Apa iku Promise lan apa wae staatose?

Question

Accepted Answer

A **Promise** is an object representing a value that may not be available yet — the eventual result of an asynchronous operation. Iku nduweni telu state:

- **pending** — inisial, durung settled.
- **fulfilled** — rampung kasuksessan (nduweni nilai).
- **rejected** — gagal (nduweni alasan/error).

Sakdene settled (fulfilled utawa rejected) iku **immutable** — ora bisa owah maneh.

```js
fetch("/api/user")
  .then(res => res.json())     // runs on fulfillment; return value flows down
  .then(user => render(user))
  .catch(err => showError(err)) // runs on ANY rejection in the chain
  .finally(() => hideSpinner()); // always runs
```

## Chaining flatten nested async

Ngasilake promise sajroning `.then` nunggu dheweke sadurunge nerusake, mula sampeyan ilangi callback nesting sing jero ("callback hell"):

```js
getUser(id)
  .then(user => getPosts(user.id)) // returns a promise → chain waits for it
  .then(posts => console.log(posts));
```

## Combinators kanggo pirang-pirang promise

```js
Promise.all([a, b, c]);     // wait for ALL; rejects fast if any fails
Promise.allSettled([a, b]); // wait for all, get each result/error
Promise.race([a, b]);       // settles with the FIRST to settle
Promise.any([a, b]);        // first to FULFILL (ignores rejections)
```

## Mengapa penting

Promises minangka landasan async JS — dulur callback nesting, menehi unified error handling via `.catch`, lan `async/await` mung luwih becik syntax ndhuwure.

Lali `.catch` (unhandled rejection) minangka bug sing umum.