Introducing Sass Modules

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Sass just launched a major new feature you might recognize from other languages: a module system. This is a big step forward for @import. one of the most-used Sass-features. While the current @import rule allows you to pull in third-party packages, and split your Sass into manageable "partials," it has a few limitations:

• @import is also a CSS feature, and the differences can be confusing
• If you @import the same file multiple times, it can slow down compilation, cause override conflicts, and generate duplicate output.
• Everything is in the global namespace, including third-party packages – so my color() function might override your existing color() function, or vice versa.
• When you use a function like color(). it’s impossible to know exactly where it was defined. Which @import does it come from?

Sass package authors (like me) have tried to work around the namespace issues by manually prefixing our variables and functions — but Sass modules are a much more powerful solution. In brief, @import is being replaced with more explicit @use and @forward rules. Over the next few years Sass @import will be deprecated, and then removed. You can still use CSS imports, but they won’t be compiled by Sass. Don’t worry, there’s a migration tool to help you upgrade!

Import files with @use

@use 'buttons';

The new @use is similar to @import. but has some notable differences:

• The file is only imported once, no matter how many times you @use it in a project.
• Variables, mixins, and functions (what Sass calls "members") that start with an underscore (_) or hyphen (-) are considered private, and not imported.
• Members from the used file (buttons.scss in this case) are only made available locally, but not passed along to future imports.
• Similarly, @extends will only apply up the chain; extending selectors in imported files, but not extending files that import this one.
• All imported members are namespaced by default.

When we @use a file, Sass automatically generates a namespace based on the file name:

@use 'buttons'; // creates a `buttons` namespace
@use 'forms'; // creates a `forms` namespace

We now have access to members from both buttons.scss and forms.scss — but that access is not transferred between the imports: forms.scss still has no access to the variables defined in buttons.scss. Because the imported features are namespaced, we have to use a new period-divided syntax to access them:

// variables: <namespace>.$variable
$btn-color: buttons.$color;
$form-border: forms.$input-border;

// functions: <namespace>.function()
$btn-background: buttons.background();
$form-border: forms.border();

// mixins: @include <namespace>.mixin()
@include buttons.submit();
@include forms.input();

We can change or remove the default namespace by adding as <name> to the import:

@use 'buttons' as *; // the star removes any namespace
@use 'forms' as 'f';

$btn-color: $color; // buttons.$color without a namespace
$form-border: f.$input-border; // forms.$input-border with a custom namespace

Using as * adds a module to the root namespace, so no prefix is required, but those members are still locally scoped to the current document.

Import built-in Sass modules

Internal Sass features have also moved into the module system, so we have complete control over the global namespace. There are several built-in modules — math, color, string, list, map, selector, and meta — which have to be imported explicitly in a file before they are used:

@use 'sass:math';
$half: math.percentage(1/2);

Sass modules can also be imported to the global namespace:

@use 'sass:math' as *;
$half: percentage(1/2);

Internal functions that already had prefixed names, like map-get or str-index. can be used without duplicating that prefix:

@use 'sass:map';
@use 'sass:string';
$map-get: map.get(('key': 'value'), 'key');
$str-index: string.index('string', 'i');

You can find a full list of built-in modules, functions, and name changes in the Sass module specification.

New and changed core features

As a side benefit, this means that Sass can safely add new internal mixins and functions without causing name conflicts. The most exciting example in this release is a sass:meta mixin called load-css(). This works similar to @use but it only returns generated CSS output, and it can be used dynamically anywhere in our code:

@use 'sass:meta';
$theme-name: 'dark';

[data-theme='#{$theme-name}'] {
  @include meta.load-css($theme-name);
}

The first argument is a module URL (like @use) but it can be dynamically changed by variables, and even include interpolation, like theme-#{$name}. The second (optional) argument accepts a map of configuration values:

// Configure the $base-color variable in 'theme/dark' before loading
@include meta.load-css(
  'theme/dark', 
  $with: ('base-color': rebeccapurple)
);

The $with argument accepts configuration keys and values for any variable in the loaded module, if it is both:

• A global variable that doesn’t start with _ or - (now used to signify privacy)
• Marked as a !default value, to be configured

// theme/_dark.scss
$base-color: black !default; // available for configuration
$_private: true !default; // not available because private
$config: false; // not available because not marked as a !default

Note that the 'base-color' key will set the $base-color variable.

