#420 Promise.finally

JavaScript’s Promise.prototype.finally executes a callback when the promise is settled (resolved or rejected).

Your task is to implement a polyfill for Promise.prototype.finally.

Requirements

• finally(callback) should run the callback once the promise settles, whether it resolves or rejects.

• The returned promise should:

  • Resolve with the original value if the promise was fulfilled.
  • Reject with the original reason if the promise was rejected.

• If the callback returns a promise, wait for it to finish before proceeding.

Example

Promise.resolve(42)
  .finally(() => console.log('done'))
  .then(value => console.log('resolved with', value));
// Output:
// done
// resolved with 42

Promise.reject('error')
  .finally(() => console.log('cleanup'))
  .catch(err => console.log('rejected with', err));
// Output:
// cleanup
// rejected with error