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Integrate Dart Sass with the asset pipeline in Rails.


>= 6.0.0
 Project Readme

Dart Sass for Rails

Sass is a stylesheet language that’s compiled to CSS. It allows you to use variables, nested rules, mixins, functions, and more, all with a fully CSS-compatible syntax.

This gem wraps the standalone executable version of the Dart version of Sass. The platform specific Dart Sass executables are distributed by sass-embedded gem.

The installer will create your default Sass input file in app/assets/stylesheets/application.scss. This is where you should import all the style files to be compiled using the @use rule. When you run rails dartsass:build, this input file will be used to generate the output in app/assets/builds/application.css. That's the output CSS that you'll include in your app. The load path for Sass is automatically configured to be app/assets/stylesheets.

If you need to configure the build process – beyond configuring the build files – you can run bundle exec dartsass to access the platform-specific executable, and give it your own build options.

When you're developing your application, you want to run Dart Sass in watch mode, so changes are automatically reflected in the generated CSS output. You can do this either by running rails dartsass:watch as a separate process, or by running ./bin/dev which uses foreman to start both the Dart Sass watch process and the rails server in development mode.


  1. Run ./bin/bundle add dartsass-rails
  2. Run ./bin/rails dartsass:install

Building in production

The dartsass:build is automatically attached to assets:precompile, so before the asset pipeline digests the files, the Dart Sass output will be generated.

Configuring builds

By default, only app/assets/stylesheets/application.scss will be built. If you'd like to change the path of this stylesheet, add additional entry points, or customize the name of the built file, use the Rails.application.config.dartsass.builds configuration hash.

# config/initializers/dartsass.rb
Rails.application.config.dartsass.builds = {
  "app/index.sass"  => "app.css",
  "site.scss"       => "site.css"

The hash key is the relative path to a Sass file in app/assets/stylesheets/ and the hash value will be the name of the file output to app/assets/builds/.

If both the hash key and the hash value are directories instead of files, it configures a directory to directory compliation, which compiles all public Sass files whose filenames do not start with underscore (_).

# config/initializers/dartsass.rb
Rails.application.config.dartsass.builds = {
  "." => "."

Configuring build options

By default, sass is invoked with ["--style=compressed", "--no-source-map"]. You can adjust these options by overwriting Rails.application.config.dartsass.build_options.

# config/initializers/dartsass.rb
Rails.application.config.dartsass.build_options << "--no-charset" << "--quiet-deps"

Importing assets from gems

dartsass:build includes application assets paths as Sass load paths. Assuming the gem has made assets visible to the Rails application, no additional configuration is required to use them.

Migrating from sass-rails

If you're migrating from sass-rails (applies to sassc-rails as well) and want to switch to dartsass-rails, follow these instructions below:

  1. Remove the sass-rails gem from the Gemfile by running

    ./bin/bundle remove sass-rails
  2. Install dartsass-rails by following the Installation instructions above

  3. Remove any references to Sass files from the Sprockets manifest file: app/assets/config/manifest.js

  4. In your continuous integration pipeline, before running any tests that interact with the browser, make sure to build the Sass files by running:

    bundle exec rails dartsass:build


Some common problems experienced by users:

LoadError: cannot load such file -- sassc

The reason for the above error is that Sprockets is trying to build Sass files but the sass-rails or sassc-rails gems are not installed. This is expected, since Dart Sass is used instead to build Sass files, and the solution is to make sure that Sprockets is not building any Sass files.

There are three reasons why this error can occur:

Sass files are referenced in the Sprockets manifest file

If any Sass files are referenced in the Sprockets manifest file (app/assets/config/manifest.js) Sprockets will try to build the Sass files and fail.


Remove any references to Sass files from the Sprockets manifest file. These are now handled by Dart Sass. If you have more Sass files than application.scss, make sure these are compiled by Dart Sass (see Configuring builds above).

Running locally

If you receive this error when running the Rails server locally and have already removed any references to Sass files from the Sprockets manifest file, the Dart Sass process is most likely not running.


Make sure the Dart Sass process is running by starting the Rails server by running: ./bin/dev.

Running continuous integration pipelines

If you receive this error when running tests that interact with the browser in a continuous integration pipeline and have removed any references to Sass files from the Sprockets manifest file, the Sass files have most likely not been built.


Add a step to the continuous integration pipeline to build the Sass files with the following command: bundle exec rails dartsass:build.


bundle exec dartsass --version


Dart Sass for Rails is released under the MIT License. Sass is released under the MIT License.