Summary

Next.js 9.3 introduces first-class SSG and SSR data-fetching APIs (getStaticProps, getStaticPaths, getServerSideProps), Preview Mode for showing CMS drafts, built-in Sass support (global and CSS modules), automatic static 404 optimization, and a ~32KB smaller runtime. All changes are backwards compatible and can be adopted incrementally.

Upgrade: run npm i next@latest react@latest react-dom@latest.

Key Points

• New data-fetching APIs

  • getStaticProps: fetch at build time (server-only; tree-shaken from client bundles). Use for CMS-driven or publicly cacheable pages that should be pre-rendered.
  • getStaticPaths: provide dynamic route paths to pre-render at build time; return fallback: false (unknown paths 404) or fallback: true (on-demand static generation with a loading state).
  • getServerSideProps: fetch on every request (SSR); runs server-side and is tree-shaken from the client.

• Preview Mode

  • Lets you bypass static output to render draft content from a CMS for previewing without changing the published static files.

• Styling & runtime

  • Built-in Sass support: import .scss for global styles and .module.scss for locally scoped CSS modules.
  • 404 now automatically served statically for faster, more reliable responses.
  • Runtime reduced by ~32KB for better baseline performance.

• Other

  • Discussions moved to GitHub Discussions for community Q&A.
  • No breaking changes; getInitialProps remains supported but new APIs are recommended for new pages.