What is Next.js Turbopack?
Why Interviewers Ask This
Foundational questions like this help interviewers calibrate the rest of the interview. A confident, accurate answer signals that you have solid Next.js basics — a prerequisite for any developer role.
Answer
Turbopack is Vercel's Rust-based build tool for JavaScript and TypeScript that is bundled with Next.js as an alternative to Webpack. Introduced in Next.js 13 as alpha, it became the default dev server bundler in Next.js 15. Why Turbopack? Webpack is fast but has limitations — large projects see slow cold starts (10-60 seconds) and HMR (Hot Module Replacement) times of several seconds. Turbopack is designed for significantly faster performance: (1) Rust implementation: Rust is much faster than JavaScript for CPU-intensive build tasks; (2) Incremental computation: Turbopack only recomputes what has changed — caches previous work at the function level; (3) Parallelism: takes full advantage of multi-core CPUs. Benchmarks: Up to 700x faster than Webpack in cold start, 10x faster in HMR for large Next.js apps. Enable in development: next dev --turbopack (or default in Next.js 15). Current status: Turbopack is stable for development (next dev) and working on production builds (next build). Production Turbopack is in beta. Limitations vs Webpack: not all Webpack loaders/plugins are supported in Turbopack (most common ones work); some advanced configurations may not be available. SWC (Speedy Web Compiler): separate from Turbopack — the Rust-based JavaScript/TypeScript compiler that replaced Babel in Next.js 12. SWC is used for code transformation; Turbopack is the bundler.
Pro Tip
Before answering, structure your response: one-line definition → real-world analogy → concrete example from a project. This makes even complex Next.js answers easy to follow.