index.js

'use strict'

exports.fromCallback = function (fn) {
  return Object.defineProperty(function (...args) {
    if (typeof args[args.length - 1] === 'function') fn.apply(this, args)
    else {
      return new Promise((resolve, reject) => {
        args.push((err, res) => (err != null) ? reject(err) : resolve(res))
        fn.apply(this, args)
      })
    }
  }, 'name', { value: fn.name })
}

exports.fromPromise = function (fn) {
  return Object.defineProperty(function (...args) {
    const cb = args[args.length - 1]
    if (typeof cb !== 'function') return fn.apply(this, args)
    else {
      args.pop()
      fn.apply(this, args).then(r => cb(null, r), cb)
    }
  }, 'name', { value: fn.name })
}