Create a function promisify(fn) that converts a Node.js-style callback function into a function that returns a promise.

The difference from the standard version is:

• If the original function explicitly returns a value, that return value should be used instead of the promise.
• If the function returns undefined, then the promise-based behavior should take place.
• The callback signature is (err, result).

Examples

function asyncAdd(a, b, cb) {
  setTimeout(() => cb(null, a + b), 100);
}

const promisedAdd = promisify(asyncAdd);

promisedAdd(2, 3).then(console.log); // 5

function syncReturn(x) {
  return x * 2;
}

const wrapped = promisify(syncReturn);

console.log(wrapped(4)); // 8, returns directly instead of a Promise