Deprecating the unload event

The unload event will be gradually deprecated by gradually changing the default so that unload handlers stop firing on pages unless a page explicitly opts in to re-enable them.

Deprecation timeline

We noted that unload behavior would likely be subject to changes as early as January 2019, when we announced our intent to implement a back/forward cache. In parallel to the implementation work, we conducted a large outreach which resulted in a significant drop of unload usage. To complement this outreach, we also started to offer ways to test the effect of deprecating unload from Chrome 115:

Following these outreach and trial phases, here is how we expect the soft deprecation to roll out:

  • A scoped phase where unload will gradually cease to function for the top 50 popular sites (reference as of the time of writing).
    • Starting with 1% of users from Chrome 120 (end of November 2023).
    • Ending with 100% of users by the end of Q3 2024
  • In addition, from Q3 2024, we intend to start a generic phase where unload will gradually cease to function on any sites, starting with 1% of users and ending with 100% of users by the end of Q1 2025.

Note that we also offer a menu of opt-out options in case this soft deprecation timeline doesn't provide sufficient time to migrate away from unload. Our goal is to use this soft deprecation to inform the timeline for the last phase (hard deprecation of unload) in which these opt-outs will be removed or reduced.

Timeline of the unload deprecation.

Background

unload was designed to fire when the document is being unloaded. In theory, it can be used to run code any time a user is navigating away from a page, or as an end of session callback.

Scenarios where this event was most commonly used include:

  • Saving user data: Save data before leaving the page.
  • Performing cleanup tasks: Closing open resources before abandoning the page.
  • Sending analytics: Sending data related to user interactions at the end of the session.

However the unload event is extremely unreliable.

On desktop Chrome and Firefox, unload is reasonably reliable but it has a negative impact on a site's performance by preventing the usage of bfcache (back/forward cache).

On mobile browsers unload often doesn't run as tabs are frequently backgrounded and then killed. For this reason browsers choose to prioritize the bfcache on mobile over unload, making them even more unreliable. Safari also uses this behaviour on desktop.

The Chrome team believe using the mobile model of prioritizing bfcache over unload on desktop would be disruptive by making it more unreliable there too, when previously this has been reasonably reliable in Chrome (and Firefox). Instead, Chrome's aim is to remove the unload event completely. Until then it will remain reliable on desktop for those who have explicitly opted-out of the deprecation.

Why deprecate the unload event?

Deprecating unload is a key step in a much bigger recognition of the web we live in now. The unload event gives a false sense of control of the app lifecycle that is increasingly untrue of how we browse the web in the modern computing world.

Mobile operating systems frequently freeze or unload web pages to conserve memory and desktop browsers are doing this more and more now too for the same reasons. Even without operating system interventions, users themselves frequently tab switch and kill old tabs without formally "leaving pages".

Removing the unload event as obselete is a recognition that we as web developers need to ensure our paradigm matches that of the real world and not depend on outdated concepts that no longer hold true—if they ever did.

Alternatives to unload events

Instead of unload it is recommended to use:

  • visibilitychange: To determine when the visibility of a page changes. This event happens when the user switches tabs, minimizes the browser window, or opens a new page. Consider the hidden state the last reliable time to save app and user data.
  • pagehide: To determine when the user has navigated away from the page. This event happens when the user navigates away from the page, reloads the page, or closes the browser window. The pagehide event is not fired when the page is simply minimized or switched to another tab. Note that, as pagehide does not make a page ineligible for the back/forward cache, it is possible a page can be restored after this event fires. If you're cleaning up any resources in this event, then they may have to be restored on page restore.

The beforeunload event has a slightly different use case to unload in that it is a cancellable event. It is often used to warn users of unsaved changes when navigating away. This event is also unrealiable as it will not fire if a background tab is killed. It is recommended to limit use of beforeunload and only add it conditionally. Instead, use the above events for most unload replacements.

For more details, see this advice on never using the unload handler.

Detect usage of unload

There are different tools to help you find appearances of the unload event on pages. This allows sites to discover if they are using this event—either in their own code, or via libraries—and so may be affected by the upcoming deprecation.

Lighthouse

Lighthouse has a no-unload-listeners audit, which warns developers if any JavaScript on their pages (including that from third-party libraries) adds an unload event listener.

