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171 lines
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171 lines
18 KiB
HTML
<article id="wikiArticle">
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<div></div>
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<p><code>yield</code> 关键字用来暂停和恢复一个生成器函数（(<a href="Reference/Statements/function*" title="function* 这种声明方式(function关键字后跟一个星号）会定义一个生成器函数 (generator function)，它返回一个 Generator 对象。"><code>function*</code></a> 或<a href="Reference/Statements/Legacy_generator_function">遗留的生成器函数</a>）。</p>
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<h2 id="Syntax" name="Syntax">语法</h2>
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<pre><code class="language-javascript">[<em>rv</em>] = <strong>yield</strong> [<em>expression</em>];</code></pre>
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<dl>
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<dt><code>expression</code></dt>
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<dd>定义通过<a href="Reference/Iteration_protocols#iterator">迭代器协议</a>从生成器函数返回的值。如果省略，则返回<code>undefined</code>。</dd>
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<dt><code>rv</code></dt>
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<dd>
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<p>返回传递给生成器的<code>next()</code>方法的可选值，以恢复其执行。</p>
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</dd>
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</dl>
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<h2 id="描述">描述</h2>
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<p><code>yield</code>关键字使生成器函数执行暂停，<code>yield</code>关键字后面的表达式的值返回给生成器的调用者。它可以被认为是一个基于生成器的版本的<code>return</code>关键字。</p>
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<p><code>yield</code>关键字实际返回一个<code>IteratorResult</code>对象，它有两个属性，<code>value</code>和<code>done</code>。<code>value</code>属性是对<code>yield</code>表达式求值的结果，而<code>done</code>是<code>false</code>，表示生成器函数尚未完全完成。</p>
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<p>一旦遇到 <code>yield</code> 表达式，生成器的代码将被暂停运行，直到生成器的 <code>next()</code> 方法被调用。每次调用生成器的<code>next()</code>方法时，生成器都会恢复执行，直到达到以下某个值：</p>
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<ul>
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<li><code>yield</code>，导致生成器再次暂停并返回生成器的新值。 下一次调用<code>next()</code>时，在<code>yield</code>之后紧接着的语句继续执行。</li>
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<li><a href="Reference/Statements/throw" title="throw语句用来抛出一个用户自定义的异常。当前函数的执行将被停止（throw之后的语句将不会执行），并且控制将被传递到调用堆栈中的第一个catch块。如果调用者函数中没有catch块，程序将会终止。"><code>throw</code></a>用于从生成器中抛出异常。这让生成器完全停止执行，并在调用者中继续执行，正如通常情况下抛出异常一样。</li>
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<li>到达生成器函数的结尾；在这种情况下，生成器的执行结束，并且<code>IteratorResult</code>给调用者返回<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>并且<code>done</code>为<code>true</code>。</li>
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<li>到达<a href="Reference/Statements/return" title="return语句终止函数的执行，并返回一个指定的值给函数调用者。"><code>return</code></a> 语句。在这种情况下，生成器的执行结束，并将<code>IteratorResult</code>返回给调用者，其值是由<code>return</code>语句指定的，并且<code>done</code> 为<code>true</code>。</li>
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</ul>
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<p>如果将参数传递给生成器的<code>next()</code>方法，则该值将成为生成器当前<code>yield</code>操作返回的值。</p>
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<p>在生成器的代码路径中的<code>yield</code>运算符，以及通过将其传递给<a href="Reference/Global_Objects/Generator/next" title="next() 方法返回一个包含属性 done 和 value 的对象。该方法也可以通过接受一个参数用以向生成器传值。"><code>Generator.prototype.next()</code></a>指定新的起始值的能力之间，生成器提供了强大的控制力。</p>
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<h2 id="Examples" name="Examples">示例</h2>
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<p>以下代码是一个生成器函数的声明。</p>
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<pre><code class="language-javascript">function* countAppleSales () {
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var saleList = [3, 7, 5];
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for (var i = 0; i < saleList.length; i++) {
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yield saleList[i];
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}
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}</code></pre>
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<p>一旦生成器函数已定义，可以通过构造一个迭代器来使用它。</p>
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<pre><code class="language-javascript">var appleStore = countAppleSales(); // Generator { }
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console.log(appleStore.next()); // { value: 3, done: false }
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console.log(appleStore.next()); // { value: 7, done: false }
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console.log(appleStore.next()); // { value: 5, done: false }
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console.log(appleStore.next()); // { value: undefined, done: true }</code></pre>
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<h2 id="Specifications" name="Specifications">规范</h2>
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<table class="standard-table">
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<thead>
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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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</thead>
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<tbody>
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<tr>
