# Confused about this .then/.finally error handling example

**URL:** <https://es.discourse.group/t/confused-about-this-then-finally-error-handling-example/2489>\
**Category:** I have questions\
**Created:** [February 6, 2026, 2:21pm UTC](https://es.discourse.group/t/confused-about-this-then-finally-error-handling-example/2489 "2026-02-06T14:21:41Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![bendtherules](https://yyz2.discourse-cdn.com/free1/user_avatar/es.discourse.group/bendtherules/32/339_2.png) [@bendtherules](https://es.discourse.group/u/bendtherules)\
**Post date:** [February 6, 2026, 2:21pm UTC](https://es.discourse.group/t/confused-about-this-then-finally-error-handling-example/2489/1 "2026-02-06T14:21:42Z")

</div>

Here is a wierd behaviour that’s been bugging me for a whole day. All the usual docs and even AI explanations don’t make sense - so i have a feeling this might be something actually explained in the spec.

This throws synchronous error -

```javascript
const taskFn = () => {
    throw new Error('sync error');
};

const {promise, resolve, reject} = Promise.withResolvers();
promise.catch(() => {});

const taskPromise = Promise.resolve().then(() => taskFn());
taskPromise
  .then(resolve, reject);
taskPromise
  .finally(() => {
  });

```

while this doesn’t -

```javascript
const taskFn = () => {
    throw new Error('sync error');
};

const {promise, resolve, reject} = Promise.withResolvers();
promise.catch(() => {});

const taskPromise = Promise.resolve().then(() => taskFn());
taskPromise
  .then(resolve, reject)
  .finally(() => {
  });

```

The only difference is that second one has `finally` chained after `then`.  
Why does this matter, since finally doesn’t handle any error?

(Sorry if it’s a very basic question)

---

<div class="post-metadata">

**Author:** ![senocular](https://yyz2.discourse-cdn.com/free1/user_avatar/es.discourse.group/senocular/32/642_2.png) [@senocular](https://es.discourse.group/u/senocular)\
**Post date:** [February 6, 2026, 3:37pm UTC](https://es.discourse.group/t/confused-about-this-then-finally-error-handling-example/2489/2 "2026-02-06T15:37:34Z")

</div>

The error in your first example is not a sync error (despite the error message 😉). Its an unhandled promise rejection.

This unhandled rejection is coming from the promise created by the `finally` method. `finally` callbacks do not handle promise rejections and instead forward them on through the chain. So the promise returned by the `finally` is also getting rejected and remains unhandled thereby creating the error. If you handled that returned promises’s rejection with a `catch` the error would go away.

The fact that you handled the error in the original `taskPromise` doesn’t prevent it from passing that error through to other promises (e.g. via `finally`) which would also have to account for the rejection being handled.

---

<div class="post-metadata">

**Author:** ![bendtherules](https://yyz2.discourse-cdn.com/free1/user_avatar/es.discourse.group/bendtherules/32/339_2.png) [@bendtherules](https://es.discourse.group/u/bendtherules)\
**Post date:** [February 6, 2026, 6:01pm UTC](https://es.discourse.group/t/confused-about-this-then-finally-error-handling-example/2489/3 "2026-02-06T18:01:40Z")

</div>

Ahh, that makes so much sense. Finally would create a new promise, which is unhandled.

Yes, the error message also added to the confusion.

Thank you.