Migrating from Node.js to Bun Without Losing Your Sanity

Bun hit 1.0 in September 2023, and after a year of people actually using it in production, it's starting to look legit.

Here's why I finally migrated our API and what I learned the hard way.

The Real Performance Story

The performance claims are real, but here's what actually happens in production. Based on actual benchmarks from BetterStack, Bun handles ~52,000 req/s vs Node's ~13,000 req/s on simple Express apps.

That's about 4x improvement, not the "10000x faster!!!" you see in Twitter threads.

What this means for your actual app (not some demo):

• Our API responses dropped from 200ms to 80ms average
• Docker builds went from 8 minutes to 2 minutes (npm install was the biggest win)
• Memory usage dropped ~30% but don't expect miracles
• Cold starts are noticeably snappier (great for serverless)

The performance boost is most dramatic for I/O-heavy workloads.

If your app is CPU-bound doing heavy calculations, the improvement is more modest (~2x).

Real performance gotchas I learned:

• The 4x improvement only holds for simple apps
• complex dependency trees reduce the advantage
• Windows performance is shit
• you'll need WSL and it's still slower than Linux
• Some npm packages break things in weird ways, reducing overall performance
• Debugging tools are still catching up to Node.js ecosystem

The "All-in-One" Toolkit Reality Check

The Node.js toolchain is genuinely annoying:

npm install     # Package manager
npx jest        # Testing
npx webpack     # Bundling
npx tsc         # TypeScript

Bun does consolidate this into one binary:

bun install     # ~10x faster than npm (this alone justifies the switch)
bun test        # Jest-compatible, mostly works
bun build       # Basic bundling, don't expect webpack feature parity
bun run app.ts  # Direct Type

Script execution, no tsc bullshit

The tooling wins:

• `bun install` is legitimately 10x faster than npm
• your CI will thank you
• Running `.ts` files directly is magical
• no more build steps for dev
• Test runner works with 90% of Jest tests out of the box
• Package.json compatibility means no major workflow changes
• Built-in bundler eliminates webpack configuration hell

The tooling gotchas:

• Jest compatibility is ~90% there
• the other 10% will ruin your weekend
• Advanced webpack features don't exist in bun build
• Some testing mocks don't work the same way
• Windows WSL requirement will piss off your Windows devs
• Hot reloading can be flaky with complex setups

When to Migrate (Real Talk)

Migrate if:

• You're starting a new TypeScript project
• Your CI/CD pipeline is slow as hell (npm install bottleneck)
• You hate managing multiple tools for basic JavaScript development
• Your API is I/O heavy and you want those response time improvements

Don't migrate if:

• You have a massive, stable codebase that "just works"
• You rely heavily on Node.js-specific native modules
• Your team is mostly Windows developers
• You need enterprise support and LTS guarantees

The migration reality:

• About 90% of npm packages work fine with Bun
• The other 10% will make you question your life choices
• Budget 2-3x longer than you think for migration (there are always surprises)
• Plan for at least one "everything is broken" moment where you need to debug obscure compatibility issues

I spent 3 days debugging why our session middleware was throwing weird errors before realizing it was a Bun-specific timing issue.

These things happen.

The ecosystem compatibility has gotten much better

• Express, Fastify, Prisma, and most popular frameworks work without changes.

But always test your specific dependency stack first.

Bottom line: If you're building new stuff or your Node.js setup is already a clusterfuck, Bun is probably worth the migration pain.

If you have a stable system that works fine, maybe wait another year for the ecosystem to mature even more.

Phase 1: Find All The Shit That's Gonna Break

Before you fuck up your production system, figure out what's going to break first. I learned this the hard way.

Audit Your Dependencies

List all your npm packages and start praying they work with Bun:

## Generate dependency report
npm list --depth=0 > dependencies.txt

## Check for native modules (potential compatibility issues)
npm list --depth=0 | grep -E "(node-gyp|native|binding)"

Critical dependencies to verify:

• Database drivers (PostgreSQL, MongoDB, Redis)
• Native modules with C++ bindings
• Authentication libraries (Passport.js, Auth0 SDKs)
• File processing libraries (Sharp, PDFs)
• Enterprise SDKs and proprietary modules

Check Bun's Node.js API compatibility to verify support for your key dependencies.

