Summary

Next.js 9.4 (2020-05-11) adds Fast Refresh by default, Incremental Static Regeneration (ISR, beta), built-in fetch for Node and browsers, .env support plus NEXT_PUBLIC_ prefix, Web Vitals reporting, absolute imports/aliases, configurable Sass options, and new CMS examples. These changes focus on developer experience, predictable static regeneration, and simpler configuration.

Key Points

• Fast Refresh (default): reliable hot reloading that preserves component state, with a redesigned error overlay showing accurate locations and editor click-to-open.
• Incremental Static Regeneration (beta): background re-generation of static pages via unstable_revalidate, serving static pages with no downtime and bounded re-generation concurrency.
• Environment variables: automatic .env loading; prefix variables with NEXT_PUBLIC_ to inline them into the browser bundle (no next.config.js needed).
• Built-in fetch polyfill: fetch() is available in both Node and browsers; remove server-side polyfills like node-fetch or isomorphic-unfetch.
• Web Vitals: export reportWebVitals(metric) from pages/_app.js to forward Core Web Vitals to analytics.
• Absolute imports & aliases: set baseUrl in jsconfig.json/tsconfig.json and use paths for custom aliases (e.g. @/design-system/*).
• Configurable Sass: use sassOptions in next.config.js to set includePaths and other compiler options.
• Extras: new CMS examples (Contentful, DatoCMS, Prismic, Sanity, TakeShape) and improved log output.

Upgrade notes

• Fast Refresh is enabled automatically; verify hot-reload behavior in your app.
• ISR is beta and uses unstable_revalidate; test re-generation logic and monitor failures.
• Remove server-side fetch polyfills and rely on built-in fetch() in data fetching functions.
• Move browser-exposed env vars to NEXT_PUBLIC_ and use .env files for local/production variables.