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https://github.com/sass/dart-sass

The reference implementation of Sass, written in Dart.
https://github.com/sass/dart-sass

css-preprocessor dart dart-sass sass

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The reference implementation of Sass, written in Dart.

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README 

        
A [Dart][dart] implementation of [Sass][sass]. **Sass makes CSS fun**.



  

    

      Sass logo

    

    

      npm statistics

    

    

      Pub version

      


      GitHub actions build status

    

    

      @SassCSS on Twitter

      


      stackoverflow

      


      Gitter

    

  



[dart]: https://www.dartlang.org

[sass]: https://sass-lang.com/



* [Using Dart Sass](#using-dart-sass)

  * [From Chocolatey or Scoop (Windows)](#from-chocolatey-or-scoop-windows)

  * [From Homebrew (macOS)](#from-homebrew-macos-or-linux)

  * [Standalone](#standalone)

  * [From npm](#from-npm)

    * [Dart Sass in the Browser](#dart-sass-in-the-browser)

    * [Legacy JavaScript API](#legacy-javascript-api)

    * [Using Sass with Jest](#using-sass-with-jest)

  * [From Pub](#from-pub)

    * [`sass_api` Package](#sass_api-package)

  * [From Source](#from-source)

  * [In Docker](#in-docker)

* [Why Dart?](#why-dart)

* [Compatibility Policy](#compatibility-policy)

  * [Browser Compatibility](#browser-compatibility)

  * [Node.js Compatibility](#nodejs-compatibility)

* [Embedded Dart Sass](#embedded-dart-sass)

  * [Usage](#usage)

* [Behavioral Differences from Ruby Sass](#behavioral-differences-from-ruby-sass)



## Using Dart Sass



There are a few different ways to install and run Dart Sass, depending on your

environment and your needs.



### From Chocolatey or Scoop (Windows)



If you use [the Chocolatey package manager](https://chocolatey.org/)

or [the Scoop package manager](https://github.com/lukesampson/scoop) for

Windows, you can install Dart Sass by running



```cmd

choco install sass

```



or



```cmd

scoop install sass

```



That'll give you a `sass` executable on your command line that will run Dart

Sass. See [the CLI docs][cli] for details.



[cli]: https://sass-lang.com/documentation/cli/dart-sass



### From Homebrew (macOS or Linux)



If you use [the Homebrew package manager](https://brew.sh/), you

can install Dart Sass by running



```sh

brew install sass/sass/sass

```



That'll give you a `sass` executable on your command line that will run Dart

Sass.



### Standalone



You can download the standalone Dart Sass archive for your operating

system—containing the Dart VM and the snapshot of the executable—from [the

GitHub release page][]. Extract it, [add the directory to your path][], restart

your terminal, and the `sass` executable is ready to run!



[the GitHub release page]: https://github.com/sass/dart-sass/releases/

[add the directory to your path]: https://katiek2.github.io/path-doc/



### From npm



Dart Sass is available, compiled to JavaScript, [as an npm package][npm]. You

can install it globally using `npm install -g sass` which will provide access to

the `sass` executable. You can also add it to your project using

`npm install --save-dev sass`. This provides the executable as well as a

library:



[npm]: https://www.npmjs.com/package/sass



```js

const sass = require('sass');



const result = sass.compile(scssFilename);



// OR



// Note that `compileAsync()` is substantially slower than `compile()`.

const result = await sass.compileAsync(scssFilename);

```



See [the Sass website][js api] for full API documentation.



[js api]: https://sass-lang.com/documentation/js-api



#### Dart Sass in the Browser



The `sass` npm package can also be run directly in the browser. It's compatible

with all major web bundlers as long as you disable renaming (such as

[`--keep-names`] in esbuild). You can also import it directly from a browser as

an ECMAScript Module without any bundling (assuming `node_modules` is served as

well):



[`--keep-names`]: https://esbuild.github.io/api/#keep-names



```html



  {

    "imports": {

      "immutable": "./node_modules/immutable/dist/immutable.es.js",

      "sass": "./node_modules/sass/sass.default.js"

    }

  }



  import * as sass from 'sass';



  console.log(sass.compileString(`

    .box {

      width: 10px + 15px;

    }

  `));



