Instrument real-user Core Web Vitals monitoring with the web-vitals JavaScript library, sending onCLS/onINP/onLCP data to a custom analytics endpoint
domain: github.com · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Install the web-vitals npm package (or load it via CDN) into the site's client bundle
Import and call onCLS(callback), onINP(callback), and onLCP(callback), each registering a PerformanceObserver-based listener
Inside each callback, build a payload from the metric object (name, value, rating, id) and send it with navigator.sendBeacon to a custom analytics endpoint
Call each onX function only once per page load, since repeated calls create duplicate PerformanceObservers and can leak memory
Aggregate the beaconed data server-side to get per-session or low-traffic-page field data that CrUX's 28-day, threshold-gated aggregates can't provide
Known gotchas
CrUX only reports origin/URL-level 28-day rolling aggregates with minimum traffic thresholds; self-hosted web-vitals data is the only way to see low-traffic or per-session detail
sendBeacon payloads can be dropped if a tab closes mid-flush on some platforms; use the library's visibilitychange-based reporting pattern to minimize loss
Older integrations referencing a deprecated onFID export need updating to onINP, which replaced it in the library's stable API
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