Window: beforeunload event

The beforeunload event is fired when the window, the document and its resources are about to be unloaded. The document is still visible and the event is still cancelable at this point.

This event enables a web page to trigger a confirmation dialog asking the user if they really want to leave the page. If the user confirms, the browser navigates to the new page, otherwise it cancels the navigation.

According to the specification, to show the confirmation dialog an event handler should call preventDefault() on the event.

To combat unwanted pop-ups, browsers may not display prompts created in beforeunload event handlers unless the page has been interacted with, or may even not display them at all.

The HTML specification states that calls to window.alert(), window.confirm(), and window.prompt() methods may be ignored during this event. See the HTML specification for more details.

The beforeunload event suffers from the same problems as the unload event.

Especially on mobile, the beforeunload event is not reliably fired. For example, the beforeunload event is not fired at all in the following scenario:

1. A mobile user visits your page.
2. The user then switches to a different app.
3. Later, the user closes the browser from the app manager.

The beforeunload event is not compatible with the back/forward cache (bfcache), because many pages using this event assume that the page will not continue to exist after the event is fired. To combat this, browsers will not place pages in the bfcache if they have beforeunload listeners, and this is bad for performance.

However, unlike the unload event, there is a legitimate use case for the beforeunload event: the scenario where the user has entered unsaved data that will be lost if the page is unloaded.

It is recommended that developers listen for beforeunload only in this scenario, and only when they actually have unsaved changes, so as to minimize the effect on performance. See the Examples section below for an example of this.

See the Page Lifecycle API guide for more information about the problems associated with the beforeunload event.

In this example a page listens for changes to a text input. If the element contains a value, it adds a listener for beforeunload. If the element is empty, it removes the listener:

const beforeUnloadListener = (event) => {
  event.preventDefault();
  return event.returnValue = "Are you sure you want to exit?";
};

const nameInput = document.querySelector("#name");

nameInput.addEventListener("input", (event) => {
  if (event.target.value !== "") {
    addEventListener("beforeunload", beforeUnloadListener, {capture: true});
  } else {
    removeEventListener("beforeunload", beforeUnloadListener, {capture: true});
  }
});

See WindowEventHandlers/onbeforeunload for more details on how various browsers handle this event.

• Related events: DOMContentLoaded, readystatechange, load, unload
• Unloading Documents — Prompt to unload a document
• Remove Custom Messages in onbeforeload Dialogs after Chrome 51
• Don't lose user and app state, use Page Visibility explains in detail why you should use visibilitychange, not beforeunload/unload.
• Page Lifecycle API gives best-practices guidance on handling page lifecycle behavior in your web applications.
• PageLifecycle.js: a JavaScript library that deals with cross-browser inconsistencies in page lifecycle behavior.
• Back/forward cache explains what the back/forward cache is, and its implications for various page lifecycle events.