Promisify

promisify.js

const promisify = func => {
  // this need to not using arrow function, otherwise the `this` context will be wrong
  return function() {
    const args = Array.from(arguments);

    return new Promise((resolve, reject) => {
      const callback = (err, data) => {
        if (error) {
          return reject(err);
        }

        return resolve(data);
      };

      args.push(callback);

      func.apply(this, args);
    });
  };
}

API.someAsyncFunc = promisify(someAsyncFunc)