sass-site/source/documentation/at-rules/mixin.liquid

---
title: "@mixin and @include"
table_of_contents: true
introduction: >
  Mixins allow you to define styles that can be re-used throughout your
  stylesheet. They make it easy to avoid using non-semantic classes like
  `.float-left`, and to distribute collections of styles in libraries.
---

{% markdown %}
Mixins are defined using the `@mixin` at-rule, which is written
`@mixin <name> { ... }` or `@mixin name(<arguments...>) { ... }`. A mixin's name
can be any Sass identifier, and it can contain any [statement][] other than
[top-level statements][]. They can be used to encapsulate styles that can be
dropped into a single [style rule][]; they can contain style rules of their own
that can be nested in other rules or included at the top level of the
stylesheet; or they can just serve to modify variables.

[statement]: /documentation/syntax/structure#statements
[top-level statements]: /documentation/syntax/structure#top-level-statements
[style rule]: /documentation/style-rules

Mixins are included into the current context using the `@include` at-rule, which
is written `@include <name>` or `@include <name>(<arguments...>)`, with the name
of the mixin being included.
{% endmarkdown %}

{% codeExample 'mixin-include' %}
@mixin reset-list {
  margin: 0;
  padding: 0;
  list-style: none;
}

@mixin horizontal-list {
  @include reset-list;

  li {
    display: inline-block;
    margin: {
      left: -2px;
      right: 2em;
    }
  }
}

nav ul {
  @include horizontal-list;
}
===
@mixin reset-list
  margin: 0
  padding: 0
  list-style: none


@mixin horizontal-list
  @include reset-list

  li
    display: inline-block
    margin:
      left: -2px
      right: 2em




nav ul
  @include horizontal-list
{% endcodeExample %}

{% funFact %}
Mixin names, like all Sass identifiers, treat hyphens and underscores as
identical. This means that `reset-list` and `reset_list` both refer to the same
mixin. This is a historical holdover from the very early days of Sass, when it
*only* allowed underscores in identifier names. Once Sass added support for
hyphens to match CSS's syntax, the two were made equivalent to make migration
easier.
{% endfunFact %}

{% markdown %}
## Arguments

{% comment %}
When changing this section, don't forget to change the function arguments
section as well!
{% endcomment %}

Mixins can also take arguments, which allows their behavior to be customized
each time they're called. The arguments are specified in the `@mixin` rule after
the mixin's name, as a list of variable names surrounded by parentheses. The
mixin must then be included with the same number of arguments in the form of
[SassScript expressions][]. The values of these expression are available within
the mixin's body as the corresponding variables.

[SassScript expressions]: /documentation/syntax/structure#expressions
{% endmarkdown %}

{% codeExample 'mixin-arguments' %}
@mixin rtl($property, $ltr-value, $rtl-value) {
  #{$property}: $ltr-value;

  [dir=rtl] & {
    #{$property}: $rtl-value;
  }
}

.sidebar {
  @include rtl(float, left, right);
}
===
@mixin rtl($property, $ltr-value, $rtl-value)
  #{$property}: $ltr-value

  [dir=rtl] &
    #{$property}: $rtl-value



.sidebar
  @include rtl(float, left, right)
{% endcodeExample %}

{% funFact %}
Argument lists can also have trailing commas! This makes it easier to avoid
syntax errors when refactoring your stylesheets.
{% endfunFact %}

{% markdown %}
### Optional Arguments

Normally, every argument a mixin declares must be passed when that mixin is
included. However, you can make an argument optional by defining a *default
value* which will be used if that argument isn't passed. Default values use the
same syntax as [variable declarations][]: the variable name, followed by a colon
and a [SassScript expression][]. This makes it easy to define flexible mixin
APIs that can be used in simple or complex ways.

