uTools-Manuals/docs/javascript/Reference/Global_Objects/Object/toString.html

<article id="wikiArticle">
<p></p><p></p>
<p><code><strong>toString()</strong></code> 方法返回一个表示该对象的字符串。</p>
<h2 id="Syntax" name="Syntax">语法</h2>
<pre><code class="language-javascript"><code><em>object</em>.toString()</code>
</code></pre>
<h3 id="返回值">返回值</h3>
<p>表示该对象的字符串。</p>
<h2 id="Description" name="Description">描述</h2>
<p>每个对象都有一个<code>toString()</code>方法，当该对象被表示为一个文本值时，或者一个对象以预期的字符串方式引用时自动调用。默认情况下，<code>toString()</code>方法被每个<code>Object</code>对象继承。如果此方法在自定义对象中未被覆盖，<code>toString()</code> 返回 "[object <em>type</em>]"，其中<code>type</code>是对象的类型。以下代码说明了这一点：</p>
<pre><code  class="language-javascript"><code>var o = new Object();
o.toString(); // returns [object Object]</code></code></pre>
<div class="note"><strong>注意：</strong>从JavaScript1.8.5开始<code>toString()</code>调用 <a href="Reference/Global_Objects/null" title="值 null 特指对象的值未设置。它是 JavaScript 基本类型 之一。"><code>null</code></a>返回<code>[object <em>Null</em>]</code>，<a href="Reference/Global_Objects/undefined" title="undefined是全局对象的一个属性。也就是说，它是全局作用域的一个变量。undefined的最初值就是原始数据类型undefined。"><code>undefined</code></a> 返回<code>[object Undefined]</code>，如第5版的ECMAScript和随后的Errata。请参阅<a href="#Using_toString()_to_detect_object_class">使用<code>toString()</code>检测对象类型</a>。</div>
<h2 id="Examples" name="Examples">示例</h2>
<h3 id="Overriding_the_default_toString_method" name="Overriding_the_default_toString_method">覆盖默认的<code>toString</code>方法</h3>
<p>可以自定义一个方法来取代默认的<span> </span><code>toString()</code><span> 方法。该</span><span> </span><code>toString()</code><span> 方法不能传入参数并且必须返回一个字符串。自定义的</span><span> </span><code>toString()</code><span> 方法可以是任何我们需要的值，但如果它附带有关对象的信息，它将变的非常有用。</span></p>
<p><span>以下代码定义了<code>Dog</code>对象类型，并创建了一个<code>Dog</code>类型的</span><code>theDog</code><span>对象：</span></p>
<pre><code  class="language-javascript">function Dog(name,breed,color,sex) {
   this.name=name;
   this.breed=breed;
   this.color=color;
   this.sex=sex;
}

var theDog = new Dog("Gabby","Lab","chocolate","female");</code></pre>
<p>如果当前的对象调用了<span> </span><code>toString()</code><span> 方法，它将会返回从</span><a href="Reference/Global_Objects/Object" title="Object 构造函数创建一个对象包装器。"><code>Object</code></a>继承下来的<span> </span><code>toString()</code><span>方法</span><span>的返回默认值：</span></p>
<pre><code  class="language-javascript"><code>theDog.toString(); // returns [object Object]</code></code></pre>
<p>下面的代码中定义了一个叫做<span> </span><code>dogToString()</code><span> 的方法来覆盖默认的</span><span> </span><code>toString()</code><span> 方法。这个方法生成一个 "</span><code>property = value;</code><span>" 形式的字符串，该字符串包含了当前对象的 </span><span>name， breed，color 和 sex 的值。</span></p>
<pre><code  class="language-javascript">Dog.prototype.toString = function dogToString() {
  var ret = "Dog " + this.name + " is a " + this.sex + " " + this.color + " " + this.breed;
  return ret;
}</code></pre>
<p>使用上述代码，任何时候在字符串上下文中使用<code>theDog.toString()</code><span>，</span><span>JavaScript 都会自动调用</span><span> </span><code>dogToString()</code><span> 方法（</span><code>dogToString()</code>可以是一个匿名函数<span>）</span><span>，并且返回以下字符串</span><span>：</span></p>
<pre><code  class="language-javascript"><code>"Dog Gabby is a female chocolate Lab"</code></code></pre>
<h3 id="Using_toString_to_detect_object_type" name="Using_toString_to_detect_object_type"><a id="Using_toString()_to_detect_object_class" name="Using_toString()_to_detect_object_class">使用<code>toString()</code>检测对象类型</a></h3>
<p>可以通过<code>toString()</code><span> 来获取每个对象的类型。为了每个对象都能通过</span><span> </span><code>Object.prototype.toString()</code><span> 来检测，需要以</span><span> </span><code>Function.prototype.call()</code><span> 或者 </span><code>Function.prototype.apply()</code><span> 的形式来调用，传递要检查的对象作为第一个参数，称为<code>thisArg</code>。</span></p>
<pre><code  class="language-javascript">var toString = Object.prototype.toString;

