Bun – The Fast JavaScript Runtime

Build and run JavaScript/TypeScript applications with Bun’s all-in-one toolkit.

Quick Start

# Install Bun (macOS, Linux, WSL)
curl -fsSL https://bun.sh/install | bash

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

# Create new project
bun init

# Run TypeScript directly (no build step!)
bun run index.ts

# Install packages (faster than npm)
bun install

# Run scripts
bun run dev

Package Management

# Install dependencies
bun install              # Install all from package.json
bun add express          # Add dependency
bun add -d typescript    # Add dev dependency
bun add -g serve         # Add global package

# Remove packages
bun remove express

# Update packages
bun update

# Run package binaries
bunx prisma generate     # Like npx but faster
bunx create-next-app

# Lockfile
bun install --frozen-lockfile  # CI mode

bun.lockb vs package-lock.json

# Bun uses binary lockfile (bun.lockb) - much faster
# To generate yarn.lock for compatibility:
bun install --yarn

# Import from other lockfiles
bun install  # Auto-detects package-lock.json, yarn.lock

Bun Runtime

Run Files

# Run any file
bun run index.ts         # TypeScript
bun run index.js         # JavaScript
bun run index.jsx        # JSX

# Watch mode
bun --watch run index.ts

# Hot reload
bun --hot run server.ts

Built-in APIs

// File I/O (super fast)
const file = Bun.file('data.json');
const content = await file.text();
const json = await file.json();
const bytes = await file.arrayBuffer();

// Write files
await Bun.write('output.txt', 'Hello, Bun!');
await Bun.write('data.json', JSON.stringify({ key: 'value' }));
await Bun.write('image.png', await fetch('https://example.com/img.png'));

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

// Glob files
const glob = new Bun.Glob('**/*.ts');
for await (const file of glob.scan('.')) {
  console.log(file);
}

HTTP Server

// Simple server
const server = Bun.serve({
  port: 3000,
  fetch(req) {
    const url = new URL(req.url);
    
    if (url.pathname === '/') {
      return new Response('Hello, Bun!');
    }
    
    if (url.pathname === '/json') {
      return Response.json({ message: 'Hello!' });
    }
    
    return new Response('Not Found', { status: 404 });
  },
});

console.log(`Server running at http://localhost:${server.port}`);

Advanced Server

Bun.serve({
  port: 3000,
  
  // Main request handler
  async fetch(req, server) {
    const url = new URL(req.url);
    
    // WebSocket upgrade
    if (url.pathname === '/ws') {
      const upgraded = server.upgrade(req, {
        data: { userId: '123' },  // Attach data to socket
      });
      if (upgraded) return undefined;
    }
    
    // Static files
    if (url.pathname.startsWith('/static/')) {
      const filePath = `./public${url.pathname}`;
      const file = Bun.file(filePath);
      if (await file.exists()) {
        return new Response(file);
      }
    }
    
    // JSON API
    if (url.pathname === '/api/data' && req.method === 'POST') {
      const body = await req.json();
      return Response.json({ received: body });
    }
    
    return new Response('Not Found', { status: 404 });
  },
  
  // WebSocket handlers
  websocket: {
    open(ws) {
      console.log('Client connected:', ws.data.userId);
      ws.subscribe('chat');  // Pub/sub
    },
    message(ws, message) {
      // Broadcast to all subscribers
      ws.publish('chat', message);
    },
    close(ws) {
      console.log('Client disconnected');
    },
  },
  
  // Error handling
  error(error) {
    return new Response(`Error: ${error.message}`, { status: 500 });
  },
});

WebSocket Client

const ws = new WebSocket('ws://localhost:3000/ws');

ws.onopen = () => {
  ws.send('Hello, server!');
};

ws.onmessage = (event) => {
  console.log('Received:', event.data);
};

Bun APIs

SQLite (Built-in)

import { Database } from 'bun:sqlite';

const db = new Database('mydb.sqlite');

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

// Insert
const insert = db.prepare('INSERT INTO users (name, email) VALUES (?, ?)');
insert.run('Alice', 'alice@example.com');

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

// All results
const allUsers = db.prepare('SELECT * FROM users').all();

// Transaction
const insertMany = db.transaction((users) => {
  for (const user of users) {
    insert.run(user.name, user.email);
  }
});

insertMany([
  { name: 'Bob', email: 'bob@example.com' },
  { name: 'Charlie', email: 'charlie@example.com' },
]);

Password Hashing (Built-in)

// Hash password
const hash = await Bun.password.hash('mypassword', {
  algorithm: 'argon2id',  // or 'bcrypt'
  memoryCost: 65536,      // 64 MB
  timeCost: 2,
});

// Verify password
const isValid = await Bun.password.verify('mypassword', hash);

Spawn Processes

// Spawn process
const proc = Bun.spawn(['ls', '-la'], {
  cwd: '/home/user',
  env: { ...process.env, MY_VAR: 'value' },
  stdout: 'pipe',
});

const output = await new Response(proc.stdout).text();
console.log(output);

