d-subrahmanyam/bun

Bun - Quick Reference

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: bun for comprehensive documentation.

When to Use This Skill

• Projects requiring high performance
• Replacement for Node.js + npm + bundler
• Fast test runner alternative to Vitest/Jest
• Quick scripts with native TypeScript

Setup

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

# Create project
bun init

# Run TypeScript directly (no compilation needed!)
bun run index.ts

# Run with watch mode
bun --watch run index.ts

Package Manager

# Install dependencies (faster than npm/pnpm)
bun install

# Add packages
bun add express zod
bun add -d typescript @types/node

# Remove
bun remove package-name

# Update
bun update

# Run scripts
bun run build
bun run dev

# Execute binary
bunx prisma generate

Workspaces

// package.json
{
  "workspaces": ["packages/*"]
}

# Install all workspace deps
bun install

# Run in specific workspace
bun run --filter @myorg/api build

Bundler

// Build for production
await Bun.build({
  entrypoints: ['./src/index.ts'],
  outdir: './dist',
  target: 'node',        // 'browser' | 'bun'
  minify: true,
  sourcemap: 'external',
  splitting: true,       // Code splitting
  format: 'esm',         // 'cjs' | 'esm'
});

// CLI
bun build ./src/index.ts --outdir ./dist --minify

Build Config

// bunfig.toml alternative - build.ts
const result = await Bun.build({
  entrypoints: ['./src/index.tsx'],
  outdir: './dist',
  target: 'browser',
  minify: {
    whitespace: true,
    identifiers: true,
    syntax: true,
  },
  define: {
    'process.env.NODE_ENV': '"production"',
  },
  external: ['react', 'react-dom'],
  loader: {
    '.png': 'file',
    '.svg': 'text',
  },
});

if (!result.success) {
  console.error('Build failed:', result.logs);
  process.exit(1);
}

Test Runner

// math.test.ts
import { describe, it, expect, beforeAll, mock } from 'bun:test';

describe('math', () => {
  it('adds numbers', () => {
    expect(1 + 2).toBe(3);
  });

  it('handles async', async () => {
    const result = await fetchData();
    expect(result).toBeDefined();
  });
});

// Mocking
const mockFn = mock(() => 42);
mockFn();
expect(mockFn).toHaveBeenCalled();

// Module mocking
mock.module('./config', () => ({
  apiUrl: 'http://test.local',
}));

# Run tests
bun test

# Watch mode
bun test --watch

# Coverage
bun test --coverage

# Filter
bun test --filter "user"

HTTP Server

// Native Bun server (fastest)
Bun.serve({
  port: 3000,
  fetch(req) {
    const url = new URL(req.url);

    if (url.pathname === '/api/health') {
      return Response.json({ status: 'ok' });
    }

    if (url.pathname === '/api/users' && req.method === 'POST') {
      const body = await req.json();
      return Response.json({ id: 1, ...body }, { status: 201 });
    }

    return new Response('Not Found', { status: 404 });
  },
  error(error) {
    return new Response(`Error: ${error.message}`, { status: 500 });
  },
});

console.log('Server running on http://localhost:3000');

With Hono (recommended for APIs)

import { Hono } from 'hono';
import { cors } from 'hono/cors';
import { logger } from 'hono/logger';

const app = new Hono();

app.use('*', logger());
app.use('/api/*', cors());

app.get('/api/users', (c) => {
  return c.json([{ id: 1, name: 'John' }]);
});

app.post('/api/users', async (c) => {
  const body = await c.req.json();
  return c.json({ id: 1, ...body }, 201);
});

export default app;

File I/O

// Read file (returns string or ArrayBuffer)
const text = await Bun.file('data.txt').text();
const json = await Bun.file('data.json').json();
const buffer = await Bun.file('image.png').arrayBuffer();

// Write file
await Bun.write('output.txt', 'Hello World');
await Bun.write('data.json', JSON.stringify(data));

// Stream large files
const file = Bun.file('large.csv');
const stream = file.stream();

for await (const chunk of stream) {
  process.stdout.write(chunk);
}

// File metadata
const file = Bun.file('data.txt');
console.log(file.size);  // bytes
console.log(file.type);  // MIME type

SQLite (Built-in)

import { Database } from 'bun:sqlite';

const db = new Database('app.db');

// Create table
db.run(`
  CREATE TABLE IF NOT EXISTS users (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    name TEXT NOT NULL,
    email TEXT UNIQUE
  )
`);

