> For the complete documentation index, see [llms.txt](https://lochiwei.gitbook.io/web/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://lochiwei.gitbook.io/web/js/async/promise.md).

# Promise

[JS](/web/js.md) ⟩ [async](/web/js/async.md) ⟩ Promise

{% hint style="success" %}
[⭐️ ES6 (2015)](/web/js/feature/es6.md) \
the <mark style="color:red;">**future**</mark>**&#x20;**<mark style="color:yellow;">**result**</mark> of an [**async**](/web/js/async.md) operation.

```javascript
let p = promise.then(onFulfilled, onRejected)
```

{% endhint %}

{% tabs %}
{% tab title="🔸 定義" %} <img src="/files/xt4RyXvx1IOi3wdCGXmk" alt="" class="gitbook-drawing">

{% hint style="success" %}
if we have：

```javascript
let p = promise.then(onResolved, onRejected)
```

we say p is：

* <mark style="color:purple;">**resolved**</mark> if **onResolved()** is called.
  * <mark style="color:green;">**fulfilled**</mark> if **onResolved()** returns a **non-Promise** value.
* <mark style="color:red;">**rejected**</mark> if <mark style="color:red;">`onRejected()`</mark> is called.
* <mark style="color:orange;">**settled**</mark> if it's <mark style="color:green;">**fulfilled**</mark> **or** <mark style="color:red;">**rejected**</mark>.
* <mark style="color:yellow;">**pending**</mark> if it's **neither** <mark style="color:green;">**fulfilled**</mark> **nor** <mark style="color:red;">**rejected**</mark>.
  {% endhint %}

{% hint style="warning" %}
Any <mark style="color:orange;">**settled**</mark> Promise has an associated <mark style="color:yellow;">**value**</mark> , which <mark style="color:red;">**won't change**</mark>.
{% endhint %}
{% endtab %}

{% tab title="🔴 主題" %}

* [using Promises](/web/js/async/promise/using-promises.md)
  * [getJSON()](/web/js/async/promise/custom-promises/getjson.md) - get JSON ([Promise](/web/js/async/promise.md)) from [URL](/web/browser/api/url.md).
* [error handling](/web/js/async/promise/error-handling.md)
  * [recoverable errors](/web/js/async/promise/error-handling/recoverable-errors.md)
  * [finally()](/web/js/async/promise/error-handling/finally.md)
* [chaining Promises](/web/js/async/promise/chaining.md)
* [Promises in parallel](/web/js/async/promise/in-parallel.md)
  {% endtab %}

{% tab title="👥 相關" %}

* [⭐️ ES2018](/web/js/feature/es2018.md) - [for-await](/web/js/grammar/statement/loop/for/await.md), [finally()](/web/js/async/promise/error-handling/finally.md)
* [await](/web/js/async/await.md)
* [await promises](/web/js/async/await/promises.md)
* [Promise.wait()](/web/js/async/promise/custom-promises/promise.wait.md)
* [Promise.inSeries()](/web/js/async/promise/chaining/promise.inseries.md)
* [Promises in parallel](/web/js/async/promise/in-parallel.md)
* [async function](/web/js/async/async/functions.md)
* [async method](/web/js/async/async/methods.md)
  {% endtab %}

{% tab title="📗 參考" %}

* [ ] JavaScript: The Definitive Guide (13.2 Promises)
* [ ] amazon.jp ⟩ [マンガでわかる JavaScript の Promise](https://www.amazon.co.jp/dp/B09XQ89M9Y/ref=olp-opf-redir?aod=1) ([Kindle Cloud Reader](https://read.amazon.co.jp/manga/B09XQ89M9Y?ref_=dbs_t_r_mwr))
  {% endtab %}

{% tab title="📘 手冊" %}

* [Promise.resolve()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/resolve)
* [Promise.reject()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/reject)
  {% endtab %}
  {% endtabs %}