BeforeUnloadEvent is an interface for the beforeunload event.

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.

Note: For security reasons, only a generic string not under the control of the webpage is shown instead of the returned string.

Event BeforeUnloadEvent
Bubbles No
Cancelable Yes
Target objects defaultView
Interface Event



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

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

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", (e) => {
  const confirmationMessage = "\\o/";

  // Gecko + IE
  (e || window.event).returnValue = confirmationMessage;

  // Safari, Chrome, and other WebKit-derived browsers
  return confirmationMessage;


HTML Standard
# the-beforeunloadevent-interface

Browser compatibility

BCD tables only load in the browser

See also