PostCSS Custom Selectors

[PostCSS Custom Selectors] lets you define @custom-selector in CSS following the [Custom Selectors Specification].

@custom-selector :--heading h1, h2, h3;

article :--heading + p {
    margin-top: 0;
}

/* becomes */

article h1 + p,article h2 + p,article h3 + p {
    margin-top: 0;
}

Usage

Add [PostCSS Custom Selectors] to your project:

npm install postcss postcss-custom-selectors --save-dev

Use it as a PostCSS plugin:

const postcss = require('postcss');
const postcssCustomSelectors = require('postcss-custom-selectors');

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

[PostCSS Custom Selectors] runs in all Node environments, with special
instructions for:

Options

preserve

The preserve option determines whether the original notation
is preserved. By default, it is not preserved.

postcssCustomSelectors({ preserve: true })

@custom-selector :--heading h1, h2, h3;

article :--heading + p {
    margin-top: 0;
}

/* becomes */

@custom-selector :--heading h1, h2, h3;

article h1 + p,article h2 + p,article h3 + p {
    margin-top: 0;
}

article :--heading + p {
    margin-top: 0;
}

importFrom

The importFrom option specifies sources where custom selectors can be
imported from, which might be CSS, JS, and JSON files, functions, and directly
passed objects.

postcssCustomSelectors({
    importFrom: 'path/to/file.css' // => @custom-selector :--heading h1, h2, h3;
});

article :--heading + p {
    margin-top: 0;
}

/* becomes */

article h1 + p, article h2 + p, article h3 + p {}

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 selectors using the customProperties or
custom-properties key.

postcssCustomSelectors({
    importFrom: [
        'path/to/file.css',
        'and/then/this.js',
        'and/then/that.json',
        {
            customSelectors: { ':--heading': 'h1, h2, h3' }
        },
        () => {
            const customProperties = { ':--heading': 'h1, h2, h3' };

            return { customProperties };
        }
    ]
});

exportTo

The exportTo option specifies destinations where custom selectors can be
exported to, which might be CSS, JS, and JSON files, functions, and directly
passed objects.

postcssCustomSelectors({
    exportTo: 'path/to/file.css' // @custom-selector :--heading h1, h2, h3;
});

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 selectors using the customProperties or
custom-properties key.

const cachedObject = { customSelectors: {} };

postcssCustomSelectors({
    exportTo: [
        'path/to/file.css',   // @custom-selector :--heading h1, h2, h3;
        'and/then/this.js',   // module.exports = { customSelectors: { ':--heading': 'h1, h2, h3' } }
        'and/then/this.mjs',  // export const customSelectors = { ':--heading': 'h1, h2, h3' } }
        'and/then/that.json', // { "custom-selectors": { ":--heading": "h1, h2, h3" } }
        cachedObject,
        customProperties => {
            customProperties    // { ':--heading': 'h1, h2, h3' }
        }
    ]
});