sass-site/source/guide.html.haml

---
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 setup.
---

- content_for :navigation do
  %h3 Topics

  %nav.sl-c-list-navigation-wrapper
    :markdown
      - [Preprocessing](#topic-1)
      - [Variables](#topic-2)
      - [Nesting](#topic-3)
      - [Partials](#topic-4)
      - [Modules](#topic-5)
      - [Mixins](#topic-6)
      - [Inheritance](#topic-7)
      - [Operators](#topic-8)
      - [Lists](#topic-9)

%section#topic-1
  :markdown
    ## 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 lets you use features that don't exist in CSS yet like variables,
    nesting, mixins, inheritance and other nifty goodies that make writing CSS
    fun again.

    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:

        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:

  = partial 'code-snippets/homepage-sass-watch'

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

    ---

%section#topic-2
  :markdown
    ## 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:

  - example do
    :plain
      $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

  :markdown
    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#topic-3
  :markdown
    ## 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:

  - example do
    :plain
      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

  :markdown
    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#topic-4
  :markdown
    ## 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 simply 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#topic-5
  %h2 Modules
  = partial 'documentation/snippets/module-system-status'

  :markdown
    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]: #topic-6
    [functions]: documentation/at-rules/function

  - example do
    :plain
      // _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;
      }

  :markdown
    Notice we're using `@use 'reset';` in the `base.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#topic-6
  :markdown
    ## 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. You can even
    pass in values to make your mixin more flexible. A good use of a mixin is
    for vendor prefixes. Here's an example for `transform`.

  - example do
    :plain
      @mixin transform($property) {
        -webkit-transform: $property;
        -ms-transform: $property;
        transform: $property;
      }
      .box { @include transform(rotate(30deg)); }
      ===
      =transform($property)
        -webkit-transform: $property
        -ms-transform: $property
        transform: $property
      .box
        +transform(rotate(30deg))

  :markdown
    To create a mixin you use the `@mixin` directive and give it a name. We've
    named our mixin `transform`. We're also using the variable `$property`
    inside the parentheses so we can pass in a transform 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#topic-7
  :markdown
    ## Extend/Inheritance

    This is one of the most useful features of Sass. Using `@extend` lets you
    share a set of CSS properties from one selector to another. It helps keep
    your Sass very DRY. 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.

  - example do
    :plain
      /* 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

  :markdown
    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#topic-8
  :markdown
    ## Operators

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

  - example do
    :plain
      .container {
        width: 100%;
      }

      article[role="main"] {
        float: left;
        width: 600px / 960px * 100%;
      }

      aside[role="complementary"] {
        float: right;
        width: 300px / 960px * 100%;
      }
      ===
      .container
        width: 100%


      article[role="main"]
        float: left
        width: 600px / 960px * 100%


      aside[role="complementary"]
        float: right
        width: 300px / 960px * 100%

  :markdown
    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.

    ---