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---
title: undefined
slug: Glossary/undefined
translation_of: Glossary/undefined
---
<p><span class="seoSummary">一个声明未定义的变量的初始值，或没有实际参数的形式参数。</span></p>
<h2 id="例子">例子</h2>
<p> </p>
<pre><code>var x; //创建一个变量，但并没有赋值
console.log("X的值是", x) //返回X的值是undefined</code></pre>
<p> </p>
<h2 id="更多">更多</h2>
<h3 id="基本知识">基本知识</h3>
<ul>
<li>{{Interwiki("wikipedia", "Undefined value")}} on Wikipedia</li>
</ul>
<h3 id="技术参考">技术参考</h3>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures">JavaScript data types and data structures</a></li>
</ul>
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