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authorPeter Bengtsson <mail@peterbe.com>2020-12-08 14:43:23 -0500
committerPeter Bengtsson <mail@peterbe.com>2020-12-08 14:43:23 -0500
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+---
+title: 解構賦值
+slug: Web/JavaScript/Reference/Operators/Destructuring_assignment
+tags:
+ - Destructuring
+ - Destructuring_assignment
+ - ECMAScript 2015
+ - ES6
+ - JavaScript
+ - Language feature
+ - Operator
+ - 解構
+translation_of: Web/JavaScript/Reference/Operators/Destructuring_assignment
+---
+<div>{{jsSidebar("Operators")}}</div>
+
+<p><strong>解構賦值</strong> (Destructuring assignment) 語法是一種 JavaScript 運算式,可以把陣列或物件中的資料解開擷取成為獨立變數。</p>
+
+<div>{{EmbedInteractiveExample("pages/js/expressions-destructuringassignment.html", "taller")}}</div>
+
+
+
+<h2 id="語法">語法</h2>
+
+<pre class="brush:js">let a, b, rest;
+[a, b] = [10, 20];
+console.log(a); // 10
+console.log(b); // 20
+
+[a, b, ...rest] = [10, 20, 30, 40, 50];
+console.log(a); // 10
+console.log(b); // 20
+console.log(rest); // [30, 40, 50]
+
+({ a, b } = { a: 10, b: 20 });
+console.log(a); // 10
+console.log(b); // 20
+
+
+// Stage 4(finished) proposal
+({a, b, ...rest} = {a: 10, b: 20, c: 30, d: 40});
+console.log(a); // 10
+console.log(b); // 20
+console.log(rest); // {c: 30, d: 40}
+</pre>
+
+<h2 id="描述">描述</h2>
+
+<p>物件與陣列運算式提供了簡單的方式,建立特定的資料組。</p>
+
+<pre class="brush: js">const x = [1, 2, 3, 4, 5];</pre>
+
+<p>解構賦值使用類似語法;不過在指定敘述式的左側,要宣告從來源變數接收解開值之變數。</p>
+
+<pre class="brush: js">const x = [1, 2, 3, 4, 5];
+const [y, z] = x;
+console.log(y); // 1
+console.log(z); // 2
+</pre>
+
+<p>Perl 和 Python 也有類似的語法和功能。</p>
+
+<h2 id="陣列解構">陣列解構</h2>
+
+<h3 id="基本變數指定敘述">基本變數指定敘述</h3>
+
+<pre class="brush: js">const foo = ['one', 'two', 'three'];
+
+const [red, yellow, green] = foo;
+console.log(red); // "one"
+console.log(yellow); // "two"
+console.log(green); // "three"
+</pre>
+
+<h3 id="宣告指派分開敍述">宣告指派分開敍述</h3>
+
+<p>變數可以在宣告式後,再透過解構賦值。</p>
+
+<pre class="brush:js">let a, b;
+
+[a, b] = [1, 2];
+console.log(a); // 1
+console.log(b); // 2
+</pre>
+
+<h3 id="預設值">預設值</h3>
+
+<p>當解構來源陣列對應的元素是 undefined 時,變數可以被設定預設值。</p>
+
+<pre class="brush: js">let a, b;
+
+[a=5, b=7] = [1];
+console.log(a); // 1
+console.log(b); // 7
+</pre>
+
+<h3 id="變數交換">變數交換</h3>
+
+<p>兩個變數可以透過一個解構指派式交換。</p>
+
+<p>沒有解構指派式時,這需要一個暫存變數來達成(或者像某些低階語言的 <a class="external" href="https://en.wikipedia.org/wiki/XOR_swap_algorithm">XOR-swap trick</a>)。</p>
+
+<pre class="brush:js">let a = 1;
+let b = 3;
+
+[a, b] = [b, a];
+console.log(a); // 3
+console.log(b); // 1
+
+const arr = [1,2,3];
