CSS Custom Properties (vars) with SASS/SCSS, a practical architecture strategy

← ← ←   5/2/2022, 4:55:20 PM | Posted by: Felippe Regazio

---

If you dont know about CSS Custom Properties yet, you really should learn about. I particularly prefer to use CSS custom properties instead of SASS variables always when possible, mostly because of its reactivity. For this post I'll assume that you're already comfortable with CSS Custom Properties and SASS Vars (with SCSS Syntax).

Sometimes the use of CSS Custom Properties is not a case of taste, but a necessity. It just fits perfectly to build themes, dynamic components, configurable pages etc. But as our page grows, also grows the maintenance complexity of the code base and the risk of create a modules labyrinth.

A common approach when dealing with SASS/SCSS variables is to create a _vars file which will hold most of our app variables, and in the case of CSS Custom Properties, also add a prefix to avoid conflicts. The idea here is the use of CSS Custom Props, but i think it fits to SASS vars as well. Here is a common pattern:

  
  $prefix: pf;

  :root {
    --#{$prefix}-primary-color: #000000; 
    // this will compile to: --pf-primary-color: #000000;
  }    
  

Now we use it along the stylesheets:

  
  @import './config/_vars';

  .btn-example {
    background-color: var(--#{$prefix}-primary-color);
  }    
  

Nice, you are separating concerns, using a prefix to avoid conflicts, maybe adding the _vars on a main file to keep your variables always available along your stylesheets. Nice, but there are some serious problems with this approach:

1. When using a prefixed CSS Custom Prop with SCSS you have to add the prefix by typing --#{$prefix}- and thats annoying. If you are making an attribution to a property, its even worse color: var(--#{$prefix}-color);.
2. You'll end up copying and pasting code a lot, and thats increments the error probability on your code base. Or someone will, eventually, add a hard coded prefix.
3. A notation like --#{$prefix}... for a variable is ugly and also a problem for new devs. They can just forget about the prefix, or they will probably dislike the syntax to just write a simple var, etc. You can think "ok, they will dislike, who cares...". Well, you should care, but in a practical manner of speaking, if your team don't like what they are doing, they will end up trying to cheat on that code just to get rid of the task as soon as possible.
4. So you decided to separate the concerns. You have the impression that you are separating concerns because you splitted the variables from your styles, and elements by modules, but you dont. All your variables are on the same file, so a simple button style is consuming variables from a file related to the entire application.
5. Then you say, "ok, ill create a variable collection files for each concern, tadá", and you end up importing all that files (which is the same), or end up having to open many files on your editor to edit a single property: you open your button stylesheet, see that the button is imported by a main file which imports some vars - or a configuration file - go to that configuration file and see that configuration file imports a _btn_vars and that file uses some helpers imported from... then you go to get a coffee. It will be a long day.
6. If you try to split your stylesheets by elements, when trying to separate an element as a module - a _buttons.scss for example - as it uses some variables you will need to import a _vars file with the entire application variables just to use some vars in a button.

So, what can we do?

I'm not saying that this approach don't fits at all. But for scalable and large applications the above architecture can become a problem very quickly, or in the best scenario it will unnecessarily increase your css files size due code repetition. So, here is my 2 cents on that subject:

First of all, define getters and setters for your CSS Custom Properties. Then use your getters and setters to deal with variables along the app providing a pattern and reliability, now you can really separate the concerns in a solid architecture. Lets see how to do it:

Declaring Custom Properties

Lets start adding a @mixin to declare a set of CSS Custom Properties. This mixin will be used to prefix our variables and add them on the code. It will also make our app really scalable and easy to maintain. If you need to modify how variables work on your app you dont need to open all the files to make changes then keep checking if something has broken, you just need to tune that mixin. Its also friendly for new devs by avoiding errors when declaring properties (syntax errors, forgetting the prefix, etc):

    
    /**
      * Use this mixin to declare a set of CSS Custom Properties.
      * The variables in $css_variables will be properly prefixed.
      * The use of this mixin is encoraged to keep a good scalability.
      *
      * Usage:
      *
      * @include cssvars((
      *  base-font-size: 65.5%,
      *  font-family: #{"HelveticaNeue", "Helvetica Neue", Helvetica, Arial, sans-serif},
      * 
      *  primary-color: #33b5e5,
      *  secondary-color: #ff500a,
      * ));
      *
      * Will result in
      *
      * root {
      *    --prefix-var-name: value;
      *    --prefix-var-name: value;
      *    --prefix-var-name: value;
      * }
      *
      */
     @mixin cssvars($css_variables, $prefix: pf) {
         :root {
             @each $name, $value in $css_variables {
                 --#{$prefix}-#{$name}: #{$value};
             }
         }
     }     
    
