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<!DOCTYPE html>
<html lang="zh-TW">
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<article>
<h2 id="13-6">13-6 Object.fromEntries()</h2>
<p>方法定義</p>
<pre><code class="language-ts">
interface ObjectConstructor {
/**
* Returns an object created by key-value entries for properties and methods
* @param entries An iterable object that contains key-value entries for properties and methods.
*/
fromEntries<T = any>(entries: Iterable<readonly [PropertyKey, T]>): { [k: string]: T };
/**
* Returns an object created by key-value entries for properties and methods
* @param entries An iterable object that contains key-value entries for properties and methods.
*/
fromEntries(entries: Iterable<readonly any[]>): any;
}
</code></pre>
<p>
我們在 ES8 中新增了 Object.entries 方法，可以將物件轉成 entry 陣列。在 ES10 則增加了一個從
entry 陣列生成物件的方法。
</p>
<p>Object.fromEntries() 靜態方法將鍵值配對列表轉換為一個物件。</p>
<pre><code class="language-js">
const entries = new Map([
['foo', 'bar'],
['baz', 42]
]);
const obj = Object.fromEntries(entries);
console.log(obj);
// Expected output: Object { foo: "bar", baz: 42 }
</code></pre>
<p>
Object.fromEntries()
方法接收一個鍵值對列表，並回傳一個新物件，該物件的屬性由這些條目給定。iterable
參數應該是實現了 @@iterator
方法的可疊代物件。該方法回傳一個可疊代物件，產生包含兩個元素的類陣列物件。第一個元素是將用作屬性鍵的值，第二個元素是要與該屬性鍵關聯的值。Object.fromEntries()
是 Object.entries() 的逆操作，只是 Object.entries() 只回傳字串鍵屬性，而
Object.fromEntries() 還可以創建符號鍵屬性。
</p>
<p>
<strong>備注：</strong>
與 Array.from() 不同的是，Object.fromEntries() 不使用 this
的值，因此在另一個構造函數上調用它不會創建該類型的物件。
</p>
<p>以下是相互轉換的範例</p>
<h3>將 Map 轉換成物件</h3>
<pre><code class="language-js">
const map = new Map([
["foo", "bar"],
["baz", 42],
]);
const obj = Object.fromEntries(map);
console.log(obj); // { foo: "bar", baz: 42 }
</code></pre>
<h3>將 Array 轉換成物件</h3>
<pre><code class="language-js">
const arr = [
["0", "a"],
["1", "b"],
["2", "c"],
];
const obj = Object.fromEntries(arr);
console.log(obj); // { 0: "a", 1: "b", 2: "c" }
</code></pre>
<h3>物件轉換</h3>
<p>
通過 Object.fromEntries、其逆操作 Object.entries()
和陣列操作方法，你可以像這樣轉換物件達到類似屬性疊代效果：
</p>
<pre><code class="language-js">
const object1 = { a: 1, b: 2, c: 3 };
const object2 = Object.fromEntries(
Object.entries(object1).map(([key, val]) => [key, val * 2]),
);
console.log(object2);
// { a: 2, b: 4, c: 6 }
</code></pre>
<h3>其他示範</h3>
<p>陣列降解。</p>
<pre><code class="language-js">
const arr = [
{ name: `A男`, age: 26 },
{ name: `B男`, age: 18 },
];
const obj3 = Object.fromEntries(arr.map(({ name, age }) => [name, age]));
console.log(obj3);
// { 'A男': 26, 'B男': 18 }
</code></pre>
<p>URLSearchParams 操作</p>
<pre><code class="language-js">
const obj4 = Object.fromEntries(new URLSearchParams(`x=1&y=2`));
console.log(obj4);
// { x: '1', y: '2' }
</code></pre>
<p>資料來源:</p>
<p>
<a
href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/fromEntries"
>
https://developer.mozilla.org/.../Object/fromEntries
</a>
</p>
<p>
<a href="https://juejin.cn/post/7023895016384757796">
https://juejin.cn/post/7023895016384757796
</a>
</p>
</article>
</main>
</body>
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