Variables are used to store a value once which will be used multiple times throughout a Sass document.

They are mostly used for controlling things such as fonts and colors but can be used for any value of any property.

Sass uses the $ symbol to make something a variable.

$font-stack: Helvetica, sans-serif
$primary-color: #000000

body
  font-family: $font-stack
  color: $primary-color

Just as in Sass, SCSS variables are used to store a value which will be used multiple times throughout a SCSS document.

Variables are mostly used to store frequently-used property values (such as fonts and colors), but can be used for any value of any property.

SCSS uses the $ symbol to declare a variable.

$font-stack: Helvetica, sans-serif;
$primary-color: #000000;

body {
  font-family: $font-stack;
  color: $primary-color;
}

You can use !default when declaring a variable if you want to assign a new value to this variable only if it hasn't been assigned yet:

$primary-color: blue;
$primary-color: red !default; // $primary-color is still "blue"
$primary-color: green;        // And now it's green.

Variables exist within a specific scope, much like in in JavaScript.

If you declare a variable outside of a block, it can be used throughout the sheet.

$blue: dodgerblue;

.main {
    background: $blue;

    p {
        background: #ffffff;
        color: $blue;
    }
}

.header {
    color: $blue;
}

If you declare a variable within a block, it can only be used in that block.

.main {
    $blue: dodgerblue;

    background: $blue;

    p {
        background: #ffffff;
        color: $blue;
    }
}

.header {
    color: $blue; // throws a variable not defined error in SASS compiler
}

Variables declared at the sheet level (outside of a block) can also be used in other sheets if they are imported.

@at-root directive can be used to localize variables.

$color: blue;

@at-root {
  $color: red;

  .a {
    color: $color;
  }
  .b {
    color: $color;
  }
}

.c {
  color: $color;
}

is compiled to:

.a {
  color: red;
}

.b {
  color: red;
}

.c {
  color: blue;
}

Variables can be used in string interpolation. This allows you to dynamically generate selectors, properties and values. And the syntax for doing so a variable is #{$variable}.

$className: widget;
$content: 'a widget';
$prop: content;

.#{$className}-class {
  #{content}: 'This is #{$content}';
}
// Compiles to

.widget-class {
  content: "This is a widget";
}

You cannot, however use it to dynamically generate names of mixins or functions.

In SCSS variables begin with $ sign, and are set like CSS properties.

$label-color: #eee;

They are only available within nested selectors where they’re defined.

#menu {
    $basic-color: #eee;
    color: $basic-color;
}

If they’re defined outside of any nested selectors, then they can be used everywhere.

$width: 5em;

#menu {
  width: $width;
}

#sidebar {
  width: $width;
}

They can also be defined with the !global flag, in which case they’re also available everywhere.

#menu {
  $width: 5em !global;
  width: $width;
}

#sidebar {
  width: $width;
}

It is important to note that variable names can use hyphens and underscores interchangeably. For example, if you define a variable called $label-width, you can access it as $label_width, and vice versa.