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beforeunload

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.

If a string is assigned to the returnValue Event property, a dialog appears asking the user for confirmation to leave the page (see example below). Some browsers display the returned string in the dialog box, but others display their own message. If no value is provided, the event is processed silently.

Note: 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.

Bubbles	No
Cancelable	Yes
Target objects	defaultView
Interface	`Event`

Properties

Property	Type	Description
`target` Read only	`EventTarget`	The event target (the topmost target in the DOM tree).
`type` Read only	`DOMString`	The type of event.
`bubbles` Read only	`Boolean`	Does the event normally bubble?
`cancelable` Read only	`Boolean`	Is it possible to cancel the event?
`returnValue`	`DOMString`	The current return value of the event (the message to show the user).

Examples

window.addEventListener("beforeunload", function (event) {
  event.returnValue = "\o/";
});

// is equivalent to
window.addEventListener("beforeunload", function (event) {
  event.preventDefault();
});

WebKit-based browsers don't follow the spec for the dialog box. An almost cross-browser working example would be close to the following:

window.addEventListener("beforeunload", function (e) {
  var confirmationMessage = "\o/";

  e.returnValue = confirmationMessage;     // Gecko, Trident, Chrome 34+
  return confirmationMessage;              // Gecko, WebKit, Chrome <34
});

Notes

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.

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.

Specifications

Specification	Status	Comment
HTML Living Standard The definition of 'beforeunload' in that specification.	Living Standard
HTML5 The definition of 'beforeunload' in that specification.	Recommendation	Initial definition

Browser compatibility

Feature	Chrome	Edge	Firefox (Gecko)	Internet Explorer	Opera	Safari (WebKit)
Basic support	1.0	(Yes)	1	4	12	3
Custom text support removed	51.0	No support	44.0 (44.0)		38	9.1