How to make (and use) a custom SASS function

The SASS (Syntactically Awesome StyleSheets) language is pretty neat. For my first use of SASS, information pills I realized that one of the requirements for our system wasn’t easily supported by the language… but SASS is written in Ruby which is really easy to extend. The docs even mentioned the ability to create custom functions. However, more about I couldn’t find any docs on how to actually create and use a custom SASS function, prosthesis so I figured I’d give a quick tutorial here of what I learned. This method is probably obvious to hardcore Ruby users, but I’d never used Ruby before. Turns out it’s pretty quick to learn. If you want to do the Matrix thing and pump the whole language into your brain like Neo in a ghetto dentist-chair, check out this Ruby crash course.

NOTE: This tutorial is targeted primarily at people using SASS via the command line (for PHP, Java, etc.), not as a Rails module.

My example: getting values from the sass command-line

I’m using SASS in a PHP environment (rather than Rails) and due to unique requirements, I need to be able to configure certain values in .scss at ‘compile’ time (referring to when the .scss is being compiled into .css).

One simple trick would be to simply write out a .scss file containing the key-value pairs. Unfortunately, the system I need to use SASS for already has tens of millions of page-requests per day so disk-writes would be a huge bottleneck (because disk i/o – even on solid state disks – is slow compared to many other methods). Custom SASS functions provided the perfect opportunity to completely skip this step. And yes: pre-generating all of the CSS files at code-deployment is out of the question because the number of possible stylesheets we need to support is intractably large.

The custom function

To create your custom SASS function, make a ruby file. We’ll call it sass_function.rb in this example. In the file, you need to define your function and then insert your module into SASS. Behold!

require 'sass'

module WikiaFunctions
def get_command_line_param(paramName, defaultResult="")
assert_type paramName, :String
retVal = defaultResult.to_s

# Look through the args given to SASS command-line
ARGV.each do |arg|
# Check if arg is a key=value pair
if arg =~ /.=./
pair = arg.split(/=/)
if(pair[0] == paramName.value)
# Found correct param name
retVal = pair[1] || defaultResult
Sass::Script::Parser.parse(retVal, 0, 0)

module Sass::Script::Functions
include WikiaFunctions

The particular SASS function in this example takes in name/value pairs defined on the sass command line and returns them if they’re there (or an optional default otherwise).

Calling SASS

Since this tutorial assumes that you’re using sass from the command-line, you’ll have to tweak the command a little bit to tell ruby to use your new module. Here is a simple example:
sass unicorn.scss unicorn.css -r sass_function.rb
That doesn’t make use of the awesomeness of our new, command-line parsing function though! So here is an example that WOULD make use of it:
sass unicorn.scss unicorn.css logoColor=#6495ED -r sass_function.rb

So now we have a function capable of reading the command-line and a command-line with some useful information in it. Now all that’s needed is some SASS code (.scss) to make use of all of that.

In this example, we’ll set the “logo” element to have a background-color that we get from the command-line (and default to white if no matching value is passed in on the command-line). Remember: this would go in SASS code such as unicorn.scss

$logoBackgroundColor: get_command_line_param("logoColor", "white");

background-color: $logoBackgroundColor;

Now we have all of the pieces:

  1. The custom SASS function (called get_command_line_param()) in sass_function.rb
  2. The code in unicorn.scss to use our function to set the style by command-line info.
  3. The command-line needed to include our custom code and to set the logoColor value.

So if we run
sass unicorn.scss unicorn.css logoColor=#6495ED -r sass_function.rb
we will have a unicorn.css which contains something like:

background-color: #6495ED;

Just what we were going for! If you try this out, let me know in the comments if it worked for you or if you have any questions.

Best of luck!

Special thanks to Nathan Weizenbaum for pointing me down the right path with this stuff. Updated on 20100729 to change the return-value of the function based on the helpful comments below. Thanks!