web-vitals

web-vitals is a small JavaScript library developed by Google that provides utilities for measuring Core Web Vitals metrics in real-world user experiences. Core Web Vitals include Largest Contentful Paint (LCP) for loading performance, First Input Delay (FID) for interactivity, and Cumulative Layout Shift (CLS) for visual stability. The library offers a simple API to capture these metrics accurately in production environments, sending data to analytics endpoints for monitoring and optimization. web-vitals uses the same measurement methodology that Google uses for Chrome User Experience Report and Search Console, ensuring consistency between development testing and production monitoring. The library is framework-agnostic and can be integrated into any website regardless of the underlying technology stack. It includes support for newer metrics like Interaction to Next Paint (INP) which is replacing FID as a Core Web Vital. Developers use web-vitals to implement real user monitoring (RUM) that tracks actual user experiences rather than synthetic lab testing alone.