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183 lines
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183 lines
15 KiB
HTML
<article id="wikiArticle">
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<div></div>
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<p><code><strong>Object.preventExtensions()</strong></code>方法让一个对象变的不可扩展,也就是永远不能再添加新的属性。</p>
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<h2 id="Syntax" name="Syntax">语法</h2>
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<pre><code class="language-javascript"><code>Object.preventExtensions(<em>obj</em>)</code></code></pre>
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<h3 id="Parameters" name="Parameters">参数</h3>
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<dl>
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<dt><code>obj</code></dt>
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<dd>将要变得不可扩展的对象。</dd>
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<dt>
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<h3 id="返回值">返回值</h3>
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</dt>
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<dd>已经不可扩展的对象。</dd>
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</dl>
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<h2 id="Description" name="Description">描述</h2>
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<p>如果一个对象可以添加新的属性,则这个对象是可扩展的。<code>Object.preventExtensions()</code>将对象标记为不再可扩展,因此它将永远不会具有超出它被标记为不可扩展的属性。注意,一般来说,不可扩展对象的属性可能仍然可被删除。尝试将新属性添加到不可扩展对象将静默失败或抛出<a href="Reference/Global_Objects/TypeError" title="TypeError(类型错误) 对象用来表示值的类型非预期类型时发生的错误。"><code>TypeError</code></a>(最常见但不排除其他情况,如在<a class="new" href="Reference/Functions_and_function_scope/Strict_mode" rel="nofollow" title="此页面仍未被本地化, 期待您的翻译!">strict mode</a>中)。</p>
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<p><code>Object.preventExtensions()</code>仅阻止添加自身的属性。但属性仍然可以添加到对象原型。</p>
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<p>一旦使其不可扩展,就无法再对象进行扩展。</p>
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<h2 id="Examples" name="Examples">例子</h2>
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<pre><code class="language-javascript">// Object.preventExtensions将原对象变的不可扩展,并且返回原对象.
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var obj = {};
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var obj2 = Object.preventExtensions(obj);
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obj === obj2; // true
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// 字面量方式定义的对象默认是可扩展的.
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var empty = {};
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Object.isExtensible(empty) //=== true
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// ...但可以改变.
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Object.preventExtensions(empty);
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Object.isExtensible(empty) //=== false
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// 使用Object.defineProperty方法为一个不可扩展的对象添加新属性会抛出异常.
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var nonExtensible = { removable: true };
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Object.preventExtensions(nonExtensible);
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Object.defineProperty(nonExtensible, "new", { value: 8675309 }); // 抛出TypeError异常
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// 在严格模式中,为一个不可扩展对象的新属性赋值会抛出TypeError异常.
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function fail()
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{
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"use strict";
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nonExtensible.newProperty = "FAIL"; // throws a TypeError
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}
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fail();
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// 一个不可扩展对象的原型是不可更改的,__proto__是个非标准魔法属性,可以更改一个对象的原型.
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var fixed = Object.preventExtensions({});
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fixed.__proto__ = { oh: "hai" }; // 抛出TypeError异常</code></pre>
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<h2 id="Notes">Notes</h2>
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<p>在 ES5 中,如果参数不是一个对象类型,将抛出一个<a href="https://developer.mozilla.orgReference/Global_Objects/TypeError" title="TypeError(类型错误) 对象用来表示值的类型非预期类型时发生的错误。"><code>TypeError</code></a>异常。在 ES2015 中,非对象参数将被视为一个不可扩展的普通对象,因此会被直接返回。</p>
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<pre><code class="language-javascript">Object.preventExtensions(1);
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// TypeError: 1 is not an object (ES5 code)
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Object.preventExtensions(1);
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// 1 (ES2015 code)</code></pre>
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<h2 id="规范" style="margin-bottom: 20px; line-height: 30px;">规范</h2>
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<table class="standard-table">
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<tbody>
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<tr>
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<th scope="col">Specification</th>
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<th scope="col">Status</th>
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<th scope="col">Comment</th>
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</tr>
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<tr>
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<td><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-15.2.3.10" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">Object.preventExtensions</small></a></td>
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<td><span class="spec-Standard">Standard</span></td>
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<td>Initial definition. Implemented in JavaScript 1.8.5.</td>
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</tr>
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<tr>
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<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-object.preventextensions" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Object.preventExtensions</small></a></td>