// Prepared statements (recommended)
const insert = db.prepare('INSERT INTO users (name, email) VALUES (?, ?)');
insert.run('John', '[email protected]');

const select = db.prepare('SELECT * FROM users WHERE id = ?');
const user = select.get(1);

// Query all
const all = db.prepare('SELECT * FROM users').all();

// Transaction
db.transaction(() => {
  insert.run('Alice', '[email protected]');
  insert.run('Bob', '[email protected]');
})();

Environment Variables

// .env file loaded automatically
const apiKey = Bun.env.API_KEY;
const port = Bun.env.PORT ?? '3000';

// process.env also works
const nodeEnv = process.env.NODE_ENV;

Shell Commands

import { $ } from 'bun';

// Simple command
const result = await $`ls -la`;
console.log(result.stdout.toString());

// With variables (auto-escaped)
const filename = 'my file.txt';
await $`cat ${filename}`;

// Piping
const files = await $`ls`.text();
const count = await $`echo ${files} | wc -l`.text();

// Error handling
try {
  await $`exit 1`;
} catch (error) {
  console.error('Command failed:', error.exitCode);
}

Configuration (bunfig.toml)

# bunfig.toml

[install]
# Registry
registry = "https://registry.npmjs.org"

# Frozen lockfile in CI
frozenLockfile = true

[run]
# Shell for scripts
shell = "bash"

[test]
# Test configuration
coverage = true
coverageDir = "coverage"

Node.js Compatibility

// Most Node.js APIs work
import { readFile } from 'fs/promises';
import { createServer } from 'http';
import path from 'path';

// Some differences
import.meta.dir;  // __dirname equivalent
import.meta.file; // __filename equivalent

// Check runtime
const isBun = typeof Bun !== 'undefined';

---

When NOT to Use This Skill

| Scenario | Use Instead | |----------|-------------| | Node.js runtime specifics | nodejs skill | | Hono framework | Framework-specific skill | | Elysia framework | Framework-specific skill | | TypeScript syntax | typescript skill | | Testing strategies | testing-vitest skill |

---

Anti-Patterns

| Anti-Pattern | Why It's Bad | Correct Approach | |--------------|--------------|------------------| | Using node_modules/.bin | Not optimized for Bun | Use bunx instead | | Ignoring compatibility | Some npm packages fail | Test compatibility | | Complex routing in Bun.serve | Hard to maintain | Use Hono or Elysia | | Not pinning versions | Breaking changes | Use bun.lockb | | Mixing package managers | Inconsistent deps | Stick to bun | | Not using built-in SQLite | Extra dependency | Use bun:sqlite | | Blocking operations | Defeats performance | Use async APIs | | Not using watch mode | Slow dev loop | Use --watch flag |

---

Quick Troubleshooting

| Issue | Cause | Solution | |-------|-------|----------| | "Module not found" | npm compatibility issue | Check Bun compatibility list | | "bun: command not found" | Not installed | Install Bun or add to PATH | | Tests fail in Bun but not Jest | Different runtime | Check Bun-specific APIs | | Slow install | Network/cache issue | Clear cache with bun pm cache | | "Cannot find package" | Wrong specifier | Use npm: prefix for npm packages | | Type errors with .ts files | tsconfig mismatch | Check Bun's default config | | Build output incorrect | Wrong target | Set target in Bun.build | | SQLite errors | Database locked | Close connections properly |

---

Performance Comparison

| Task | Bun | Node.js | |------|-----|---------| | Install deps | ~2s | ~15s | | Run TS file | <100ms | ~500ms (tsx) | | HTTP requests/sec | ~100k | ~40k | | Test execution | ~200ms | ~2s |

Production Readiness

# Dockerfile
FROM oven/bun:1 AS base
WORKDIR /app

# Install deps
FROM base AS deps
COPY package.json bun.lockb ./
RUN bun install --frozen-lockfile

# Build
FROM base AS build
COPY --from=deps /app/node_modules ./node_modules
COPY . .
RUN bun run build

# Production
FROM base AS production
COPY --from=build /app/dist ./dist
COPY --from=build /app/package.json ./

USER bun
EXPOSE 3000
CMD ["bun", "run", "dist/index.js"]

Checklist

• [ ] Bun installed (v1.0+)
• [ ] bunfig.toml configured
• [ ] Test suite with bun:test
• [ ] Build script configured
• [ ] Docker multi-stage for production
• [ ] npm package compatibility verified

---

Reference Documentation

• Bun Docs

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: bun for comprehensive documentation.