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sass-site/source/documentation/at-rules/import.md
Jonny Gerig Meyer e3e903a8f1
Convert at-rules docs to markdown
2023-06-21 15:20:03 -04:00

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title table_of_contents introduction
@import true Sass extends CSS's [`@import` rule](https://developer.mozilla.org/en-US/docs/Web/CSS/@import) with the ability to import Sass and CSS stylesheets, providing access to [mixins](/documentation/at-rules/mixin), [functions](/documentation/at-rules/function), and [variables](/documentation/variables) and combining multiple stylesheets' CSS together. Unlike plain CSS imports, which require the browser to make multiple HTTP requests as it renders your page, Sass imports are handled entirely during compilation.

Sass imports have the same syntax as CSS imports, except that they allow multiple imports to be separated by commas rather than requiring each one to have its own @import. Also, in the indented syntax, imported URLs aren't required to have quotes.

{% headsUp %} The Sass team discourages the continued use of the @import rule. Sass will gradually phase it out over the next few years, and eventually remove it from the language entirely. Prefer the @use rule instead. (Note that only Dart Sass currently supports @use. Users of other implementations must use the @import rule instead.)

{{ "What's Wrong With `@import`?" | markdown }}

The @import rule has a number of serious issues:

  • @import makes all variables, mixins, and functions globally accessible. This makes it very difficult for people (or tools) to tell where anything is defined.

  • Because everything's global, libraries must add a prefix to all their members to avoid naming collisions.

  • @extend rules are also global, which makes it difficult to predict which style rules will be extended.

  • Each stylesheet is executed and its CSS emitted every time it's @imported, which increases compilation time and produces bloated output.

  • There was no way to define private members or placeholder selectors that were inaccessible to downstream stylesheets.

The new module system and the @use rule address all these problems.

{{ 'How Do I Migrate?' | markdown }}

We've written a migration tool that automatically converts most @import-based code to @use-based code in a flash. Just point it at your entrypoints and let it run!

{% endheadsUp %}

{% codeExample 'import' %} // foundation/_code.scss code { padding: .25em; line-height: 0; }

// foundation/_lists.scss ul, ol { text-align: left;

& & {
  padding: {
    bottom: 0;
    left: 0;
  }
}

}

// style.scss @import 'foundation/code', 'foundation/lists';

// foundation/_code.sass code padding: .25em line-height: 0

// foundation/_lists.sass ul, ol text-align: left

& &
  padding:
    bottom: 0
    left: 0

// style.sass @import foundation/code, foundation/lists

code { padding: .25em; line-height: 0; }

ul, ol { text-align: left; } ul ul, ol ol { padding-bottom: 0; padding-left: 0; } {% endcodeExample %}

When Sass imports a file, that file is evaluated as though its contents appeared directly in place of the @import. Any mixins, functions, and variables from the imported file are made available, and all its CSS is included at the exact point where the @import was written. What's more, any mixins, functions, or variables that were defined before the @import (including from other @imports) are available in the imported stylesheet.

{% headsUp %} If the same stylesheet is imported more than once, it will be evaluated again each time. If it just defines functions and mixins, this usually isn't a big deal, but if it contains style rules they'll be compiled to CSS more than once. {% endheadsUp %}

Finding the File

It wouldn't be any fun to write out absolute URLs for every stylesheet you import, so Sass's algorithm for finding a file to import makes it a little easier. For starters, you don't have to explicitly write out the extension of the file you want to import; @import "variables" will automatically load variables.scss, variables.sass, or variables.css.

{% headsUp %} To ensure that stylesheets work on every operating system, Sass imports files by URL, not by file path. This means you need to use forward slashes, not backslashes, even when you're on Windows. {% endheadsUp %}

Load Paths

All Sass implementations allow users to provide load paths: paths on the filesystem that Sass will look in when resolving imports. For example, if you pass node_modules/susy/sass as a load path, you can use @import "susy" to load node_modules/susy/sass/susy.scss.

Imports will always be resolved relative to the current file first, though. Load paths will only be used if no relative file exists that matches the import. This ensures that you can't accidentally mess up your relative imports when you add a new library.

{% funFact %} Unlike some other languages, Sass doesn't require that you use ./ for relative imports. Relative imports are always available. {% endfunFact %}

Partials

As a convention, Sass files that are only meant to be imported, not compiled on their own, begin with _ (as in _code.scss). These are called partials, and they tell Sass tools not to try to compile those files on their own. You can leave off the _ when importing a partial.

