Recursively walk a directory in TypeScript

walk.ts

import fs from 'fs';
import path from 'path';

/**
 * Recursively walk a directory asynchronously and obtain all file names (with full path).
 *
 * @param dir Folder name you want to recursively process
 * @param done Callback function, returns all files with full path.
 * @param filter Optional filter to specify which files to include, 
 *   e.g. for json files: (f: string) => /.json$/.test(f)
 * @see https://stackoverflow.com/questions/5827612/node-js-fs-readdir-recursive-directory-search/50345475#50345475
 */
const walk = (
  dir: string,
  done: (err: Error | null, results?: string[]) => void,
  filter?: (f: string) => boolean
) => {
  let results: string[] = [];
  fs.readdir(dir, { withFileTypes: true }, (err: Error | null, list: string[]) => {
    if (err) {
      return done(err);
    }
    let pending = list.length;
    if (!pending) {
      return done(null, results);
    }
    list.forEach((file: string) => {
      file = path.resolve(dir, file);
      fs.stat(file, (err2, stat) => {
        if (stat.isDirectory()) {
          walk(file, (err3, res) => {
            if (res) {
              results = results.concat(res);
            }
            if (!--pending) {
              done(null, results);
            }
          }, filter);
        } else {
          if (typeof filter === 'undefined' || (filter && filter(file))) {
            results.push(file);
          }
          if (!--pending) {
            done(null, results);
          }
        }
      });
    });
  });
};

@TheTrashAcc @keyoti Thanks for your suggestions - I've improved the gist with them!