A small example for a script walking through a path to find all files in the folder and nested subfolders. Implemented only using callbacks

listNestedFiles.js

import { readdir, stat } from 'fs'
import { join } from 'path'

function listNestedFilesRec (path, state, cb) {
  state.ops++
  readdir(path, (err, files) => {
    state.ops--

    if (err) {
      return cb(err)
    }
    for (const f of files) {
      const newPath = join(path, f)

      state.ops++
      stat(newPath, (err, stats) => {
        state.ops--

        if (err) {
          return cb(err)
        }

        if (stats.isDirectory()) {
          // continue recursively
          listNestedFilesRec(newPath, state, cb)
        } else if (stats.isFile()) {
          // add the file to the list of files
          state.files.push(newPath)
        }

        if (state.ops === 0) {
          // no more pending operations
          return cb(null, state.files)
        }
      })
    }
  })
}

export function listNestedFiles (path, cb) {
  const state = { files: [], ops: 0 }
  listNestedFilesRec(path, state, cb)
}