Index Files

{% compatibility 'dart: true', 'libsass: "3.6.0"', 'ruby: "3.6.0"' %}{% endcompatibility %}

If you write an _index.scss or _index.sass in a folder, when the folder itself is imported that file will be loaded in its place.

{% codeExample 'index-files' %} // foundation/_code.scss code { padding: .25em; line-height: 0; }

// foundation/_lists.scss ul, ol { text-align: left;

& & {
  padding: {
    bottom: 0;
    left: 0;
  }
}

}

// foundation/_index.scss @import 'code', 'lists';

// style.scss @import 'foundation';

// foundation/_code.sass code padding: .25em line-height: 0

// foundation/_lists.sass ul, ol text-align: left

& &
  padding:
    bottom: 0
    left: 0

// foundation/_index.sass @import code, lists

// style.sass @import foundation

code { padding: .25em; line-height: 0; }

ul, ol { text-align: left; } ul ul, ol ol { padding-bottom: 0; padding-left: 0; } {% endcodeExample %}

Custom Importers

All Sass implementations provide a way to define custom importers, which control how @imports locate stylesheets:

Nesting

Imports are usually written at the top level of a stylesheet, but they don't have to be. They can nested within style rules or plain CSS at-rules as well. The imported CSS is nested in that context, which makes nested imports useful for scoping a chunk of CSS to a particular element or media query. Note that top-level mixins, functions, and variables defined in the nested import are still defined globally, though.

{% codeExample 'nesting' %} // _theme.scss pre, code { font-family: 'Source Code Pro', Helvetica, Arial; border-radius: 4px; }

// style.scss .theme-sample { @import "theme"; }

// _theme.sass pre, code font-family: 'Source Code Pro', Helvetica, Arial border-radius: 4px

// style.sass .theme-sample @import theme

.theme-sample pre, .theme-sample code { font-family: 'Source Code Pro', Helvetica, Arial; border-radius: 4px; } {% endcodeExample %}

{% funFact %} Nested imports are very useful for scoping third-party stylesheets, but if you're the author of the stylesheet you're importing, it's usually a better idea to write your styles in a mixin and include that mixin in the nested context. A mixin can be used in more flexible ways, and it's clearer when looking at the imported stylesheet how it's intended to be used.

{% endfunFact %}

{% headsUp %} The CSS in nested imports is evaluated like a mixin, which means that any parent selectors will refer to the selector in which the stylesheet is nested.

{% codeExample 'parent-selector' %} // _theme.scss ul li { $padding: 16px; padding-left: $padding; [dir=rtl] & { padding: { left: 0; right: $padding; } } } --- // style.scss .theme-sample { @import "theme"; } === // _theme.sass ul li $padding: 16px padding-left: $padding [dir=rtl] & padding: left: 0 right: $padding --- // style.sass .theme-sample @import theme === .theme-sample ul li { padding-left: 16px; } [dir=rtl] .theme-sample ul li { padding-left: 0; padding-right: 16px; } {% endcodeExample %} {% endheadsUp %}

Importing CSS

{% compatibility 'dart: "1.11.0"', 'libsass: "partial"', 'ruby: false' %} LibSass supports importing files with the extension .css, but contrary to the specification they're treated as SCSS files rather than being parsed as CSS. This behavior has been deprecated, and an update is in the works to support the behavior described below. {% endcompatibility %}

In addition to importing .sass and .scss files, Sass can import plain old .css files. The only rule is that the import must not explicitly include the .css extension, because that's used to indicate a plain CSS @import.

{% codeExample 'import-css' %} // code.css code { padding: .25em; line-height: 0; }

// style.scss @import 'code';

// code.css code { padding: .25em; line-height: 0; }

// style.sass @import code

code { padding: .25em; line-height: 0; } {% endcodeExample %}

CSS files imported by Sass don't allow any special Sass features. In order to make sure authors don't accidentally write Sass in their CSS, all Sass features that aren't also valid CSS will produce errors. Otherwise, the CSS will be rendered as-is. It can even be extended!

Plain CSS @imports

{% compatibility 'dart: true', 'libsass: "partial"', 'ruby: true' %} By default, LibSass handles plain CSS imports correctly. However, any custom importers will incorrectly apply to plain-CSS @import rules, making it possible for those rules to load Sass files.