There are two more sass:meta functions that are new: module-variables() and module-functions(). Each returns a map of member names and values from an already-imported module. These accept a single argument matching the module namespace:

@use 'forms';

$form-vars: module-variables('forms');
// (
//   button-color: blue,
//   input-border: thin,
// )

$form-functions: module-functions('forms');
// (
//   background: get-function('background'),
//   border: get-function('border'),
// )

Several other sass:meta functions — global-variable-exists(), function-exists(), mixin-exists(), and get-function() — will get additional $module arguments, allowing us to inspect each namespace explicitly.

Adjusting and scaling colors

The sass:color module also has some interesting caveats, as we try to move away from some legacy issues. Many of the legacy shortcuts like lighten(). or adjust-hue() are deprecated for now in favor of explicit color.adjust() and color.scale() functions:

// previously lighten(red, 20%)
$light-red: color.adjust(red, $lightness: 20%);

// previously adjust-hue(red, 180deg)
$complement: color.adjust(red, $hue: 180deg);

Some of those old functions (like adjust-hue) are redundant and unnecessary. Others — like lighten. darken. saturate. and so on — need to be re-built with better internal logic. The original functions were based on adjust(). which uses linear math: adding 20% to the current lightness of red in our example above. In most cases, we actually want to scale() the lightness by a percentage, relative to the current value:

// 20% of the distance to white, rather than current-lightness + 20
$light-red: color.scale(red, $lightness: 20%);

Once fully deprecated and removed, these shortcut functions will eventually re-appear in sass:color with new behavior based on color.scale() rather than color.adjust(). This is happening in stages to avoid sudden backwards-breaking changes. In the meantime, I recommend manually checking your code to see where color.scale() might work better for you.

Configure imported libraries

Third-party or re-usable libraries will often come with default global configuration variables for you to override. We used to do that with variables before an import:

// _buttons.scss
$color: blue !default;

// old.scss
$color: red;
@import 'buttons';

Since used modules no longer have access to local variables, we need a new way to set those defaults. We can do that by adding a configuration map to @use:

@use 'buttons' with (
  $color: red,
  $style: 'flat',
);

This is similar to the $with argument in load-css(). but rather than using variable-names as keys, we use the variable itself, starting with $.

I love how explicit this makes configuration, but there’s one rule that has tripped me up several times: a module can only be configured once, the first time it is used. Import order has always been important for Sass, even with @import. but those issues always failed silently. Now we get an explicit error, which is both good and sometimes surprising. Make sure to @use and configure libraries first thing in any "entrypoint" file (the central document that imports all partials), so that those configurations compile before other @use of the libraries.

It’s (currently) impossible to "chain" configurations together while keeping them editable, but you can wrap a configured module along with extensions, and pass that along as a new module.

Pass along files with @forward

We don’t always need to use a file, and access its members. Sometimes we just want to pass it along to future imports. Let’s say we have multiple form-related partials, and we want to import all of them together as one namespace. We can do that with @forward:

// forms/_index.scss
@forward 'input';
@forward 'textarea';
@forward 'select';
@forward 'buttons';

Members of the forwarded files are not available in the current document and no namespace is created, but those variables, functions, and mixins will be available when another file wants to @use or @forward the entire collection. If the forwarded partials contain actual CSS, that will also be passed along without generating output until the package is used. At that point it will all be treated as a single module with a single namespace:

// styles.scss
@use 'forms'; // imports all of the forwarded members in the `forms` namespace

Note: if you ask Sass to import a directory, it will look for a file named index or _index)

By default, all public members will forward with a module. But we can be more selective by adding show or hide clauses, and naming specific members to include or exclude:

// forward only the 'input' border() mixin, and $border-color variable
@forward 'input' show border, $border-color;

// forward all 'buttons' members *except* the gradient() function
@forward 'buttons' hide gradient;

Note: when functions and mixins share a name, they are shown and hidden together.

In order to clarify source, or avoid naming conflicts between forwarded modules, we can use as to prefix members of a partial as we forward:

// forms/_index.scss
// @forward "<url>" as <prefix>-*;
// assume both modules include a background() mixin
@forward 'input' as input-*;
@forward 'buttons' as btn-*;

// style.scss
@use 'forms';
@include forms.input-background();
@include forms.btn-background();

And, if we need, we can always @use and @forward the same module by adding both rules:

@forward 'forms';
@use 'forms';

That’s particularly useful if you want to wrap a library with configuration or any additional tools, before passing it along to your other files. It can even help simplify import paths:

// _tools.scss
// only use the library once, with configuration
@use 'accoutrement/sass/tools' with (
  $font-path: '../fonts/',
);
// forward the configured library with this partial
@forward 'accoutrement/sass/tools';

// add any extensions here...


// _anywhere-else.scss
// import the wrapped-and-extended library, already configured
@use 'tools';

Both @use and @forward must be declared at the root of the document (not nested), and at the start of the file. Only @charset and simple variable definitions can appear before the import commands.

