documentation/node_modules/@docusaurus/cssnano-preset/src/remove-overridden-custom-properties/index.ts

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

import type {Plugin, Rule, Node} from 'postcss';

const isRule = (node: Node | undefined): node is Rule => node?.type === 'rule';

/**
 * This PostCSS plugin will remove duplicate/same custom properties (which are
 * actually overridden ones) **only** from `:root` selector.
 *
 * Depending on the presence of an `!important` rule in value of custom
 * property, the following actions will happen:
 *
 * - If the same custom properties do **not** have an `!important` rule, then
 * all of them will be removed except for the last one (which will actually be
 * applied).
 * - If the same custom properties have at least one `!important` rule, then
 * only those properties that do not have this rule will be removed.
 */
function creator(): Plugin {
  return {
    postcssPlugin: 'postcss-remove-overridden-custom-properties',
    Declaration(decl) {
      if (!isRule(decl.parent) || decl.parent.selector !== ':root') {
        return;
      }

      const sameProperties = decl.parent.nodes.filter(
        (n) => 'prop' in n && n.prop === decl.prop,
      );
      const hasImportantProperties = sameProperties.some(
        (p) => 'important' in p,
      );

      const overriddenProperties = hasImportantProperties
        ? sameProperties.filter((p) => !('important' in p))
        : sameProperties.slice(0, -1);

      overriddenProperties.map((p) => p.remove());
    },
  };
}

creator.postcss = true as const;

export default creator;