---
title: XMLHttpRequest.onreadystatechange
slug: Web/API/XMLHttpRequest/onreadystatechange
tags:
  - XHR
  - XMLHttpRequest
  - 参考
  - 处理程序
  - 属性
  - 接口
translation_of: Web/API/XMLHttpRequest/onreadystatechange
---
<div>{{APIRef}}</div>

<p>只要 <code>readyState</code> 属性发生变化，就会调用相应的<a href="/zh-CN/docs/Web/API/EventHandler">处理函数</a><font face="consolas, Liberation Mono, courier, monospace">。</font>这个回调函数会被用户线程所调用。<strong><code>XMLHttpRequest.onreadystatechange</code></strong> 会在 {{domxref("XMLHttpRequest")}} 的{{domxref("XMLHttpRequest.readyState", "readyState")}} 属性发生改变时触发 {{event("readystatechange")}} 事件的时候被调用。</p>

<div class="warning">
<p><strong>警告</strong>：这个方法不该用于同步的requests对象,并且不能在内部(C++)代码中使用.</p>
</div>

<p>当一个 <code>XMLHttpRequest</code> 请求被 <a href="/en-US/docs/Web/API/XMLHttpRequest/abort">abort()</a> 方法取消时，其对应的 <code>readystatechange</code> 事件不会被触发。</p>

<div class="note">
<p>UPDATE: 在下面的浏览器版本中会触发 (Firefox 51.0.1, Opera 43.0.2442.991, Safari 10.0.3 (12602.4.8), Chrome 54.0.2840.71, Edge, IE11). 例子在 <a href="https://jsfiddle.net/merksam/ve5oc0gn/">here</a> - 重新加载几次页面即可。</p>
</div>

<h2 id="Syntax" name="Syntax">语法</h2>

<pre class="syntaxbox"><em>XMLHttpRequest</em>.onreadystatechange = <em>callback</em>;</pre>

<h3 id="取值">取值</h3>

<ul>
 <li>当 <code>readyState</code> 的值改变的时候，<code><em>callback</em></code> 函数会被调用。</li>
</ul>

<h2 id="Example" name="Example">示例</h2>

<pre class="brush: js">var xhr= new XMLHttpRequest(),
    method = "GET",
    url = "https://developer.mozilla.org/";

xhr.open(<em>method</em>, <em>url</em>, true);
xhr.onreadystatechange = function () {
  if(xhr.readyState === XMLHttpRequest.DONE &amp;&amp; xhr.status === 200) {
    console.log(xhr.responseText)
  }
}
xhr.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">注释</th>
  </tr>
  <tr>
   <td>{{SpecName('XMLHttpRequest', '#handler-xhr-onreadystatechange')}}</td>
   <td>{{Spec2('XMLHttpRequest')}}</td>
   <td>WHATWG living standard</td>
  </tr>
 </tbody>
</table>

<h2 id="浏览器兼容性">浏览器兼容性</h2>

<p>{{CompatibilityTable}}</p>

<div id="compat-desktop">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Chrome</th>
   <th>Firefox (Gecko)</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatChrome(1)}}</td>
   <td>{{CompatGeckoDesktop(1.0)}}</td>
   <td>{{CompatIe(7)}}<sup>[1]</sup></td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatSafari(1.2)}}</td>
  </tr>
 </tbody>
</table>
</div>

<div id="compat-mobile">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Android</th>
   <th>Chrome for Android</th>
   <th>Firefox Mobile (Gecko)</th>
   <th>IE Mobile</th>
   <th>Opera Mobile</th>
   <th>Safari Mobile</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatUnknown}}</td>
   <td>1.0</td>
   <td>{{CompatVersionUnknown}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
   <td>{{CompatUnknown}}</td>
  </tr>
 </tbody>
</table>
</div>

<p>[1] IE 5 和 6可以通过使用 <code>ActiveXObject()</code> 支持ajax。</p>