Maps in Sass hold pairs of keys and values, and make it easy to look up a value by its corresponding key. They’re written (<expression>: <expression>, <expression>: <expression>). The expression before the : is the key, and the expression after is the value associated with that key. The keys must be unique, but the values may be duplicated. Unlike lists, maps must be written with parentheses around them. A map with no pairs is written ().

Maps allow any Sass values to be used as their keys. The == operator is used to determine whether two keys are the same.

Using Maps

Since maps aren’t valid CSS values, they don’t do much of anything on their own. That’s why Sass provides a bunch of functions to create maps and access the values they contain.

Look Up a Value

Maps are all about associating keys and values, so naturally there’s a way to get the value associated with a key: the map.get($map, $key) function! This function returns the value in the map associated with the given key. It returns null if the map doesn’t contain the key.

$font-weights: ("regular": 400, "medium": 500, "bold": 700);

@debug map.get($font-weights, "medium"); // 500
@debug map.get($font-weights, "extra-bold"); // null

$font-weights: ("regular": 400, "medium": 500, "bold": 700)

@debug map.get($font-weights, "medium")  // 500
@debug map.get($font-weights, "extra-bold")  // null

Do Something for Every Pair

This doesn’t actually use a function, but it’s still one of the most common ways to use maps. The @each rule evaluates a block of styles for each key/value pair in a map. The key and the value are assigned to variables so they can easily be accessed in the block.

$icons: ("eye": "\f112", "start": "\f12e", "stop": "\f12f");

@each $name, $glyph in $icons {
  .icon-#{$name}:before {
    display: inline-block;
    font-family: "Icon Font";
    content: $glyph;
  }
}

$icons: ("eye": "\f112", "start": "\f12e", "stop": "\f12f")

@each $name, $glyph in $icons
  .icon-#{$name}:before
    display: inline-block
    font-family: "Icon Font"
    content: $glyph

@charset "UTF-8";
.icon-eye:before {
  display: inline-block;
  font-family: "Icon Font";
  content: "";
}

.icon-start:before {
  display: inline-block;
  font-family: "Icon Font";
  content: "";
}

.icon-stop:before {
  display: inline-block;
  font-family: "Icon Font";
  content: "";
}

Add to a Map

It’s also useful to add new pairs to a map, or to replace the value for an existing key. The map.set($map, $key, $value) function does this: it returns a copy of $map with the value at $key set to $value.

@use "sass:map";

$font-weights: ("regular": 400, "medium": 500, "bold": 700);

@debug map.set($font-weights, "extra-bold", 900);
// ("regular": 400, "medium": 500, "bold": 700, "extra-bold": 900)
@debug map.set($font-weights, "bold", 900);
// ("regular": 400, "medium": 500, "bold": 900)

@use "sass:map"

$font-weights: ("regular": 400, "medium": 500, "bold": 700)

@debug map.set($font-weights, "extra-bold": 900)
// ("regular": 400, "medium": 500, "bold": 700, "extra-bold": 900)
@debug map.set($font-weights, "bold", 900)
// ("regular": 400, "medium": 500, "bold": 900)

Instead of setting values one-by-one, you can also merge two existing maps using map.merge($map1, $map2).

@use "sass:map";

$light-weights: ("lightest": 100, "light": 300);
$heavy-weights: ("medium": 500, "bold": 700);

@debug map.merge($light-weights, $heavy-weights);
// ("lightest": 100, "light": 300, "medium": 500, "bold": 700)

@use "sass:map"

$light-weights: ("lightest": 100, "light": 300)
$heavy-weights: ("medium": 500, "bold": 700)

@debug map.merge($light-weights, $heavy-weights)
// ("lightest": 100, "light": 300, "medium": 500, "bold": 700)

If both maps have the same keys, the second map’s values are used in the map that gets returned.

@use "sass:map";

$weights: ("light": 300, "medium": 500);

@debug map.merge($weights, ("medium": 700));
// ("light": 300, "medium": 700)

@use "sass:map";

$weights: ("light": 300, "medium": 500)

@debug map.merge($weights, ("medium": 700))
// ("light": 300, "medium": 700)

Note that because Sass maps are immutable, map.set() and map.merge() do not modify the original list.

Immutability

Maps in Sass are immutable, which means that the contents of a map value never changes. Sass’s map functions all return new maps rather than modifying the originals. Immutability helps avoid lots of sneaky bugs that can creep in when the same map is shared across different parts of the stylesheet.

You can still update your state over time by assigning new maps to the same variable, though. This is often used in functions and mixins to track configuration in a map.

@use "sass:map";

$prefixes-by-browser: ("firefox": moz, "safari": webkit, "ie": ms);

@mixin add-browser-prefix($browser, $prefix) {
  $prefixes-by-browser: map.merge($prefixes-by-browser, ($browser: $prefix)) !global;
}

@include add-browser-prefix("opera", o);
@debug $prefixes-by-browser;
// ("firefox": moz, "safari": webkit, "ie": ms, "opera": o)

@use "sass:map"

$prefixes-by-browser: ("firefox": moz, "safari": webkit, "ie": ms)

@mixin add-browser-prefix($browser, $prefix)
  $prefixes-by-browser: map.merge($prefixes-by-browser, ($browser: $prefix)) !global


@include add-browser-prefix("opera", o)
@debug $prefixes-by-browser
// ("firefox": moz, "safari": webkit, "ie": ms, "opera": o)