Promise ni nini na ni maamkati gani anayo?

Question

Accepted Answer

**Promise** ni kitu kinachorejezesha thamani ambayo inaweza kuwa haipatikani bado — matokeo ya mwisho ya operesheni isiyo-tarajibu. Ina maamkati matatu:

- **pending** — wa kwanza, bado hajasuluhishwa.
- **fulfilled** — kumalizika kwa njia salama (ina thamani).
- **rejected** — kupata hitilafu (ina sababu/kosa).

Mara tu itakaposuluhisha (fulfilled au rejected) ina **immutable** — haiwezi kubadilika tena.

```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
```

## Mnyororo unatasaza async iliyofungwa

Kurudi Promise ndani ya `.then` kinachanga hadi itakaposuluhisha kabla ya kuendelea, kwa hivyo unakuepuka callbacks zilizofungwa sana ("callback hell"):

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

## Wachanganyaji kwa promises nyingi

```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)
```

## Kwa nini inakusudiwa

Promises ni msingi wa async JS — zinabadilisha callback-nesting, zinatoa muhtasari wa kukamatiana na hitilafu kupitia `.catch`, na `async/await` ni syntax nzuri tu juu yake.

Kusahau `.catch` (kumkataalia bila kukamatiana) ni hitilafu ya kawaida.