Promisifying Error-First Asynchronous Callbacks in JavaScript

I have been writing a lot about Promises in JavaScript the past couple of weeks. What happens however, when you wish to use an asynchronous function which does not return a Promise, such as those found in Node? It is actually pretty simple to abstract away the error-first asynchronous function callback into a Promise we can handle.

const promisify = (fn) => (...args) =>
  new Promise((res, rej) =>
    fn(...args, (err, val) => err ? rej(err) : res(val)));

We are now able to wrap a function which uses the error-first callback paradigm into a Promise, as demoed below.

promisify(fs.readFile)('./hello.txt', 'utf8')
  .then(console.log)
  .catch(console.error);
// Hello, world!