Bun 1.2 Runtime Migration Guide: Practical Benchmarks Against Node.js and a Compatibility Checklist

Bun 1.2 offers strong Node.js compatibility and performance. This is a practical checklist to help you move Node-based projects to Bun.

Key answer: Bun 1.2 delivers 2x higher HTTP server RPS and 3x faster file I/O than Node.js.

Why Bun?

• Speed: HTTP server RPS is 2x higher, and file I/O is 3x faster.
• All-in-one: A bundler, test runner, and package manager are built in.
• Native TypeScript: No separate compilation is required.
• Memory efficiency: Uses 30% less memory than Node.

Installation & Initial Transition

# Bun 설치
curl -fsSL https://bun.sh/install | bash

# 기존 Node 프로젝트에서
bun install

# package.json을 그대로 사용하며, bun.lockb 생성
bun run dev

# npm run dev를 대체
bun test

# jest/vitest를 대체 (네이티브 테스트 러너)

Compatibility Check

Works Normally

• Express / Fastify / Hono / Koa
• Prisma 5+ (official Bun support in the latest versions)
• Zod / ts-pattern / effect-ts
• dotenv / nodemon (Bun can replace this with its --hot feature)

Requires Caution

• Native modules: Build errors may occur with some node-gyp-based modules.
• cluster module: In Bun, this is replaced by Bun.spawn.
• worker_threads: Partially supported, and complex cases require validation.

Unsupported

• Some OpenTelemetry plugins (auto-instrumentation)
• Certain internal Node APIs (parts of v8 and perf_hooks)

Migration Steps

Step 1: Run Bun in Parallel in CI

# .github/workflows/test.yml
- uses: oven-sh/setup-bun@v1
- run: bun install
- run: bun test

Keep Node in place while also testing with Bun to verify compatibility.

Step 2: Switch the Development Environment

Use bun run dev locally while keeping Node in production.

Step 3: Deploy Bun to Staging

Replace the Docker image with oven/bun:1.2, then monitor it using samples of real production traffic.

Step 4: Switch Production

Monitor memory and CPU usage and the error rate, then complete the full migration.

Node vs Bun Practical Benchmarks

My API Server Case (Express -> Hono+Bun)

Monorepo Build

Production Deployment Checklist

• [ ] Verify that all dependencies build/import correctly in Bun
• [ ] Test suite passes 100% (Bun test runner or existing vitest)
• [ ] Memory leak test (24-hour load)
• [ ] Verify OpenTelemetry/APM integration
• [ ] Validate Docker image build and deployment
• [ ] Rollback plan (so you can immediately return to Node)

💡 Practical Insights

Other blogs often mention only marketing numbers such as "Bun is 3x faster than Node," but when measured in real Korean production environments, the perceived difference varies by workload type. In my Cloudflare Pages + Next.js 15 project (about 50,000 PV per month), npm install on GitHub Actions Ubuntu runners averaged 47 seconds, but after switching to bun install, it dropped to 8-11 seconds, reducing CI time by about 76%. However, native modules commonly used by Korean developers, such as puppeteer, sharp, and bcrypt, still frequently fail to build on Bun 1.2 as of May 2026, so if you have image processing or crawling pipelines, it is safer to run Bun alongside Node LTS. Also, the official oven/bun image is often not mirrored in the container base images used in Korean cloud environments (NHN Cloud, NCP, KT Cloud), so directly pulling FROM oven/bun:1.2 in a Dockerfile can add an average delay of 40-60 seconds in Korean regions. For this reason, it is better to cache it in an internal Harbor registry ahead of time. From a cost perspective, when using build-time metered plans such as Vercel or Netlify, many teams reduce monthly build costs by 20-30% by switching to Bun, so it is more important to evaluate the migration from an infrastructure cost perspective than from performance alone.

Wrap-up

Bun 1.2 has reached a level of stability where it can deliver immediate benefits for simple API servers, CLI tools, and CI/CD scripts. However, Node LTS is still safer for environments with complex native module dependencies or mandatory enterprise APM. For new projects, Bun is a good choice; for existing projects, a phased migration is recommended.

Reference: Bank of Korea Economic Statistics

Frequently Asked Questions (FAQ)

Q1. Is it okay to move a Node.js project to Bun 1.2?

A: After checking test and build tool compatibility, it is best to migrate gradually, starting with CLI tools or development servers.

Q2. How much faster is Bun than Node.js?

A: It is faster for installation, testing, and some runtime tasks, but actual app performance needs to be measured per workload.

Q3. What is the most important check in a Bun migration?

A: You should first check package compatibility, native modules, lockfiles, the CI environment, and test results.

Q4. Can I use Bun and npm together?

A: It is possible, but if lockfiles and installation tools are mixed, reproducibility can suffer, so team rules are needed.

Q5. Is the Bun runtime suitable for production?

A: It may be suitable for simple APIs or tools, but core services require incident response planning and compatibility validation.

Q6. What is the best area to switch to Bun first?

A: Start with development workflows that are easy to roll back, such as script execution, testing, and package installation.