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<td><span class="spec-Standard">Standard</span></td>
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<td> </td>
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</tr>
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<tr>
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<td><a class="external" href="https://tc39.github.io/ecma262/#sec-object.preventextensions" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">Object.preventExtensions</small></a></td>
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<td><span class="spec-Draft">Draft</span></td>
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<td> </td>
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</tr>
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</tbody>
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</table>
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<h2 id="浏览器兼容">浏览器兼容</h2>
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<div class="hidden">The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out <a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">https://github.com/mdn/browser-compat-data</a> and send us a pull request.</div>
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<p></p><div class="bc-data"><a class="bc-github-link external" href="https://github.com/mdn/browser-compat-data" rel="noopener">Update compatibility data on GitHub</a><table class="bc-table bc-table-js"><thead><tr class="bc-platforms"><td></td><th class="bc-platform-desktop" colspan="6"><span>Desktop</span></th><th class="bc-platform-mobile" colspan="7"><span>Mobile</span></th><th class="bc-platform-server" colspan="1"><span>Server</span></th></tr><tr class="bc-browsers"><td></td><th class="bc-browser-chrome"><span class="bc-head-txt-label bc-head-icon-chrome">Chrome</span></th><th class="bc-browser-edge"><span class="bc-head-txt-label bc-head-icon-edge">Edge</span></th><th class="bc-browser-firefox"><span class="bc-head-txt-label bc-head-icon-firefox">Firefox</span></th><th class="bc-browser-ie"><span class="bc-head-txt-label bc-head-icon-ie">Internet Explorer</span></th><th class="bc-browser-opera"><span class="bc-head-txt-label bc-head-icon-opera">Opera</span></th><th class="bc-browser-safari"><span class="bc-head-txt-label bc-head-icon-safari">Safari</span></th><th class="bc-browser-webview_android"><span class="bc-head-txt-label bc-head-icon-webview_android">Android webview</span></th><th class="bc-browser-chrome_android"><span class="bc-head-txt-label bc-head-icon-chrome_android">Chrome for Android</span></th><th class="bc-browser-edge_mobile"><span class="bc-head-txt-label bc-head-icon-edge_mobile">Edge Mobile</span></th><th class="bc-browser-firefox_android"><span class="bc-head-txt-label bc-head-icon-firefox_android">Firefox for Android</span></th><th class="bc-browser-opera_android"><span class="bc-head-txt-label bc-head-icon-opera_android">Opera for Android</span></th><th class="bc-browser-safari_ios"><span class="bc-head-txt-label bc-head-icon-safari_ios">Safari on iOS</span></th><th class="bc-browser-samsunginternet_android"><span class="bc-head-txt-label bc-head-icon-samsunginternet_android">Samsung Internet</span></th><th class="bc-browser-nodejs"><span class="bc-head-txt-label bc-head-icon-nodejs">Node.js</span></th></tr></thead><tbody><tr><th scope="row"><code>preventExtensions</code></th><td class="bc-supports-yes bc-browser-chrome"><span class="bc-browser-name">Chrome</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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6</td><td class="bc-supports-yes bc-browser-edge"><span class="bc-browser-name">Edge</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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12</td><td class="bc-supports-yes bc-browser-firefox"><span class="bc-browser-name">Firefox</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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4</td><td class="bc-supports-yes bc-browser-ie"><span class="bc-browser-name">IE</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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9</td><td class="bc-supports-yes bc-browser-opera"><span class="bc-browser-name">Opera</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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12</td><td class="bc-supports-yes bc-browser-safari"><span class="bc-browser-name">Safari</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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5.1</td><td class="bc-supports-yes bc-browser-webview_android"><span class="bc-browser-name">WebView Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td><td class="bc-supports-yes bc-browser-chrome_android"><span class="bc-browser-name">Chrome Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td><td class="bc-supports-yes bc-browser-edge_mobile"><span class="bc-browser-name">Edge Mobile</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td><td class="bc-supports-yes bc-browser-firefox_android"><span class="bc-browser-name">Firefox Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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4</td><td class="bc-supports-yes bc-browser-opera_android"><span class="bc-browser-name">Opera Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td><td class="bc-supports-yes bc-browser-safari_ios"><span class="bc-browser-name">Safari iOS</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td><td class="bc-supports-yes bc-browser-samsunginternet_android"><span class="bc-browser-name">Samsung Internet Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td><td class="bc-supports-yes bc-browser-nodejs"><span class="bc-browser-name">nodejs</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td></tr><tr><th scope="row">ES2015 behavior for non-object argument</th><td class="bc-supports-yes bc-browser-chrome"><span class="bc-browser-name">Chrome</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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44</td><td class="bc-supports-unknown bc-browser-edge"><span class="bc-browser-name">Edge</span><abbr title="Compatibility unknown; please update this.">
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?