  

Now instead of doing and maintain this:

  
  :root {
    --#{$prefix}-base-font-size: 65.5%;
    --#{$prefix}-font-family: "HelveticaNeue", "Helvetica Neue", Helvetica, Arial, sans-serif;
  
    --#{$prefix}-primary-color: #33b5e5;
    --#{$prefix}-secondary-color: #ff500a;
  }    
  

You will do and maintain this:

  
  @include cssvars((
    base-font-size: 65.5%,
    font-family: #{"HelveticaNeue", "Helvetica Neue", Helvetica, Arial, sans-serif},
  
    primary-color: #33b5e5,
    secondary-color: #ff500a,
  ));   
  

The result will be the same:

  
  :root {
    --pf-base-font-size: 65.5%;
    --pf-font-family: "HelveticaNeue", "Helvetica Neue", Helvetica, Arial, sans-serif;
    --pf-primary-color: #33b5e5;
    --pf-secondary-color: #ff500a;
  }    
  

You can also override the prefix passing another one as the second argument. Note that for all the functions and mixins showed here, the $prefix will appear as param with a default value. I prefer to use it that way, but you can put it out and use a single $prefix var if you prefer.

Maybe you can add a new argument to override the ":root" selector creating scoped custom properties declarations, i dont know... You can tweak this mixin to fit your style and needings, the idea here is to have a setter for your Custom Properties that will allow you to have a pattern and single source of truth when dealing with your variables, leading to a solid application growth.

Getting a Custom property

Lets create a getter to retrieve our CSS Custom Properties avoiding weird syntax and dense code for simple things:

  
  /**
    * Retrieve a css variable value with prefix
    *
    * Usage
    *
    * .selector {
    *   color: cssva(primary-color);
    * }
    *
    * Will result in
    *
    * .selector {
    *    color: var(--prefix-primary-color);
    * }
    */
   @function cssvar($name, $prefix: pm) {
       @return var(--#{$prefix}-#{$name});
   }
  

Now, instead of this:

  
  :root {
    --#{$prefix}-button-height: 40px;
    --#{$prefix}-button-color: #ffffff;
    --#{$prefix}-button-background: #000000;
  
    .button-primary {
      height: var(--#{$prefix}-button-height);
      line-height: var(--#{$prefix}-button-height);
      color: var(--#{$prefix}-button-color);
      background-color: var(--#{$prefix}-button-background);
    }
  }    
  

We get this:

  
  @include cssvars((
    button-height: 40px,
    button-color: #ffffff,
    button-background: #000000,
  ));
  
  .btn-primary {
    height: cssvar(button-height);
    line-height: cssvar(button-height);
    color: cssvar(button-color);
    background-color: cssvar(button-background);
  }    
  

Which will result in this:

  
  :root {
    --pm-button-height: 40px;
    --pm-button-color: #ffffff;
    --pm-button-background: #000000;
  }
  
  .btn-primary {
    height: var(--pm-button-height);
    line-height: var(--pm-button-height);
    color: var(--pm-button-color);
    background-color: var(--pm-button-height);
  }    
  

The syntax, readability and maintainment where improved. Also be aware that when we talk about prefix here, we are talking about prefixes for variable names like --prefix-color:blue, not css native properties like -moz-*.

Updating a Custom Property

Lets say we want to add a big button now. Simple, we just override the value of our button-height var to a bigger one in a class. So, we will need a mixin that updates (overrides) a custom property value. To do it in a clean way, add this mixin:

  
  @mixin cssvar ($name, $value: '', $prefix: pm) {
    --#{$prefix}-#{$name}: #{$value};
  }  
  

Now, instead of this:

  
  @include cssvars((...));

  .btn-primary {
    height: cssvar(button-height);
    line-height: cssvar(button-height);
    color: cssvar(button-color);
    background-color: cssvar(button-background);
    &--big {
      // ** LOOK HERE **
      --#{$prefix}-button-height: 56px;
    }
  }    
  

We get this:

  
  @include cssvars((...));

  .btn-primary {
    height: cssvar(button-height);
    line-height: cssvar(button-height);
    color: cssvar(button-color);
    background-color: cssvar(button-background);
    &--big {
      // ** LOOK HERE **
      @include cssvar(button-height, 56px);
    }
  }    
  

With the following result:

  
  :root { ...vars }

  .btn-primary {
    height: var(--pm-button-height);
    line-height: var(--pm-button-height);
    color: var(--pm-button-color);
    background-color: var(--pm-button-height);
  }
  
