Before Promises/async–await were common, many JS APIs used a callback-last, error-first style: (…args, callback), where callback has the signature (err, value).

Implement promisify(fn) which:

• Accepts a function fn that expects its final argument to be (err, value) => void.
• Returns a new function that returns a Promise instead of requiring a callback.
• On success (callback called with null or undefined error), the Promise resolves with the value.
• On failure (callback called with an error), the Promise rejects with that error.
• It should preserve this context when the returned function is called (i.e., use fn.call(this,…)).
• If fn throws synchronously, the Promise should reject with that error.

Example

// Callback-last, error-first function
function foo(url, options, callback) {
  fakeRequest(url, options)
    .then((data) => callback(null, data))
    .catch((err) => callback(err));
}

const promisifiedFoo = promisify(foo);
// Now usable with async/await:
const data = await promisifiedFoo("example.com", { q: 1 });