+[arr[2], arr[1]] = [arr[1], arr[2]];
+console.log(arr); // [1,3,2]
+
+</pre>
+
+<h3 id="解析自函式回傳的陣列">解析自函式回傳的陣列</h3>
+
+<p>一直以來函式都可以回傳陣列,而解構指派式可以讓回傳的值更加簡潔。</p>
+
+<p>在這個例子, <code>f()</code> 回傳 <code>[1, 2]</code> ,接著透過一個解構指派式解析。</p>
+
+<pre class="brush:js">function f() {
+ return [1, 2];
+}
+
+let a, b;
+[a, b] = f();
+console.log(a); // 1
+console.log(b); // 2
+</pre>
+
+<h3 id="忽略某些回傳值">忽略某些回傳值</h3>
+
+<p>你可以忽略某些回傳值:</p>
+
+<pre class="brush:js">function f() {
+ return [1, 2, 3];
+}
+
+const [a, , b] = f();
+console.log(a); // 1
+console.log(b); // 3
+</pre>
+
+<p>當然你也可以忽略全部回傳值:</p>
+
+<pre class="brush:js">[,,] = f();
+</pre>
+
+<h3 id="把矩陣剩餘部分解構到一個變數">把矩陣剩餘部分解構到一個變數</h3>
+
+<p>解構一個陣列時,你可以透過其餘元素(rest pattern)將來源剩下之元素指派到一個變數:</p>
+
+<pre class="brush: js">const [a, ...b] = [1, 2, 3];
+console.log(a); // 1
+console.log(b); // [2, 3]</pre>
+
+<p>要注意的是,當左邊函式裡使用其餘解構,同時使用結尾逗號,這樣會拋出例外 {{jsxref("SyntaxError")}} :</p>
+
+<pre class="brush: js example-bad">const [a, ...b,] = [1, 2, 3];
+
+// SyntaxError 語法錯誤: 其餘元素不可以跟隨結尾逗號
+// 需要把其餘運算子放在最後的元素
+</pre>
+
+<h3 id="從正則運算式的比對結果取值">從正則運算式的比對結果取值</h3>
+
+<p>當正則運算式的方法 <code><a href="/zh-TW/docs/Web/JavaScript/Reference/Global_Objects/RegExp/exec">exec()</a></code> 比對到一個值,其回傳陣列中的第一個值是相符的完整字串,後績的則是比對到正則運算式每組括號內的部分。當你沒需要利用第一個完整比對結果時,解構指派式讓你更簡單的取出後績元素。</p>
+
+<pre class="brush:js">function parseProtocol(url) {
+ const parsedURL = /^(\w+)\:\/\/([^\/]+)\/(.*)$/.exec(url);
+ if (!parsedURL) {
+ return false;
+ }
+ console.log(parsedURL); // ["https://developer.mozilla.org/en-US/Web/JavaScript", "https", "developer.mozilla.org", "en-US/Web/JavaScript"]
+
+ const [, protocol, fullhost, fullpath] = parsedURL;
+ return protocol;
+}
+
+console.log(parseProtocol('https://developer.mozilla.org/en-US/Web/JavaScript')); // "https"
+</pre>
+
+<h2 id="物件解構">物件解構</h2>
+
+<h3 id="基本指派式">基本指派式</h3>
+
+<pre class="brush: js">const o = {p: 42, q: true};
+const {p, q} = o;
+
+console.log(p); // 42
+console.log(q); // true
+</pre>
+
+<h3 id="無宣告指派">無宣告指派</h3>
+
+<p>變數可以在宣告式後,再透過解構進行指派。</p>
+
+<pre class="brush:js">let a, b;
+
+({a, b} = {a:1, b:2});</pre>
+
+<div class="note">
+<p><strong>注意</strong>:當針對物件進行解構,而該句式沒有進行宣告時,指派式外必須加上括號 <code>( ... )</code> 。</p>
+
+<p><code>{a, b} = {a: 1, b: 2}</code> 不是有效的獨立語法,因為左邊的 <code>{a, b}</code> 被視為程式碼區塊而非物件。</p>
+
+<p>然而,<code>({a, b} = {a: 1, b: 2})</code> 是有效的,如同 <code>const {a, b} = {a: 1, b: 2}</code></p>
+
+<p><code>( ... )</code> 表達式前句需要以分號結束,否則可能把上一句視為函式隨即執行。</p>
+</div>
+
+<h3 id="指派到新的變數名稱">指派到新的變數名稱</h3>
+
+<p>物件中的屬性可以解構並擷取到名稱跟該屬性不一樣的變數。</p>
+
+<pre class="brush: js">const o = {p: 42, q: true};
+const {p: foo, q: bar} = o;
