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---
title: XMLHttpRequestEventTarget.onloadstart
slug: Web/API/XMLHttpRequestEventTarget/onloadstart
translation_of: Web/API/XMLHttpRequestEventTarget/onloadstart
---
<div>{{APIRef("XMLHttpRequest")}}</div>
<p><strong><code>XMLHttpRequestEventTarget.onloadstart</code></strong> 在{{domxref("XMLHttpRequest")}} 开始传送数据时被调用</p>
<h2 id="Syntax" name="Syntax">语法</h2>
<pre class="syntaxbox"><em>XMLHttpRequest</em>.onloadstart = <em>callback</em>;</pre>
<h3 id="值">值</h3>
<ul>
<li><code><em>callback</em></code> 是开始传送数据时被调用的回调函数</li>
</ul>
<h2 id="Example" name="Example">示例</h2>
<pre class="brush: js">var xmlhttp = new XMLHttpRequest(),
method = 'GET',
url = 'https://developer.mozilla.org/';
xmlhttp.open(<em>method</em>, <em>url</em>, true);
xmlhttp.onloadstart = function () {
console.log("Download underway");
};
xmlhttp.send();
</pre>
<h2 id="规范">规范</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">规范</th>
<th scope="col">状态</th>
<th scope="col">说明</th>
</tr>
<tr>
<td>{{SpecName('XMLHttpRequest', '#handler-xhr-onloadstart')}}</td>
<td>{{Spec2('XMLHttpRequest')}}</td>
<td>WHATWG 现存标准</td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<p>{{Compat("api.XMLHttpRequestEventTarget.onloadstart")}}</p>
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