Window: unload event

Developers should avoid using this event. See "Usage notes" below.

The unload event is fired when the document or a child resource is being unloaded.

Bubbles No
Cancelable No
Interface Event
Event handler property onunload

It is fired after:

The document is in the following state:

  • All the resources still exist (img, iframe etc.)
  • Nothing is visible anymore to the end user
  • UI interactions are ineffective (window.open, alert, confirm, etc.)
  • An error won't stop the unloading workflow

Please note that the unload event also follows the document tree: parent frame unload will happen before child frame unload (see example below).

Usage notes

Developers should avoid using this event.

Especially on mobile, the unload event is not reliably fired. For example, the unload 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.

Also, the unload 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, some browsers (such as Firefox) will not place pages in the bfcache if they have unload listeners, and this is bad for performance. Others, such as Chrome, will not fire the unload when a user navigates away.

The best event to use to signal the end of a user's session is the visibilitychange event. In browsers that don't support visibilitychange the next-best alternative is the pagehide event, which is also not fired reliably, but which is bfcache-compatible.

If you're specifically trying to detect page unload events, it's best to listen for the pagehide event.

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

Examples

<!DOCTYPE html>
<html>
  <head>
    <title>Parent Frame</title>
    <script>
      window.addEventListener('beforeunload', function(event) {
        console.log('I am the 1st one.');
      });
      window.addEventListener('unload', function(event) {
        console.log('I am the 3rd one.');
      });
    </script>
  </head>
  <body>
    <iframe src="child-frame.html"></iframe>
  </body>
</html>

Below, the content of child-frame.html:

<!DOCTYPE html>
<html>
  <head>
    <title>Child Frame</title>
    <script>
      window.addEventListener('beforeunload', function(event) {
        console.log('I am the 2nd one.');
      });
      window.addEventListener('unload', function(event) {
        console.log('I am the 4th and last one…');
      });
    </script>
  </head>
  <body></body>
</html>

When the parent frame is unloaded, events will be fired in the order described by the console.log() messages.

Specifications

Specification Status Comment
HTML Living Standard
The definition of 'unload' in that specification.
Living Standard

Browser compatibility

BCD tables only load in the browser

See also