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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


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

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

Webkit-based 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;                                //Webkit, Safari, Chrome etc.

Browser compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
User interaction required for dialog box 60 ? ? 47 ?
Feature Android Webview Chrome for Android Firefox Mobile (Gecko) Firefox OS IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes) ?
User interaction required for dialog box 60 60 ? ? ? 47 ?

See also

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 Colaboradores en esta página: jpmedley, fscholz, kscarfone, rabimba
 Última actualización por: jpmedley,