BeforeUnloadEvent

The beforeunload event is fired when the window, the document and its resources are about to be unloaded.

When a non-empty 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. Some implementations only show the dialog box if the frame or any embedded frame receives a user gesture or user interaction. See Browser compatibility for more information.

Bubbles No
Cancelable Yes
Target objects defaultView
Interface Event

Examples

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

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

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

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

  (e || window.event).returnValue = confirmationMessage;     // Gecko + IE
  return confirmationMessage;                                /* Safari, Chrome, and other
                                                              * WebKit-derived browsers */
});

Specifications

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
BeforeUnloadEvent
30
12
1.5
4
17
7
≤37
30
4
18
7
3.0
returnValue
30
12
1.5
9
17
7
≤37
30
4
18
7
3.0
user_interaction
60
18
44
No
47
11
60
60
44
44
11
8.0

See also

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/API/BeforeUnloadEvent