Summary

Next.js 9.1.7 focuses on runtime size reduction, clearer production build output, compatibility improvements, and developer ergonomics. Most changes are non‑breaking: upgrade by running npm i next@latest react@latest react-dom@latest.

Key Points

• Client bundle size reductions (typical 3–4%, up to 6–8%+; ~7.5 kB saved on a hello-world app) through optimized JSX output and smaller polyfills.
• New built-in differential polyfills for fetch(), URL, and Object.assign (removes many common dependency polyfills and avoids shipping bytes to modern browsers).
• Redesigned production next build CLI: displays gzipped page sizes (excludes shared files) and classifies pages as Server, Static, or SSG; size thresholds colored for quick performance signaling (<130 kB green, 130–170 kB yellow, >170 kB red).
• Page-loading improvements (CSS prioritized over JS, lower-priority/idle-time prefetching) to improve FCP and TTI.
• Support for modern JS features including Optional Chaining (?.) and Nullish Coalescing (??) via the compiler.
• next export now deploys to Vercel with zero config (just use next build && next export in your build script).
• Full Next.js client runtime is React Strict Mode compatible and opt-in via module.exports = { reactStrictMode: true } in next.config.js.
• Automated compatibility testing run against React’s nightly (next) builds to surface future regressions.

Upgrade notes

• Non‑breaking release; update with npm i next@latest react@latest react-dom@latest and test.

Quick action items for engineers

• Run the upgrade in staging and smoke-test pages for size/perf regressions.
• Remove duplicate polyfill dependencies (e.g., whatwg-fetch, unfetch, url, object-assign) where unnecessary.
• Consider enabling reactStrictMode incrementally to catch warnings early and prepare for future optimizations.