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+---
+title: 'ReferenceError: reference to undefined property "x"'
+slug: Web/JavaScript/Reference/Errors/Undefined_prop
+tags:
+  - Errors
+  - JavaScript
+  - ReferenceError
+  - Strict Mode
+  - 严格模式
+translation_of: Web/JavaScript/Reference/Errors/Undefined_prop
+---
+<div>{{jsSidebar("Errors")}}</div>
+
+<h2 id="信息">信息</h2>
+
+<pre class="syntaxbox">ReferenceError: reference to undefined property "x" (Firefox)
+</pre>
+
+<h2 id="错误类型">错误类型</h2>
+
+<p>仅在 <a href="/zh-CN/docs/Web/JavaScript/Reference/Strict_mode">strict mode</a> 下出现 {{jsxref("ReferenceError")}} 警告。</p>
+
+<h2 id="哪里出错了">哪里出错了?</h2>
+
+<p>脚本尝试去访问一个不存在的对象属性。<a href="/en-US/docs/Web/JavaScript/Reference/Operators/Property_Accessors">property accessors</a> 页面描述了两种访问属性的方法。</p>
+
+<p>引用未定义属性的错误仅出现在 <a href="/zh-CN/docs/Web/JavaScript/Reference/Strict_mode">strict mode </a>代码中。在非严格代码中，对不存在的属性的访问将被忽略。</p>
+
+<h2 id="示例">示例</h2>
+
+<h3 id="无效的">无效的</h3>
+
+<p>本例中，<code>bar</code> 属性是未定义的，隐藏 <code>ReferenceError</code> 会出现。</p>
+
+<pre class="brush: js example-bad">"use strict";
+
+var foo = {};
+foo.bar; // ReferenceError: reference to undefined property "bar"
+</pre>
+
+<h3 id="无效的_2">无效的</h3>
+
+<p>为了避免错误，您需要向对象添加 <code>bar</code> 的定义或在尝试访问 <code>bar</code> 属性之前检查 <code>bar</code> 属性的存在；一种检查的方式是使用 {{jsxref("Object.prototype.hasOwnProperty()")}} 方法。如下所示：</p>
+
+<pre class="brush: js example-good">"use strict";
+
+var foo = {};
+
+// Define the bar property
+
+foo.bar = "moon";
+console.log(foo.bar); // "moon"
+
+// Test to be sure bar exists before accessing it
+
+if (foo.hasOwnProperty("bar") {
+  console.log(foo.bar);
+}</pre>
+
+<h2 id="相关">相关</h2>
+
+<ul>
+ <li><a href="/en-US/docs/Web/JavaScript/Reference/Strict_mode">Strict mode</a></li>
+</ul>