uTools-Manuals/docs/javascript/Reference/Global_Objects/Symbol/unscopables.html

<article id="wikiArticle">
<div></div>
<p><strong><code>Symbol.unscopables </code></strong>指用于指定对象值，其对象自身和继承的从关联对象的 with 环境绑定中排除的属性名称。</p>
<div><table class="standard-table">
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<th class="header" colspan="2"><code>Symbol.unscopables</code> 属性的属性特性：</th>
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<tbody>
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<td>writable</td>
<td>false</td>
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<td>enumerable</td>
<td>false</td>
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<td>configurable</td>
<td>false</td>
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</tbody>
</table></div>
<h2 id="描述">描述</h2>
<p><code>可以在任何对象上定义 @@unscopables</code> symbol (<code>Symbol.unscopables</code>)<code>，用于排除属性名称并与 with 环境绑定在一起作为词法变量公开。 请注意，如果使用 </code><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode">Strict mode</a><code>，语句将不可用，并且可能也不需要 symbol。</code></p>
<p>在 <code>unscopables</code> 对象上设置属性为 true，将使其 <em>unscopable</em> 并且因此该属性也将不会在词法环境变量中出现。 如果设置属性为 <code>false</code> ，则将使其可 <code>scopable</code> 并且该属性会出现在词法环境变量中。</p>
<h2 id="示例">示例</h2>
<p>下列的代码可兼容 ES5 及以下版本。然而，在 ECMAScript 2015 (ES6) 或其后续版本中，<a href="Reference/Global_Objects/Array/keys" title="keys() 方法返回一个包含数组中每个索引键的Array Iterator对象。"><code>Array.prototype.keys()</code></a> 方法才会出现。意味着内部 <code>with</code> 环境“关键字” 存在该方法，但变量中不会存在。 也就是说，当 <code>unscopable</code>s symbol 被展示时，内置的<code>unscopables </code>设置是由 <a href="Reference/Global_Objects/Array/@@unscopables" title="Symbol属性 @@unscopable 包含了所有 ES2015 (ES6) 中新定义的且并未被更早的 ECMAScript 标准收纳的属性名。这些属性并不包含在 with 语句绑定的环境中"><code>Array.prototype[@@unscopables]</code></a> 展示并实现的， 一些 Array 的方法 将作为 scoped 放入 <code>with</code> 语句中。</p>
<pre><code  class="language-javascript">var keys = [];

with(Array.prototype) {
  keys.push("something");
}

Object.keys(Array.prototype[Symbol.unscopables]); 
// ["copyWithin", "entries", "fill", "find", "findIndex", 
//  "includes", "keys", "values"]
</code></pre>
<p>你也可以为你自己的对象设置 <code>unscopables</code> 。</p>
<pre><code  class="language-javascript">var obj = { 
  foo: 1, 
  bar: 2 
};

obj[Symbol.unscopables] = { 
  foo: false, 
  bar: true 
};

with(obj) {
  console.log(foo); // 1
  console.log(bar); // ReferenceError: bar is not defined
}
</code></pre>
<h2 id="规范">规范</h2>
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<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
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<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-symbol.unscopables" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Symbol.unscopables</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>首次定义</td>
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<td><a class="external" href="https://tc39.github.io/ecma262/#sec-symbol.unscopables" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">Symbol.unscopables</small></a></td>
<td><span class="spec-Draft">Draft</span></td>
<td><span class="op_dict_text2">起草</span></td>
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</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<p></p><div class="blockIndicator warning"><strong><a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">We're converting our compatibility data into a machine-readable JSON format</a></strong>.
            This compatibility table still uses the old format,
            because we haven't yet converted the data it contains.
            <strong><a class="new" href="/zh-CN/docs/MDN/Contribute/Structures/Compatibility_tables" rel="nofollow">Find out how you can help!</a></strong></div>
<div class="htab">
<a id="AutoCompatibilityTable" name="AutoCompatibilityTable"></a>
<ul>
<li class="selected"><a>Desktop</a></li>
<li><a>Mobile</a></li>
</ul>
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<div id="compat-desktop">
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<th>Feature</th>
<th>Chrome</th>
<th>Firefox (Gecko)</th>
<th>Internet Explorer/Edge</th>
<th>Opera</th>
<th>Safari</th>
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<td>Basic support</td>
<td>38</td>
<td><a href="/en-US/Firefox/Releases/48" title="Released on 2016-08-02.">48</a> (48)</td>
<td>12</td>
<td><span style="color: #f00;">未实现</span></td>
<td>9</td>
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</div>
<div id="compat-mobile">
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<th>Feature</th>
<th>Android</th>
<th>Chrome for Android</th>
<th>Firefox Mobile (Gecko)</th>
<th>IE/Edge Mobile</th>
<th>Opera Mobile</th>
<th>Safari Mobile</th>
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<td>Basic support</td>
<td><span style="color: #f00;">未实现</span></td>
<td><span style="color: #f00;">未实现</span></td>
<td>48.0 (48)</td>
<td>12</td>
<td><span style="color: #f00;">未实现</span></td>
<td>9</td>
</tr>
</tbody>
</table>
</div>
<h2 id="相关链接">相关链接</h2>
<ul>
<li><a href="Reference/Global_Objects/Array/@@unscopables" title="Symbol属性 @@unscopable 包含了所有 ES2015 (ES6) 中新定义的且并未被更早的 ECMAScript 标准收纳的属性名。这些属性并不包含在 with 语句绑定的环境中"><code>Array.prototype[@@unscopables]</code></a></li>
<li><code><a href="/en-US/docs/Web/JavaScript/Reference/Statements/with">with</a></code> statement (not available in <a href="/en-US/docs/Web/JavaScript/Reference/Strict_mode">Strict mode</a>)</li>
</ul>
</article>