```



Or from a CDN:



```html



  {

    "imports": {

      "immutable": "https://unpkg.com/immutable@^4.0.0",

      "sass": "https://unpkg.com/sass@^1.63.0/sass.default.js"

    }

  }



  import * as sass from 'sass';



  console.log(sass.compileString(`

    .box {

      width: 10px + 15px;

    }

  `));



```



Or even bundled with all its dependencies:



```html



  import * as sass from 'https://jspm.dev/sass';



  console.log(sass.compileString(`

    .box {

      width: 10px + 15px;

    }

  `));



```



Since the browser doesn't have access to the filesystem, the [`compile()`] and

`compileAsync()` functions aren't available for it. If you want to load other

files, you'll need to pass a [custom importer] to [`compileString()`] or

[`compileStringAsync()`]. The [legacy API] is also not supported in the browser.



[`compile()`]: https://sass-lang.com/documentation/js-api/functions/compile

[`compileAsync()`]: https://sass-lang.com/documentation/js-api/functions/compileAsync

[custom importer]: https://sass-lang.com/documentation/js-api/interfaces/StringOptionsWithImporter#importer

[`compileString()`]: https://sass-lang.com/documentation/js-api/functions/compileString

[`compileStringAsync()`]: https://sass-lang.com/documentation/js-api/functions/compileStringAsync

[legacy API]: #legacy-javascript-api



#### Legacy JavaScript API



Dart Sass also supports an older JavaScript API that's fully compatible with

[Node Sass] (with a few exceptions listed below), with support for both the

[`render()`] and [`renderSync()`] functions. This API is considered deprecated

and will be removed in Dart Sass 2.0.0, so it should be avoided in new projects.



[Node Sass]: https://github.com/sass/node-sass

[`render()`]: https://sass-lang.com/documentation/js-api/functions/render

[`renderSync()`]: https://sass-lang.com/documentation/js-api/functions/renderSync



Sass's support for the legacy JavaScript API has the following limitations:



* Only the `"expanded"` and `"compressed"` values of [`outputStyle`] are

  supported.



* Dart Sass doesn't support the [`precision`] option. Dart Sass defaults to a

  sufficiently high precision for all existing browsers, and making this

  customizable would make the code substantially less efficient.



* Dart Sass doesn't support the [`sourceComments`] option. Source maps are the

  recommended way of locating the origin of generated selectors.



[`outputStyle`]: https://sass-lang.com/documentation/js-api/interfaces/LegacySharedOptions#outputStyle

[`precision`]: https://github.com/sass/node-sass#precision

[`sourceComments`]: https://github.com/sass/node-sass#sourcecomments



#### Using Sass with Jest



If you're using [Jest] to run your tests, be aware that it has a [longstanding

bug] where its default test environment breaks JavaScript's built-in

[`instanceof` operator]. Dart Sass's JS package uses `instanceof` fairly

heavily, so in order to avoid breaking Sass you'll need to install

[`jest-environment-node-single-context`] and add `testEnvironment:

'jest-environment-node-single-context'` to your Jest config.



[Jest]: https://jestjs.io/

[longstanding bug]: https://github.com/facebook/jest/issues/2549

[`instanceof` operator]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof

[`jest-environment-node-single-context`]: https://www.npmjs.com/package/jest-environment-node-single-context



### From Pub



If you're a Dart user, you can install Dart Sass globally using `pub global

activate sass`, which will provide a `sass` executable. You can also add it to

your pubspec and use it as a library. We strongly recommend importing it with

the prefix `sass`:



```dart

import 'package:sass/sass.dart' as sass;



void main(List args) {

  print(sass.compile(args.first));

}

```



See [the Dart API docs][api] for details.



[api]: https://www.dartdocs.org/documentation/sass/latest/sass/sass-library.html



#### `sass_api` Package



Dart users also have access to more in-depth APIs via the [`sass_api` package].

This provides access to the Sass AST and APIs for resolving Sass loads without

running a full compilation. It's separated out into its own package so that it

can increase its version number independently of the main `sass` package.



[`sass_api` package]: https://pub.dev/packages/sass_api



### From Source



Assuming you've already checked out this repository:



1. [Install Dart](https://www.dartlang.org/install). If you download an archive

   manually rather than using an installer, make sure the SDK's `bin` directory

   is on your `PATH`.



