sass-site/source/documentation/variables.html.md.erb

---
title: Variables
table_of_contents: true
introduction: >
  Sass variables are simple: you assign a value to a name that begins with `$`,
  and then you can refer to that name instead of the value itself. But despite
  their simplicity, they're one of the most useful tools Sass brings to the
  table. Variables make it possible to reduce repetition, do complex math,
  configure libraries, and much more.
---

A variable declaration looks a lot like a [property declaration][]: it's written
`<variable>: <expression>`. Unlike a property, which can only be declared in a
style rule or at-rule, variables can be declared anywhere you want. To use a
variable, just include it in a value.

[property declaration]: style-rules/declarations

<% example do %>
  $base-color: #c6538c;
  $border-dark: rgba($base-color, 0.88);

  .alert {
    border: 1px solid $border-dark;
  }
  ===
  $base-color: #c6538c
  $border-dark: rgba($base-color, 0.88)

  .alert
    border: 1px solid $border-dark
<% end %>

<% heads_up do %>
  CSS has [variables of its own][], which are totally different than Sass
  variables. Know the differences!

  [variables of its own]: style-rules/declarations#custom-properties

  * Sass variables are all compiled away by Sass. CSS variables are included in
    the CSS output.

  * CSS variables can have different values for different elements, but Sass
    variables only have one value at a time.

  * Sass variables are *imperative*, which means if you use a variable and then
    change its value, the earlier use will stay the same. CSS variables are
    *declarative*, which means if you change the value, it'll affect both
    earlier uses and later uses.

  <% example do %>
    $variable: value 1;
    .rule-1 {
      value: $variable;
    }

    $variable: value 2;
    .rule-2 {
      value: $variable;
    }
    ===
    $variable: value 1
    .rule-1
      value: $variable


    $variable: value 2
    .rule-2
      value: $variable
  <% end %>
<% end %>

<% fun_fact do %>
  Sass variables, like all Sass identifiers, treat hyphens and underscores as
  identical. This means that `$font-size` and `$font_size` both refer to the
  same variable. 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.
<% end %>

## Default Values

Normally when you assign a value to a variable, if that variable already had a
value, its old value is overwritten. But if you're writing a Sass library, you
might want to allow your users to configure your library's variables before you
use them to generate CSS.

To make this possible, Sass provides the `!default` flag. This assigns a value
to a variable *only if* that variable isn't defined or its value is [`null`][].
Otherwise, the existing value will be used.

[`null`]: values/null

### Configuring Modules

<%= partial 'snippets/module-system-status' %>

Variables defined with `!default` can be configured when loading a module with
the [`@use` rule][]. Sass libraries often use `!default` variables to allow
their users to configure the library's CSS.

[`@use` rule]: at-rules/use

To load a module with configuration, write `@use <url> with (<variable>:
<value>, <variable>: <value>)`. The configured values will override the
variables' default values. Only variables written at the top level of the
stylesheet with a `!default` flag can be configured.

<%= partial '../code-snippets/example-use-with' %>

## Built-in Variables

Variables that are defined by a [built-in module] cannot be modified.

[built-in module]: modules

<% example(autogen_css: false) do %>
  @use "sass:math" as math;

  // This assignment will fail.
  math.$pi: 0;
  ===
  @use "sass:math" as math

  // This assignment will fail.
  math.$pi: 0
<% end %>

## Scope

Variables declared at the top level of a stylesheet are *global*. This means
that they can be accessed anywhere in their module after they've been declared.
But that's not true for all variables. Those declared in blocks (curly braces in
SCSS or indented code in Sass) are usually *local*, and can only be accessed
within the block they were declared.

<% example do %>
  $global-variable: global value;

  .content {
    $local-variable: local value;
    global: $global-variable;
    local: $local-variable;
  }

  .sidebar {
    global: $global-variable;

    // This would fail, because $local-variable isn't in scope:
    // local: $local-variable;
  }
  ===
  $global-variable: global value

  .content
    $local-variable: local value
    global: $global-variable
    local: $local-variable


  .sidebar
    global: $global-variable

    // This would fail, because $local-variable isn't in scope:
    // local: $local-variable
<% end %>

### Shadowing

Local variables can even be declared with the same name as a global variable. If
this happens, there are actually two different variables with the same name: one
local and one global. This helps ensure that an author writing a local variable
doesn't accidentally change the value of a global variable they aren't even
aware of.

