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11-3.html
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<!DOCTYPE html>
<html lang="zh-TW">
<head>
<meta charset="UTF-8" />
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<header>
<nav>
<h1>
<span id="toggle-menu"></span>
<a href="index.html"></a>
</h1>
</nav>
</header>
<main>
<aside>
<nav></nav>
</aside>
<article>
<h2 id="11-3">11-3 Trailing Commas</h2>
<p>
以往早期的 Javascript 版本在陣列和物件屬性最後加上逗號是會出錯。另外 function
放置參數的括號內也會出錯。
</p>
<h3>Array</h3>
<p>
版本控制上可以獲得好處。如下範例所示，在看差異時不會連同新增刪減的逗號也一併標記進去，增加可讀性。
</p>
<pre><code class="language-diff">
[
'foo',
+ 'baz',
'bar',
- 'baz',
]
</code></pre>
<p>結尾逗號不會影響陣列長度</p>
<pre><code class="language-js">
const arr = [
1,
2,
3,
];
arr; // [1, 2, 3]
arr.length; // 3
</code></pre>
<p>連續結尾逗號代表新增空白元素。</p>
<pre><code class="language-js">
const arr = [1, 2, 3, , ,];
arr.length; // 5
arr[3]; // undefined
</code></pre>
<h3>Objects</h3>
<p>物件屬性不可連續使用結尾逗號</p>
<pre><code class="language-js">
const object = {
foo: 'bar',
baz: 'qwerty',
age: 42,
};
const object2 = {
foo: 'bar',
baz: 'qwerty',
age: 42,,
};
// SyntaxError: Unexpected token ','
</code></pre>
<p>ES6 以後允許 function 使用結尾逗號。以下幾個版本的函式結果相等。</p>
<pre><code class="language-js">
function f(p) {}
function f(p,) {}
(p) => {};
(p,) => {};
</code></pre>
<p>其他案例: class 及 物件內 function</p>
<pre><code class="language-js">
class C {
one(a,) {}
two(a, b,) {}
}
const obj = {
one(a,) {},
two(a, b,) {},
};
</code></pre>
<h3>Function calls</h3>
<pre><code class="language-js">
f(p);
f(p,);
Math.max(10, 20);
Math.max(10, 20,);
</code></pre>
<h3>Illegal trailing commas</h3>
<pre><code class="language-js error-code">
function f(,) {} // SyntaxError: missing formal parameter
(,) => {}; // SyntaxError: expected expression, got ','
f(,) // SyntaxError: expected expression, got ','
function f(...p,) {} // SyntaxError: parameter after rest parameter
(...p,) => {} // SyntaxError: expected closing parenthesis, got ','
</code></pre>
<h3>Trailing commas in destructuring</h3>
<pre><code class="language-js">
// array destructuring with trailing comma
[a, b,] = [1, 2];
// object destructuring with trailing comma
const o = {
p: 42,
q: true,
};
const { p, q, } = o;
</code></pre>
<p>也無法用在展開運算子</p>
<pre><code class="language-js error-code">
const [a, ...b,] = [1, 2, 3];
// SyntaxError: rest element may not have a trailing comma
</code></pre>
<h3>Trailing commas in JSON</h3>
<p>JSON 不支援。</p>
<pre><code class="language-js error-code">
JSON.parse('[1, 2, 3, 4, ]');
JSON.parse('{"foo" : 1, }');
// SyntaxError JSON.parse: unexpected character
// at line 1 column 14 of the JSON data
</code></pre>
<pre><code class="language-js correct-code">
JSON.parse('[1, 2, 3, 4 ]');
JSON.parse('{"foo" : 1 }');
</code></pre>
<h3>Trailing commas in named imports and named exports</h3>
<h3>Named imports</h3>
<pre><code class="language-js">
import {
A,
B,
C,
} from 'D';
import { X, Y, Z, } from 'W';
import { A as B, C as D, E as F, } from 'Z';
</code></pre>
<h3>Named exports</h3>
<pre><code class="language-js ">
export {
A,
B,
C,
};
export { A, B, C, };
export { A as B, C as D, E as F, };
</code></pre>
<h3>Quantifier prefix</h3>
<pre><code class="language-js">
/x{2}/; // Exactly 2 occurrences of 'x'; equivalent to /xx/
/x{2,}/; // At least 2 occurrences of 'x'; equivalent to /xx+/
/x{2,4}/; // 2 to 4 occurrences of 'x'; equivalent to /xxx?x?/
</code></pre>
</article>
</main>
</body>
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</html>