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.

Bubbles No
Cancelable Yes
Interface Event
Event handler property onbeforeunload

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.

However note that not all browsers support this method, and some instead require the event handler to implement one of two legacy methods:

  • assigning a string to the event's returnValue property
  • returning a string from the event handler.

Some browsers used to display the returned string in the confirmation dialog, enabling the event handler to display a custom message to the user. However, this is deprecated and no longer supported in most browsers.

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.

Usage notes

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.

Examples

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});
  }
});

Specifications

Specification Status Comment
HTML Living Standard
The definition of 'beforeunload' in that specification.
Living Standard
HTML5
The definition of 'beforeunload' in that specification.
Recommendation Initial definition

Browser compatibility

BCD tables only load in the browser

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

See also