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---
title: 'TypeError: can''t access property "x" of "y"'
slug: Web/JavaScript/Reference/Errors/Cant_access_property
translation_of: Web/JavaScript/Reference/Errors/Cant_access_property
---
<div>{{jsSidebar("Errors")}}</div>
<h2 id="消息">消息</h2>
<pre class="syntaxbox">TypeError: Unable to get property {x} of undefined or null reference (Edge)
TypeError: can't access property {x} of {y} (Firefox)
TypeError: {y} is undefined, can't access property {x} of it (Firefox)
TypeError: {y} is null, can't access property {x} of it (Firefox)
Examples:
TypeError: x is undefined, can't access property "prop" of it
TypeError: x is null, can't access property "prop" of it
TypeError: can't access property "prop" of undefined
TypeError: can't access property "prop" of null
</pre>
<h2 id="错误类型">错误类型</h2>
<p>{{jsxref("TypeError")}}.</p>
<h2 id="哪里出错了">哪里出错了?</h2>
<p>访问了 {{jsxref("undefined")}} 或{{jsxref("null")}} 值的属性.</p>
<h2 id="例子">例子</h2>
<h3 id="无效情形">无效情形</h3>
<pre class="brush: js example-bad">// 对于undefined 以及 null 值， substring 方法无法正常工作
var foo = undefined;
foo.substring(1); // TypeError: x is undefined, can't access property "substring" of it
var foo = null;
foo.substring(1); // TypeError: x is null, can't access property "substring" of it
</pre>
<h3 id="解决问题">解决问题</h3>
<p>为了解决<code>undefined</code> 或 <code>null</code> 值的空指针问题，你可以使用 <a href="/en-US/docs/Web/JavaScript/Reference/Operators/typeof">typeof</a> 操作符, 比如.</p>
<pre class="brush: js">if (typeof foo !== 'undefined') {
// 现在已经确认foo已定义，可以进一步操作.
}</pre>
<h2 id="参考更多">参考更多</h2>
<ul>
<li>{{jsxref("undefined")}}</li>
<li>{{jsxref("null")}}</li>
</ul>
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