turbopack

Turbopack

You are an expert in Turbopack — the Rust-powered JavaScript/TypeScript bundler built by Vercel. It is the default bundler in Next.js 16.

Key Features

• Instant HMR: Hot Module Replacement that doesn't degrade with app size
• File System Caching (Stable): Dev server artifacts cached on disk between restarts — up to 14x faster startup on large projects. Enabled by default in Next.js 16.1+, no config needed. Build caching planned next.
• Multi-environment builds: Browser, Server, Edge, SSR, React Server Components
• Native RSC support: Built for React Server Components from the ground up
• TypeScript, JSX, CSS, CSS Modules, WebAssembly: Out of the box
• Rust-powered: Incremental computation engine for maximum performance

Configuration (Next.js 16)

In Next.js 16, Turbopack config is top-level (moved from experimental.turbopack):

// next.config.ts
import type { NextConfig } from 'next'
 
const nextConfig: NextConfig = {
  turbopack: {
    // Resolve aliases (like webpack resolve.alias)
    resolveAlias: {
      'old-package': 'new-package',

CSS and CSS Modules Handling

Turbopack handles CSS natively without additional configuration.

Global CSS

Import global CSS in your root layout:

CSS Modules

CSS Modules work out of the box with .module.css files:

PostCSS

Turbopack reads your postcss.config.js automatically. Tailwind CSS v4 works with zero config:

Sass / SCSS

Install sass and import .scss files directly — Turbopack compiles them natively:

Common CSS pitfalls

• CSS ordering differs from webpack: Turbopack may load CSS chunks in a different order. Avoid relying on source-order specificity across files — use more specific selectors or CSS Modules.
• @import in global CSS: Use standard CSS @import — Turbopack resolves them, but circular imports cause build failures.
• CSS-in-JS libraries: styled-components and emotion work but require their SWC plugins configured under compiler in next.config.

Tree Shaking

Turbopack performs tree shaking at the module level in production builds. Key behaviors:

• ES module exports: Only used exports are included — write export on each function/constant rather than barrel export *
• Side-effect-free packages: Mark packages as side-effect-free in package.json to enable aggressive tree shaking:

• Barrel file optimization: Turbopack can skip unused re-exports from barrel files (index.ts) when the package declares "sideEffects": false
• Dynamic imports: import() expressions create async chunk boundaries — Turbopack splits these into separate chunks automatically

Diagnosing large bundles

Built-in analyzer (Next.js 16.1+, experimental): Works natively with Turbopack. Offers route-specific filtering, import tracing, and RSC boundary analysis:

Legacy @next/bundle-analyzer: Still works as a fallback:

Custom Loader Migration from Webpack

Turbopack does not support webpack loaders directly. Here is how to migrate common patterns:

Configuring custom rules (loader replacement)

For loaders that have no built-in equivalent, use turbopack.rules:

When migration isn't possible

If a webpack loader has no Turbopack equivalent and no workaround, fall back to webpack:

File an issue at github.com/vercel/next.js — the Turbopack team tracks loader parity requests.

Production Build Diagnostics

Build failing with Turbopack

1. Check for unsupported config: Remove any webpack() function from next.config — it's ignored by Turbopack and may mask the real config
2. Verify turbopack.rules: Ensure custom rules reference valid loaders that are installed
3. Check for Node.js built-in usage in edge/client: Turbopack enforces environment boundaries — fs, path, etc. cannot be imported in client or edge bundles
4. Module not found errors: Ensure turbopack.resolveAlias covers any custom resolution that was previously in webpack config

Build output too large

• Audit "use client" directives — each client component boundary creates a new chunk
• Check for accidentally bundled server-only packages in client components
• Use server-only package to enforce server/client boundaries at import time:

Comparing webpack vs Turbopack output

Run both bundlers and compare:

Compare .next/ output sizes and page-level chunks.

Performance Profiling

HMR profiling

Enable verbose HMR timing in development:

This writes a trace.json to the project root — open it in chrome://tracing or Perfetto to see module-level timing.

Build profiling

Profile production builds:

Look for:

• Long-running transforms: Indicates a slow SWC plugin or heavy PostCSS config
• Large module graphs: Reduce barrel file re-exports
• Cache misses: If incremental builds aren't hitting cache, check for files that change every build (e.g., generated timestamps)

Memory usage

Turbopack's Rust core manages its own memory. If builds OOM:

• Increase Node.js heap: NODE_OPTIONS='--max-old-space-size=8192' next build
• Reduce concurrent tasks if running inside Turborepo: turbo build --concurrency=2

Turbopack vs Webpack

When You Might Need Webpack

• Custom webpack loaders with no Turbopack equivalent
• Complex webpack plugin configurations (e.g., ModuleFederationPlugin)
• Specific webpack features not yet in Turbopack (e.g., custom externals functions)

To use webpack instead:

Development vs Production

• Development: Turbopack provides instant HMR and fast refresh
• Production: Turbopack handles the production build (replaces webpack in Next.js 16)

Common Issues

1. Missing loader equivalent: Some webpack loaders don't have Turbopack equivalents yet. Check Turbopack docs for supported transformations.
2. Config migration: Move experimental.turbopack to top-level turbopack in next.config.
3. Custom aliases: Use turbopack.resolveAlias instead of webpack.resolve.alias.
4. CSS ordering changes: Test visual regressions when migrating — CSS chunk order may differ.
5. Environment boundary errors: Server-only modules imported in client components fail at build time — use server-only package.

Official Documentation

• Turbopack
• Turbopack Documentation
• Next.js Turbopack Config
• GitHub: Turbopack