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

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);
}

Usage

Add PostCSS Custom Properties to your project:

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

Use PostCSS Custom Properties to process your CSS:

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

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

Or use it as a PostCSS plugin:

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

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

PostCSS Custom Properties runs in all Node environments, with special instructions for:

Node	PostCSS CLI	Webpack	Create React App	Gulp	Grunt

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' // => :root { --color: red }
});

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',   // :root { --color: red; }
    'and/then/this.js',   // module.exports = { customProperties: { '--color': 'red' } }
    'and/then/that.json', // { "custom-properties": { "--color": "red" } }
    {
      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' // :root { --color: red; }
});

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',   // :root { --color: red; }
    'and/then/this.js',   // module.exports = { customProperties: { '--color': 'red' } }
    'and/then/this.mjs',  // export const customProperties = { '--color': 'red' } }
    'and/then/that.json', // { "custom-properties": { "--color": "red" } }
    cachedObject,
    customProperties => {
      customProperties    // { '--color': 'red' }
    }
  ]
});

See example exports written to CSS, JS, MJS, and JSON.