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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.

See also

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 Last updated by: gwokae,