Moving to modules

In order to test the new syntax, I built a new open source Sass library (Cascading Color Systems) and a new website for my band — both still under construction. I wanted to understand modules as both a library and website author. Let’s start with the "end user" experience of writing site styles with the module syntax…

Maintaining and writing styles

Using modules on the website was a pleasure. The new syntax encourages a code architecture that I already use. All my global configuration and tool imports live in a single directory (I call it config), with an index file that forwards everything I need:

// config/_index.scss
@forward 'tools';
@forward 'fonts';
@forward 'scale';
@forward 'colors';

As I build out other aspects of the site, I can import those tools and configurations wherever I need them:

// layout/_banner.scss
@use '../config';

.page-title {
  @include config.font-family('header');
}

This even works with my existing Sass libraries, like Accoutrement and Herman, that still use the old @import syntax. Since the @import rule will not be replaced everywhere overnight, Sass has built in a transition period. Modules are available now, but @import will not be deprecated for another year or two — and only removed from the language a year after that. In the meantime, the two systems will work together in either direction:

• If we @import a file that contains the new @use/@forward syntax, only the public members are imported, without namespace.
• If we @use or @forward a file that contains legacy @import syntax, we get access to all the nested imports as a single namespace.

That means you can start using the new module syntax right away, without waiting for a new release of your favorite libraries: and I can take some time to update all my libraries!

Migration tool

Upgrading shouldn’t take long if we use the Migration Tool built by Jennifer Thakar. It can be installed with Node, Chocolatey, or Homebrew:

npm install -g sass-migrator
choco install sass-migrator
brew install sass/sass/migrator

This is not a single-use tool for migrating to modules. Now that Sass is back in active development (see below), the migration tool will also get regular updates to help migrate each new feature. It’s a good idea to install this globally, and keep it around for future use.

The migrator can be run from the command line, and will hopefully be added to third-party applications like CodeKit and Scout as well. Point it at a single Sass file, like style.scss. and tell it what migration(s) to apply. At this point there’s only one migration called module:

# sass-migrator <migration> <entrypoint.scss...>
sass-migrator module style.scss

By default, the migrator will only update a single file, but in most cases we’ll want to update the main file and all its dependencies: any partials that are imported, forwarded, or used. We can do that by mentioning each file individually, or by adding the --migrate-deps flag:

sass-migrator --migrate-deps module style.scss

For a test-run, we can add --dry-run --verbose (or -nv for short), and see the results without changing any files. There are a number of other options that we can use to customize the migration — even one specifically for helping library authors remove old manual namespaces — but I won’t cover all of them here. The migration tool is fully documented on the Sass website.

Updating published libraries

I ran into a few issues on the library side, specifically trying to make user-configurations available across multiple files, and working around the missing chained-configurations. The ordering errors can be difficult to debug, but the results are worth the effort, and I think we’ll see some additional patches coming soon. I still have to experiment with the migration tool on complex packages, and possibly write a follow-up post for library authors.

The important thing to know right now is that Sass has us covered during the transition period. Not only can imports and modules work together, but we can create "import-only" files to provide a better experience for legacy users still @importing our libraries. In most cases, this will be an alternative version of the main package file, and you’ll want them side-by-side: <name>.scss for module users, and <name>.import.scss for legacy users. Any time a user calls @import <name>, it will load the .import version of the file:

// load _forms.scss
@use 'forms';

// load _forms.input.scss
@import 'forms';

This is particularly useful for adding prefixes for non-module users:

// _forms.import.scss
// Forward the main module, while adding a prefix
@forward "forms" as forms-*;

Upgrading Sass

You may remember that Sass had a feature-freeze a few years back, to get various implementations (LibSass, Node Sass, Dart Sass) all caught up, and eventually retired the original Ruby implementation. That freeze ended last year, with several new features and active discussions and development on GitHub – but not much fanfare. If you missed those releases, you can get caught up on the Sass Blog:

• CSS Imports and CSS Compatibility (Dart Sass v1.11)
• Content Arguments and Color Functions (Dart Sass v1.15)

Dart Sass is now the canonical implementation, and will generally be the first to implement new features. If you want the latest, I recommend making the switch. You can install Dart Sass with Node, Chocolatey, or Homebrew. It also works great with existing gulp-sass build steps.

Much like CSS (since CSS3), there is no longer a single unified version-number for new releases. All Sass implementations are working from the same specification, but each one has a unique release schedule and numbering, reflected with support information in the beautiful new documentation designed by Jina.

Sass Modules are available as of October 1st, 2019 in Dart Sass 1.23.0.

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