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The beforeunload
event is fired when the window, the document and its resources are about to be unloaded.
When a string is assigned to the returnValue
Event property, a dialog box appears, asking the users for confirmation to leave the page (see example below). When no value is provided, the event is processed silently.
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
When this event returns a non-void value, the user is prompted to confirm the page unload. In most browsers, the return value of the event is displayed in this dialog. In Firefox 4 and later the returned string is not displayed to the user. Instead, 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.
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 mobile browsers ignore the result of the event (that is, they do not ask the user for confirmation). Firefox has a hidden preference in about:config to do the same. In essence this means the user always confirms that the document may be unloaded.
Specifications
Specification | Status | Comment |
---|---|---|
WHATWG HTML Living Standard The definition of 'beforeunload' in that specification. |
Living Standard | |
HTML5 The definition of 'beforeunload' in that specification. |
Recommendation | Initial definition |