--- 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>