+
+console.log(foo); // 42
+console.log(bar); // true</pre>
+
+<p>舉例來說, <code>const {p: foo} = o</code> 把物件 <code>o</code> 裡名為 <code>p</code> 的屬性解出並指派到一個名為 <code>foo</code> 的本地變數。</p>
+
+<h3 id="預設值_2">預設值</h3>
+
+<p>當解構物件中對應的值是 <code>undefined</code> 時,變數可以設定預設值。</p>
+
+<pre class="brush: js">const {a = 10, b = 5} = {a: 3};
+
+console.log(a); // 3
+console.log(b); // 5</pre>
+
+<h3 id="指定新的變數名稱及預設值">指定新的變數名稱及預設值</h3>
+
+<p>屬性 1) 可以從物件中被解開,且被指定一個不同名稱的變數及 2) 同時指定一個預設值,在解開的值為 <code>undefined</code> 時使用。</p>
+
+<pre class="brush: js">const {a:aa = 10, b:bb = 5} = {a: 3};
+
+console.log(aa); // 3
+console.log(bb); // 5
+</pre>
+
+<h3 id="從作為函式參數的物件中提出某屬性的值">從作為函式參數的物件中提出某屬性的值</h3>
+
+<pre class="brush:js">const user = {
+ id: 42,
+ displayName: 'jdoe',
+ fullName: {
+ firstName: 'John',
+ lastName: 'Doe'
+ }
+};
+
+function userId({id}) {
+ return id;
+}
+
+function whois({displayName, fullName: {firstName: name}}) {
+ return `${displayName} is ${name}`;
+}
+
+console.log(userId(user)); // 42
+console.log(whois(user)); // "jdoe is John"</pre>
+
+<p>這樣從 user 物件中提出了 <code>id</code>, <code>displayName</code> 和 <code>firstName</code> 並且印出。</p>
+
+<h3 id="設定函式參數的預設值">設定函式參數的預設值</h3>
+
+<pre class="brush: js">function drawChart({size = 'big', coords = {x: 0, y: 0}, radius = 25} = {}) {
+ console.log(size, coords, radius);
+ // do some chart drawing
+}
+
+drawChart({
+ coords: {x: 18, y: 30},
+ radius: 30
+});</pre>
+
+<div class="note">
+<p>在上述函式 <strong><code>drawChart</code></strong> 中,左方之解構式被指派到一個空物件: <code>{size = 'big', coords = {x: 0, y: 0}, radius = 25} = {}</code> 。你也可以略過填寫右方的指派式。不過,當你沒有使用右方指派式時,函式在呼叫時會找出最少一個參數。透過上述形式,你可以直接不使用參數的呼叫 <code><strong>drawChart()</strong></code> 。當你希望在呼叫這個函式時不傳送參數,這個設計會帶來方便。而另一個設計則能讓你確保函式必須傳上一個物件作為參數。</p>
+</div>
+
+<h3 id="巢狀物件或陣列的解構">巢狀物件或陣列的解構</h3>
+
+<pre class="brush:js">const metadata = {
+ title: 'Scratchpad',
+ translations: [
+ {
+ locale: 'de',
+ localization_tags: [],
+ last_edit: '2014-04-14T08:43:37',
+ url: '/de/docs/Tools/Scratchpad',
+ title: 'JavaScript-Umgebung'
+ }
+ ],
+ url: '/en-US/docs/Tools/Scratchpad'
+};
+
+let {
+ title: englishTitle, // rename
+ translations: [
+ {
+ title: localeTitle, // rename
+ },
+ ],
+} = metadata;
+
+console.log(englishTitle); // "Scratchpad"
+console.log(localeTitle); // "JavaScript-Umgebung"</pre>
+
+<h3 id="循環取出的解構">循環取出的解構</h3>
+
+<pre class="brush: js">const people = [
+ {
+ name: 'Mike Smith',
+ family: {
+ mother: 'Jane Smith',
+ father: 'Harry Smith',
+ sister: 'Samantha Smith'
+ },
+ age: 35
+ },
+ {
+ name: 'Tom Jones',
+ family: {
+ mother: 'Norah Jones',
