files/ja/web/api/filereader/onerror/index.html


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---
title: onerror
slug: Web/API/FileReader/onerror
translation_of: Web/API/FileReader/onerror
---
<p><a href="/en-US/docs/Web/API/FileReader">FileReader</a> の onerror ハンドラは、Error オブジェクトではなく Event オブジェクトをパラメータとして受け取りますが、エラーは FileReader オブジェクトから <code><a href="/en-US/docs/Web/API/FileReader/error">instanceOfFileReader.error</a></code> のようにアクセスすることができます。</p>

<pre class="brush:js; line-numbers  language-js notranslate"><code class="language-js"><span class="comment token">// </span></code>&lt;input type="file" onchange="onChange(event)"&gt; からのコールバック<code class="language-js">
<span class="keyword token">function</span> <span class="function token">onChange</span><span class="punctuation token">(</span>event<span class="punctuation token">)</span> <span class="punctuation token">{</span>
  <span class="keyword token">var</span> file <span class="operator token">=</span> event<span class="punctuation token">.</span>target<span class="punctuation token">.</span>files<span class="punctuation token">[</span><span class="number token">0</span><span class="punctuation token">]</span><span class="punctuation token">;</span>
  <span class="keyword token">var</span> reader <span class="operator token">=</span> <span class="keyword token">new</span> <span class="class-name token">FileReader</span><span class="punctuation token">(</span><span class="punctuation token">)</span><span class="punctuation token">;</span>
  reader<span class="punctuation token">.onerror </span><span class="operator token">=</span> <span class="keyword token">function</span><span class="punctuation token">(</span>event<span class="punctuation token">)</span> <span class="punctuation token">{
    alert("</span></code>ファイルの読み込みに失敗しました。<code class="language-js"><span class="punctuation token">\n\n" + reader.error);</span>
    <span class="punctuation token">reader.abort(); // </span></code>(...これは onerror ハンドラで何か有用なことをするのでしょうか？)<code class="language-js">
  <span class="punctuation token">}</span><span class="punctuation token">;</span>

  reader<span class="punctuation token">.</span><span class="function token">readAsText</span><span class="punctuation token">(</span>file<span class="punctuation token">)</span><span class="punctuation token">;</span>
<span class="punctuation token">}</span></code></pre>