Back/forward cache (bfcache) restores and pagehide

Into the back/forward cache

When the user goes to another page, the browser can keep the current page in the back/forward cache (bfcache). The page gets a pagehide event, and its persisted property is true. The tracker changes the state to frozen, with the trigger pagehide.

When the browser does not keep the page, persisted is false. Then the tracker changes the state to terminated. All three engines have the persisted property (browser support).

Out of the back/forward cache

When the user goes back to the page, the browser restores it from the cache. The page gets a pageshow event, and its persisted property is true. The tracker changes the state to active or passive, with the trigger pageshow. The state is active when document.hasFocus() gives true.

No new navigation entry comes with a restore. Thus the pageshow transition is the signal of a new page view.

A restore is always a transition

Chromium restores a page with resume, visibilitychange and then pageshow. Thus the state is already visible when pageshow comes. The tracker still gives a transition with the trigger pageshow. Then from and to are the same, for example active to active.

Thus the subscribers get a transition for each restore. A subscriber finds a restore from its trigger, not from its states:

getPageLifecycle().subscribe(({ trigger }) => {
    if (trigger === "pageshow") startNewPageView();
});

summarizeTransitions() gives wasRestoredFromBFCache: true for a list of transitions that contains a pageshow transition. The restore recipe shows a full example.

No beforeunload listener

The tracker does not listen for beforeunload, for two causes:

• A script can cancel that event. Then a live page has the state terminated.
• A beforeunload listener can make the page ineligible for the back/forward cache.

pagehide versus unload

Do not use the unload event to send data. Chrome deprecated unload in steps: 1% of sites in version 146 (March 2026), 40% in version 150, and 100% in version 154 (2026-09-22). From Chrome 154, Chrome does not start unload listeners.

Use pagehide and visibilitychange instead. The tracker uses both: a page that becomes hidden gets the hidden state, and a page that goes away gets frozen or terminated.

Why a page is not restored

Chromium gives the causes in notRestoredReasons (gradual rollout from Chrome 123). Firefox and Safari do not have it. The strings of the reasons change over time:

• Chrome removed the reason websocket in version 149. From that version, pages with open WebSockets can go into the back/forward cache, because Chrome disconnects the WebSockets.
• Chrome removed the reason unload-listener in version 154, together with the unload event.

Try a restore on this site

The home page shows the transitions of your visit. Leave the page with its link to another page, and come back with the Back button. If the browser restores the page from the back/forward cache, the diagram shows frozen and then a pageshow transition.