Web/API/Window/beforeunload event

The 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.

Bubbles	No
Cancelable	Yes
Interface	Event
Event handler property	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(), and window.prompt() methods may be ignored during this event. See the HTML specification for more details.

Note: You shouldn't use the beforeunload event with sendBeacon(). See the Navigator.sendBeacon() page for more details and best practices.

Examples

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();
  // Older browsers supported custom message
  event.returnValue = '';
});

Specifications

Specification	Status	Comment
HTML Living StandardThe definition of 'beforeunload' in that specification.	Living Standard
HTML5The definition of 'beforeunload' in that specification.	Recommendation	Initial definition

Browser compatibility

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.

Update compatibility data on GitHub

	Desktop						Mobile
	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 Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support 4	Opera Full support 12	Safari Full support 3	WebView Android Full support 1	Chrome Android Full support 18	Firefox Android Full support 4	Opera Android Full support 12	Safari iOS Full support 1	Samsung Internet Android Full support 1.0
Custom text support Deprecated'Non-standard'	Chrome No support ? — 51	Edge No support No	Firefox No support ? — 44	IE Full support Yes	Opera No support ? — 38	Safari No support ? — 9	WebView Android No support ? — 51	Chrome Android No support ? — 51	Firefox Android No support ? — 44	Opera Android No support ? — 41	Safari iOS No support No	Samsung Internet Android No support ? — 5.0
Activation using event.returnValue = "string"; Deprecated'	Chrome Full support 30	Edge Full support 12	Firefox Full support Yes	IE Full support Yes	Opera ?	Safari ?	WebView Android Full support Yes	Chrome Android Full support Yes	Firefox Android Full support Yes	Opera Android Full support Yes	Safari iOS No support No	Samsung Internet Android Full support Yes
Activation using event.preventDefault()	Chrome No support No	Edge No support 12 — 79	Firefox Full support Yes	IE Full support 9	Opera No support No	Safari Full support 11	WebView Android No support No	Chrome Android No support No	Firefox Android Full support Yes	Opera Android No support No	Safari iOS No support No	Samsung Internet Android No support No
Activation using return "string"; Deprecated'	Chrome Full support 1	Edge Full support 12	Firefox Full support 1	IE Full support Yes	Opera Full support 12	Safari Full support 3	WebView Android Full support Yes	Chrome Android Full support Yes	Firefox Android Full support Yes	Opera Android Full support Yes	Safari iOS No support No	Samsung Internet Android Full support Yes

Legend

Full support  

Full support

No support  

No support

Compatibility unknown  

Compatibility unknown

Non-standard. Expect poor cross-browser support.'

Non-standard. Expect poor cross-browser support.

Deprecated. Not for use in new websites.'

Deprecated. Not for use in new websites.

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

See also

• Related events: DOMContentLoaded, readystatechange, load, unload
• Unloading Documents — Prompt to unload a document
• Remove Custom Messages in onbeforeload Dialogs after Chrome 51

Window: beforeunload event by Mozilla Contributors is licensed under CC-BY-SA 2.5.