Using Functional Promises library

auth-with-FP.js

// Functional River Pattern
// Original: https://github.com/justsml/escape-from-callback-mountain/blob/master/src/auth.fp.js
const FP           = require('functional-promises')
const {hashString} = require('./lib/crypto')
const users        = require('./lib/users')

function auth({username = '', password = ''}) {
  return FP
    .resolve({username, password})
    .then(_checkArgs)
    .then(_loginUser)
    .then(_checkUser)
    .thenIf(
      user => user.admin,
      user => _loadAdmin(user))
}

// You could still use async/await here - tiny functions are the key:
async function _loadAdmin(user) {
  const admin = await admin.findOneAsync({ id: user.id });
  user.adminPermissions = admin.permissions;
  return user;
}

function _checkArgs({username = '', password = ''}) {
  if (username.length < 1) throw new Error('Enter valid username')
  if (password.length < 6) throw new Error('Enter valid Password')
  return {username, password}
}

function _loginUser({username, password}) {
  return Promise.resolve(password)
    .then(hashString)
    .then(password => users.getOne({username, password}))
}

function _checkUser(user) {
  if (user && user._id) return user
  throw new Error('No User found. Check credentials and try again.')
}

module.exports = {auth, _checkArgs, _loginUser, _checkUser}