Lighthouse audit showing unload handlers in use

Chrome DevTools

Chrome DevTools includes a back-foward-cache audit to help you identify issues that may prevent your page from being eligible for back/forward cache, including the usage of the unload handler.

To test back/forward cache, follow these steps:

  1. On your page, open DevTools, then navigate to Application > Background services > Back/forward cache.

  2. Click Test back/forward cache Chrome automatically takes you to chrome://terms/ and back to your page. Alternatively, you can click the browser's back and forward buttons.

If your page isn't eligible for back/forward caching, the Back/forward cache tab shows you a list of issues. Under Actionable, you can see if you are using unload:

Chrome DevTools Back/forward cache testing tool showing an unload handler was used

Reporting API

The Reporting API can be used to in conjuction with a read-only Permission Policy to detect usage of unload from your website users.

For more details see usUsing Reporting API to find unloads

Bfcache notRestoredReasons API

The notRestoredReasons property—added to the PerformanceNavigationTiming class—reports information on whether documents were blocked from using the bfcache on navigation, and why. Usage instructions can be found here. This is an example of how the response object warning of an existing unload listener looks like:

{
   blocked: true,
   children: [],
   id: "",
   name: "",
   reasons: [ "Internal Error", "Unload handler" ],
   src: "",
   url: "a.com"
}

Control access to unload

Chrome will deprecate the unload event gradually. In the meantime, you can use different tools to control this behavior and prepare for the upcoming deprecation. Keep in mind that you should not rely on these techniques in the long term, and you should plan to migrate to the alternatives instead as soon as possible.

The following options allow you to enable or disable unload handlers to test how your site would work without them so you can prepare for the upcoming deprecation. There are different types of policies:

  • Permissions Policy: This is a platform API for site owners to control access to features, at a site or an individual page level, via the usage of HTTP headers.
  • Enterprise policies: Tools for IT admins to configure Chrome for their organization or business. They can be configured via an admin panel, like the Google Admin Console.
  • Chrome flags: This allows an individual developer to change the unload deprecation setting to test impact on various sites.

Permissions Policy

A Permissions Policy has been added from Chrome 115 to allow sites to opt-out of using unload handlers and immediately benefit from the bfcache to improve site performance. See these examples on how to set this for your site. This allows sites to get ahead of the unload deprecation.

This will be extended in Chrome 117 to allow sites to do the reverse, and to opt-in to continuing to try to fire unload handlers, as Chrome changes the default for these to not fire in future. See these examples on how to continue to allow unload handlers to fire for your site. This opt-in will not remain forever and should be used to allow time for sites to migrate away from unload handlers.

Enterprise policy

Enterprises that have software that depends on the unload event to function correctly can use the ForcePermissionPolicyUnloadDefaultEnabled policy to prevent the gradual deprecation for devices under their control. By enabling this policy, unload will continue to default to enabled for all origins. A page may still set a stricter policy if it wants. Like the Permissions Policy opt-out, this is a tool to mitigate potential breaking changes, but it should not be used indefinitely.

Chrome flags and command line switches

As well as the enterprise policy, you can disable the deprecation for individual users via the Chrome flags and command lines swtiches:

Setting chrome://flags/#deprecate-unload this to enabled will bring forward the deprecation default and prevent unload handlers from firing. They can still be overridden on a site-by-site basis via Permissions Policy, but will continue to fire by default.

These settings can be also be controlled by command line switches.

Options comparison

The following table summarizes the different uses of the options discussed previously:

Bring deprecation forward Bring deprecation forward (with exceptions) Prevent deprecation to secure time for a migration
Permissions Policy
(applies to pages/sites)
Yes Yes Yes
Enterprise policy
(applies to devices)
No No Yes
Chrome flags
(applies to individual users)
Yes No No
Chrome command line switches
(applies to individual users)
Yes No Yes

Conclusion

unload handlers are being deprecated. They have been unreliable for a long time and are not guaranteed to be fired on all cases where a document gets destroyed. Additionally, unload handlers are incompatible with bfcache.

Sites that currently make use of unload handlers should prepare for this upcoming deprecation by testing for any existing unload handlers, removing or migrating them or, as a last resort, delaying the deprecation if more time is needed.

Acknowledgements

Thanks to Kenji Baheux, Fergal Daly, Adriana Jara, and Jeremy Wagner for help reviewing this article.

Hero image by Anja Bauermann on Unsplash