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.
PostCSS Custom Properties 

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

:root {
--color: red;
}
h1 {
color: var(--color);
}
/* becomes */
:root {
--color: red;
}
h1 {
color: red;
color: var(--color);
}
Note: This plugin only processes variables that are defined in the :root
selector.
Usage
Add PostCSS Custom Properties to your project:
npm install postcss-custom-properties --save-dev
Use PostCSS Custom Properties as a PostCSS plugin:
const postcss = require('postcss');
const postcssCustomProperties = require('postcss-custom-properties');
postcss([
postcssCustomProperties()
]).process(YOUR_CSS );
PostCSS Custom Properties runs in all Node environments, with special instructions for:
Options
preserve
The preserve
option determines whether Custom Properties and properties using
custom properties should be preserved in their original form. By default, both
of these are preserved.
postcssCustomProperties({
preserve: false
});
:root {
--color: red;
}
h1 {
color: var(--color);
}
/* becomes */
h1 {
color: red;
}
importFrom
The importFrom
option specifies sources where Custom Properties can be imported
from, which might be CSS, JS, and JSON files, functions, and directly passed
objects.
postcssCustomProperties({
importFrom: 'path/to/file.css'
});
h1 {
color: var(--color);
}
/* becomes */
h1 {
color: red;
}
Multiple sources can be passed into this option, and they will be parsed in the
order they are received. JavaScript files, JSON files, functions, and objects
will need to namespace Custom Properties using the customProperties
or
custom-properties
key.
postcssCustomProperties({
importFrom: [
'path/to/file.css',
'and/then/this.js',
'and/then/that.json',
{
customProperties: { '--color': 'red' }
},
() => {
const customProperties = { '--color': 'red' };
return { customProperties };
}
]
});
See example imports written in CSS,
JS, and JSON.
exportTo
The exportTo
option specifies destinations where Custom Properties can be exported
to, which might be CSS, JS, and JSON files, functions, and directly passed
objects.
postcssCustomProperties({
exportTo: 'path/to/file.css'
});
Multiple destinations can be passed into this option, and they will be parsed
in the order they are received. JavaScript files, JSON files, and objects will
need to namespace Custom Properties using the customProperties
or
custom-properties
key.
const cachedObject = { customProperties: {} };
postcssCustomProperties({
exportTo: [
'path/to/file.css',
'and/then/this.js',
'and/then/this.mjs',
'and/then/that.json',
'and/then/that.scss',
cachedObject,
customProperties => {
customProperties
}
]
});
See example exports written to CSS,
JS, MJS,
JSON and SCSS.