use Async lib

genAccountAsync.js

const async = require('async');
const bcrypt = require('bcrypt');
const MongoClient = require('mongodb').MongoClient;

const NAME = 'mockuser';
const PWD = 'mockpwd';
const NUM = 5;
const URL = 'mongodb://localhost:27017/myApp';
const USERS = 'users';
const FOLDERS = 'folders';

function mockNames(name, num = 3) {
  let count = 1;
  return Array(num).fill(NAME).map(name => (`${name}-${count++}`));
}

function salt(cb) {
  bcrypt.genSalt(2, (err, salt) => {
    if(err) return cb(err);
    cb(null, salt);
  });
}

function hash(salt, cb) {
  bcrypt.hash(PWD, salt, (err, hash)=> {
    if(err) return cb(err);
    cb(null, hash);
  });
}

function connMongo(hash, cb) {
  MongoClient.connect(URL, (err, db) => {
    if(err) return cb(err);
    cb(null, hash, db);
  });
}

function addUsers(hash, db, cb) {
  console.log('insert users to DB');
  const names =  mockNames(NAME, NUM);
  const users = names
    .map(name => ({
      name,
      pwd: hash
    }));
  db.collection(USERS)
    .insertMany(users, (err, result) => {
      if(err) return cb(err);
      cb(null, db, names, result);
    });
}

function addRootFolder(db, names, addUsersResult, cb) {
  // {"owner" : "mockUser", "dir-1" : { "name" : "暫存區", "childs" : [] }
  const rootFolders = names.map(name => ({
    owner: name,
    'dir-1': {
      name: '暫存區',
      childs: []
    }
  }));

  db.collection(FOLDERS)
    .insertMany(rootFolders, (err, addFolderResult) => {
      if (err) return cb(err);
      cb(null, db, addUsersResult, addFolderResult);
    });
}

async.waterfall([
  salt,
  hash,
  connMongo,
  addUsers,
  addRootFolder
], (err, db, addUsersResult, addFolderResult) => {
  if (err) console.log(err);
  else {
    db.close();
    console.log(addUsersResult);
    console.log(addFolderResult);
  }
});