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.

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/";

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


HTML Standard
# the-beforeunloadevent-interface

Browser compatibility

BCD tables only load in the browser

See also