{% endcompatibility %}

Because @import is also defined in CSS, Sass needs a way of compiling plain CSS @imports without trying to import the files at compile time. To accomplish this, and to ensure SCSS is as much of a superset of CSS as possible, Sass will compile any @imports with the following characteristics to plain CSS imports:

  • Imports where the URL ends with .css.
  • Imports where the URL begins http:// or https://.
  • Imports where the URL is written as a url().
  • Imports that have media queries.

{% codeExample 'plain-css-imports' %} @import "theme.css"; @import "http://fonts.googleapis.com/css?family=Droid+Sans"; @import url(theme); @import "landscape" screen and (orientation: landscape);

@import "theme.css" @import "http://fonts.googleapis.com/css?family=Droid+Sans" @import url(theme) @import "landscape" screen and (orientation: landscape) {% endcodeExample %}

Interpolation

Although Sass imports can't use interpolation (to make sure it's always possible to tell where mixins, functions, and variables come from), plain CSS imports can. This makes it possible to dynamically generate imports, for example based on mixin parameters.

{% codeExample 'interpolation' %} @mixin google-font($family) { @import url("http://fonts.googleapis.com/css?family=#{$family}"); }

@include google-font("Droid Sans");

@mixin google-font($family) @import url("http://fonts.googleapis.com/css?family=#{$family}")

@include google-font("Droid Sans") {% endcodeExample %}

Import and Modules

{% render 'doc_snippets/module-system-status' %}

Sass's module system integrates seamlessly with @import, whether you're importing a file that contains @use rules or loading a file that contains imports as a module. We want to make the transition from @import to @use as smooth as possible.

Importing a Module-System File

When you import a file that contains @use rules, the importing file has access to all members (even private members) defined directly in that file, but not any members from modules that file has loaded. However, if that file contains @forward rules, the importing file will have access to forwarded members. This means that you can import a library that was written to be used with the module system.

{% headsUp %} When a file with @use rules is imported, all the CSS transitively loaded by those is included in the resulting stylesheet, even if it's already been included by another import. If you're not careful, this can result in bloated CSS output! {% endheadsUp %}

Import-Only Files

An API that makes sense for @use might not make sense for @import. For example, @use adds a namespace to all members by default so you can safely use short names, but @import doesn't so you might need something longer. If you're a library author, you may be concerned that if you update your library to use the new module system, your existing @import-based users will break.

To make this easier, Sass also supports import-only files. If you name a file <name>.import.scss, it will only be loaded for imports, not for @uses. This way, you can retain compatibility for @import users while still providing a nice API for users of the new module system.

{% codeExample 'import-only-files', false %} // _reset.scss

// Module system users write @include reset.list(). @mixin list() { ul { margin: 0; padding: 0; list-style: none; } }

// _reset.import.scss

// Legacy import users can keep writing @include reset-list(). @forward "reset" as reset-*;

// _reset.sass

// Module system users write @include reset.list(). @mixin list() ul margin: 0 padding: 0 list-style: none

// _reset.import.sass

// Legacy import users can keep writing @include reset-list(). @forward "reset" as reset-* {% endcodeExample %}

Configuring Modules Through Imports

{% compatibility 'dart: "1.24.0"', 'libsass: false', 'ruby: false' %}{% endcompatibility %}

You can configure modules that are loaded through an @import by defining global variables prior the @import that first loads that module.

{% codeExample 'configuring-modules' %} // _library.scss $color: blue !default;

a { color: $color; }

// _library.import.scss @forward 'library' as lib-*;

// style.sass $lib-color: green; @import "library";

$color: blue !default

a color: $color

// _library.import.sass @forward 'library' as lib-*

// style.sass $lib-color: green @import "library"

a { color: green; } {% endcodeExample %}

{% headsUp %} Modules are only loaded once, so if you change the configuration after you @import a module for the first time (even indirectly), the change will be ignored if you @import the module again. {% endheadsUp %}

Loading a Module That Contains Imports

When you use @use (or @forward) load a module that uses @import, that module will contain all the public members defined by the stylesheet you load and everything that stylesheet transitively imports. In other words, everything that's imported is treated as though it were written in one big stylesheet.

This makes it easy to convert start using @use in a stylesheet even before all the libraries you depend on have converted to the new module system. Be aware, though, that if they do convert their APIs may well change!