The onbeforeunload property of the WindowEventHandlers mixin is the EventHandler for processing beforeunload events. These events fire when a window is about to unload its resources. At this point, the document is still visible and the event is still cancelable.

Note: To combat unwanted pop-ups, some browsers don't display prompts created in beforeunload event handlers unless the page has been interacted with. Moreover, some don't display them at all.


window.addEventListener("beforeunload", function(event) { ... });
window.onbeforeunload = function(event) { ... };

Typically, it is better to use window.addEventListener() and the beforeunload event, instead of onbeforeunload.


This example prompts the user before unloading.

The HTML specification states that authors should use the Event.preventDefault() method instead of using Event.returnValue to prompt the user.

window.addEventListener('beforeunload', function (e) {
  // Cancel the event
  e.preventDefault(); // If you prevent default behavior in Mozilla Firefox prompt will always be shown
  // Chrome requires returnValue to be set
  e.returnValue = '';

Guarantee the browser unload by removing the returnValue property of the event

window.addEventListener('beforeunload', function (e) {
  // the absence of a returnValue property on the event will guarantee the browser unload happens
  delete e['returnValue'];


When your page uses JavaScript to render content, the JavaScript may stop when leaving and then navigating back to the page. You can bind to window.onbeforeunload to prevent the browser from fully caching the page. If you do so, JavaScript in the page will be triggered on the subsequent return visit and update the content as desired.


The event was originally introduced by Microsoft in Internet Explorer 4 and standardized in the HTML5 specification.

Specification Status Comment
HTML Living Standard
The definition of 'onbeforeunload' in that specification.
Living Standard
HTML 5.1
The definition of 'GlobalEventHandlers' in that specification.
The definition of 'GlobalEventHandlers' in that specification.

Browser compatibility

ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
onbeforeunloadChrome Full support 1Edge Full support 12Firefox Full support 1IE Full support 4Opera Full support 12Safari Full support 3WebView Android Full support 1Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 12Safari iOS Full support 1Samsung Internet Android Full support 1.0
Custom text support
Chrome No support ? — 51Edge No support NoFirefox No support ? — 44IE Full support YesOpera No support ? — 38Safari No support ? — 9WebView Android No support ? — 51Chrome Android No support ? — 51Firefox Android No support ? — 44Opera Android No support ? — 41Safari iOS No support NoSamsung Internet Android No support ? — 5.0


Full support
Full support
No support
No support
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.

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

When this event returns (or sets the returnValue property to) a value other than null or undefined, the user will be prompted to confirm the page unload. In older browsers, the return value of the event is displayed in this dialog. Starting with Firefox 44, Chrome 51, Opera 38, and Safari 9.1, a generic string not under the control of the webpage will be shown instead of the returned string. For example:

  • Firefox displays the string, "This page is asking you to confirm that you want to leave - data you have entered may not be saved." (see bug 588292).
  • Chrome displays the string, "Do you want to leave this site? Changes you made may not be saved." (see Chrome Platform Status).

Internet Explorer does not respect the null return value and will display this to users as "null" text. You have to use undefined to skip the prompt.

In some browsers, calls to window.alert(), window.confirm(), and window.prompt() may be ignored during this event. See the HTML specification for more details.

Note also, that various browsers ignore the result of the event and do not ask the user for confirmation at all. In such cases, the document will always be unloaded automatically. Firefox has a switch named dom.disable_beforeunload in about:config to enable this behaviour. As of Chrome 60, the confirmation will be skipped if the user has not performed a gesture in the frame or page since it was loaded. Pressing F5 in the page seems to count as user interaction, whereas mouse-clicking the refresh arrow or pressing F5 with Chrome DevTools focused does not count as user interaction (as of Chrome 81).