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+---
+title: 'TypeError: can''t access property "x" of "y"'
+slug: Web/JavaScript/Reference/Errors/Cant_access_property
+tags:
+  - Error
+  - JavaScript
+  - TypeError
+translation_of: Web/JavaScript/Reference/Errors/Cant_access_property
+---
+<div>{{jsSidebar("Errors")}}</div>
+
+<h2 id="メッセージ">メッセージ</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="エラータイプ">エラータイプ</h2>
+
+<p>{{jsxref("TypeError")}}。</p>
+
+<h2 id="何がうまくいかなかったのか？">何がうまくいかなかったのか？</h2>
+
+<p>{{jsxref("undefined")}} か {{jsxref("null")}} に対してプロパティアクセスを行いました。</p>
+
+<h2 id="例">例</h2>
+
+<h3 id="無効なケース">無効なケース</h3>
+
+<pre class="brush: js example-bad">// undefined and null cases on which the substring method won't work
+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="問題解決">問題解決</h3>
+
+<p><code>undefined</code> か <code>null</code> の null pointer アクセスを修正するには、たとえば <a href="/ja/docs/Web/JavaScript/Reference/Operators/typeof">typeof</a> 演算子を使用できます。</p>
+
+<pre class="brush: js">if (typeof foo !== 'undefined') {
+  // Now we know that foo is defined, we are good to go.
+}</pre>
+
+<h2 id="関連項目">関連項目</h2>
+
+<ul>
+ <li>{{jsxref("undefined")}}</li>
+ <li>{{jsxref("null")}}</li>
+</ul>