The **es‑feature‑tests** library provides a lightweight runtime suite for detecting which modern JavaScript (ES 6+) language features and APIs a browser or Node environment actually supports. It’s intended for front‑end developers (or build tools) who want to conditionally serve native ES code, polyfills, or transpiled bundles based on real feature tests rather than user‑agent sniffing, and it includes a CLI (`testify`) that scans source files to generate the exact set of tests needed for a given codebase.