sass-site/source/guide.md

---
layout: has_navigation
title: Sass Basics
introduction: >
  Before you can use Sass, you need to set it up on your project. If you want to
  just browse here, go ahead, but we recommend you go install Sass first. [Go
  here](/install) if you want to learn how to get everything set up.
navigation: |
  <h3>Topics</h3>

  <nav class="sl-c-list-navigation-wrapper">

  - [Preprocessing](#preprocessing)
  - [Variables](#variables)
  - [Nesting](#nesting)
  - [Partials](#partials)
  - [Modules](#modules)
  - [Mixins](#mixins)
  - [Inheritance](#inheritance)
  - [Operators](#operators)

  </nav>
---

<section id="preprocessing">
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<span id="topic-1"></span>

## Preprocessing

CSS on its own can be fun, but stylesheets are getting larger, more complex, and
harder to maintain. This is where a preprocessor can help. Sass has features
that don't exist in CSS yet like nesting, mixins, inheritance, and other nifty
goodies that help you write robust, maintainable CSS.

Once you start tinkering with Sass, it will take your preprocessed Sass file and
save it as a normal CSS file that you can use in your website.

The most direct way to make this happen is in your terminal. Once Sass is
installed, you can compile your Sass to CSS using the `sass` command. You'll
need to tell Sass which file to build from, and where to output CSS to. For
example, running `sass input.scss output.css` from your terminal would take a
single Sass file, `input.scss`, and compile that file to `output.css`.

You can also watch individual files or directories with the `--watch` flag. The
watch flag tells Sass to watch your source files for changes, and re-compile CSS
each time you save your Sass. If you wanted to watch (instead of manually build)
your `input.scss` file, you'd just add the watch flag to your command, like so:

```shellsession
sass --watch input.scss output.css
```

You can watch and output to directories by using folder paths as your input and
output, and separating them with a colon. In this example:

```shellsession
sass --watch app/sass:public/stylesheets
```

Sass would watch all files in the `app/sass` folder for changes, and compile CSS
to the `public/stylesheets` folder.

{% funFact %}
  Sass has two syntaxes! The SCSS syntax (`.scss`) is used most commonly. It's
  a superset of CSS, which means all valid CSS is also valid SCSS. The
  indented syntax (`.sass`) is more unusual: it uses indentation rather than
  curly braces to nest statements, and newlines instead of semicolons to
  separate them. All our examples are available in both syntaxes.
{% endfunFact %}

</section>

<hr>

<section id="variables">
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<span id="topic-2"></span>

## Variables

Think of variables as a way to store information that you want to reuse
throughout your stylesheet. You can store things like colors, font stacks, or
any CSS value you think you'll want to reuse. Sass uses the `$` symbol to make
something a variable. Here's an example:

{% codeExample 'variables' %}
  $font-stack: Helvetica, sans-serif;
  $primary-color: #333;

  body {
    font: 100% $font-stack;
    color: $primary-color;
  }
  ===
  $font-stack: Helvetica, sans-serif
  $primary-color: #333

  body
    font: 100% $font-stack
    color: $primary-color
{% endcodeExample %}

When the Sass is processed, it takes the variables we define for the
`$font-stack` and `$primary-color` and outputs normal CSS with our variable
values placed in the CSS. This can be extremely powerful when working with brand
colors and keeping them consistent throughout the site.

</section>

<hr>

<section id="nesting">
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<span id="topic-3"></span>

## Nesting

When writing HTML you've probably noticed that it has a clear nested and visual
hierarchy. CSS, on the other hand, doesn't.

Sass will let you nest your CSS selectors in a way that follows the same visual
hierarchy of your HTML. Be aware that overly nested rules will result in
over-qualified CSS that could prove hard to maintain and is generally considered
bad practice.

With that in mind, here's an example of some typical styles for a site's
navigation:

{% codeExample 'nesting' %}
  nav {
    ul {
      margin: 0;
      padding: 0;
      list-style: none;
    }

    li { display: inline-block; }

    a {
      display: block;
      padding: 6px 12px;
      text-decoration: none;
    }
  }
  ===
  nav
    ul
      margin: 0
      padding: 0
      list-style: none

    li
      display: inline-block

    a
      display: block
      padding: 6px 12px
      text-decoration: none
{% endcodeExample %}

You'll notice that the `ul`, `li`, and `a` selectors are nested inside the `nav`
selector. This is a great way to organize your CSS and make it more readable.

</section>

<hr>

<section id="partials">
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<span id="topic-4"></span>

## Partials

You can create partial Sass files that contain little snippets of CSS that you
can include in other Sass files. This is a great way to modularize your CSS and
help keep things easier to maintain. A partial is a Sass file named with a
leading underscore. You might name it something like `_partial.scss`. The
underscore lets Sass know that the file is only a partial file and that it
should not be generated into a CSS file. Sass partials are used with the `@use`
rule.

</section>

<hr>

<section id="modules">
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<span id="topic-5"></span>

## Modules

{% render 'doc_snippets/module-system-status' %}

You don't have to write all your Sass in a single file. You can split it up
however you want with the `@use` rule. This rule loads another Sass file as a
*module*, which means you can refer to its variables, [mixins][], and
[functions][] in your Sass file with a namespace based on the filename. Using a
file will also include the CSS it generates in your compiled output!