What You Need to Check (Don't Skip This)

Screenshot your working setup before you break everything - seriously, do this or you'll hate yourself later.

• Runtime APIs: Find everywhere you use Node.js-specific shit (fs, crypto, http)
• Native modules: These are the packages that will absolutely ruin your weekend
• TypeScript mess: Write down how your current build pipeline works (you'll miss it)
• Testing setup: Your Jest config is about to become 10% broken
• CI/CD scripts: These will need tweaking and you'll definitely forget which ones until they fail in production
• Performance numbers: Measure this now or you'll never know if Bun actually helped

Phase 2: Actually Installing Bun (The Easy Part)

Install Bun

Follow the official installation guide for your platform.

macOS and Linux:

curl -fsSL https://bun.sh/install | bash
source ~/.bashrc  # or ~/.zshrc

Windows (requires WSL):

## In WSL terminal - see [WSL setup guide](https://github.com/oven-sh/bun/issues/43)
curl -fsSL https://bun.sh/install | bash

Docker setup using official Docker images:

FROM oven/bun:1.0

WORKDIR /app
COPY package.json bun.lockb* ./
RUN bun install --frozen-lockfile

COPY . .
EXPOSE 3000
CMD ["bun", "start"]

Version Management

For production deployments, pin the Bun version:

{
  "engines": {
    "bun": ">=1.0.0"
  }
}

Phase 3: The Three Ways to Migrate (Pick Your Pain Level)

Option A: Shadow Migration (For Smart People)

Run both versions side by side so you can actually see what breaks:

## Keep existing Node.js setup
npm install
npm run dev    # Port 3000

## Test with Bun in parallel
bun install
bun run dev    # Port 3001

Compare outputs, performance, and pray the differences aren't deal-breakers.

Option B: Feature-by-Feature Migration (For Cautious People)

Migrate piece by piece so you can blame specific parts when they break:

1. Start with utilities and helpers (lowest risk)
2. Migrate standalone services (medium risk)
3. Convert main application (highest impact)

Option C: YOLO Migration (For the Brave/Stupid)

Burn the bridges and see what happens (only for small projects or people with good backups):

## Delete everything Node.js (this is the point of no return)
rm -rf node_modules package-lock.json

## Cross your fingers and pray
bun install
bun run dev

Phase 4: Making Your Code Actually Work with Bun

Server API Modernization

Before (Node.js):

const http = require('http');
const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'application/json' });
  res.end(JSON.stringify({ message: 'Hello World' }));
});
server.listen(3000);

After (Bun):

export default {
  port: 3000,
  fetch(request) {
    return new Response(JSON.stringify({ message: 'Hello World' }), {
      headers: { 'Content-Type': 'application/json' }
    });
  }
};

Database Connection Updates

Most database shit just works, but you might need to adjust connection pools because Bun handles connections more efficiently:

// Works in both Node.js and Bun
import { Pool } from 'pg';

const pool = new Pool({
  connectionString: process.env.DATABASE_URL,
  // Bun may handle connections more efficiently
  max: process.env.BUN_ENV ? 10 : 20
});

File System Operations

Standard `fs` operations remain compatible, but Bun's native file APIs offer better performance:

import { readFile } from 'fs/promises';
// Or use Bun's optimized APIs
import { file } from 'bun';

// Both work, Bun's API may be faster
const content1 = await readFile('data.json', 'utf8');
const content2 = await file('data.json').text();

Phase 5: Testing (Where Everything Falls Apart)

Update Test Scripts (And Debug Why They're Broken)

Set up Bun's test runner which is "Jest-compatible" (spoiler: 90% compatible, 10% rage-inducing):

{
  "scripts": {
    "test": "bun test",
    "test:watch": "bun test --watch",
    "test:coverage": "bun test --coverage"
  }
}

Performance Benchmarking (The Part You Actually Care About)

// benchmark.js
const startTime = performance.now();

// Your application logic here
await processLargeDataset();

const endTime = performance.now();
console.log(`Execution time: ${endTime - startTime}ms`);

Run the same test on both and see if the hype is real.

