Node.js to Bun Migration Guide - AI-Optimized Intelligence

Performance Reality Check

Actual Performance Gains

• HTTP throughput: 4x improvement (13,000 → 52,000 req/s) for simple Express apps
• Real-world API response times: 200ms → 80ms average (60% improvement)
• Memory usage: 30% reduction, not miraculous but measurable
• Docker build times: 8 minutes → 2 minutes (primary benefit from bun install speed)
• Cold starts: Microseconds vs 2-5 seconds (massive serverless advantage)

Performance Degradation Scenarios

• Complex dependency trees: Reduce 4x advantage significantly
• Windows performance: Requires WSL, still slower than Linux
• CPU-bound workloads: Only ~2x improvement, not 4x
• Breaking native modules: Can reduce overall performance below Node.js

Critical Failure Scenarios

Migration Breaking Points

• 10% of npm packages break: Budget 2-3x longer than expected for debugging
• Native modules with C++ bindings: Will completely fail
• Jest compatibility: 90% works, 10% requires complete test rewrites
• Windows development: WSL requirement will anger Windows developers
• Enterprise LTS requirements: Fast iteration cycle unsuitable for risk-averse environments

Production Failure Examples

• Session middleware timing errors: Requires different session store implementation
• Docker health checks fail: Bun starts faster than expected, breaking timing assumptions
• CI environment variables: Need Bun-specific path adjustments
• Auth library incompatibilities: bcrypt specifically breaks, requires bcryptjs replacement

Resource Investment Requirements

Migration Time Investment

• Simple migration: Budget 2-3x longer than initial estimate
• Complex codebases: Expect "everything is broken" debugging sessions
• Testing phase: 5-10% of test suite requires rewrites
• Production deployment: Plan for at least one rollback scenario

Expertise Requirements

• Docker containerization: Must understand multi-stage builds and Alpine packages
• Database connection optimization: Need to adjust pool sizes (Bun handles connections more efficiently)
• Performance monitoring: Required to prove migration value to stakeholders

Financial Impact

• Infrastructure cost reduction: Real example shows $800/month → $400/month (50% savings)
• Developer time savings: Eliminate webpack configuration and TypeScript build steps
• CI cost reduction: 10x faster bun install significantly reduces build minutes

Decision Support Matrix

Migrate When

• Starting new TypeScript projects
• CI/CD pipeline bottlenecked by npm install (5+ minute builds)
• I/O-heavy APIs needing response time improvements
• Hatred of managing multiple tools (webpack, Jest, tsc, etc.)
• Serverless deployments requiring fast cold starts

Don't Migrate When

• Massive stable codebase that "just works"
• Heavy reliance on Node.js-specific native modules
• Primarily Windows development team
• Enterprise requiring LTS guarantees and support
• CPU-bound applications (minimal benefit)

Risk vs Reward Assessment

• High reward scenarios: New projects, slow CI, TypeScript-heavy codebases
• High risk scenarios: Production systems with custom native modules
• Break-even point: Migration effort justified if CI build times >3 minutes
• Success probability: ~90% for standard web APIs, ~60% for complex native module usage

Implementation Reality

Configuration That Actually Works

Production Dockerfile:

FROM oven/bun:1.0-slim as base
WORKDIR /app

FROM base as deps
COPY package.json bun.lockb* ./
RUN bun install --frozen-lockfile --production

FROM base as runner
COPY --from=deps /app/node_modules ./node_modules
COPY . .
RUN addgroup --system --gid 1001 bunjs
RUN adduser --system --uid 1001 bunjs
USER bunjs
EXPOSE 3000
CMD ["bun", "start"]

Resource Limits (Kubernetes):

resources:
  requests:
    memory: "64Mi"    # Reduced from Node.js 128Mi
    cpu: "100m"
  limits:
    memory: "128Mi"   # Reduced from Node.js 256Mi
    cpu: "200m"

Breaking Changes That Will Ruin Your Weekend

Package Replacements Required:

• node-sass → sass (Dart Sass)
• bcrypt → bcryptjs
• ts-node → Direct .ts execution
• jest.doMock() → Requires test rewrites

File System Behavior Differences:

• Different timing behavior affects session middleware
• File permission handling varies from Node.js
• Path resolution edge cases exist

Critical Warnings

What Official Documentation Doesn't Tell You

Ecosystem Compatibility Lies:

• "90% npm package compatibility" is misleading for enterprise codebases
• Native module failures cascade to break seemingly unrelated functionality
• Windows WSL requirement is a dealbreaker for many teams

Testing Reality:

• Jest "compatibility" means 10% of complex mocks break
• Snapshot tests have different formatting
• Custom transformers for unusual file types fail

Production Deployment Gotchas:

• Health checks fail due to faster startup (not slower)
• Memory monitoring thresholds need adjustment
• Error logging patterns differ from Node.js

Rollback Requirements

Essential Rollback Preparation:

# Keep package-lock.json alongside bun.lockb
# Use feature flags for runtime switching
# Tag Docker images for instant reversion
# Monitor error rates closely for first week

Rollback Trigger Indicators:

• Error rates increase >10% post-deployment
• Critical package compatibility failures in production
• Performance gains don't materialize (indicates configuration issues)

Tool Quality Assessment

What Actually Works Well

• bun install: Legitimately 10x faster, single biggest migration benefit
• TypeScript execution: Finally run .ts files directly without build steps
• Docker builds: Dramatic improvement due to dependency installation speed
• Basic Express/Fastify apps: Usually work without modification

What's Still Problematic

• Advanced webpack features: Don't exist in bun build
• Complex Jest mocking: Requires significant test rewrites
• Windows development: WSL requirement creates team friction
• Enterprise native modules: High failure rate with proprietary packages

Community Support Quality

• GitHub issues: Active but fast iteration means breaking changes
• Documentation: Good for basic cases, lacking for edge cases
• Stack Overflow: Limited compared to Node.js ecosystem
• Enterprise support: Nonexistent, community-driven only

Migration Success Criteria

Quantifiable Success Metrics

• Response times: Should achieve 60-70% improvement or troubleshoot
• Memory usage: Expect 30-50% reduction
• Build times: Should see 60-80% improvement in CI
• Error rates: Must remain stable or decrease

Acceptable Trade-offs

• 10% test rewriting effort: Acceptable for 4x performance gains
• Windows development friction: Acceptable if team primarily Mac/Linux
• Reduced enterprise support: Acceptable for non-critical applications
• Learning curve for debugging: Acceptable given tooling consolidation benefits

Failure Indicators

• Migration taking >2x estimated time: Indicates compatibility issues
• Performance gains <2x: Suggests configuration problems
• Error rates increase: Indicates fundamental incompatibilities
• Team productivity decrease: Suggests insufficient preparation or training

Operational Intelligence Summary

This migration succeeds when:

1. You're building new or rebuilding existing systems
2. Your CI pipeline is already painful (>3 minute builds)
3. You have TypeScript-heavy codebases
4. Your team is comfortable with fast-moving ecosystems

This migration fails when:

1. You underestimate the 10% of packages that break everything
2. You attempt it on massive legacy codebases
3. Your team requires Windows-native development
4. You need enterprise LTS guarantees

The realistic timeline: Budget 2-3x your initial estimate, plan for one complete rollback scenario, and measure success over months, not weeks.