  .btn-primary--big {
    --pm-button-height: 56px;
  }    
  

Now we have a powerful toolset :)

An organization proposal

Now that we have a cool set of helpers, lets use it! Here is my tip:

1. Create a _helpers file with the function and mixins showed here, allowing you to deal with the CSS Custom Properties easily.
2. Create a _config file to hold you application global custom properties, the ones that concerns to the entire application, like primary and secondary colors, font-family, base-font-size, container-width, etc.
3. Now Create single stylesheets by concern, and keep them as autonomous universes. For example, to buttons you must create a _buttons.scss that holds all the variables, selectors, properties, styles, etc. If you have a large set of buttons, put the files in a "buttons" folder and split the variations in new files joining them in a main file on the buttons folder. Same for Forms, Typography, etc etc.
4. Call your _helpers, _config, and all the application modules (buttons, forms, typo...) in a main file. You can also create a core.css that holds only the _helpers and _config, then compile the modules stylesheets separately, then you add the core and the modules you want.

At the end you will have a folder/file tree almost like that:

  
  styles
    config
        _helpers.scss
        _config.scss
    modules
        _buttons.scss
        _form.scss
        _typography.scss
        _tables.scss
    main.scss 
  

At runtime, as we are talking about CSS Custom Properties, once declared in the :root {}, all variables will be globally available in its context. You can use them anywhere without myriads of @imports. They are just available. Of course this is sometimes a bad thing depending on your codebase size and can harm your performance - or if you dont need dynamic vars you can use SASS vars, but keeping them in its right place. What i mean is that you can easily tweak this architecture to scope your custom props.

Our main.scss would be something almost like that:

  
  @import './config/_helpers';
  @import './config/_config';
  
  @import './modules/_typography';
  @import './modules/_buttons';
  @import './modules/_form';
  @import './modules/_tables';
  
  // etc    
  

And this would be a module:

  
  @include cssvars((
      btn-primary-text-color: #ffffff,
      btn-secondary-text-color: #ffffff,
      btn-border-radius: 4px,
      btn-text-transform: uppercase,
      btn-font-size: 12px,
      btn-font-weight: 600,
      btn-height: 40px,
      btn-padding: 0 30px,
  ));
  
  .button, button,
  input[type="button"],
  input[type="submit"],
  input[type="reset"] {
          // { hard coded properties }
      height: cssvar(btn-height);
      padding: cssvar(btn-padding);
      line-height: cssvar(btn-height);
      font-size: cssvar(btn-font-size);
      font-weight: cssvar(btn-font-weight);
      letter-spacing: cssvar(btn-font-weight);
      color: cssvar(primary-color);
      border-radius: cssvar(btn-border-radius);
      border: 1px solid cssvar(primary-color);
      text-transform: cssvar(btn-text-transform);
      &.focus, &:focus,
      &.hover, &:hover {
          // { ... whatever you want }
      }
      &.button-block {
          display: block;
          width: 100%;
      }
      &.button-primary {
          color: cssvar(btn-primary-text-color);
          background-color: cssvar(primary-color);
          border-color: cssvar(primary-color);
      }
      &.button-secondary {
          color: cssvar(btn-secondary-text-color);
          background-color: cssvar(secondary-color);
          border-color: cssvar(secondary-color);
      }
      &.button--big {
          @include cssvar(btn-height, 56px);
      }
  }  
  

This is just an example. A button can be far more complex. But note that all the things the button needs is in itself. And when imported, its Custom Properties will be globally available on the context. Also look that we are using our helpers to provide a pattern and reliability.

How that solves the first 6 Problems

At the beginning of this post i talked about 6 common problems in SCSS code bases. Here is how that approach will solve them:

Now that we have a nice and clean syntax to deal with prefixed props there is no need to copy paste codes to gain speed, its easy and fast to use our helpers, change and maintain our css props is really fun and they are also prefixed and standardized.

Its easy and safe to onboard new devs on the code base. When talking about vars we basically have 3 functions to deal with variables and to explain to them. The _config file holds the app globals and the another vars are on the module itself:

  
  // setter
  @include cssvars((...));
  
  // updater
  @include cssvar(name, value);
  
  // getter fn
  prop: cssvar(name);    
  

There is a real separation of concerns. We keep any module as a single universe, declaring everything there and only providing the output. If we need to split a module, it will become a folder with a main exporter file, no need to keep fishing files on the project trying to discover where are all the peaces.

Conclusion

The function and mixins showed here are a demonstration of how to provide an architecture to a solid growth. But there is no silver bullet, you can of course change it to fit your needings, change names, arguments, its just a spark of an idea, you can adapt as you need, the idea is help application to expand without pain :)