stylesheets. The second one is optional and it's for a preprocessor: .scss for Sass.

require 'sassc'
require 'hanami/assets'

Hanami::Assets.configure do
  compile true

  sources << [

And in a template you can use the stylesheet helper:

<%= stylesheet 'reset', 'main' %>

Your public directory will look like this:

% tree public
└── assets
    ├── reset.css
    └── main.css

Preprocessors engines

Hanami::Assets uses Tilt to provide support for the most common preprocessors, such as Sass (including sassc-ruby), Less, ES6, JSX, CoffeScript, Opal, Handlebars, JBuilder.

In order to use one or more of them, be sure to add the corresponding gem to your Gemfile and require the library.

EcmaScript 6

We strongly suggest you use EcmaScript 6 for your next project. It isn't fully supported yet by browsers, but it's the future of JavaScript.

As of today, you need to 'transpile' ES6 code into ES5, which current browsers understand. The most popular tool for this is Babel, which we support.


Hanami::Assets ships with an executable (hanami-assets), which can be used to precompile assets and make them cacheable by browsers (via checksum suffix).

NOTE: If you're using Hanami::Assets with the full Hanami framework, you should use bundle exec hanami assets precompile instead of hanami-assets.

Let's say we have an application that has a main file that requires the entire codebase (config/environment.rb), a gem that brings in Ember.js code, and the following sources:

% tree .
├── apps
│   ├── admin
│   │   ├── assets
│   │   │   └── js
│   │   │       ├── application.js
│   │   │       └── zepto.js
# ...
│   ├── metrics
│   │   ├── assets
│   │   │   └── javascripts
│   │   │       └── dashboard.js
# ...
│   └── web
│       ├── assets
│       │   ├── images
│       │   │   └── bookshelf.jpg
│       │   └── javascripts
│       │       └── application.js
# ...
│       └── vendor
│           └── assets
│               └── javascripts
│                   └── jquery.js
└── config
    └── environment.rb

In order to deploy, we can run:

bundle exec hanami-assets --config=config/environment.rb

It will output:

tree public
├── assets
│   ├── admin
│   │   ├── application-28a6b886de2372ee3922fcaf3f78f2d8.js
│   │   ├── application.js
│   │   ├── ember-b2d6de1e99c79a0e52cf5c205aa2e07a.js
│   │   ├── ember-source-e74117fc6ba74418b2601ffff9eb1568.js
│   │   ├── ember-source.js
│   │   ├── ember.js
│   │   ├── zepto-ca736a378613d484138dec4e69be99b6.js
│   │   └── zepto.js
│   ├── application-d1829dc353b734e3adc24855693b70f9.js
│   ├── application.js
│   ├── bookshelf-237ecbedf745af5a477e380f0232039a.jpg
│   ├── bookshelf.jpg
│   ├── ember-b2d6de1e99c79a0e52cf5c205aa2e07a.js
│   ├── ember-source-e74117fc6ba74418b2601ffff9eb1568.js
│   ├── ember-source.js
│   ├── ember.js
│   ├── jquery-05277a4edea56b7f82a4c1442159e183.js
│   ├── jquery.js
│   └── metrics
│       ├── dashboard-7766a63ececc63a7a629bfb0666e9c62.js
│       ├── dashboard.js
│       ├── ember-b2d6de1e99c79a0e52cf5c205aa2e07a.js
│       ├── ember-source-e74117fc6ba74418b2601ffff9eb1568.js
│       ├── ember-source.js
│       └── ember.js
└── assets.json


Minification is a process that shrinks file size in production, by removing unnecessary spaces and characters. The goal of this step is to have lighter assets, which will be served faster to browsers.

Hanami supports JavaScript and stylesheet minifiers.

Because this framework relies on external gems for minification, this feature is turned off by default.

To use minification, we need to specify which gem we want to use and add it to our Gemfile.

JavaScript Compressors

Hanami can use the following compressors (aka minifiers) for JavaScript.

Hanami::Assets.configure do
  javascript_compressor :uglifier
Stylesheet Compressors

Hanami can use the following compressors (aka minifiers) for stylesheets.

  • :builtin - Ruby based compressor. It doesn't require any external gem. It's fast, but not an efficient compressor.
  • :yui - YUI Compressor, it depends on yui-compressor gem and it requires Java 1.4+
  • :sass - Sass, it depends on sassc gem
Hanami::Assets.configure do
  stylesheet_compressor :sass
Custom Compressors

We can specify our own minifiers:

Hanami::Assets.configure do
  javascript_compressor MyJavascriptCompressor.new
  stylesheet_compressor MyStylesheetCompressor.new

Fingerprint Mode

This is a mode that can be activated via configuration and it's suitable for production environments. When generating files, it adds a string to the end of each file name, which is a checksum of its contents. This lets you leverage caching while still ensuring that clients get the most up-to-date assets (this is known as cache busting).

Hanami::Assets.configure do
  fingerprint true

Once turned on, it will look at /public/assets.json, and helpers such as javascript will return a relative URL that includes the fingerprint of the asset.

<%= javascript 'application' %>
<script src="/assets/application-d1829dc353b734e3adc24855693b70f9.js" type="text/javascript"></script>

Subresource Integrity (SRI) Mode

This is a mode that can be activated via the configuration and it's suitable for production environments.

Hanami::Assets.configure do
  subresource_integrity true

Once turned on, it will look at /public/assets.json, and helpers such as javascript will include an integrity and crossorigin attribute.

<%= javascript 'application' %>
<script src="/assets/application-d1829dc353b734e3adc24855693b70f9.js" type="text/javascript" integrity="sha384-oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxy9rx7HNQlGYl1kPzQho1wx4JwY8wC" crossorigin="anonymous"></script>

CDN Mode

A Hanami project can serve assets via a Content Delivery Network (CDN).

Hanami::Assets.configure do
  scheme 'https'
  host   '123.cloudfront.net'
  port   443
  cdn    true

From now on, helpers will return the absolute URL for the asset, hosted on the CDN specified.

<%= javascript 'application' %>
<script src="https://123.cloudfront.net/assets/application-d1829dc353b734e3adc24855693b70f9.js" type="text/javascript"></script>

Standalone mode

If you're using hanami-assets without hanami, you must explicitly boot the framework with:

Hanami::Assets.configure do
  # ...


Hanami::Assets.configure do
  # ...

# ...


Third party gems

Developers can maintain gems that distribute assets for Hanami. For instance hanami-ember or hanami-jquery.

To do this, inside your gem you have tell Hanami::Assets where to look for assets:

# lib/hanami/jquery.rb
Hanami::Assets.sources << '/path/to/jquery'

Running tests

  • Make sure you have one of ExecJS supported runtime on your machine.
  • Java 1.4+ (for YUI Compressor and Google Closure Compiler)
bundle exec rake test


Hanami::Assets uses Semantic Versioning 2.0.0


  1. Fork it ( https://github.com/hanami/assets/fork )
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request

Copyright © 2014-2021 Luca Guidi – Released under MIT License

This project was formerly known as Lotus (lotus-assets).