2. [Install Buf]. This is used to build the protocol buffers for the [embedded

   compiler].



3. In this repository, run `dart pub get`. This will install Dart Sass's

   dependencies.



4. Run `dart run grinder protobuf`. This will download and build the embedded

   protocol definition.



5. Run `dart bin/sass.dart path/to/file.scss`.



[Install Buf]: https://docs.buf.build/installation

[embedded compiler]: #embedded-dart-sass



That's it!



### In Docker



You can install and run Dart Sass within Docker using the following Dockerfile

commands:



```Dockerfile

# Dart stage

FROM bufbuild/buf AS buf

FROM dart:stable AS dart



# Add your scss files

COPY --from=another_stage /app /app



# Include Protocol Buffer binary

COPY --from=buf /usr/local/bin/buf /usr/local/bin/



WORKDIR /dart-sass

RUN git clone https://github.com/sass/dart-sass.git . && \

  dart pub get && \

  dart run grinder protobuf

# This is where you run sass.dart on your scss file(s)

RUN dart ./bin/sass.dart /app/sass/example.scss /app/public/css/example.css

```



## Why Dart?



Dart Sass has replaced Ruby Sass as the canonical implementation of the Sass

language. We chose Dart because it presented a number of advantages:



* It's fast. The Dart VM is highly optimized, and getting faster all the time

  (for the latest performance numbers, see [`perf.md`][perf]). It's much faster

  than Ruby, and close to par with C++.



* It's portable. The Dart VM has no external dependencies and can compile

  applications into standalone snapshot files, so we can distribute Dart Sass as

  only three files (the VM, the snapshot, and a wrapper script). Dart can also

  be compiled to JavaScript, which makes it easy to distribute Sass through npm,

  which the majority of our users use already.



* It's easy to write. Dart is a higher-level language than C++, which means it

  doesn't require lots of hassle with memory management and build systems. It's

  also statically typed, which makes it easier to confidently make large

  refactors than with Ruby.



* It's friendlier to contributors. Dart is substantially easier to learn than

  Ruby, and many Sass users in Google in particular are already familiar with

  it. More contributors translates to faster, more consistent development.



[perf]: https://github.com/sass/dart-sass/blob/master/perf.md



## Compatibility Policy



For the most part, Dart Sass follows [semantic versioning][]. We consider all of

the following to be part of the versioned API:



[semantic versioning]: https://semver.org/



* The Sass language semantics implemented by Dart Sass.

* The Dart API.

* The JavaScript API.

* The command-line interface.



Because Dart Sass has a single version that's shared across the Dart,

JavaScript, and standalone distributions, this may mean that we increment the

major version number when there are in fact no breaking changes for one or more

distributions. However, we will attempt to limit the number of breaking changes

we make and group them in as few releases as possible to minimize churn. We

strongly encourage users to use [the changelog][] for a full understanding of

all the changes in each release.



[the changelog]: https://github.com/sass/dart-sass/blob/master/CHANGELOG.md



There is one exception where breaking changes may be made outside of a major

version revision. It is occasionally the case that CSS adds a feature that's

incompatible with existing Sass syntax in some way. Because Sass is committed to

full CSS compatibility, we occasionally need to break compatibility with old

Sass code in order to remain compatible with CSS.



In these cases, we will first release a version of Sass that emits deprecation

warnings for any stylesheets whose behavior will change. Then, at least three

months after the release of a version with these deprecation warnings, we will

release a minor version with the breaking change to the Sass language semantics.



### Browser Compatibility



In general, we consider any change to Dart Sass's CSS output that would cause

that CSS to stop working in a real browser to be a breaking change. However,

there are some cases where such a change would have substantial benefits and

would only negatively affect a small minority of rarely-used browsers. We don't

want to have to block such a change on a major version release.



As such, if a change would break compatibility with less than 2% of the global

market share of browser according to [StatCounter GlobalStats][], we may release

a minor version of Dart Sass with that change.



