Window: beforeunload event

在此页

范例
规范
浏览器兼容性
另请参阅
相关话题

beforeunload event is fired when the window, the document and its resources are about to be unloaded. The document is still visible and the event is still cancelable at this point.

冒泡	No
可取消	Yes
接口	`事件`
事件处理程序特性	`onbeforeunload`

This event enables a web page to trigger a confirmation dialog asking the user if they really want to leave the page. If the user confirms, the browser navigates to the new page, otherwise it cancels the navigation.

According to the specification, to show the confirmation dialog an event handler should call preventDefault() on the event.

However note that not all browsers support this method, and some instead require the event handler to implement one of two legacy methods:

assigning a string to the event's returnValue property
returning a string from the event handler.

Some browsers used to display the returned string in the confirmation dialog, enabling the event handler to display a custom message to the user. However, this is deprecated and no longer supported in most browsers.

To combat unwanted pop-ups, browsers may not display prompts created in beforeunload event handlers unless the page has been interacted with, or may even not display them at all.

Attaching an event handler/listener to window or document 's beforeunload event prevents browsers from using in-memory page navigation caches, like Firefox's Back-Forward cache or WebKit's Page Cache .

The HTML specification states that calls to window.alert() , window.confirm() ，和 window.prompt() methods may be ignored during this event. See the HTML specification 了解更多细节。

范例

The HTML specification states that authors should use the Event.preventDefault() method instead of using Event.returnValue . However, this is not supported by all browsers.

window.addEventListener('beforeunload', (event) => {
  // Cancel the event as stated by the standard.
  event.preventDefault();
  // Chrome requires returnValue to be set.
  event.returnValue = '';
});

规范

规范	状态	注释
HTML 实时标准 The definition of 'beforeunload' in that specification.	实时标准
HTML5 The definition of 'beforeunload' in that specification.	推荐	初始定义

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	桌面						移动
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`beforeunload` event	Chrome 1	Edge 12	Firefox 1	IE 4	Opera 12	Safari 3	WebView Android 1	Chrome Android 18	Firefox Android 4	Opera Android 12	Safari iOS 1	Samsung Internet Android 1.0
Custom text support 弃用非标	Chrome ? — 51	Edge No	Firefox ? — 44	IE Yes	Opera ? — 38	Safari ? — 9	WebView Android ? — 51	Chrome Android ? — 51	Firefox Android ? — 44	Opera Android ? — 41	Safari iOS No	Samsung Internet Android ? — 5.0
Activation using `event.returnValue = "string";` 弃用	Chrome 30	Edge 12	Firefox Yes	IE Yes	Opera ?	Safari ?	WebView Android Yes	Chrome Android Yes	Firefox Android Yes	Opera Android Yes	Safari iOS No	Samsung Internet Android Yes
Activation using `event.preventDefault()`	Chrome No	Edge 12 — 79	Firefox Yes	IE 9	Opera No	Safari 11	WebView Android No	Chrome Android No	Firefox Android Yes	Opera Android No	Safari iOS No	Samsung Internet Android No
Activation using `return "string";` 弃用	Chrome 1	Edge 12	Firefox 1	IE Yes	Opera 12	Safari 3	WebView Android Yes	Chrome Android Yes	Firefox Android Yes	Opera Android Yes	Safari iOS No	Samsung Internet Android Yes

图例

完整支持
不支持
兼容性未知 ?
非标。预期跨浏览器支持较差。
弃用。不要用于新网站。

见 WindowEventHandlers/onbeforeunload for more details on how various browsers handle this event.

另请参阅

相关事件： DOMContentLoaded , readystatechange , load , unload
Unloading Documents — Prompt to unload a document
Remove Custom Messages in onbeforeload Dialogs after Chrome 51

元数据

最后修改： Aug 5, 2020

在此页

范例

规范

浏览器兼容性

图例

另请参阅

相关话题