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---
title: WindowEventHandlers.onbeforeunload
slug: Web/API/WindowEventHandlers/onbeforeunload
translation_of: Web/API/WindowEventHandlers/onbeforeunload
---
<div>
<div>{{APIRef("HTML DOM")}}</div>
</div>
<div> </div>
<p><code><strong>WindowEventHandlers.onbeforeunload</strong></code> 事件处理函数包含的代码将在 {{event("beforeunload")}} 发出时被执行。当 window 准备释放它的资源时，该事件被触发。此时 document 仍然可见，且事件是仍然可被取消的。</p>
<div class="note">
<p><strong>注意:</strong> 为了避免不必要的弹出窗口，除非页面已经有过互动，否则可能不会显示beforeunload创建的询问窗口。对于特定的浏览器列表，请参阅浏览器兼容性部分。</p>
</div>
<h2 id="Syntax" name="Syntax">语法</h2>
<pre class="syntaxbox">window.onbeforeunload = <var>funcRef</var></pre>
<ul>
<li><code>funcRef</code> 是函数或函数表达式的引用。</li>
<li>这个函数应该设置一个字符串到事件对象的 returnValue 属性上，且返回该字符串。</li>
</ul>
<h2 id="Example" name="Example">示例</h2>
<pre class="brush:js">window.onbeforeunload = function(e) {
var dialogText = 'Dialog text here';
e.returnValue = dialogText;
return dialogText;
};
</pre>
<h2 id="注意">注意</h2>
<p>当事件返回了一个非空值时，将需要用户确认是否 unload 页面。在大部分浏览器中，事件的返回值将在对话框中显示。在 Firefox 4 及以后，返回值将不会显示给用户。作为替代，Firefox将会显示"This page is asking you to confirm that you want to leave - data you have entered may not be saved." 请查看{{bug("588292")}}.</p>
<p>Since 25 May 2011, the HTML5 specification states that calls to {{domxref("window.alert()")}}, {{domxref("window.confirm()")}}, and {{domxref("window.prompt()")}} methods may be ignored during this event. See the <a href="http://www.w3.org/TR/html5/webappapis.html#user-prompts">HTML5 specification</a> for more details.</p>
<p>Note also that various mobile browsers ignore the result of the event (that is, they do not ask the user for confirmation). Firefox has a hidden preference in about:config to do the same. In essence this means the user always confirms that the document may be unloaded.</p>
<p>You <em>can</em> and <em>should</em> handle this event through {{domxref("EventTarget.addEventListener","window.addEventListener()")}} and the {{event("beforeunload")}} event. More documentation is available there.</p>
<h2 id="Specifications">Specifications</h2>
<p>The event was originally introduced by Microsoft in Internet Explorer 4 and standardized in the HTML5 specification.</p>
<table class="standard-table">
<thead>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
</thead>
<tbody>
<tr>
<td>{{SpecName('HTML WHATWG', '#windoweventhandlers', 'GlobalEventHandlers')}}</td>
<td>{{Spec2('HTML WHATWG')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName('HTML5.1', '#windoweventhandlers', 'GlobalEventHandlers')}}</td>
<td>{{Spec2('HTML5.1')}}</td>
<td> </td>
</tr>
<tr>
<td>{{SpecName("HTML5 W3C", "#windoweventhandlers", "GlobalEventHandlers")}}</td>
<td>{{Spec2('HTML5 W3C')}}</td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<div>{{CompatibilityTable}}</div>
<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 (WebKit)</th>
</tr>
<tr>
<td>Basic support</td>
<td>{{CompatChrome(1.0)}}</td>
<td>1</td>
<td>4</td>
<td>12</td>
<td>3</td>
</tr>
<tr>
<td>Custom text support removed</td>
<td>{{CompatChrome(51.0)}}</td>
<td>{{CompatGeckoMobile("44.0")}}</td>
<td> </td>
<td>38</td>
<td><a href="https://developer.apple.com/library/mac/releasenotes/General/WhatsNewInSafari/Articles/Safari_9_1.html">9.1</a></td>
</tr>
</tbody>
</table>
</div>
<div id="compat-mobile">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Android</th>
<th>Android Webview</th>
<th>Firefox Mobile (Gecko)</th>
<th>IE Phone</th>
<th>Opera Mobile</th>
<th>Safari Mobile</th>
<th>Chrome for Android</th>
</tr>
<tr>
<td>Basic support</td>
<td>{{CompatUnknown}}</td>
<td>{{CompatVersionUnknown}}</td>
<td>{{CompatUnknown}}</td>
<td>{{CompatUnknown}}</td>
<td>{{CompatUnknown}}</td>
<td>(no) <a href="https://bugs.webkit.org/show_bug.cgi?id=19324">defect</a></td>
<td>{{CompatVersionUnknown}}</td>
</tr>
<tr>
<td>Custom text support removed</td>
<td>{{CompatUnknown}}</td>
<td>{{CompatChrome(51.0)}}</td>
<td>{{CompatGeckoMobile("44.0")}}</td>
<td> </td>
<td> </td>
<td> </td>
<td>{{CompatChrome(51.0)}}</td>
</tr>
</tbody>
</table>
</div>
<h2 id="See also" name="See also">See also</h2>
<ul>
<li><a href="http://msdn.microsoft.com/en-us/library/ms536907(VS.85).aspx">MSDN: onbeforeunload event</a></li>
</ul>
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