Bun

When to Use

User needs Bun expertise — fast JavaScript/TypeScript runtime, bundler, and package manager. Agent handles migration from Node, bundling for web/server, and troubleshooting compatibility issues.

Quick Reference

Runtime Compatibility Traps

• process.nextTick timing differs from Node — race conditions appear that didn't exist before, use queueMicrotask for cross-runtime code
• __dirname and __filename don't exist in ESM — use import.meta.dir and import.meta.file, forgetting causes ReferenceError
• fs.watch misses events that Node catches — file watcher scripts silently miss changes, add polling fallback
• child_process.spawn options subset — some stdio configurations silently ignored, test subprocess code explicitly
• cluster module not supported — app crashes immediately if code uses cluster, must refactor to workers
• vm module partial — sandboxed code may escape or behave differently, security implications

Bundler Traps

• --target=browser strips Node APIs silently — build succeeds, then runtime crashes on fs, path, etc.
• --splitting requires --format=esm — error message doesn't mention this, just fails cryptically
• Everything bundled by default — server code bundles node_modules, use --external:package for server deps
• Tree-shaking assumes no side effects — code with side effects may be removed, add "sideEffects": false to package.json or lose code
• CSS imports work differently than webpack — url() paths resolve wrong, test in actual browser
• --minify mangles names aggressively — debugging production crashes is harder, use --minify-syntax for safer minification

Package Manager Traps

• bun.lockb is binary format — can't diff, can't merge, Git conflicts require delete and regenerate
• Peer dependencies auto-installed unlike npm — version conflicts appear silently, different versions than npm would pick
• bun install resolves differently than npm — "works on my machine" when teammate uses npm
• Workspaces link: protocol behaves differently — imports from workspace packages may fail
• bun add modifies package.json formatting — unwanted diff noise in commits
• No npm audit equivalent — security vulnerabilities not surfaced automatically

TypeScript Traps

• Bun runs TypeScript directly without tsc — type errors don't stop execution, bugs ship to production
• Type-only imports may be kept — bundle size larger than expected
• tsconfig.json paths work differently — imports that worked in Node+tsc may fail
• Decorators experimental — behavior may differ from tsc, especially with legacy decorators

Testing Traps

• bun test has different assertion API — tests written for Jest need adaptation
• Mock timing differs — tests that pass in Jest may fail or flake
• No native coverage like c8/nyc — need different tooling
• Snapshot format incompatible with Jest — can't share snapshots between runners

Hot Reload Traps

• bun --hot doesn't reload native modules — changes require restart
• State preserved across reloads — bugs from stale state hard to debug
• WebSocket connections not re-established — clients appear connected but dead