---
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>

<h2 id="语法">语法</h2>

<pre class="syntaxbox"><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">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 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">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>