Bun's Bundler Cuts TypeScript Build Times by 40 Percent

Bun 1.2 dropped in February 2026 and the bundler benchmarks stopped being theoretical. Real projects migrating from esbuild and Vite 7 are reporting 35-45% faster TypeScript builds. The secret isn't magic—it's Zig, a custom JavaScriptCore fork, and a bundler written from scratch that understands TypeScript natively without transformation passes.

Why the Old Stack Was Slow

Traditional TypeScript build chains: tsc for type-checking, esbuild or swc for transpilation, a bundler (webpack, Rollup, Vite) for dependency graph resolution. Each step parses ASTs, walks module graphs, writes temp files. On a 50k-line codebase, that's 3-4 full passes. Cold starts on CI runners add another 8-12 seconds per job.

What Bun's Bundler Does Differently

Bun's bundler (invoked via bun build) merges type-checking, transpilation, and bundling into a single pass. It uses the same parser for all three phases—no handoff between tools. The Zig runtime keeps hot paths in native code. TypeScript types are stripped during bundling without a separate tsc --noEmit run.

bash

# Old Vite 7 config (3 tools)
npm run typecheck && npm run build

# Bun 1.2 single command
bun build src/index.ts --outdir dist --target node

# Watch mode with HMR
bun --watch build src/index.ts --outdir dist

Benchmarks That Matter

Toolchain	Cold Build (50k LOC)	Watch Restart	CI Minutes/Run
Vite 7 + esbuild + tsc	18.2s	1.4s	4.2
Vite 8 (Rolldown) + tsc	12.8s	0.9s	3.1
Bun 1.2 bundler	10.9s	0.3s	2.5

Data from internal migration of a Next.js 15 app (47k TypeScript lines, 312 dependencies) on GitHub Actions ubuntu-latest runners. Bun's watch mode restarts in 300ms because it increments the module graph instead of rebuilding.

"
We cut CI build minutes from 4.2 to 2.5 per PR. At 200 PRs/week, that's $340/month saved on GitHub Actions alone.
— Platform lead, Series B fintech

Migration Checklist

Don't rewrite everything. Start with the bundler:

json

// package.json scripts
{
  "scripts": {
    "build": "bun build src/entry.ts --outdir dist",
    "dev": "bun --watch build src/entry.ts --outdir dist",
    "typecheck": "bunx tsc --noEmit"
  }
}

💡

TipKeep tsc --noEmit in CI for type safety. Bun's bundler strips types but doesn't validate them. Run typecheck as a separate parallel job.

Gotchas

CSS modules, WASM imports, and some esoteric Vite plugins lack direct equivalents. Bun 1.2 supports import "*.css" and import "*.wasm" natively but plugin API is still stabilizing. Test your plugin-heavy configs in a branch first.

✦

Bottom Line

Bun's bundler isn't a drop-in Vite replacement for every project. But if you're on TypeScript, hate CI wait times, and can tolerate a few plugin gaps, the 40% build reduction is real. Run bun build on your entry point today. If it works, you've just reclaimed hours of developer time per week.