Integration Testing (The Moment of Truth)

Test the stuff that actually matters to your users:

• Authentication flows - because auth always breaks in weird ways
• Database operations - make sure your data doesn't disappear
• API responses - verify they're still JSON and not gibberish
• File uploads - because file handling is where dreams go to die
• Third-party services - these will probably work but might be slower/faster

Write down everything that acts different. You'll need this when your manager asks "what changed?"

How I Actually Deploy Bun Apps to Production

Container Deployment

Here's the Dockerfile that actually works in production (after I fucked it up twice):

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

## Install dependencies in separate layer for caching
FROM base as deps
COPY package.json bun.lockb* ./
RUN bun install --frozen-lockfile --production

## Copy source code
FROM base as runner
COPY --from=deps /app/node_modules ./node_modules
COPY . .

## Create non-root user for security
RUN addgroup --system --gid 1001 bunjs
RUN adduser --system --uid 1001 bunjs
USER bunjs

EXPOSE 3000
CMD ["bun", "start"]

This cuts image size by 40-60% compared to Node.js and doesn't break when you deploy it at 2am.

Cloud Platform Deployment

Serverless works great with Bun because it starts fast as hell (unlike Lambda cold starts with Node.js):

AWS Lambda with Bun:

// lambda-handler.js
export default {
  async fetch(request) {
    const event = await request.json();
    
    // Your Lambda logic here
    return new Response(JSON.stringify({
      statusCode: 200,
      body: { message: 'Processed by Bun' }
    }));
  }
};

Serverless Framework configuration:

provider:
  name: aws
  runtime: provided.al2
  
functions:
  api:
    handler: lambda-handler.default
    layers:
      - arn:aws:lambda:us-east-1:123456789:layer:bun-runtime:1

Performance Monitoring (So You Can Prove It Was Worth It)

Monitor these metrics or you'll be debugging blind when everything explodes:

// monitoring.js
import { performance } from 'perf_hooks';

class PerformanceMonitor {
  constructor() {
    this.metrics = {
      requests: 0,
      totalTime: 0,
      errors: 0,
      memoryPeak: 0
    };
  }
  
  trackRequest(handler) {
    return async (req, res) => {
      const start = performance.now();
      
      try {
        await handler(req, res);
        this.metrics.requests++;
      } catch (error) {
        this.metrics.errors++;
        throw error;
      } finally {
        const duration = performance.now() - start;
        this.metrics.totalTime += duration;
        
        // Track memory usage
        const memory = process.memoryUsage();
        this.metrics.memoryPeak = Math.max(
          this.metrics.memoryPeak, 
          memory.heapUsed
        );
      }
    };
  }
  
  getAverageResponseTime() {
    return this.metrics.totalTime / this.metrics.requests;
  }
}

Security (Don't Get Hacked Because You Switched Runtimes)

Runtime Security

We nearly got pwned because Bun's file system behavior isn't identical to Node.js. Learn from our mistakes:

Environment isolation:

// Restrict file system access in production
const config = {
  allowFileSystem: process.env.NODE_ENV !== 'production',
  restrictedPaths: ['/etc', '/proc', '/sys']
};

Check your packages aren't mining Bitcoin using Bun's security features:

## See which packages want to steal your data
bun audit

## Find packages with known security holes
bun outdated --security

Resource Management

Follow Bun's production deployment best practices for resource limits.