toString.call(new Date); // [object Date]
toString.call(new String); // [object String]
toString.call(Math); // [object Math]

//Since JavaScript 1.8.5
toString.call(undefined); // [object Undefined]
toString.call(null); // [object Null]
</code></pre>
<h2 id="规范" style="margin-bottom: 20px; line-height: 30px;">规范</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%201st%20edition,%20June%201997.pdf" hreflang="en" lang="en" rel="noopener" title="ECMAScript 1st Edition (ECMA-262)">ECMAScript 1st Edition (ECMA-262)</a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>Initial definition. Implemented in JavaScript 1.0.</td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-15.2.4.2" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">Object.prototype.toString</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>Call on <a href="Reference/Global_Objects/null" title="值 null 特指对象的值未设置。它是 JavaScript 基本类型 之一。"><code>null</code></a> returns <code>[object <em>Null</em>]</code>, and <a href="Reference/Global_Objects/undefined" title="undefined是全局对象的一个属性。也就是说，它是全局作用域的一个变量。undefined的最初值就是原始数据类型undefined。"><code>undefined</code></a> returns <code>[object <em>Undefined</em>]</code></td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-object.prototype.tostring" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Object.prototype.toString</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://tc39.github.io/ecma262/#sec-object.prototype.tostring" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">Object.prototype.toString</small></a></td>
<td><span class="spec-Draft">Draft</span></td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容" style="margin-bottom: 20px; line-height: 30px;">浏览器兼容</h2>
<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>
<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>toString</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">
<span>Full support</span>
</abbr>
              Yes</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">
<span>Full support</span>
</abbr>
              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">
<span>Full support</span>
</abbr>
              1</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">
<span>Full support</span>
</abbr>
              Yes</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">
<span>Full support</span>
</abbr>
              Yes</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">
<span>Full support</span>
</abbr>
              Yes</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">
<span>Full support</span>
</abbr>
              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">
<span>Full support</span>
</abbr>
              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">
<span>Full support</span>
</abbr>
              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">
<span>Full support</span>
</abbr>
              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">
<span>Full support</span>
</abbr>
              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">
<span>Full support</span>
</abbr>
              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">
<span>Full support</span>
</abbr>
              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">
<span>Full support</span>
</abbr>
              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">
<abbr class="bc-level bc-level-yes only-icon" title="Full support">
<span>Full support</span>
                  
                </abbr></span></dt><dd>Full support</dd></dl></section></div><p></p>
<h2 id="See_also" name="See_also" style="margin-bottom: 20px; line-height: 30px;">相关链接</h2>
<ul>
<li><a href="Reference/Global_Objects/Object/toSource" title="toSource()方法返回一个表示对象源代码的字符串。"><code>Object.prototype.toSource()</code></a></li>
<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>
<li><a href="Reference/Global_Objects/Number/toString" title="toString() 方法返回指定 Number 对象的字符串表示形式。"><code>Number.prototype.toString()</code></a></li>
</ul>
</article>