[StatCounter GlobalStats]: https://gs.statcounter.com/



### Node.js Compatibility



We consider dropping support for a given version of Node.js to be a breaking

change *as long as* that version is still supported by Node.js. This means that

releases listed as Current, Active LTS, or Maintenance LTS according to [the

Node.js release page][]. Once a Node.js version is out of LTS, Dart Sass

considers itself free to break support if necessary.



[the Node.js release page]: https://nodejs.org/en/about/previous-releases



## Embedded Dart Sass



Dart Sass includes an implementation of the compiler side of the [Embedded Sass

protocol]. It's designed to be embedded in a host language, which then exposes

an API for users to invoke Sass and define custom functions and importers.



[Embedded Sass protocol]: https://github.com/sass/sass/blob/main/spec/embedded-protocol.md



### Usage



* `sass --embedded` starts the embedded compiler and listens on stdin.

* `sass --embedded --version` prints `versionResponse` with `id = 0` in JSON and

  exits.



The `--embedded` command-line flag is not available when you install Dart Sass

as an [npm package]. No other command-line flags are supported with

`--embedded`.



[npm package]: #from-npm



## Behavioral Differences from Ruby Sass



There are a few intentional behavioral differences between Dart Sass and Ruby

Sass. These are generally places where Ruby Sass has an undesired behavior, and

it's substantially easier to implement the correct behavior than it would be to

implement compatible behavior. These should all have tracking bugs against Ruby

Sass to update the reference behavior.



1. `@extend` only accepts simple selectors, as does the second argument of

   `selector-extend()`. See [issue 1599][].



2. Subject selectors are not supported. See [issue 1126][].



3. Pseudo selector arguments are parsed as ``s rather than

   having a more limited custom parsing. See [issue 2120][].



4. The numeric precision is set to 10. See [issue 1122][].



5. The indented syntax parser is more flexible: it doesn't require consistent

   indentation across the whole document. See [issue 2176][].



6. Colors do not support channel-by-channel arithmetic. See [issue 2144][].



7. Unitless numbers aren't `==` to unit numbers with the same value. In

   addition, map keys follow the same logic as `==`-equality. See

   [issue 1496][].



8. `rgba()` and `hsla()` alpha values with percentage units are interpreted as

   percentages. Other units are forbidden. See [issue 1525][].



9. Too many variable arguments passed to a function is an error. See

   [issue 1408][].



10. Allow `@extend` to reach outside a media query if there's an identical

    `@extend` defined outside that query. This isn't tracked explicitly, because

    it'll be irrelevant when [issue 1050][] is fixed.



11. Some selector pseudos containing placeholder selectors will be compiled

    where they wouldn't be in Ruby Sass. This better matches the semantics of

    the selectors in question, and is more efficient. See [issue 2228][].



12. The old-style `:property value` syntax is not supported in the indented

    syntax. See [issue 2245][].



13. The reference combinator is not supported. See [issue 303][].



14. Universal selector unification is symmetrical. See [issue 2247][].



15. `@extend` doesn't produce an error if it matches but fails to unify. See

    [issue 2250][].



16. Dart Sass currently only supports UTF-8 documents. We'd like to support

    more, but Dart currently doesn't support them. See [dart-lang/sdk#11744][],

    for example.



[issue 1599]: https://github.com/sass/sass/issues/1599

[issue 1126]: https://github.com/sass/sass/issues/1126

[issue 2120]: https://github.com/sass/sass/issues/2120

[issue 1122]: https://github.com/sass/sass/issues/1122

[issue 2176]: https://github.com/sass/sass/issues/2176

[issue 2144]: https://github.com/sass/sass/issues/2144

[issue 1496]: https://github.com/sass/sass/issues/1496

[issue 1525]: https://github.com/sass/sass/issues/1525

[issue 1408]: https://github.com/sass/sass/issues/1408

[issue 1050]: https://github.com/sass/sass/issues/1050

[issue 2228]: https://github.com/sass/sass/issues/2228

[issue 2245]: https://github.com/sass/sass/issues/2245

[issue 303]: https://github.com/sass/sass/issues/303

[issue 2247]: https://github.com/sass/sass/issues/2247

[issue 2250]: https://github.com/sass/sass/issues/2250

[dart-lang/sdk#11744]: https://github.com/dart-lang/sdk/issues/11744



Disclaimer: this is not an official Google product.