Callbackify: adhoc callbackify

callbackify.js

const callbackify = fn => (...args) => {
  const callback = args[args.length - 1];
  const fnArgs = args.slice(0, -1);
  fn(...fnArgs)
    .then(value => {
      callback(undefined, value);
    })
    .catch(error => {
      callback(error);
    });
};