The WindowEventHandlers.onbeforeunload event handler property contains the code executed when the beforeunload is sent. This event fires when a window is about to unload its resources. 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; some don't display them at all. For a list of specific browsers, see the Browser_compatibility section.

Syntax

window.onbeforeunload = funcRef
  • funcRef is a reference to a function or a function expression.
  • The function should assign a string value to the returnValue property of the Event object and return the same string.

Example

window.onbeforeunload = function(e) {
  var dialogText = 'Dialog text here';
  e.returnValue = dialogText;
  return dialogText;
};

Notes

When this event returns (or sets the returnValue property to) a value other than null or undefined, the user is prompted to confirm the page unload. In some browsers, the return value of the event is displayed in this dialog. Starting with Firefox 4, 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 and Chrome Platform Status.

Since 25 May 2011, the HTML5 specification states that calls to window.alert(), window.confirm(), and window.prompt() methods may be ignored during this event. See the HTML5 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. The document will always be unloaded automatically. Firefox has a switch named dom.disable_beforeunload in about:config to enable this behaviour.

You can and should handle this event through window.addEventListener() and the beforeunload event. More documentation is available there.

Binding to this event can be used to prevent the browser from fully caching the page in cases where content is rendered by javascript. In certain circumstances when returning to a page that has executed javascript in order to populate content, you may find the javascript not running upon the return visit when navigating back. If window.onbeforeunload has been bound (and thus triggered when leaving that page) javascript in the page will be triggered on the subsequent return visit and therefore update the content.

Specifications

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 'GlobalEventHandlers' in that specification.
Living Standard  
HTML 5.1
The definition of 'GlobalEventHandlers' in that specification.
Recommendation  
HTML5
The definition of 'GlobalEventHandlers' in that specification.
Recommendation  

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 1 (Yes) 1 4 12 3
Custom text support removed 51 No support 44.0 (44.0) ? 38 9.1
Feature Android Webview Chrome for Android Edge Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) ? ? (Yes) (no) defect
Custom text support removed 51 51 No support 44.0 (44.0) ? 38 ?

See also