<% example do %>
  $variable: global value;

  .content {
    $variable: local value;
    value: $variable;
  }

  .sidebar {
    value: $variable;
  }
  ===
  $variable: global value

  .content
    $variable: local value
    value: $variable


  .sidebar
    value: $variable
<% end %>

If you need to set a global variable's value from within a local scope (such as
in a mixin), you can use the `!global` flag. A variable declaration flagged as
`!global` will *always* assign to the global scope.

<% example do %>
  $variable: first global value;

  .content {
    $variable: second global value !global;
    value: $variable;
  }

  .sidebar {
    value: $variable;
  }
  ===
  $variable: first global value

  .content
    $variable: second global value !global
    value: $variable


  .sidebar
    value: $variable
<% end %>

<% heads_up do %>
  <% impl_status dart: '2.0.0', libsass: false, ruby: false do %>
    Older Sass versions allowed `!global` to be used for a variable that
    doesn't exist yet. This behavior was deprecated to make sure each stylesheet
    declares the same variables no matter how it's executed.
  <% end %>

  The `!global` flag may only be used to set a variable that has already been
  declared at the top level of a file. It *may not* be used to declare a new
  variable.

<% end %>

### Flow Control Scope

Variables declared in [flow control rules][] have special scoping rules: they
don't shadow variables at the same level as the flow control rule. Instead, they
just assign to those variables. This makes it much easier to conditionally
assign a value to a variable, or build up a value as part of a loop.

[flow control rules]: at-rules/control

<% example do %>
  $dark-theme: true !default;
  $primary-color: #f8bbd0 !default;
  $accent-color: #6a1b9a !default;

  @if $dark-theme {
    $primary-color: darken($primary-color, 60%);
    $accent-color: lighten($accent-color, 60%);
  }

  .button {
    background-color: $primary-color;
    border: 1px solid $accent-color;
    border-radius: 3px;
  }
  ===
  $dark-theme: true !default
  $primary-color: #f8bbd0 !default
  $accent-color: #6a1b9a !default

  @if $dark-theme
    $primary-color: darken($primary-color, 60%)
    $accent-color: lighten($accent-color, 60%)


  .button
    background-color: $primary-color
    border: 1px solid $accent-color
    border-radius: 3px
<% end %>

<% heads_up do %>
  Variables in flow control scope can assign to existing variables in the outer
  scope, but they can't declare new variables there. Make sure the variable is
  already declared before you assign to it, even if you need to declare it as
  `null`.
<% end %>

## Advanced Variable Functions

The Sass core library provides a couple advanced functions for working with
variables. The [`meta.variable-exists()` function][] returns whether a variable
with the given name exists in the current scope, and the
[`meta.global-variable-exists()` function][] does the same but only for the
global scope.

[`meta.variable-exists()` function]: modules/meta#variable-exists
[`meta.global-variable-exists()` function]: modules/meta#global-variable-exists

<% heads_up do %>
  Users occasionally want to use interpolation to define a variable name based
  on another variable. Sass doesn't allow this, because it makes it much harder
  to tell at a glance which variables are defined where. What you *can* do,
  though, is define a [map][] from names to values that you can then access
  using variables.

  [map]: values/maps

  <% example do %>
    @use "sass:map";

    $theme-colors: (
      "success": #28a745,
      "info": #17a2b8,
      "warning": #ffc107,
    );

    .alert {
      // Instead of $theme-color-#{warning}
      background-color: map.get($theme-colors, "warning");
    }
    ===
    @use "sass:map"

    $theme-colors: ("success": #28a745, "info": #17a2b8, "warning": #ffc107)

    .alert
      // Instead of $theme-color-#{warning}
      background-color: map.get($theme-colors, "warning")
    ===
    .alert {
      background-color: #ffc107;
    }
  <% end %>
<% end %>