Next.js and Turbopack

Next.js 16+ uses Turbopack by default for local development: an incremental bundler written in Rust that significantly speeds up dev startup and hot updates.

When to Use

Turbopack (default dev): Use for day-to-day development. Faster cold start and HMR, especially in large apps.
Webpack (legacy dev): Use only if you hit a Turbopack bug or rely on a webpack-only plugin in dev. Disable with --webpack (or --no-turbopack depending on your Next.js version; check the docs for your release).
Production: Production build behavior (next build) may use Turbopack or webpack depending on Next.js version; check the official Next.js docs for your version.

Use when: developing or debugging Next.js 16+ apps, diagnosing slow dev startup or HMR, or optimizing production bundles.

How It Works

Turbopack: Incremental bundler for Next.js dev. Uses file-system caching so restarts are much faster (e.g. 5–14x on large projects).
Default in dev: From Next.js 16, next dev runs with Turbopack unless disabled.
File-system caching: Restarts reuse previous work; cache is typically under .next; no extra config needed for basic use.
Bundle Analyzer (Next.js 16.1+): Experimental Bundle Analyzer to inspect output and find heavy dependencies; enable via config or experimental flag (see Next.js docs for your version).

Examples

Commands

next dev
next build
next start

Usage

Run next dev for local development with Turbopack. Use the Bundle Analyzer (see Next.js docs) to optimize code-splitting and trim large dependencies. Prefer App Router and server components where possible.

Middleware File Naming

Next.js 16 introduced proxy.ts as the middleware filename, replacing the older middleware.ts convention:

Next.js 16+: use proxy.ts at the project root
Pre-Next.js 16: use middleware.ts at the project root

The filename change is tied to the Next.js version, not to which bundler (Turbopack or webpack) is in use. Always check the official docs for the version you are reviewing.

Do not flag proxy.ts as a misnamed or missing middleware file in Next.js 16 projects. The file is correct and intentional. Suggesting a rename to middleware.ts will break middleware execution.

Reference: Next.js proxy docs

Best Practices

Stay on a recent Next.js 16.x for stable Turbopack and caching behavior.
If dev is slow, ensure you're on Turbopack (default) and that the cache isn't being cleared unnecessarily.
For production bundle size issues, use the official Next.js bundle analysis tooling for your version.

nextjs-turbopack