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</abbr></td><td class="bc-supports-yes bc-browser-firefox"><span class="bc-browser-name">Firefox</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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35</td><td class="bc-supports-yes bc-browser-ie"><span class="bc-browser-name">IE</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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11</td><td class="bc-supports-yes bc-browser-opera"><span class="bc-browser-name">Opera</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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31</td><td class="bc-supports-yes bc-browser-safari"><span class="bc-browser-name">Safari</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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9</td><td class="bc-supports-yes bc-browser-webview_android"><span class="bc-browser-name">WebView Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td><td class="bc-supports-yes bc-browser-chrome_android"><span class="bc-browser-name">Chrome Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td><td class="bc-supports-unknown bc-browser-edge_mobile"><span class="bc-browser-name">Edge Mobile</span><abbr title="Compatibility unknown; please update this.">
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?
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</abbr></td><td class="bc-supports-yes bc-browser-firefox_android"><span class="bc-browser-name">Firefox Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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35</td><td class="bc-supports-yes bc-browser-opera_android"><span class="bc-browser-name">Opera Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td><td class="bc-supports-yes bc-browser-safari_ios"><span class="bc-browser-name">Safari iOS</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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9</td><td class="bc-supports-yes bc-browser-samsunginternet_android"><span class="bc-browser-name">Samsung Internet Android</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td><td class="bc-supports-yes bc-browser-nodejs"><span class="bc-browser-name">nodejs</span><abbr class="bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr>
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Yes</td></tr></tbody></table><section class="bc-legend" id="sect1"><h3 class="offscreen" id="Legend">Legend</h3><dl><dt><span class="bc-supports-yes bc-supports">
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<abbr class="bc-level bc-level-yes only-icon" title="Full support">
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<span>Full support</span>
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</abbr></span></dt><dd>Full support</dd><dt><span class="bc-supports-unknown bc-supports">
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<abbr class="bc-level bc-level-unknown only-icon" title="Compatibility unknown">
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<span>Compatibility unknown</span>
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</abbr></span></dt><dd>Compatibility unknown</dd></dl></section></div><p></p>
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<h2 id="See_also" name="See_also">相关链接</h2>
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<ul>
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<li><a href="Reference/Global_Objects/Object/isExtensible" title="Object.isExtensible() 方法判断一个对象是否是可扩展的(是否可以在它上面添加新的属性)。"><code>Object.isExtensible()</code></a></li>
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<li><a href="Reference/Global_Objects/Object/seal" title="The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request."><code>Object.seal()</code></a></li>
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<li><a href="Reference/Global_Objects/Object/isSealed" title="Object.isSealed() 方法判断一个对象是否被密封。"><code>Object.isSealed()</code></a></li>
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<li><a href="Reference/Global_Objects/Object/freeze" title="Object.freeze() 方法可以冻结一个对象。一个被冻结的对象再也不能被修改;冻结了一个对象则不能向这个对象添加新的属性,不能删除已有属性,不能修改该对象已有属性的可枚举性、可配置性、可写性,以及不能修改已有属性的值。此外,冻结一个对象后该对象的原型也不能被修改。freeze() 返回和传入的参数相同的对象。"><code>Object.freeze()</code></a></li>
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<li><a href="Reference/Global_Objects/Object/isFrozen" title="Object.isFrozen()方法判断一个对象是否被冻结。"><code>Object.isFrozen()</code></a></li>
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<li><a href="Reference/Global_Objects/Reflect/preventExtensions" title="静态方法 Reflect.preventExtensions() 方法阻止新属性添加到对象 例如:防止将来对对象的扩展被添加到对象中)。该方法与 Object.preventExtensions()相似,但有一些不同点。详情可见 differences。"><code>Reflect.preventExtensions()</code></a></li>
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</ul>
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</article> |