get an array of subfolders given a path

get-subdirectories.js

const { promisify } = require('util');
const readFile = promisify(fs.readFile);
const lstat = promisify(fs.lstat);
const readDir = promisify(fs.readdir);

const isDirectory = source => {
  return lstat(source).then(stats =>
    Promise.resolve(stats.isDirectory() ? source : false)
  );
};

const getDirectories = function getDirectories(source) {
  return readDir(source).then(async children => {
    const promises = children.map(name =>
      isDirectory(path.resolve(source, name))
    );
    /*
      Array.filter is synchronous, so we have to wait
      for all lstat operations to finish before we can proceed
    */
    const results = await Promise.all(promises);
    return results.filter(r => r);
  });
};

const getDirectoriesRecursive = async function getDirectoriesRecursive(source) {
  const dirs = await getDirectories(source);
  return dirs.reduce(async (accPromise, cur) => {
    /*
      In async reductions, the accumulator is a promise,
      so any easy workaround is to just await it.
    */
    const acc = await accPromise;
    const subdirs = await getDirectoriesRecursive(cur);
    return acc.concat(cur, subdirs);
  }, []);
};