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<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">Yield</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/#" hreflang="en" lang="en" rel="noopener">ECMAScript Latest Draft (ECMA-262)<br/><small lang="zh-CN">Yield</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="Browser_compatibility" name="Browser_compatibility">浏览器兼容性</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>yield</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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39</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 bc-has-history"><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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26<div class="bc-icons"><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> </div><section class="bc-history" id="sect1"><dl><dt class="bc-supports-yes bc-supports"><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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26<div class="bc-icons"><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> </div></dt><dd><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> Starting with Firefox 33, the parsing of the <code>yield</code> expression has been updated to conform with the ES2015 specification.</dd></dl></section></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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10</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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39</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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39</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 bc-has-history"><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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26<div class="bc-icons"><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> </div><section class="bc-history" id="sect2"><dl><dt class="bc-supports-yes bc-supports"><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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26<div class="bc-icons"><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> </div></dt><dd><abbr class="only-icon" title="See implementation notes"><span>Notes</span><i class="ic-footnote"></i></abbr> Starting with Firefox 33, the parsing of the <code>yield</code> expression has been updated to conform with the ES2015 specification.</dd></dl></section></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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10</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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4.0</td><td class="bc-supports-yes bc-browser-nodejs bc-has-history"><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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4.0.0<div class="bc-icons"></div><section class="bc-history" id="sect3"><dl><dt class="bc-supports-yes bc-supports"><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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4.0.0<div class="bc-icons"></div></dt><dd></dd><dt class="bc-supports-yes bc-supports"><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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0.12<div class="bc-icons"><abbr class="only-icon" title="User must explicitly enable this feature."><span>Disabled</span><i class="ic-disabled"></i></abbr> </div></dt><dd><abbr class="only-icon" title="User must explicitly enable this feature."><span>Disabled</span><i class="ic-disabled"></i></abbr> From version 0.12: this feature is behind the <code>--harmony</code> runtime flag.</dd></dl></section></td></tr><tr><th scope="row"><code>IteratorResult</code> object instead of throwing</th><td class="bc-supports-unknown bc-browser-chrome"><span class="bc-browser-name">Chrome</span><abbr title="Compatibility unknown; please update this.">
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?
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</abbr></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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29</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-unknown bc-browser-opera"><span class="bc-browser-name">Opera</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-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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10</td><td class="bc-supports-unknown bc-browser-webview_android"><span class="bc-browser-name">WebView Android</span><abbr title="Compatibility unknown; please update this.">
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?
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</abbr></td><td class="bc-supports-unknown bc-browser-chrome_android"><span class="bc-browser-name">Chrome Android</span><abbr title="Compatibility unknown; please update this.">
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?
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</abbr></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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29</td><td class="bc-supports-unknown bc-browser-opera_android"><span class="bc-browser-name">Opera Android</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-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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10</td><td class="bc-supports-unknown bc-browser-samsunginternet_android"><span class="bc-browser-name">Samsung Internet Android</span><abbr title="Compatibility unknown; please update this.">
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?
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</abbr></td><td class="bc-supports-unknown bc-browser-nodejs"><span class="bc-browser-name">nodejs</span><abbr title="Compatibility unknown; please update this.">
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?
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</abbr></td></tr></tbody></table><section class="bc-legend" id="sect4"><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><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><dt><abbr class="only-icon" title="See implementation notes."><span>See implementation notes.</span><i class="ic-footnote"></i></abbr></dt><dd>See implementation notes.</dd><dt><abbr class="only-icon" title="User must explicitly enable this feature."><span>User must explicitly enable this feature.</span><i class="ic-disabled"></i></abbr></dt><dd>User must explicitly enable this feature.</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="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/The_Iterator_protocol">The Iterator protocol</a></li>
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<li><a href="Reference/Statements/function*" title="function* 这种声明方式(function关键字后跟一个星号）会定义一个生成器函数 (generator function)，它返回一个 Generator 对象。"><code>function*</code></a></li>
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<li><a href="Reference/Operators/function*" title="function*关键字可以在表达式内部定义一个生成器函数。"><code>function* expression</code></a></li>
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<li><a href="Reference/Operators/yield*" title="yield* 表达式用于委托给另一个generator 或可迭代对象。"><code>yield*</code></a></li>
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</ul>
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</article> |