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1.69.5 • Public • Published

Embedded Sass Host

This package is an alternative to the sass package. It supports the same JS API as sass and is maintained by the same team, but where the sass package is pure JavaScript, sass-embedded is instead a JavaScript wrapper around a native Dart executable. This means sass-embedded will generally be much faster especially for large Sass compilations, but it can only be installed on the platforms that Dart supports: Windows, Mac OS, and Linux.

Despite being different packages, both sass and sass-embedded are considered "Dart Sass" since they have the same underlying implementation. Since the first stable release of the sass-embedded package, both packages are released at the same time and share the same version number.


This package provides the same JavaScript API as the sass package, and can be used as a drop-in replacement:

const sass = require('sass-embedded');

const result = sass.compile(scssFilename);

// OR

const result = await sass.compileAsync(scssFilename);

Unlike the sass package, the asynchronous API in sass-embedded will generally be faster than the synchronous API since the Sass compilation logic is happening in a different process.

See the Sass website for full API documentation.

Legacy API

The sass-embedded package also supports the 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.

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

  • Only the "expanded" and "compressed" values of outputStyle are supported.

  • The sass-embedded package 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.

  • The sass-embedded package doesn't support the [sourceComments] option. Source maps are the recommended way of locating the origin of generated selectors.

  • The sass-embedded package doesn't support the indentWidth, indentType, or linefeed options. It implements the legacy API as a wrapper around the new API, and the new API has dropped support for these options.

How Does It Work?

The sass-embedded runs the Dart Sass embedded compiler as a separate executable and uses the Embedded Sass Protocol to communicate with it over its stdin and stdout streams. This protocol is designed to make it possible not only to start a Sass compilation, but to control aspects of it that are exposed by an API. This includes defining custom importers, functions, and loggers, all of which are invoked by messages from the embedded compiler back to the host.

Although this sort of two-way communication with an embedded process is inherently asynchronous in Node.js, this package supports the synchronous compile() API using a custom synchronous message-passing library that's implemented with the Atomics.wait() primitive. We hope to release this library as a stand-alone package at some point in the future.

Disclaimer: this is not an official Google product.




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  • nex3
  • sassbot