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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

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 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

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung Internet
Basic supportChrome Full support YesEdge ? Firefox Full support YesIE Full support YesOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile ? Firefox Android Full support YesOpera Android Full support YesSafari iOS Full support YesSamsung Internet Android ?
User interaction required for dialog boxChrome Full support 60Edge ? Firefox ? IE ? Opera Full support 47Safari ? WebView Android Full support 60Chrome Android Full support 60Edge Mobile ? Firefox Android ? Opera Android Full support 47Safari iOS ? Samsung Internet Android ?

Legend

Full support  
Full support
Compatibility unknown  
Compatibility unknown

See also

Etiquetas y colaboradores del documento

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