DartSass

March 25, 2026 · View on GitHub

Install and run Dart Sass using Elixir.

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Installation

Add DartSass to your project's dependencies in mix.exs and run mix deps.get.

If your application builds assets in production, configure DartSass as a runtime application in development:

def deps do
  [
    {:dart_sass, "~> 0.7", runtime: Mix.env() == :dev}
  ]
end

If your application's assets are precompiled during development, configure DartSass as a dependency for the development environment only:

def deps do
  [
    {:dart_sass, "~> 0.7", only: :dev}
  ]
end

Next, update your application's config/config.exs to set a Dart Sass version:

config :dart_sass, version: "1.97.3"

You may now install Dart Sass by running:

mix sass.install

Invoke the sass executable by running:

mix sass default assets/css/app.scss priv/static/assets/app.css

Additional load paths may be specified using the --load-path flag:

mix sass default assets/css/app.scss --load-path=assets/node_modules/bulma priv/static/assets/app.css

Note

The sass executable may be installed to _build/sass-<arch>. In most cases, running Dart Sass requires the portable Dart virtual machine (_build/dart-<arch>) and the Sass snapshot (_build/sass.snapshot-<arch>) where <arch> is your system's architecture (e.g. linux-arm64).

Configuring profiles

DartSass requires an execution profile as its first argument. You may define multiple execution profiles using the current directory, the environment, and default arguments:

config :dart_sass,
  version: "1.97.3",
  default: [
    args: ~w(css/app.scss ../priv/static/assets/app.css),
    cd: Path.expand("../assets", __DIR__)
  ]

Invoking mix sass default appends the task arguments to the ones configured above.

Using with Phoenix

Note

Using DartSass with Phoenix requires Phoenix v1.5.10 or newer.

First, add Phoenix as a dependency to your application's mix.exs:

def deps do
  [
    {:phoenix, "~> 1.7.14"},
    {:dart_sass, "~> 0.7", runtime: Mix.env() == :dev}
  ]
end

Next, configure DartSass to use assets/css/app.scss as the input file and set the output file to ../priv/static/assets/app.css:

config :dart_sass,
  version: "1.97.3",
  default: [
    args: ~w(css/app.scss ../priv/static/assets/app.css),
    cd: Path.expand("../assets", __DIR__)
  ]

Note

If your application uses esbuild, remove import "../css/app.css" from your application's assets/js/app.js. This change will prevent esbuild from also generating CSS files.

Note

Be sure to add assets (alongside other files from priv/static) to Plug.Static's only filter in your application's endpoint.ex file.

plug Plug.Static,
  at: "/",
  from: :my_app,
  only: ~w(assets favicon.ico robots.txt)

In development mode, configure Dart Sass' --watch flag in your application's config/dev.exs file:

config :my_app,
  # …
  watchers: [
    sass: {
      DartSass,
      :install_and_run,
      [:default, ~w(--embed-source-map --source-map-urls=absolute --watch)]
    }
  ]

The configuration above also enables embedded source maps using aboslute URLs. Consult the Dart Sass Command-Line Interface documentation for a complete list and description of supported options.

Note

When using the --watch option, the sass process is invoked using a Bash script to ensure graceful termination of the sass process when stdin closes. This script is not invoked on Windows platforms, so using the --watch option may result in orphaned processes.

Finally, in your application's mix.exs, create or update the assets.deploy alias to include sass (in this example, configuring the output style):

defp aliases do
  [
    # …
    "assets.deploy": [
      "esbuild default --minify",
      "sass default --no-source-map --style=compressed",
      "phx.digest"
    ],
    # …
  ]
end

Acknowledgements

This package is based on Wojtek Mach's and José Valim's excellent esbuild installer.

License

DartSass is freely available under the MIT License.