[mixins]: #mixins
[functions]: /documentation/at-rules/function

{% codeExample 'modules' %}
  // _base.scss
  $font-stack: Helvetica, sans-serif;
  $primary-color: #333;

  body {
    font: 100% $font-stack;
    color: $primary-color;
  }
  ---
  // styles.scss
  @use 'base';

  .inverse {
    background-color: base.$primary-color;
    color: white;
  }
  ===
  // _base.sass
  $font-stack: Helvetica, sans-serif
  $primary-color: #333

  body
    font: 100% $font-stack
    color: $primary-color
  ---
  // styles.sass
  @use 'base'

  .inverse
    background-color: base.$primary-color
    color: white
  ===
  body {
    font: 100% Helvetica, sans-serif;
    color: #333;
  }

  .inverse {
    background-color: #333;
    color: white;
  }
{% endcodeExample %}

Notice we're using `@use 'base';` in the `styles.scss` file. When you use a file
you don't need to include the file extension. Sass is smart and will figure it
out for you.

</section>

<hr>

<section id="mixins">
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<span id="topic-6"></span>

## Mixins

Some things in CSS are a bit tedious to write, especially with CSS3 and the many
vendor prefixes that exist. A mixin lets you make groups of CSS declarations
that you want to reuse throughout your site. It helps keep your Sass very DRY.
You can even pass in values to make your mixin more flexible. Here's an example
for `theme`.

{% codeExample 'mixins' %}
  @mixin theme($theme: DarkGray) {
    background: $theme;
    box-shadow: 0 0 1px rgba($theme, .25);
    color: #fff;
  }

  .info {
    @include theme;
  }
  .alert {
    @include theme($theme: DarkRed);
  }
  .success {
    @include theme($theme: DarkGreen);
  }
  ===
  @mixin theme($theme: DarkGray)
    background: $theme
    box-shadow: 0 0 1px rgba($theme, .25)
    color: #fff

  .info
    @include theme

  .alert
    @include theme($theme: DarkRed)

  .success
    @include theme($theme: DarkGreen)
{% endcodeExample %}

To create a mixin you use the `@mixin` directive and give it a name. We've named
our mixin `theme`. We're also using the variable `$theme` inside the parentheses
so we can pass in a `theme` of whatever we want. After you create your mixin,
you can then use it as a CSS declaration starting with `@include` followed by
the name of the mixin.

</section>

<hr>

<section id="inheritance">
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<span id="topic-7"></span>

## Extend/Inheritance

Using `@extend` lets you share a set of CSS properties from one selector to
another. In our example we're going to create a simple series of messaging for
errors, warnings and successes using another feature which goes hand in hand
with extend, placeholder classes. A placeholder class is a special type of class
that only prints when it is extended, and can help keep your compiled CSS neat
and clean.

{% codeExample 'extend-inheritance' %}
  /* This CSS will print because %message-shared is extended. */
  %message-shared {
    border: 1px solid #ccc;
    padding: 10px;
    color: #333;
  }

  // This CSS won't print because %equal-heights is never extended.
  %equal-heights {
    display: flex;
    flex-wrap: wrap;
  }

  .message {
    @extend %message-shared;
  }

  .success {
    @extend %message-shared;
    border-color: green;
  }

  .error {
    @extend %message-shared;
    border-color: red;
  }

  .warning {
    @extend %message-shared;
    border-color: yellow;
  }
  ===
  /* This CSS will print because %message-shared is extended. */
  %message-shared
    border: 1px solid #ccc
    padding: 10px
    color: #333

  // This CSS won't print because %equal-heights is never extended.
  %equal-heights
    display: flex
    flex-wrap: wrap

  .message
    @extend %message-shared

  .success
    @extend %message-shared
    border-color: green

  .error
    @extend %message-shared
    border-color: red

  .warning
    @extend %message-shared
    border-color: yellow
{% endcodeExample %}

What the above code does is tells `.message`, `.success`, `.error`, and
`.warning` to behave just like `%message-shared`. That means anywhere that
`%message-shared` shows up, `.message`, `.success`, `.error`, & `.warning` will
too. The magic happens in the generated CSS, where each of these classes will
get the same CSS properties as `%message-shared`. This helps you avoid having to
write multiple class names on HTML elements.

You can extend most simple CSS selectors in addition to placeholder classes in
Sass, but using placeholders is the easiest way to make sure you aren't
extending a class that's nested elsewhere in your styles, which can result in
unintended selectors in your CSS.

Note that the CSS in `%equal-heights` isn't generated, because `%equal-heights`
is never extended.

</section>

<hr>

<section id="operators">
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<span id="topic-8"></span>

## Operators

Doing math in your CSS is very helpful. Sass has a handful of standard math
operators like `+`, `-`, `*`, `math.div()`, and `%`. In our example we're going
to do some simple math to calculate widths for an `article` and `aside`.

{% codeExample 'operators' %}
  @use "sass:math";

  .container {
    display: flex;
  }

  article[role="main"] {
    width: math.div(600px, 960px) * 100%;
  }

  aside[role="complementary"] {
    width: math.div(300px, 960px) * 100%;
    margin-left: auto;
  }
  ===
  @use "sass:math"

  .container
    display: flex

  article[role="main"]
    width: math.div(600px, 960px) * 100%

  aside[role="complementary"]
    width: math.div(300px, 960px) * 100%
    margin-left: auto
{% endcodeExample %}

We've created a very simple fluid grid, based on 960px. Operations in Sass let
us do something like take pixel values and convert them to percentages without
much hassle.

</section>