A little function to convert a callback-style function to Promises

callback-to-promise.js

// usage: promisify(function([args..., cb]) { /* do stuff with args then cb */ })()
exports.promisify = function(fn) {
  return function(...args) {
    return new Promise((resolve, reject) => {
      fn(...args, (err, result) => {
        if (err) return reject(err);
        resolve(result);
      });
    });
  };
};