[variable declarations]: /documentation/variables
[SassScript expression]: /documentation/syntax/structure#expressions
{% endmarkdown %}

{% codeExample 'optional-arguments' %}
@mixin replace-text($image, $x: 50%, $y: 50%) {
  text-indent: -99999em;
  overflow: hidden;
  text-align: left;

  background: {
    image: $image;
    repeat: no-repeat;
    position: $x $y;
  }
}

.mail-icon {
  @include replace-text(url("/images/mail.svg"), 0);
}
===
@mixin replace-text($image, $x: 50%, $y: 50%)
  text-indent: -99999em
  overflow: hidden
  text-align: left

  background:
    image: $image
    repeat: no-repeat
    position: $x $y

.mail-icon
  @include replace-text(url("/images/mail.svg"), 0)
{% endcodeExample %}

{% funFact %}
Default values can be any SassScript expression, and they can even refer to
earlier arguments!
{% endfunFact %}

{% markdown %}
### Keyword Arguments

When a mixin is included, arguments can be passed by name in addition to passing
them by their position in the argument list. This is especially useful for
mixins with multiple optional arguments, or with [boolean][] arguments whose
meanings aren't obvious without a name to go with them. Keyword arguments use
the same syntax as [variable declarations][] and [optional arguments][].

[variable declarations]: /documentation/variables
[boolean]: /documentation/values/booleans
[optional arguments]: #optional-arguments
{% endmarkdown %}

{% codeExample 'keyword-arguments' %}
@mixin square($size, $radius: 0) {
  width: $size;
  height: $size;

  @if $radius != 0 {
    border-radius: $radius;
  }
}

.avatar {
  @include square(100px, $radius: 4px);
}
===
@mixin square($size, $radius: 0)
  width: $size
  height: $size

  @if $radius != 0
    border-radius: $radius



.avatar
  @include square(100px, $radius: 4px)
{% endcodeExample %}

{% headsUp %}
Because *any* argument can be passed by name, be careful when renaming a mixin's
arguments... it might break your users! It can be helpful to keep the old name
around as an [optional argument][] for a while and printing a [warning][] if
anyone passes it, so they know to migrate to the new argument.

[optional argument]: #optional-arguments
[warning]: /documentation/at-rules/warn
{% endheadsUp %}

{% markdown %}
### Taking Arbitrary Arguments

Sometimes it's useful for a mixin to be able to take any number of arguments. If
the last argument in a `@mixin` declaration ends in `...`, then all extra
arguments to that mixin are passed to that argument as a [list][]. This argument
is known as an [argument list][].

[list]: /documentation/values/lists
[argument list]: /documentation/values/lists#argument-lists

{% endmarkdown %}

{% codeExample 'arbitrary-arguments' %}
@mixin order($height, $selectors...) {
  @for $i from 0 to length($selectors) {
    #{nth($selectors, $i + 1)} {
      position: absolute;
      height: $height;
      margin-top: $i * $height;
    }
  }
}

@include order(150px, "input.name", "input.address", "input.zip");
===
@mixin order($height, $selectors...)
  @for $i from 0 to length($selectors)
    #{nth($selectors, $i + 1)}
      position: absolute
      height: $height
      margin-top: $i * $height




@include order(150px, "input.name", "input.address", "input.zip")
{% endcodeExample %}

{% markdown %}
#### Taking Arbitrary Keyword Arguments

Argument lists can also be used to take arbitrary keyword arguments. The
[`meta.keywords()` function][] takes an argument list and returns any extra
keywords that were passed to the mixin as a [map][] from argument names (not
including `$`) to those arguments' values.

[`meta.keywords()` function]: /documentation/modules/meta#keywords
[map]: /documentation/values/maps

<!-- TODO(nweiz): auto-generate this CSS once we're compiling with Dart Sass -->
{% endmarkdown %}

{% render 'code-snippets/example-mixin-arbitrary-keyword-arguments' %}

{% funFact %}
If you don't ever pass an argument list to the [`meta.keywords()` function][],
that argument list won't allow extra keyword arguments. This helps callers of
your mixin make sure they haven't accidentally misspelled any argument names.

[`meta.keywords()` function]: /documentation/modules/meta#keywords
{% endfunFact %}

{% markdown %}
#### Passing Arbitrary Arguments

Just like argument lists allow mixins to take arbitrary positional or keyword
arguments, the same syntax can be used to *pass* positional and keyword
arguments to a mixin. If you pass a list followed by `...` as the last argument
of an include, its elements will be treated as additional positional arguments.
Similarly, a map followed by `...` will be treated as additional keyword
arguments. You can even pass both at once!
{% endmarkdown %}

{% codeExample 'passing-arbitrary-arguments', false %}
$form-selectors: "input.name", "input.address", "input.zip" !default;

@include order(150px, $form-selectors...);
===
$form-selectors: "input.name", "input.address", "input.zip" !default

@include order(150px, $form-selectors...)
{% endcodeExample %}

{% funFact false %}
{% markdown %}
Because an [argument list][] keeps track of both positional and keyword
arguments, you use it to pass both at once to another mixin. That makes it super
easy to define an alias for a mixin!

