promisify

Converts an asynchronous function to return a promise.

• Use currying to return a function returning a Promise that calls the original function.
• Use the rest operator (...) to pass in all the parameters.
• Note: In Node 8+, you can use util.promisify.

const promisify = func => (...args) =>
  new Promise((resolve, reject) =>
    func(...args, (err, result) => (err ? reject(err) : resolve(result)))
  );

const delay = promisify((d, cb) => setTimeout(cb, d));
delay(2000).then(() => console.log('Hi!')); // Promise resolves after 2s