add-classes.mjs

import { selectAll } from 'hast-util-select';

export default (additions) => {
  const adders = Object.entries(additions).map(adder);
  return (node) => adders.forEach((a) => a(node));
};

const adder = ([selector, className]) => {
  const writer = write(className);
  return (node) => selectAll(selector, node).forEach(writer);
};

const write =
  (className) =>
  ({ properties }) => {
    if (!properties.className) properties.className = className;
    else properties.className += ` ${className}`;
  };