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.

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

// is equivalent to
window.addEventListener("beforeunload", (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", (e) => {
  const confirmationMessage = "\\o/";

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

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

• DOMContentLoaded
• readystatechange
• load
• beforeunload
• unload
• Unloading Documents — Prompt to unload a document