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Closes #361
307 lines
8.6 KiB
Plaintext
307 lines
8.6 KiB
Plaintext
---
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title: Variables
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table_of_contents: true
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introduction: >
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Sass variables are simple: you assign a value to a name that begins with `$`,
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and then you can refer to that name instead of the value itself. But despite
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their simplicity, they're one of the most useful tools Sass brings to the
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table. Variables make it possible to reduce repetition, do complex math,
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configure libraries, and much more.
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---
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A variable declaration looks a lot like a [property declaration][]: it's written
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`<variable>: <expression>`. Unlike a property, which can only be declared in a
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style rule or at-rule, variables can be declared anywhere you want. To use a
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variable, just include it in a value.
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[property declaration]: style-rules/declarations
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<% example do %>
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$base-color: #c6538c;
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$border-dark: rgba($base-color, 0.88);
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.alert {
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border: 1px solid $border-dark;
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}
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===
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$base-color: #c6538c
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$border-dark: rgba($base-color, 0.88)
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.alert
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border: 1px solid $border-dark
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<% end %>
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<% heads_up do %>
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CSS has [variables of its own][], which are totally different than Sass
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variables. Know the differences!
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[variables of its own]: style-rules/declarations#custom-properties
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* Sass variables are all compiled away by Sass. CSS variables are included in
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the CSS output.
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* CSS variables can have different values for different elements, but Sass
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variables only have one value at a time.
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* Sass variables are *imperative*, which means if you use a variable and then
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change its value, the earlier use will stay the same. CSS variables are
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*declarative*, which means if you change the value, it'll affect both
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earlier uses and later uses.
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<% example do %>
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$variable: value 1;
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.rule-1 {
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value: $variable;
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}
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$variable: value 2;
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.rule-2 {
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value: $variable;
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}
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===
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$variable: value 1
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.rule-1
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value: $variable
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$variable: value 2
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.rule-2
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value: $variable
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<% end %>
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<% end %>
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<% fun_fact do %>
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Sass variables, like all Sass identifiers, treat hyphens and underscores as
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identical. This means that `$font-size` and `$font_size` both refer to the
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same variable. This is a historical holdover from the very early days of Sass,
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when it *only* allowed underscores in identifier names. Once Sass added
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support for hyphens to match CSS's syntax, the two were made equivalent to
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make migration easier.
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<% end %>
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## Default Values
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Normally when you assign a value to a variable, if that variable already had a
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value, its old value is overwritten. But if you're writing a Sass library, you
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might want to allow your users to configure your library's variables before you
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use them to generate CSS.
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To make this possible, Sass provides the `!default` flag. This assigns a value
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to a variable *only if* that variable isn't defined or its value is [`null`][].
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Otherwise, the existing value will be used.
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[`null`]: values/null
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### Configuring Modules
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<%= partial 'snippets/module-system-status' %>
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Variables defined with `!default` can be configured when loading a module with
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the [`@use` rule][]. Sass libraries often use `!default` variables to allow
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their users to configure the library's CSS.
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[`@use` rule]: at-rules/use
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To load a module with configuration, write `@use <url> with (<variable>:
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<value>, <variable>: <value>)`. The configured values will override the
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variables' default values. Only variables written at the top level of the
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stylesheet with a `!default` flag can be configured.
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<%= partial '../code-snippets/example-use-with' %>
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## Scope
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Variables declared at the top level of a stylesheet are *global*. This means
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that they can be accessed anywhere in their module after they've been declared.
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But that's not true for all variables. Those declared in blocks (curly braces in
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SCSS or indented code in Sass) are usually *local*, and can only be accessed
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within the block they were declared.
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<% example do %>
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$global-variable: global value;
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.content {
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$local-variable: local value;
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global: $global-variable;
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local: $local-variable;
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}
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.sidebar {
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global: $global-variable;
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// This would fail, because $local-variable isn't in scope:
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// local: $local-variable;
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}
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===
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$global-variable: global value
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.content
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$local-variable: local value
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global: $global-variable
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local: $local-variable
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.sidebar
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global: $global-variable
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// This would fail, because $local-variable isn't in scope:
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// local: $local-variable
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<% end %>
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### Shadowing
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Local variables can even be declared with the same name as a global variable. If
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this happens, there are actually two different variables with the same name: one
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local and one global. This helps ensure that an author writing a local variable
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doesn't accidentally change the value of a global variable they aren't even
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aware of.