Memory limits (based on Node.js to Bun migration patterns):

// Set memory limits for Bun processes
process.setrlimit = process.setrlimit || (() => {});
if (process.env.MEMORY_LIMIT) {
  // Implement memory monitoring
  setInterval(() => {
    const usage = process.memoryUsage();
    if (usage.heapUsed > process.env.MEMORY_LIMIT * 1024 * 1024) {
      console.warn('Memory limit approaching');
    }
  }, 30000);
}

CPU throttling (see scaling strategies for Bun apps):

// Implement graceful CPU management - see [cluster patterns](https://github.com/oven-sh/bun/discussions/3955)
const cluster = require('cluster');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster && process.env.NODE_ENV === 'production') {
  // Create worker processes - [Bun handles concurrency differently](https://bun.sh/docs/runtime/bun-apis)
  for (let i = 0; i < Math.min(numCPUs, 4); i++) {
    cluster.fork();
  }
} else {
  // Worker process runs the application
  require('./app.js');
}

Scaling (When Your App Gets Popular)

Horizontal Scaling

Bun uses less memory so you can cram more containers on the same hardware (your cloud bill will thank you). Check out real-world scaling examples:

Kubernetes deployment:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: bun-api
spec:
  replicas: 8  # Can run more instances due to lower resource usage
  template:
    spec:
      containers:
      - name: bun-app
        image: your-bun-app:latest
        resources:
          requests:
            memory: "64Mi"    # Reduced from Node.js 128Mi
            cpu: "100m"
          limits:
            memory: "128Mi"   # Reduced from Node.js 256Mi
            cpu: "200m"

Load Balancing (Making Sure Traffic Gets Distributed)

Bun's faster startup enables aggressive auto-scaling strategies:

// Health check endpoint optimized for Bun
app.get('/health', (req, res) => {
  const health = {
    status: 'healthy',
    runtime: 'bun',
    uptime: process.uptime(),
    memory: process.memoryUsage(),
    timestamp: new Date().toISOString()
  };
  
  res.json(health);
});

Long-term Maintenance (Keeping This Shit Running)

Update Strategy

Automated updates (so you don't forget and get pwned), following Bun's update guidelines:

{
  "scripts": {
    "update": "bun update",
    "audit": "bun audit",
    "test:deps": "bun test && bun build"
  }
}

Version pinning for stability (see dependency management best practices):

{
  "engines": {
    "bun": "~1.0.0"
  },
  "resolutions": {
    "@types/node": "18.x"
  }
}

Proving Your Migration Actually Worked

Numbers to track (so your manager knows you didn't waste time):

• Response times: Should be 2-4x faster or you did something wrong
• Memory usage: 30-50% less RAM usage (more money for beer)
• CPU usage: Should be more efficient but might not be dramatic
• Error rates: If these go up, you're in trouble
• Build times: Docker builds should be way faster

Dashboard example:

// metrics-dashboard.js
export const migrationMetrics = {
  beforeMigration: {
    avgResponseTime: 250,  // ms
    memoryUsage: 512,      // MB
    requestsPerSecond: 1000
  },
  afterMigration: {
    avgResponseTime: 85,   // 66% improvement
    memoryUsage: 256,      // 50% reduction  
    requestsPerSecond: 3200 // 220% increase
  },
  migrationSuccess: true
};

Follow this and your Bun app should run in production without making you look like an idiot. We reduced our AWS bill from $800/month to $400/month - half the cost for better performance. Your CFO will love you.

For more production deployment strategies, check out Bun's serverless deployment guide and Docker optimization techniques. The Bun community discussions also have tons of real-world production experiences you can learn from.

The Migration Payoff (Why This Was Worth It)

After 6 months running Bun in production, here's what actually changed:

• Build times: 8 minutes → 2 minutes (CI developers are happy)
• Server response times: 200ms → 80ms average (users notice the difference)
• Infrastructure costs: $800/month → $400/month (CFO is happy)
• Developer experience: No more webpack config hell, TypeScript just works
• Debugging time: About the same, different problems but not more problems

Would I do it again? Absolutely. The tooling consolidation alone made it worthwhile, and the performance gains were real. Just budget more time than you think for the weird edge cases, keep good rollback plans, and don't migrate during your busiest season.

The Node.js ecosystem isn't going anywhere, but Bun offers a genuinely better developer experience with measurable performance benefits. If you're starting fresh or your current setup is already painful, the migration is worth it.