+ father: 'Richard Jones',
+ brother: 'Howard Jones'
+ },
+ age: 25
+ }
+];
+
+for (const {name: n, family: {father: f}} of people) {
+ console.log('Name: ' + n + ', Father: ' + f);
+}
+
+// "Name: Mike Smith, Father: Harry Smith"
+// "Name: Tom Jones, Father: Richard Jones"
+</pre>
+
+<h3 id="以物件演算屬性名稱解構">以物件演算屬性名稱解構</h3>
+
+<p>物件演算屬性名稱(像是在 <a href="/zh-TW/docs/Web/JavaScript/Reference/Operators/Object_initializer#Computed_property_names">object literals</a>)可以在解構指派式使用。</p>
+
+<pre class="brush: js">let key = 'z';
+let {[key]: foo} = {z: 'bar'};
+
+console.log(foo); // "bar"
+</pre>
+
+<h3 id="在物件解構時使用其餘變數">在物件解構時使用其餘變數</h3>
+
+<p><a class="external external-icon" href="https://github.com/tc39/proposal-object-rest-spread">ECMAScript 中的其餘/展開屬性</a>在 proposal (stage 4) 新增了在解構式內使用<a href="/zh-TW/docs/Web/JavaScript/Reference/Functions/rest_parameters">其餘 (rest)</a> 語法的定義。其餘屬性可以收集解構式中沒有指定的屬性值。</p>
+
+<pre class="brush: js">let {a, b, ...rest} = {a: 10, b: 20, c: 30, d: 40}
+a; // 10
+b; // 20
+rest; // { c: 30, d: 40 }</pre>
+
+<h3 id="不符合_JavaScript_識別字的屬性名稱">不符合 JavaScript 識別字的屬性名稱</h3>
+
+<p>解構賦值可以透過另一個符合 JavaScript <a href="/zh-TW/docs/Glossary/Identifier">識別字</a>的變數名稱來解出不符合識別字的屬性。</p>
+
+<pre class="brush: js">const foo = { 'fizz-buzz': true };
+const { 'fizz-buzz': fizzBuzz } = foo;
+
+console.log(fizzBuzz); // "true"
+</pre>
+
+<h3 id="混合使用矩陣及物件解構">混合使用矩陣及物件解構</h3>
+
+<p>矩陣及物件解構可以混合進行。與例來說,你只需要使用下列 <code>props</code> 矩陣中第三個元素之物件中的 <code>name</code> 屬性,你可以如下面的例子進行解構:</p>
+
+<pre class="brush: js">const props = [
+ { id: 1, name: 'Fizz'},
+ { id: 2, name: 'Buzz'},
+ { id: 3, name: 'FizzBuzz'}
+];
+
+const [,, { name }] = props;
+
+console.log(name); // "FizzBuzz"
+</pre>
+
+<h3 id="物件解構時的原型鏈追溯">物件解構時的原型鏈追溯</h3>
+
+<p>在進行物件解構時,如果一個屬性不在其當下存取,將會透過原型鏈 (prototype chain) 來進行追溯。</p>
+
+<pre>let obj = {self: '123'};
+obj.__proto__.prot = '456';
+const {self, prot} = obj;
+// self "123"
+// prot "456"(Access to the prototype chain)</pre>
+
+<h2 id="規範">規範</h2>
+
+<table class="standard-table">
+ <tbody>
+ <tr>
+ <th scope="col">規範</th>
+ </tr>
+ <tr>
+ <td>{{SpecName('ESDraft', '#sec-destructuring-assignment', 'Destructuring assignment')}}</td>
+ </tr>
+ </tbody>
+</table>
+
+<h2 id="瀏覽器相容性">瀏覽器相容性</h2>
+
+<div>
+
+
+<p>{{Compat("javascript.operators.destructuring")}}</p>
+</div>
+
+<h2 id="參見">參見</h2>
+
+<ul>
+ <li><a href="/zh-TW/docs/Web/JavaScript/Reference/Operators/Assignment_Operators">Assignment operators</a></li>
+ <li><a href="https://hacks.mozilla.org/2015/05/es6-in-depth-destructuring/">"ES6 in Depth: Destructuring" on hacks.mozilla.org</a></li>
+</ul>