--- title: Maps table_of_contents: true introduction: > 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 `(: , : )`. The [expression](../syntax/structure#expressions) 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](lists), maps *must* be written with parentheses around them. A map with no pairs is written `()`. --- <% fun_fact do %> Astute readers may note that an empty map, `()`, is written the same as an empty list. That's because it counts as both a map and a list. In fact, *all* maps count as lists! Every map counts as a list that contains a two-element list for each key/value pair. For example, `(1: 2, 3: 4)` counts as `(1 2, 3 4)`. <% end %> Maps allow any Sass values to be used as their keys. The [`==` operator][] is used to determine whether two keys are the same. [`==` operator]: ../operators/equality <% heads_up do %> Most of the time, it's a good idea to use [quoted strings][] rather than [unquoted strings][] for map keys. This is because some values, such as color names, may *look* like unquoted strings but actually be other types. To avoid confusing problems down the line, just use quotes! [quoted strings]: strings#quoted [unquoted strings]: strings#unquoted <% end %> ## 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. With [functions]: ../functions/map ### 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, $key)` function][]! This function returns the value in the map associated with the given key. It returns [`null`][] if the map doesn't contain the key. the [`map-get($map, $key)` function]: ../functions/map#map-get [`null`]: null <%= partial 'code-snippets/example-map-get' %> ### Do Something for Every Pair This doesn't actually use a function, but it's still one of the most common ways to use maps. The [`@each` rule][] evaluates a block of styles for each key/value pair in a map. The key and the value are assigned to variables so they can easily be accessed in the block. The [`@each` rule]: ../at-rules/control/each <%= partial 'code-snippets/example-each-map' %> ### Add to a Map It's also useful to add new pairs to a map, or to replace the value for an existing key. The [`map-merge($map1, $map2)` function][] does this: it returns a new map that contains all the key/value pairs in *both* arguments. [`map-merge($map1, $map2)` function]: ../functions/map#map-merge <% example(autogen_css: false) do %> $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 // ) === $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 // ) <% end %> `map-merge()` is often used with an inline map to add a single key/value pair. <% example(autogen_css: false) do %> $font-weights: ("regular": 400, "medium": 500, "bold": 700); @debug map-merge($font-weights, ("extra-bold": 900)); // ("regular": 400, "medium": 500, "bold": 700, "extra-bold": 900) === $font-weights: ("regular": 400, "medium": 500, "bold": 700) @debug map-merge($font-weights, ("extra-bold": 900)) // ("regular": 400, "medium": 500, "bold": 700, "extra-bold": 900) <% end %> If both maps have the same keys, the second map's values are used in the map that gets returned. <% example(autogen_css: false) do %> $font-weights: ("regular": 400, "medium": 500, "bold": 700); @debug map-merge($font-weights, ("medium": 600)); // ("regular": 400, "medium": 600, "bold": 700) === $font-weights: ("regular": 400, "medium": 500, "bold": 700) @debug map-merge($font-weights, ("medium": 600)) // ("regular": 400, "medium": 600, "bold": 700) <% end %> Note that because Sass maps are [immutable][], `map-merge()` doesn't modify the original list. [immutable]: #immutability ## 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. <% example(autogen_css: false) do %> $prefixes-by-browser: ("firefox": moz, "safari": webkit, "ie": ms); @mixin add-browser-prefix($browser, $prefix) { $prefixes: map-merge($prefixes-by-browser, ($browser: $prefix)); } @include add-browser-prefix("opera", o); @debug $prefixes-by-browser; // ("firefox": moz, "safari": webkit, "ie": ms, "opera": o) === $prefixes-by-browser: ("firefox": moz, "safari": webkit, "ie": ms) @mixin add-browser-prefix($browser, $prefix) $prefixes: map-merge($prefixes-by-browser, ($browser: $prefix)) @include add-browser-prefix("opera", o) @debug $prefixes-by-browser // ("firefox": moz, "safari": webkit, "ie": ms, "opera": o) <% end %>