sass-site/source/documentation/interpolation.liquid

---
title: Interpolation
table_of_contents: true
introduction: >
  Interpolation can be used almost anywhere in a Sass stylesheet to embed the result of a [SassScript expression](../syntax/structure#expressions) into a chunk
  of CSS. Just wrap an expression in `#{}` in any of the following places:
---
{% markdown %}
* [Selectors in style rules](style-rules#interpolation)
* [Property names in declarations](style-rules/declarations#interpolation)
* [Custom property values](style-rules/declarations#custom-properties)
* [CSS at-rules](at-rules/css)
* [`@extend`s](at-rules/extend)
* [Plain CSS `@import`s](at-rules/import#plain-css-imports)
* [Quoted or unquoted strings](values/strings)
* [Special functions](syntax/special-functions)
* [Plain CSS function names](at-rules/function#plain-css-functions)
* [Loud comments](syntax/comments)
{% endmarkdown %}

{% codeExample 'interpolation' %}
@mixin corner-icon($name, $top-or-bottom, $left-or-right) {
  .icon-#{$name} {
    background-image: url("/icons/#{$name}.svg");
    position: absolute;
    #{$top-or-bottom}: 0;
    #{$left-or-right}: 0;
  }
}

@include corner-icon("mail", top, left);
===
@mixin corner-icon($name, $top-or-bottom, $left-or-right)
  .icon-#{$name}
    background-image: url("/icons/#{$name}.svg")
    position: absolute
    #{$top-or-bottom}: 0
    #{$left-or-right}: 0



@include corner-icon("mail", top, left)
{% endcodeExample %}

{% markdown %}
## In SassScript

{% # Arguments are (in order): `dart`, `libsass`, `ruby`, optional feature name, additional details within %}
{% compatibility true, false, '4.0.0 (unreleased)', 'Modern Syntax' %}
LibSass and Ruby Sass currently use an older syntax for parsing interpolation in SassScript. For most practical purposes it works the same, but it can behave strangely around [operators][]. See [this document][] for details.

[operators]: ../operators
[this document]:   https://github.com/sass/language/blob/master/accepted/free-interpolation.md#old-interpolation-rules
{% endcompatibility %}

Interpolation can be used in SassScript to inject SassScript into [unquoted strings][]. This is particularly useful when dynamically generating names (for example for animations), or when using [slash-separated values][]. Note that interpolation in SassScript always returns an unquoted string.

[unquoted strings]: ../values/strings#unquoted
[slash-separated values]: ../operators/numeric#slash-separated-values
{% endmarkdown %}

{% codeExample 'interpolation-sass-script' %}
@mixin inline-animation($duration) {
  $name: inline-#{unique-id()};

  @keyframes #{$name} {
    @content;
  }

  animation-name: $name;
  animation-duration: $duration;
  animation-iteration-count: infinite;
}

.pulse {
  @include inline-animation(2s) {
    from { background-color: yellow }
    to { background-color: red }
  }
}
===
@mixin inline-animation($duration)
  $name: inline-#{unique-id()}

  @keyframes #{$name}
    @content


  animation-name: $name
  animation-duration: $duration
  animation-iteration-count: infinite


.pulse
  @include inline-animation(2s)
    from
      background-color: yellow
    to
      background-color: red
{% endcodeExample %}

{% funFact %}
Interpolation is useful for injecting values into strings, but other than that it's rarely necessary in SassScript expressions. You definitely *don't* need
it to just use a variable in a property value. Instead of writing `color: #{$accent}`, you can just write `color: $accent`!
{% endfunFact %}

{% headsUp %}
{% markdown %}
It's almost always a bad idea to use interpolation with numbers. Interpolation
returns unquoted strings that can't be used for any further math, and it avoids Sass's built-in safeguards to ensure that units are used correctly.

Sass has powerful [unit arithmetic][] that you can use instead. For example, instead of writing `#{$width}px`, write `$width * 1px`—or better yet, declare the `$width` variable in terms of `px` to begin with. That way if `$width` already has units, you'll get a nice error message instead of compiling bogus CSS.

[unit arithmetic]: ../values/numbers#units
{% endmarkdown %}
{% endheadsUp %}

{% markdown %}
## Quoted Strings

In most cases, interpolation injects the exact same text that would be used if the expression were used as a [property value][]. But there is one exception: the quotation marks around quoted strings are removed (even if those quoted strings are in lists). This makes it possible to write quoted strings that contain syntax that's not allowed in SassScript (like selectors) and interpolate
them into style rules.

[property value]: ../style-rules/declarations
{% endmarkdown %}

{% codeExample 'quoted-strings' %}
.example {
  unquoted: #{"string"};
}
===
.example
  unquoted: #{"string"}
{% endcodeExample %}

{% headsUp %}
{% markdown %}
While it's tempting to use this feature to convert quoted strings to unquoted strings, it's a lot clearer to use the [`string.unquote()` function][]. Instead of `#{$string}`, write `string.unquote($string)`!

[`string.unquote()` function]: ../modules/string#unquote
{% endmarkdown %}
{% endheadsUp %}