thunk to promise

thunkToPromise.js

/**
 * 
 * @param fn
 * @returns {Promise}
 * eg: thunkToPromise(fs.readdir, path).then()
 */
function thunkToPromise(fn) {
  var ctx = this
  var args = Array.prototype.slice.call(arguments, 1)

  return new Promise(function (resolve, reject) {
    args.push(function (err, res) {
      if (err) reject(err)
      resolve(res)
    })
    fn.apply(ctx, args)
  })
}