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authorPeter Bengtsson <mail@peterbe.com>2020-12-08 14:40:17 -0500
committerPeter Bengtsson <mail@peterbe.com>2020-12-08 14:40:17 -0500
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+---
+title: FileReader.readAsDataURL()
+slug: Web/API/FileReader/readAsDataURL
+tags:
+ - API
+ - File API
+translation_of: Web/API/FileReader/readAsDataURL
+---
+<div>{{APIRef("File API")}}</div>
+
+<p><code>readAsDataURL</code> 方法会读取指定的 {{domxref("Blob")}} 或 {{domxref("File")}} 对象。读取操作完成的时候,{{domxref("FileReader.readyState","readyState")}} 会变成已完成<code>DONE</code>,并触发 {{event("loadend")}} 事件,同时 {{domxref("FileReader.result","result")}} 属性将包含一个<code>data:</code>URL格式的字符串(base64编码)以表示所读取文件的内容。</p>
+
+<h2 id="语法">语法</h2>
+
+<pre class="syntaxbox"><em>instanceOfFileReader</em>.readAsDataURL(blob);</pre>
+
+<h3 id="参数">参数</h3>
+
+<dl>
+ <dt><code>blob</code></dt>
+ <dd>即将被读取的 {{domxref("Blob")}} 或 {{domxref("File")}} 对象。</dd>
+</dl>
+
+<h2 id="示例">示例</h2>
+
+<h3 id="HTML">HTML</h3>
+
+<pre class="brush: html">&lt;input type="file" onchange="previewFile()"&gt;&lt;br&gt;
+&lt;img src="" height="200" alt="Image preview..."&gt;</pre>
+
+<h3 id="JavaScript">JavaScript</h3>
+
+<pre class="brush: js">function previewFile() {
+  var preview = document.querySelector('img');
+  var file    = document.querySelector('input[type=file]').files[0];
+  var reader  = new FileReader();
+
+  reader.addEventListener("load", function () {
+    preview.src = reader.result;
+  }, false);
+
+  if (file) {
+    reader.readAsDataURL(file);
+  }
+}</pre>
+
+<h3 id="演示">演示</h3>
+
+<p>{{EmbedLiveSample("Example", "100%", 240)}}</p>
+
+<h2 id="读取多个文件的例子">读取多个文件的例子</h2>
+
+<h3 id="HTML_2">HTML</h3>
+
+<pre class="brush: html">&lt;input id="browse" type="file" onchange="previewFiles()" multiple&gt;
+&lt;div id="preview"&gt;&lt;/div&gt;</pre>
+
+<h3 id="JavaScript_2">JavaScript</h3>
+
+<pre class="brush: js">function previewFiles() {
+
+ var preview = document.querySelector('#preview');
+ var files = document.querySelector('input[type=file]').files;
+
+ function readAndPreview(file) {
+
+ // 确保 `file.name` 符合我们要求的扩展名
+ if ( /\.(jpe?g|png|gif)$/i.test(file.name) ) {
+ var reader = new FileReader();
+
+ reader.addEventListener("load", function () {
+ var image = new Image();
+ image.height = 100;
+ image.title = file.name;
+ image.src = this.result;
+ preview.appendChild( image );
+ }, false);
+
+ reader.readAsDataURL(file);
+ }
+
+ }
+
+ if (files) {
+ [].forEach.call(files, readAndPreview);
+ }
+
+}
+</pre>
+
+<div class="note"><strong>注意:</strong> Internet Explorer  10 之前的版本并不支持  <a href="/en-US/docs/Web/API/FileReader"><code>FileReader()</code></a> 。关于图片文件预览的兼容性解决方案,可以查看 <a class="internal" href="https://mdn.mozillademos.org/files/3699/crossbrowser_image_preview.html" title="crossbrowser_image_preview.html">crossbrowser possible solution for image preview</a> 或者 <a href="https://mdn.mozillademos.org/files/3698/image_upload_preview.html">this more powerful example</a> 。</div>
+
+<h2 id="规范">规范</h2>
+
+<table class="standard-table">
+ <tbody>
+ <tr>
+ <th scope="col">Specification</th>
+ <th scope="col">Status</th>
+ <th scope="col">Comment</th>
+ </tr>
+ <tr>
+ <td>{{SpecName("File API", "#FileReader-interface", "FileReader")}}</td>
+ <td>{{Spec2("File API")}}</td>
+ <td>Initial definition</td>
+ </tr>
+ </tbody>
+</table>
+
+<h2 id="浏览器兼容性">浏览器兼容性</h2>
+
+<p>{{Compat("api.FileReader.readAsDataURL")}}</p>
+
+<h2 id="参见">参见</h2>
+
+<ul>
+ <li>{{domxref("FileReader")}}</li>
+ <li>{{domxref("URL.createObjectURL()")}}</li>
+</ul>
+
+<p>{{APIRef("File API")}}</p>