postcss-custom-properties

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

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

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

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

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

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

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

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

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' // => :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" } }
    'and/then/that.scss', // $color: red;
    cachedObject,
    customProperties => {
      customProperties    // { '--color': 'red' }
    }
  ]
});

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