Simple promisify.

promisify.js

function promisify(fn) {
  return function(...args) {
    return new Promise((resolve, reject) => {
      fn.call(this, ...args, (err, res) => {
        if (err) reject(err);
        else resolve(res);
      });
    });
  };
};

function respond(subject, done) {
  setTimeout(() => done(null, `hello ${subject}`), 2000);
}

let respondWithPromise = promisify(respond);
respondWithPromise('world').then((val) => console.log(val));