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

<article id="wikiArticle">
<div></div>
<h2 id="Summary" name="Summary">概述</h2>
<p><code><strong>valueOf()</strong></code> 方法返回当前 symbol 对象所包含的 symbol 原始值。</p>
<h2 id="Syntax" name="Syntax">语法</h2>
<pre><code class="language-javascript"><var>symbol.valueOf();
</var></code></pre>
<h2 id="Description" name="Description">描述</h2>
<p>在 JavaScript 中，虽然大多数类型的对象在某些操作下都会自动的隐式调用自身的 <code>valueOf()</code> 方法或者 <code>toString()</code> 方法来将自己转换成一个原始值，但 symbol 对象不会这么干，symbol 对象无法隐式转换成对应的原始值：</p>
<pre><code  class="language-javascript">Object(Symbol("foo")) + "bar";
// TypeError: can't convert symbol object to primitive
// 无法隐式的调用 valueOf() 方法

Object(Symbol("foo")).valueOf() + "bar";
// TypeError:  can't convert symbol to string
// 手动调用 valueOf() 方法，虽然转换成了原始值，但 symbol 原始值不能转换为字符串

Object(Symbol("foo")).toString() + "bar";
// "Symbol(foo)bar"，需要手动调用 toString() 方法才行
</code></pre>
<h2 id="规范">规范</h2>
<table class="standard-table">
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<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
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<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-symbol.prototype.valueof" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Symbol.prototype.valueOf</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>Initial definition.</td>
</tr>
</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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<th>Feature</th>
<th>Chrome</th>
<th>Firefox (Gecko)</th>
<th>Internet Explorer</th>
<th>Opera</th>
<th>Safari</th>
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<td>Basic support</td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><a href="/en-US/Firefox/Releases/33" title="Released on 2014-10-14.">33.0</a> (33.0)</td>
<td><span style="color: #f00;">未实现</span></td>
<td><span style="color: #f00;">未实现</span></td>
<td><span style="color: #f00;">未实现</span></td>
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</div>
<div id="compat-mobile">
<table class="compat-table">
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<th>Feature</th>
<th>Android</th>
<th>Chrome for Android</th>
<th>Firefox Mobile (Gecko)</th>
<th>IE Mobile</th>
<th>Opera Mobile</th>
<th>Safari Mobile</th>
</tr>
<tr>
<td>Basic support</td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td>33.0 (33.0)</td>
<td><span style="color: #f00;">未实现</span></td>
<td><span style="color: #f00;">未实现</span></td>
<td><span style="color: #f00;">未实现</span></td>
</tr>
</tbody>
</table>
</div>
<h2 id="See_Also" name="See_Also">相关链接</h2>
<ul>
<li><a href="Reference/Global_Objects/Object/valueOf" title="valueOf() 方法返回指定对象的原始值。"><code>Object.prototype.valueOf()</code></a></li>
</ul>
</article>