dart-sass/README.md

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

<table>
  <tr>
    <td>
      <img width="118px" alt="Sass logo" src="https://rawgit.com/sass/sass-site/master/source/assets/img/logos/logo.svg" />
    </td>
    <td valign="middle">
      <a href="https://www.npmjs.com/package/sass"><img width="100%" alt="npm statistics" src="https://nodei.co/npm/sass.png?downloads=true"></a>
    </td>
    <td valign="middle">
      <a href="https://pub.dartlang.org/packages/sass"><img alt="Pub version" src="https://img.shields.io/pub/v/sass.svg"></a>
      <br>
      <a href="https://travis-ci.org/sass/dart-sass"><img alt="Travis build status" src="https://api.travis-ci.org/sass/dart-sass.svg?branch=master"></a>
      <br>
      <a href="https://ci.appveyor.com/project/nex3/dart-sass"><img alt="Appveyor build status" src="https://ci.appveyor.com/api/projects/status/84rl9hvu8uoecgef?svg=true"></a>
    </td>
  </tr>
</table>

[dart]: https://www.dartlang.org
[sass]: https://sass-lang.com/

* [Using Dart Sass](#using-dart-sass)
  * [From Chocolatey (Windows)](#from-chocolatey-windows)
  * [From Homebrew (OS X)](#from-homebrew-os-x)
  * [Standalone](#standalone)
  * [From npm](#from-npm)
  * [From Pub](#from-pub)
  * [From Source](#from-source)
* [JavaScript API](#javascript-api)
* [Why Dart?](#why-dart)
* [Compatibility Policy](#compatibility-policy)
* [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 (Windows)

If you use [the Chocolatey package manager](https://chocolatey.org/) for
Windows, you can install Dart Sass by running

```cmd
choco install sass
```

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

### From Homebrew (OS X)

If you use [the Homebrew package manager](https://brew.sh/) for Mac OS X, 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
var sass = require('sass');

sass.render({file: scss_filename}, function(err, result) { /* ... */ });

// OR

var result = sass.renderSync({file: scss_filename});
```

[See below](#javascript-api) for details on Dart Sass's JavaScript API.

### 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<String> 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

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

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

That's it!

## JavaScript API

When installed via npm, Dart Sass supports a JavaScript API that's fully
compatible with [Node Sass][] (with a few exceptions listed below), with support
for both the `render()` and `renderSync()` functions. See [the Sass
website][js api] for full API documentation!

[Node Sass]: https://github.com/sass/node-sass
[js api]: https://sass-lang.com/documentation/js-api

Note however that by default, **`renderSync()` is more than twice as fast as
`render()`** due to the overhead of asynchronous callbacks. To avoid this
performance hit, `render()` can use the [`fibers`][fibers] package to call
asynchronous importers from the synchronous code path. To enable this, pass the
`Fiber` class to the `fiber` option:

[fibers]: https://www.npmjs.com/package/fibers

```js
var sass = require("sass");
var Fiber = require("fibers");

sass.render({
  file: "input.scss",
  importer: function(url, prev, done) {
    // ...
  },
  fiber: Fiber
}, function(err, result) {
  // ...
});
```

Both `render()` and `renderSync()` support the following options:

* [`data`](https://github.com/sass/node-sass#data)
* [`file`](https://github.com/sass/node-sass#file)
* [`functions`](https://github.com/sass/node-sass#functions--v300---experimental)
* [`importer`](https://github.com/sass/node-sass#importer--v200---experimental)
* [`includePaths`](https://github.com/sass/node-sass#includepaths)
* [`indentType`](https://github.com/sass/node-sass#indenttype)
* [`indentWidth`](https://github.com/sass/node-sass#indentwidth)
* [`indentedSyntax`](https://github.com/sass/node-sass#indentedsyntax)
* [`linefeed`](https://github.com/sass/node-sass#linefeed)
* [`omitSourceMapUrl`](https://github.com/sass/node-sass#omitsourcemapurl)
* [`outFile`](https://github.com/sass/node-sass#outfile)
* [`sourceMapContents`](https://github.com/sass/node-sass#sourcemapcontents)
* [`sourceMapEmbed`](https://github.com/sass/node-sass#sourcemapembed)
* [`sourceMapRoot`](https://github.com/sass/node-sass#sourcemaproot)
* [`sourceMap`](https://github.com/sass/node-sass#sourcemap)
* Only the `"expanded"` and `"compressed"` values of
  [`outputStyle`](https://github.com/sass/node-sass#outputstyle) are supported.

No support is intended for the following options:

* [`precision`](https://github.com/sass/node-sass#precision). Dart Sass defaults
  to a sufficiently high precision for all existing browsers, and making this
  customizable would make the code substantially less efficient.

* [`sourceComments`](https://github.com/sass/node-sass#sourcecomments). Source
  maps are the recommended way of locating the origin of generated selectors.

## 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]: http://gs.statcounter.com/

## 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 `<declaration-value>`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.