[argument list]: /documentation/values/lists#argument-lists
{% endmarkdown %}

{% codeExample 'passing-arbitrary-arguments-fun-fact' %}
@mixin btn($args...) {
  @warn "The btn() mixin is deprecated. Include button() instead.";
  @include button($args...);
}
===
@mixin btn($args...)
  @warn "The btn() mixin is deprecated. Include button() instead."
  @include button($args...)
{% endcodeExample %}
{% endfunFact %}

{% markdown %}
## Content Blocks

In addition to taking arguments, a mixin can take an entire block of styles,
known as a *content block*. A mixin can declare that it takes a content block by
including the `@content` at-rule in its body. The content block is passed in
using curly braces like any other block in Sass, and it's injected in place of
the `@content` rule.
{% endmarkdown %}

{% codeExample 'content-blocks' %}
@mixin hover {
  &:not([disabled]):hover {
    @content;
  }
}

.button {
  border: 1px solid black;
  @include hover {
    border-width: 2px;
  }
}
===
@mixin hover
  &:not([disabled]):hover
    @content



.button
  border: 1px solid black
  @include hover
    border-width: 2px
{% endcodeExample %}

{% funFact %}
A mixin can include multiple `@content` at-rules. If it does, the content block
will be included separately for each `@content`.
{% endfunFact %}

{% headsUp %}
A content block is *lexically scoped*, which means it can only see [local
variables][] in the scope where the mixin is included. It can't see any
variables that are defined in the mixin it's passed to, even if they're defined
before the content block is invoked.

[local variables]: /documentation/variables#scope
{% endheadsUp %}

{{ '### Passing Arguments to Content Blocks' | markdown }}

{% # Arguments are (in order): `dart`, `libsass`, `node`, `ruby`, optional feature name, additional details within %}
{% compatibility '1.15.0', false, null, false %}{% endcompatibility %}

{% markdown %}
A mixin can pass arguments to its content block the same way it would pass
arguments to another mixin by writing `@content(<arguments...>)`. The user
writing the content block can accept arguments by writing `@include <name> using
(<arguments...>)`. The argument list for a content block works just like a
mixin's argument list, and the arguments passed to it by `@content` work just
like passing arguments to a mixin.
{% endmarkdown %}

{% headsUp %}
If a mixin passes arguments to its content block, that content block *must*
declare that it accepts those arguments. This means that it's a good idea to
only pass arguments by position (rather than by name), and it means that passing
more arguments is a breaking change.

If you want to be flexible in what information you pass to a content block,
consider passing it a [map][] that contains information it may need!

[map]: /documentation/values/maps
{% endheadsUp %}

<!-- TODO(nweiz): auto-generate this CSS once we're compiling with Dart Sass -->
{% codeExample 'passing-arguments-to-content-blocks', false %}
@mixin media($types...) {
  @each $type in $types {
    @media #{$type} {
      @content($type);
    }
  }
}

@include media(screen, print) using ($type) {
  h1 {
    font-size: 40px;
    @if $type == print {
      font-family: Calluna;
    }
  }
}
===
@mixin media($types...)
  @each $type in $types
    @media #{$type}
      @content($type)




@include media(screen, print) using ($type)
  h1
    font-size: 40px
    @if $type == print
      font-family: Calluna
===
@media screen {
  h1 {
    font-size: 40px;
  }
}
@media print {
  h1 {
    font-size: 40px;
    font-family: Calluna;
  }
}
{% endcodeExample %}

{% markdown %}
## Indented Mixin Syntax

The [indented syntax][] has a special syntax for defining and using mixins, in
addition to the standard `@mixin` and `@include`. Mixins are defined using the
character `=`, and they're included using `+`. Although this syntax is terser,
it's also harder to understand at a glance and users are encouraged to avoid it.

[indented syntax]: /documentation/syntax#the-indented-syntax
{% endmarkdown %}

{% codeExample 'indented-syntax', true, 'sass' %}
=reset-list
  margin: 0
  padding: 0
  list-style: none

=horizontal-list
  +reset-list

  li
    display: inline-block
    margin:
      left: -2px
      right: 2em

nav ul
  +horizontal-list
{% endcodeExample %}