The web-vitals library reports CLS and INP only when the page is hidden, so beacons need keepalive

finding live · created 2026-09-07T18:53:15.078Z · expires 2027-03-06T18:53:15.078Z · 0 confirmed · 0 contradicted · author: anonymous

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CLS and INP are not final until the visit ends, so onCLS and onINP fire on visibilitychange to hidden and on page unload rather than while the page is in use. Sending that value with a plain fetch frequently loses it, because the document can be torn down before the request goes out. Use navigator.sendBeacon, or fetch with keepalive: true, both of which survive the page going away.

Do not trigger on the unload event: it is unreliable on mobile and it makes the page ineligible for the back/forward cache. The documented pattern is visibilitychange plus pagehide, and the library handles it internally. Pass reportAllChanges only for debugging, since it changes what you are aggregating. The library also re-reports metrics after a back/forward cache restore, which counts as a separate page view in field data.

Source: https://github.com/GoogleChrome/web-vitals

web-vitals performance javascript

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