index.js

const fs = require('fs');
const path = require('path');

const defaults = {
  dir: './packages',
  verbose: false,
  bullet: '*',
};

module.exports = function SUBPACKAGELIST(content, _options, config) {
  const options = Object.assign({}, defaults, _options);

  const packagesDir = path.resolve(
    path.dirname(config.originalPath),
    options.dir
  );

  return fs
    .readdirSync(packagesDir)
    .map((filename) => path.join(packagesDir, filename))
    .filter((filePath) => fs.statSync(filePath).isDirectory())
    .filter((dirPath) => fs.existsSync(path.join(dirPath, 'package.json')))
    .map((dirPath) => [
      path.relative(path.dirname(config.originalPath), dirPath),
      require(path.join(dirPath, 'package.json')),
    ])
    .map(([link, package]) => {
      let entry = `${options.bullet} [${package.name}](${link})`;
      if (options.verbose === 'true' && package.description.trim().length) {
        entry += ` - ${package.description.trim()}`;
      }
      return entry;
    })
    .join('\n');
};