Functions are defined using the @function at-rule, which is written @function <name>(<arguments...>) { ... }. A function’s name can be any Sass identifier. It can only contain universal statements, as well as the @return at-rule which indicates the value to use as the result of the function call. Functions are called using the normal CSS function syntax.

@function pow($base, $exponent) {
  $result: 1;
  @for $_ from 1 through $exponent {
    $result: $result * $base;
  }
  @return $result;
}

.sidebar {
  float: left;
  margin-left: pow(4, 3) * 1px;
}

@function pow($base, $exponent)
  $result: 1
  @for $_ from 1 through $exponent
    $result: $result * $base

  @return $result


.sidebar
  float: left
  margin-left: pow(4, 3) * 1px

.sidebar {
  float: left;
  margin-left: 64px;
}

Arguments

Arguments allow functions’ behavior to be customized each time they’re called. The arguments are specified in the @function rule after the function’s name, as a list of variable names surrounded by parentheses. The function must be called with the same number of arguments in the form of SassScript expressions. The values of these expression are available within the function’s body as the corresponding variables.

Optional Arguments

Normally, every argument a function declares must be passed when that function is included. However, you can make an argument optional by defining a default value which will be used if that arguments 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 function APIs that can be used in simple or complex ways.

@function invert($color, $amount: 100%) {
  $inverse: change-color($color, $hue: hue($color) + 180);
  @return mix($inverse, $color, $amount);
}

$primary-color: #036;
.header {
  background-color: invert($primary-color, 80%);
}

@function invert($color, $amount: 100%)
  $inverse: change-color($color, $hue: hue($color) + 180)
  @return mix($inverse, $color, $amount)


$primary-color: #036
.header
  background-color: invert($primary-color, 80%)

.header {
  background-color: #523314;
}

Keyword Arguments

When a function is called, arguments can be passed by name in addition to passing them by their position in the argument list. This is especially useful for functions 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.

$primary-color: #036;
.banner {
  background-color: $primary-color;
  color: scale-color($primary-color, $lightness: +40%);
}

$primary-color: #036
.banner
  background-color: $primary-color
  color: scale-color($primary-color, $lightness: +40%)

.banner {
  background-color: #036;
  color: #0a85ff;
}

Taking Arbitrary Arguments

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

@function sum($numbers...) {
  $sum: 0;
  @each $number in $numbers {
    $sum: $sum + $number;
  }
  @return $sum;
}

.micro {
  width: sum(50px, 30px, 100px);
}

@function sum($numbers...)
  $sum: 0
  @each $number in $numbers
    $sum: $sum + $number

  @return $sum


.micro
  width: sum(50px, 30px, 100px)

.micro {
  width: 180px;
}

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 function as a map from argument names (not including $) to those arguments’ values.

Passing Arbitrary Arguments

Just like argument lists allow functions to take arbitrary positional or keyword arguments, the same syntax can be used to pass positional and keyword arguments to a function. If you pass a list followed by ... as the last argument of a function call, 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!

$widths: 50px, 30px, 100px;
.micro {
  width: min($widths...);
}

$widths: 50px, 30px, 100px
.micro
  width: min($widths...)

.micro {
  width: 30px;
}

@function fg($args...) {
  @warn "The fg() function is deprecated. Call foreground() instead.";
  @return foreground($args...);
}

@function fg($args...)
  @warn "The fg() function is deprecated. Call foreground() instead."
  @return foreground($args...)

@return

The @return at-rule indicates the value to use as the result of calling a function. It’s only allowed within a @function body, and each @function must end with a @return.

When a @return is encountered, it immediately ends the function and returns its result. Returning early can be useful for handling edge-cases or cases where a more efficient algorithm is available without wrapping the entire function in an @else block.

@use "sass:string";

@function str-insert($string, $insert, $index) {
  // Avoid making new strings if we don't need to.
  @if string.length($string) == 0 {
    @return $insert;
  }

  $before: string.slice($string, 0, $index);
  $after: string.slice($string, $index);
  @return $before + $insert + $after;
}

@use "sass:string"

@function str-insert($string, $insert, $index)
  // Avoid making new strings if we don't need to.
  @if string.length($string) == 0
    @return $insert


  $before: string.slice($string, 0, $index)
  $after: string.slice($string, $index)
  @return $before + $insert + $after

Other Functions

In addition to user-defined function, Sass provides a substantial core library of built-in functions that are always available to use. Sass implementations also make it possible to define custom functions in the host language. And of course, you can always call plain CSS functions (even ones with weird syntax).

Plain CSS Functions

Any function call that’s not either a user-defined or built-in function is compiled to a plain CSS function (unless it uses Sass argument syntax). The arguments will be compiled to CSS and included as-is in the function call. This ensures that Sass supports all CSS functions without needing to release new versions every time a new one is added.

@debug var(--main-bg-color); // var(--main-bg-color)

$primary: #f2ece4;
$accent: #e1d7d2;
@debug radial-gradient($primary, $accent); // radial-gradient(#f2ece4, #e1d7d2)

@debug var(--main-bg-color)  // var(--main-bg-color)

$primary: #f2ece4
$accent: #e1d7d2
@debug radial-gradient($primary, $accent)  // radial-gradient(#f2ece4, #e1d7d2)