Promisify a function with callback

Promisify.js

// Needs ... notation to be available
const promisify = (fn) => {
  return (...args) => {
    return new Promise((resolve, reject)=>{
      fn(...args, function(err, res){
        if(err){
          return reject(err);
        }
        return resolve(res);
      })
    })
  }
}


// Polified version
"use strict";
var promisify = function promisify(fn) {
  return function () {
    for (var _len = arguments.length, args = Array(_len), _key = 0; _key < _len; _key++) {
      args[_key] = arguments[_key];
    }

    return new Promise(function (resolve, reject) {
      fn.apply(undefined, args.concat([function (err, res) {
        if (err) {
          return reject(err);
        }
        return resolve(res);
      }]));
    });
  };
};