Bun JavaScript Runtime: Technical Reference

What Bun Is

• JavaScript runtime built in Zig using Safari's JavaScriptCore engine
• Node.js alternative created by Jarred Sumner to solve npm install performance issues
• Released stable v1.0 in September 2023
• All-in-one toolkit: runtime, package manager, bundler, test runner

Performance Specifications

HTTP Server Performance

• 3x faster than Node.js for web servers
• API response times: Node.js 340ms → Bun 120ms (no code changes)
• Uses less memory than Node.js

Package Installation Speed

• Parallel downloads vs npm's sequential approach
• Binary lockfiles vs JSON parsing
• Global cache with instant copying to new projects
• npm install: 3+ minutes → Bun: seconds
• Test suite: 45 seconds → 12 seconds

Configuration Requirements

Installation

# Linux/macOS
curl -fsSL https://bun.sh/install | bash

# Windows
powershell -c "irm bun.sh/install.ps1 | iex"

System Requirements

• Linux kernel 5.6+
• Modern x64/ARM processors with AVX2 support
• CRITICAL: Older CPUs without AVX2 will fail with "Illegal instruction (core dumped)"

Basic Project Setup

bun init                  # Zero config initialization
bun add express           # Fast package installation
bun run index.ts          # Native TypeScript execution

Node.js Compatibility Matrix

What Works

• Core modules: fs, http, crypto
• Popular packages: Express, React, Fastify, database drivers
• Framework support: Next.js, Svelte, most frameworks
• Simple migration: Change "node" to "bun" in package.json

What Breaks

• Native modules: sharp, node-canvas, any C++ bindings
• Filesystem magic packages
• Experimental Node.js features
• Abandoned packages

Critical Failure Modes

Windows Platform Issues

• File access denied errors despite admin privileges
• Windows Defender false positives on fast file writes
• Solution: Use WSL2 or expect debugging overhead

Docker Deployment

• Critical: Must use --init flag or containers die with exit 143
• Signal forwarding broken without init process
• Memory limits trigger OOMkill at ~70% instead of limit

Hot Reload Instability

• Crashes randomly in v1.0.x versions
• Workaround: Restart and continue development
• WebSocket connections maintained when working

Native Module Dependencies

Error: Cannot find module '@img/sharp-libvips-linux-x64'

• No compatibility layer for native modules
• Time cost: 4+ hours debugging before realizing limitation
• Solution: Keep Node.js for fallback

Built-in Capabilities

Included Tools

• HTTP server with Bun.serve()
• Database drivers: PostgreSQL, SQLite, Redis
• File operations with Bun.file()
• Password hashing (no bcrypt compilation)
• MIME type detection
• Test runner with coverage and mocking

Security Features

• postinstall scripts require explicit trust
• Popular packages (lodash) auto-approved
• Prevents malware during package installation

Production Deployment Intelligence

Companies Using It

• Twitter/X uses Bun for infrastructure services
• Released after 1+ years of stability testing

When to Use Bun

• New projects: Full benefit from modern tooling
• Normal dependency stack: Express, React, standard libraries
• Performance-critical APIs: 3x HTTP performance gains

When to Avoid Bun

• Heavy native module dependencies
• Complex webpack configurations
• Legacy projects with experimental Node features
• Windows-first development without WSL2

Resource Requirements

Migration Time Investment

• Simple projects: Change one line in package.json
• Complex projects: Test all dependencies first
• Native module projects: Plan Node.js fallback strategy
• Debugging time: Limited Stack Overflow resources

Expertise Requirements

• Basic usage: Same as Node.js knowledge
• Troubleshooting: Discord community support required
• Advanced debugging: VS Code integration feels incomplete

Breaking Points and Limits

UI Performance Degradation

• Large span counts (>1000) can break debugging interfaces
• Makes distributed transaction debugging impossible

Memory and CPU Constraints

• CPU-intensive tasks show minimal improvement
• Docker memory limits behave unexpectedly
• AVX2 processor requirement eliminates older hardware

Decision Support Matrix

Use Case	Recommendation	Risk Level
New API development	Use Bun	Low
React/Express projects	Use Bun	Low
Image processing apps	Avoid (sharp incompatibility)	High
Windows development	Use WSL2 or avoid	Medium
Legacy Node.js migration	Test extensively first	Medium
Docker production	Use with --init flag	Medium

Community and Support Quality

Documentation Quality

• Official docs are comprehensive and clear
• Framework-specific guides available
• Active GitHub discussions (80k+ stars)

Support Channels

• GitHub Issues: 500+ compatibility issues tracked
• Discord Community: Active real-time support
• Stack Overflow: Limited answers (3 vs 500 for Node.js)

Known Issue Tracking

• Windows-specific issues labeled and tracked
• Package compatibility database maintained
• Docker-specific gotchas documented

Real-World Implementation Cost

Time Investment

• Setup: Minutes vs hours for Node.js toolchains
• Learning curve: Minimal for existing Node.js developers
• Migration debugging: Budget 4+ hours for native module issues

Hidden Costs

• Limited ecosystem knowledge
• Reduced third-party tool integration
• Potential fallback to Node.js for edge cases

ROI Indicators

• 3x API performance improvement
• Significantly faster development iteration
• Reduced build pipeline complexity
• Developer productivity gains from fast package installs