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A Dart implementation of Sass. Sass makes CSS fun again.

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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 for Windows, you can install Dart Sass by running

choco install sass -prerelease

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 for Mac OS X, you can install Dart Sass by running

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 Sass library—from the release page. Extract it, add the directory to your path, and the dart-sass executable is ready to run!

To add the directory to your path on Windows, open the Control Panel, then search for and select "edit environment variables". Find the variable named PATH, click Edit, add ;C:\path\to\dart-sass to the end of the value, then click OK.

On more Unix-y systems, edit your shell configuration file (usually ~/.bashrc or ~/.profile) and add at the end:

export PATH=$PATH:/path/to/dart-sass

Regardless of your OS, you'll need to restart your terminal in order for this configuration to take effect.

From npm

Dart Sass is available, compiled to JavaScript, as an npm package. 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:

var sass = require('sass');

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

// OR

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

See below 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 ^1.0.0-alpha, which will provide a dart-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:

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

void main(List<String> args) {
  print(sass.compile(args.first));
}

See the Dart API docs for details.

From Source

Assuming you've already checked out this repository:

  1. Install Dart. 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 aims to be compatible with Node Sass. Full compatibility is a work in progress, but Dart Sass currently supports the render() and renderSync() functions. 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 package to call asynchronous importers from the synchronous code path. To enable this, pass the Fiber class to the fiber option:

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:

No support is intended for the following options:

  • 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. 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). 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.

Compatibility Policy

For the most part, Dart Sass follows semantic versioning. We consider all of the following to be part of the versioned API:

  • 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.

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.

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.

Disclaimer: this is not an official Google product.