postcss-custom-properties

What is postcss-custom-properties?

The postcss-custom-properties npm package is a PostCSS plugin that allows you to use CSS Custom Properties (also known as CSS variables) in environments that do not support them natively. It transforms CSS variables into static values based on your configurations, making it easier to maintain themes and styles dynamically across your project.

What are postcss-custom-properties's main functionalities?

Transform CSS Custom Properties

This feature allows the transformation of CSS custom properties into their corresponding static values. It is useful for supporting older browsers that do not understand CSS variables.

/* Input CSS */
:root {
  --main-color: red;
}

a {
  color: var(--main-color);
}

/* Output CSS */
a {
  color: red;
}

Preserve option

With the preserve option set to true, the plugin outputs both the transformed static value and the original variable. This is useful for progressive enhancement.

/* Input CSS */
:root {
  --main-color: red;
}

a {
  color: var(--main-color);
}

/* Output CSS with preserve: true */
a {
  color: red;
  color: var(--main-color);
}

Other packages similar to postcss-custom-properties

cssnext

cssnext is a PostCSS plugin that allows you to use future CSS features today. It includes support for CSS custom properties among other features. Compared to postcss-custom-properties, cssnext offers a broader range of CSS features but might be heavier due to its comprehensive nature.

postcss-preset-env

postcss-preset-env lets you convert modern CSS into something most browsers can understand, determining the polyfills you need based on your targeted browsers or runtime environments. It includes handling of CSS custom properties as part of its feature set, similar to postcss-custom-properties, but is more configurable and includes various stages of CSS specifications.

README

PostCSS Custom Properties

npm install postcss-custom-properties --save-dev

PostCSS Custom Properties lets you use Custom Properties in CSS, following the CSS Custom Properties specification.

:root {
	--color-blue-dark: rgb(0, 61, 184);
	--color-blue-light: rgb(0, 217, 255);
	--color-pink: rgb(255, 192, 211);
	--text-color: var(--color-pink);
}

.element {
	/* custom props */
	--border-color: var(--color-blue-light);

	/* props */
	border: 1px solid var(--border-color);
	color: var(--text-color);
}

.element--dark {
	--border-color: var(--color-blue-dark);
}

/* becomes */

:root {
	--color-blue-dark: rgb(0, 61, 184);
	--color-blue-light: rgb(0, 217, 255);
	--color-pink: rgb(255, 192, 211);
	--text-color: var(--color-pink);
}

.element {
	/* custom props */
	--border-color: var(--color-blue-light);

	/* props */
	border: 1px solid rgb(0, 217, 255);
	border: 1px solid var(--border-color);
	color: rgb(255, 192, 211);
	color: var(--text-color);
}

.element--dark {
	--border-color: var(--color-blue-dark);
}

Note:

• Only processes variables that were defined in the :root or html selector.
• Locally defined variables will be used as fallbacks only within the same rule, but not elsewhere.
• Fallback values in var() will be used if the variable was not defined in the :root or html selector.

Usage

Add PostCSS Custom Properties to your project:

npm install postcss postcss-custom-properties --save-dev

Use it as a PostCSS plugin:

const postcss = require('postcss');
const postcssCustomProperties = require('postcss-custom-properties');

postcss([
	postcssCustomProperties(/* pluginOptions */)
]).process(YOUR_CSS /*, processOptions */);

Options

preserve

The preserve option determines whether properties using custom properties should be preserved in their original form. By default these are preserved.

Custom property declarations are always preserved only var() functions can be omitted.

postcssCustomProperties({ preserve: false })

:root {
	--color-blue-dark: rgb(0, 61, 184);
	--color-blue-light: rgb(0, 217, 255);
	--color-pink: rgb(255, 192, 211);
	--text-color: var(--color-pink);
}

.element {
	/* custom props */
	--border-color: var(--color-blue-light);

	/* props */
	border: 1px solid var(--border-color);
	color: var(--text-color);
}

.element--dark {
	--border-color: var(--color-blue-dark);
}

/* becomes */

:root {
	--color-blue-dark: rgb(0, 61, 184);
	--color-blue-light: rgb(0, 217, 255);
	--color-pink: rgb(255, 192, 211);
	--text-color: var(--color-pink);
}

.element {
	/* custom props */
	--border-color: var(--color-blue-light);

	/* props */
	border: 1px solid var(--border-color);
	color: rgb(255, 192, 211);
}

.element--dark {
	--border-color: var(--color-blue-dark);
}

Modular CSS Processing

If you're using Modular CSS such as, CSS Modules, postcss-loader or vanilla-extract to name a few, you'll probably notice that custom properties are not being resolved. This happens because each file is processed separately so unless you import the custom properties definitions in each file, they won't be resolved.

To overcome this, we recommend using the PostCSS Global Data plugin which allows you to pass a list of files that will be globally available. The plugin won't inject any extra code in the output but will provide the context needed to resolve custom properties.

For it to run it needs to be placed before the PostCSS Custom Properties plugin.

const postcss = require('postcss');
const postcssCustomProperties = require('postcss-custom-properties');
const postcssGlobalData = require('@csstools/postcss-global-data');

postcss([
	postcssGlobalData({
		files: [
			'path/to/your/custom-selectors.css'
		]
	}),
	postcssCustomProperties(/* pluginOptions */)
]).process(YOUR_CSS /*, processOptions */);