Sass Basics

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:

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.

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:

SCSS

$font-stack:    Helvetica, sans-serif;
$primary-color: #333;


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

Sass

$font-stack:    Helvetica, sans-serif;
$primary-color: #333

body
  font: 100% $font-stack
  color: $primary-color

CSS

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

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.

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:

SCSS

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

Sass

nav
  ul
    margin: 0
    padding: 0
    list-style: none

  li
    display: inline-block

  a
    display: block
    padding: 6px 12px
    text-decoration: none

CSS

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

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.

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 @import directive.

CSS has an import option that lets you split your CSS into smaller, more maintainable portions. The only drawback is that each time you use @import in CSS it creates another HTTP request. Sass builds on top of the current CSS @import but instead of requiring an HTTP request, Sass will take the file that you want to import and combine it with the file you're importing into so you can serve a single CSS file to the web browser.

Let's say you have a couple of Sass files, _reset.scss and base.scss. We want to import _reset.scss into base.scss.

SCSS

// _reset.scss
html,
body,
ul,
ol {
  margin:  0;
  padding: 0;
}

// base.scss
@import 'reset';
body {
  font: 100% Helvetica, sans-serif;
  background-color: #efefef;
}

Sass

// _reset.sass
html,
body,
ul,
ol
  margin:  0
  padding: 0

// base.sass
@import reset
body
  font: 100% Helvetica, sans-serif
  background-color: #efefef

CSS

html,
body,
ul,
ol {
  margin:  0;
  padding: 0;
}
body {
  font: 100% Helvetica, sans-serif;
  background-color: #efefef;
}

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

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.

SCSS

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

Sass

=transform($property)
  -webkit-transform: $property
  -ms-transform: $property
  transform: $property
.box
  +transform(rotate(30deg))

CSS

.box {
  -webkit-transform: rotate(30deg);
  -ms-transform: rotate(30deg);
  transform: rotate(30deg);
}

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.

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.

SCSS

/* 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;
}

Sass

/* 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

CSS

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

.success {
  border-color: green;
}

.error {
  border-color: red;
}

.warning {
  border-color: yellow;
}                   

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.

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.

SCSS

.container {
  width: 100%;
}

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

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

Sass

.container
  width: 100%
  
article[role="main"]
  float: left
  width: 600px / 960px * 100%
  
aside[role="complementary"]
  float: right
  width: 300px / 960px * 100%

CSS

.container {
  width: 100%;
}
    
article[role="main"] {
  float: left;
  width: 62.5%;
}
    
aside[role="complementary"] {
  float: right;
  width: 31.25%;
}

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.