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+---
+title: 逻辑空赋值 (??=)
+slug: Web/JavaScript/Reference/Operators/Logical_nullish_assignment
+translation_of: Web/JavaScript/Reference/Operators/Logical_nullish_assignment
+---
+<div>{{jsSidebar("Operators")}}</div>
+
+<p>逻辑空赋值运算符 (<code>x ??= y</code>) 仅在 <code>x</code> 是 {{Glossary("nullish")}} (<code>null</code> 或 <code>undefined</code>) 时对其赋值。</p>
+
+<div>{{EmbedInteractiveExample("pages/js/expressions-logical-nullish-assignment.html")}}</div>
+
+<div class="hidden">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 <a href="https://github.com/mdn/interactive-examples">https://github.com/mdn/interactive-examples</a> and send us a pull request.</div>
+
+<h2 id="语法">语法</h2>
+
+<pre class="syntaxbox notranslate"><em>expr1</em> ??= <em>expr2</em>
+</pre>
+
+<h2 id="描述">描述</h2>
+
+<h3 id="语法短路求值">语法短路求值</h3>
+
+<p><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator">空值合并</a>运算符从左至右求值，其使用以下规则测试是否可能进行语法短路求值：</p>
+
+<p><code>(结果非 null 或 undefined 的表达式) ?? expr</code> 被短路求值为左侧表达式，当左侧证明为既非 <code>null</code> 也非 <code>undefined</code>.</p>
+
+<p>语法短路意味着 <code><em>expr</em></code> 部分<strong>尚未被求值</strong>，因此任何与其求值产生的相关副作用都不会生效（例如，如果 <code><em>expr</em></code> 是一个函数调用，则该调用将不会发生）。</p>
+
+<p>逻辑空赋值的语法短路也意味着 <code>x ??= y</code> 等价于：</p>
+
+<pre class="brush: js notranslate">x ?? (x = y);</pre>
+
+<p>而不等价于如下的表达式，因为其一定会发生赋值：</p>
+
+<pre class="brush: js notranslate example-bad">x = x ?? y;
+</pre>
+
+<h2 id="例子">例子</h2>
+
+<h3 id="使用逻辑空赋值">使用逻辑空赋值</h3>
+
+<pre class="brush: js notranslate">function config(options) {
+  options.duration ??= 100;
+  options.speed ??= 25;
+  return options;
+}
+
+config({ duration: 125 }); // { duration: 125, speed: 25 }
+config({}); // { duration: 100, speed: 25 }
+</pre>
+
+<h2 id="规范">规范</h2>
+
+<table class="standard-table">
+ <thead>
+  <tr>
+   <th scope="col">规范</th>
+  </tr>
+ </thead>
+ <tbody>
+  <tr>
+  </tr>
+  <tr>
+   <td>{{SpecName('Logical Assignment', '#sec-assignment-operators', 'Assignment operators')}}</td>
+  </tr>
+ </tbody>
+</table>
+
+<h2 id="浏览器兼容性">浏览器兼容性</h2>
+
+
+
+<p>{{Compat("javascript.operators.logical_nullish_assignment")}}</p>
+
+<h2 id="相关链接">相关链接</h2>
+
+<ul>
+ <li><a href="/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator">The nullish coalescing operator (<code>??</code>)</a></li>
+ <li>{{Glossary("Nullish")}}</li>
+ <li>{{Glossary("Truthy")}}</li>
+ <li>{{Glossary("Falsy")}}</li>
+</ul>