// Spawn sync
const result = Bun.spawnSync(['echo', 'hello']);
console.log(result.stdout.toString());

// Shell command
const { stdout } = Bun.spawn({
  cmd: ['sh', '-c', 'echo $HOME'],
  stdout: 'pipe',
});

Hashing & Crypto

// Hash strings
const hash = Bun.hash('hello world');  // Wyhash (fast)

// Crypto hashes
const sha256 = new Bun.CryptoHasher('sha256');
sha256.update('data');
const digest = sha256.digest('hex');

// One-liner
const md5 = Bun.CryptoHasher.hash('md5', 'data', 'hex');

// HMAC
const hmac = Bun.CryptoHasher.hmac('sha256', 'secret-key', 'data', 'hex');

Bundler

# Bundle for browser
bun build ./src/index.ts --outdir ./dist

# Bundle options
bun build ./src/index.ts \
  --outdir ./dist \
  --minify \
  --sourcemap \
  --target browser \
  --splitting \
  --entry-naming '[dir]/[name]-[hash].[ext]'

Build API

const result = await Bun.build({
  entrypoints: ['./src/index.ts'],
  outdir: './dist',
  minify: true,
  sourcemap: 'external',
  target: 'browser',  // 'bun' | 'node' | 'browser'
  splitting: true,
  naming: {
    entry: '[dir]/[name]-[hash].[ext]',
    chunk: '[name]-[hash].[ext]',
    asset: '[name]-[hash].[ext]',
  },
  external: ['react', 'react-dom'],
  define: {
    'process.env.NODE_ENV': JSON.stringify('production'),
  },
  loader: {
    '.png': 'file',
    '.svg': 'text',
  },
});

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

Testing

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

describe('Math operations', () => {
  test('addition', () => {
    expect(1 + 1).toBe(2);
  });
  
  test('array contains', () => {
    expect([1, 2, 3]).toContain(2);
  });
  
  test('object matching', () => {
    expect({ name: 'Alice', age: 30 }).toMatchObject({ name: 'Alice' });
  });
  
  test('async test', async () => {
    const result = await Promise.resolve(42);
    expect(result).toBe(42);
  });
  
  test('throws error', () => {
    expect(() => {
      throw new Error('fail');
    }).toThrow('fail');
  });
});

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

// Mock modules
mock.module('./database', () => ({
  query: mock(() => [{ id: 1 }]),
}));

# Run tests
bun test

# Watch mode
bun test --watch

# Specific file
bun test user.test.ts

# Coverage
bun test --coverage

Node.js Compatibility

// Most Node.js APIs work out of the box
import fs from 'fs';
import path from 'path';
import { createServer } from 'http';
import express from 'express';

// Bun implements Node.js APIs
const data = fs.readFileSync('file.txt', 'utf-8');
const fullPath = path.join(__dirname, 'file.txt');

// Express works!
const app = express();
app.get('/', (req, res) => res.send('Hello!'));
app.listen(3000);

Node.js vs Bun APIs

// Node.js way
import { readFile } from 'fs/promises';
const content = await readFile('file.txt', 'utf-8');

// Bun way (faster)
const content = await Bun.file('file.txt').text();

// Node.js crypto
import crypto from 'crypto';
const hash = crypto.createHash('sha256').update('data').digest('hex');

// Bun way (faster)
const hash = Bun.CryptoHasher.hash('sha256', 'data', 'hex');

Environment Variables

// .env file support (built-in, no dotenv needed!)
// .env
// DATABASE_URL=postgres://localhost/db
// API_KEY=secret

// Access env vars
const dbUrl = Bun.env.DATABASE_URL;
const apiKey = process.env.API_KEY;  // Also works

// bunfig.toml for Bun config
// [run]
// preload = ["./setup.ts"]

HTTP Client

// Fetch (optimized in Bun)
const response = await fetch('https://api.example.com/data', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'Authorization': 'Bearer token',
  },
  body: JSON.stringify({ key: 'value' }),
});

const data = await response.json();

// Streaming response
const response = await fetch('https://api.example.com/stream');
const reader = response.body?.getReader();

while (true) {
  const { done, value } = await reader!.read();
  if (done) break;
  console.log(new TextDecoder().decode(value));
}

Project Structure

my-bun-project/
├── src/
│   ├── index.ts          # Entry point
│   ├── routes/
│   │   └── api.ts
│   └── lib/
│       └── database.ts
├── tests/
│   └── index.test.ts
├── public/
│   └── static files
├── package.json
├── bunfig.toml           # Bun config (optional)
├── tsconfig.json
└── .env

bunfig.toml

[install]
# Use exact versions by default
exact = true

# Registry
registry = "https://registry.npmjs.org"

[run]
# Scripts to run before `bun run`
preload = ["./instrumentation.ts"]

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

[bundle]
# Default bundle config
minify = true
sourcemap = "external"

Performance Comparison

Resources

• Bun Docs: https://bun.sh/docs
• Bun API Reference: https://bun.sh/docs/api
• Bun Discord: https://bun.sh/discord
• GitHub: https://github.com/oven-sh/bun