also here's a promisify that blows bluebird out of the water.

promisify.js

function promisify (fn, thisArg = null) {
  return (...args) => {
    return new Promise((resolve, reject) => {
      const callback = (err, res) => {
        if (err) return reject(err);
        return resolve(res);
      }
      fn.apply(thisArg, [...args, callback]);
    });
  }
}