The Rust-based toolchain that replaced Babel for transforming and bundling your code.
Next.js uses SWC — a Rust-based compiler — by default instead of Babel, for transforming JSX and TypeScript and for minifying production output. Because SWC is compiled native code rather than a JavaScript-based transform pipeline, it's dramatically faster at the same job, which matters directly for dev server startup time and full production build times as a codebase grows.
Turbopack, a newer Rust-based bundler used in development mode, plays a different role than SWC: SWC transforms individual files, while Turbopack handles bundling and incremental compilation with fine-grained, function-level caching, which is what makes Fast Refresh feel near-instant even in large apps. A custom Babel config is still supported as an explicit opt-out, but using it disables some of the SWC-specific optimizations Next.js otherwise applies automatically.
What you'll walk away knowing