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120 lines
11 KiB
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<article id="wikiArticle">
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<div></div>
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<div><strong><code>new.target</code></strong>属性允许你检测函数或构造方法是否是通过<a href="Reference/Operators/new">new</a>运算符被调用的。在通过<a href="Reference/Operators/new">new</a>运算符被初始化的函数或构造方法中，<code>new.target</code>返回一个指向构造方法或函数的引用。在普通的函数调用中，<code>new.target</code> 的值是<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>。</div>
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<h2 id="语法">语法</h2>
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<pre><code class="language-javascript">new.target</code></pre>
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<h2 id="描述">描述</h2>
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<p><code>new.target</code>语法由一个关键字"<code>new</code>"，一个点，和一个属性名"<font face="Consolas, Liberation Mono, Courier, monospace">target</font>"组成。通常"<code>new.</code>"<code>的</code>作用是提供属性访问的上下文，但这里"<code>new.</code>"其实不是一个真正的对象。不过在构造方法调用中，<code>new.target</code>指向被<code>new</code>调用的构造函数，所以"<code>new.</code>"成为了一个虚拟上下文。</p>
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<p><code>new.target</code>属性适用于所有函数访问的元属性。在 <a class="external" href="http://www.javascripttutorial.net/es6/javascript-arrow-function/" rel="noopener">arrow functions</a> 中，<code>new.target</code> 指向最近的外层函数的<code>new.target</code>（An arrow function expression does not have its own this, arguments, super , or new.target) 。</p>
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<h2 id="示例">示例</h2>
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<h3 id="函数调用中的_new.target">函数调用中的 new.target</h3>
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<p>在普通的函数调用中（和作为构造函数来调用相对），<code>new.target</code>的值是<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>。这使得你可以检测一个函数是否是作为构造函数通过<a href="Reference/Operators/new">new</a>被调用的。</p>
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<pre><code class="language-javascript">function Foo() {
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if (!new.target) throw "Foo() must be called with new";
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console.log("Foo instantiated with new");
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}
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Foo(); // throws "Foo() must be called with new"
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new Foo(); // logs "Foo instantiated with new"
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</code></pre>
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<h3 id="构造方法中的_new.target">构造方法中的 new.target</h3>
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<p>在类的构造方法中，<code>new.target</code>指向直接被<code>new</code>执行的构造函数。并且当一个父类构造方法在子类构造方法中被调用时，情况与之相同。</p>
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<pre><code class="language-javascript">class A {
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constructor() {
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console.log(new.target.name);
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}
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}
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class B extends A { constructor() { super(); } }
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var a = new A(); // logs "A"
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var b = new B(); // logs "B"
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class C { constructor() { console.log(new.target); } }
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class D extends C { constructor() { super(); } }
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var c = new C(); // logs class C{constructor(){console.log(new.target);}}
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var d = new D(); // logs class D extends C{constructor(){super();}}
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</code></pre>
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<p class="summary">从上面类 C 和 D 的例子可以看出来，new.target 指向的是初始化类的类定义。比如当 D 通过 new 初始化的时候，打印出了 D 的类定义，C 的例子与之类似，打印出的是 C 的类定义。</p>
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<h2 id="规范">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-262/6.0/#sec-built-in-function-objects" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Built-in Function Objects</small></a></td>
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<td><span class="spec-Standard">Standard</span></td>
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<td>Initial definition.</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-built-in-function-objects" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">Built-in Function Objects</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>new.target</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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46</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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Yes</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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41</td><td class="bc-supports-no bc-browser-ie"><span class="bc-browser-name">IE</span><abbr class="bc-level-no only-icon" title="No support">
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<span>No support</span>
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</abbr>
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No</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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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">
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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-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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46</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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46</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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41</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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5.0</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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5.0.0</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-no bc-supports">
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<abbr class="bc-level bc-level-no only-icon" title="No support">
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<span>No support</span>
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</abbr></span></dt><dd>No support</dd></dl></section></div><p></p>
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<h2 id="相关链接">相关链接</h2>
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<ul>
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<li><a href="Reference/Functions">Functions</a></li>
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<li><a href="Reference/Classes">Classes</a></li>
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<li><code><a href="Reference/Operators/new">new</a></code></li>
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<li><code><a href="Reference/Operators/this">this</a></code></li>
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</ul>
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</article> |