Promisify: adhoc custom promisify

promisify.js

const CUSTOM = Symbol("promisify.custom");

const promisify = fn => {
  if (fn[CUSTOM]) {
    return fn[CUSTOM];
  }

  return (...args) => new Promise((resolve, reject) => {
    fn(...args, (error, value) => {
      if (error) {
        reject(error);
      } else {
        resolve(value);
      }
    });
  });
};

promisify.custom = CUSTOM;