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<% example do %>
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$variable: global value;
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.content {
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$variable: local value;
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value: $variable;
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}
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.sidebar {
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value: $variable;
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}
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===
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$variable: global value
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.content
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$variable: local value
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value: $variable
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.sidebar
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value: $variable
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<% end %>
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If you need to set a global variable's value from within a local scope (such as
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in a mixin), you can use the `!global` flag. A variable declaration flagged as
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`!global` will *always* assign to the global scope.
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<% example do %>
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$variable: first global value;
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.content {
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$variable: second global value !global;
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value: $variable;
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}
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.sidebar {
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value: $variable;
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}
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===
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$variable: first global value
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.content
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$variable: second global value !global
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value: $variable
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.sidebar
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value: $variable
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<% end %>
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<% heads_up do %>
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<% impl_status dart: '2.0.0', libsass: false, ruby: false do %>
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Older Sass versions allowed `!global` to be used for a variable that
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doesn't exist yet. This behavior was deprecated to make sure each stylesheet
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declares the same variables no matter how it's executed.
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<% end %>
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The `!global` flag may only be used to set a variable that has already been
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declared at the top level of a file. It *may not* be used to declare a new
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variable.
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<% end %>
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### Flow Control Scope
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Variables declared in [flow control rules][] have special scoping rules: they
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don't shadow variables at the same level as the flow control rule. Instead, they
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just assign to those variables. This makes it much easier to conditionally
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assign a value to a variable, or build up a value as part of a loop.
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[flow control rules]: at-rules/control
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<% example do %>
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$dark-theme: true !default;
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$primary-color: #f8bbd0 !default;
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$accent-color: #6a1b9a !default;
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@if $dark-theme {
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$primary-color: darken($primary-color, 60%);
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$accent-color: lighten($accent-color, 60%);
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}
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.button {
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background-color: $primary-color;
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border: 1px solid $accent-color;
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border-radius: 3px;
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}
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===
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$dark-theme: true !default
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$primary-color: #f8bbd0 !default
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$accent-color: #6a1b9a !default
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@if $dark-theme
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$primary-color: darken($primary-color, 60%)
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$accent-color: lighten($accent-color, 60%)
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.button
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background-color: $primary-color
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border: 1px solid $accent-color
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border-radius: 3px
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<% end %>
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<% heads_up do %>
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Variables in flow control scope can assign to existing variables in the outer
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scope, but they can't declare new variables there. Make sure the variable is
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already declared before you assign to it, even if you need to declare it as
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`null`.
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<% end %>
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## Advanced Variable Functions
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The Sass core library provides a couple advanced functions for working with
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variables. The [`variable-exists()` function][] returns whether a variable with
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the given name exists in the current scope, and the [`global-variable-exists()`
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function][] does the same but only for the global scope.
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[`variable-exists()` function]: functions/meta#variable-exists
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[`global-variable-exists()` function]: functions/meta#global-variable-exists
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<% heads_up do %>
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Users occasionally want to use interpolation to define a variable name based
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on another variable. Sass doesn't allow this, because it makes it much harder
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to tell at a glance which variables are defined where. What you *can* do,
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though, is define a [map][] from names to values that you can then access
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using variables.
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[map]: values/maps
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<% example do %>
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$theme-colors: (
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"success": #28a745,
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"info": #17a2b8,
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"warning": #ffc107,
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);
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.alert {
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// Instead of $theme-color-#{warning}
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background-color: map-get($theme-colors, "warning");
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}
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===
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$theme-colors: ("success": #28a745, "info": #17a2b8, "warning": #ffc107)
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.alert
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// Instead of $theme-color-#{warning}
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background-color: map-get($theme-colors, "warning")
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<% end %>
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<% end %>
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