Bun

All-in-one JavaScript/TypeScript toolkit: runtime, package manager, test runner, bundler.

Quick Navigation

When to Use Bun

• Running TypeScript/JSX without build step
• Fast HTTP server with native routing
• Headless browser automation with native input events
• SQLite database (embedded, no deps)
• WebSocket server/client
• S3-compatible storage (AWS, R2, MinIO)
• Redis caching/pub-sub
• Cross-platform shell scripts
• In-process cron scheduling
• Markdown parsing (v1.3.8+)
• Native library calls via FFI

Core Advantages

• 4x faster startup than Node.js
• Native TypeScript/JSX — no tsconfig needed
• ESM + CommonJS — both work seamlessly
• Web APIs built-in — fetch, WebSocket, etc.
• 30x faster installs than npm

Quick Start

# Run TypeScript directly
bun run index.ts

# Install packages
bun install

# Run package.json script
bun run dev

# Execute package binary
bunx cowsay "Hello"

# Run tests
bun test

# Build for production
bun build ./index.ts --outdir ./dist

# Bundle analysis for LLMs (v1.3.8+)
bun build ./index.ts --metafile-md --outdir ./dist

Critical Rules

Release Highlights (1.4.x)

• Rust rewrite: first release of the Rust port. Lower peak memory (mimalloc allocator, 13-48% less under load), ~2x faster startup, 5x lower idle CPU, and 1,500+ more passing Node.js test-suite tests.
• HTTP/2 & HTTP/3: Bun.serve() serves HTTP/1.1 and HTTP/2 on the same port via ALPN over TLS (v1.4.1); HTTP/3 is experimental via http3: true. fetch() can use HTTP/2/3 clients behind feature flags.
• Serve static directories: routes: { "/static/*": { dir: "./public" } } with sendfile, ETag, Range, conditional requests, and index.html.
• CLI: bun run --parallel for concurrent scripts; bun audit fix, bun dedupe, bun prune, bun pm diff, bun pm licenses for package maintenance.
• bun test: --parallel, --isolate, --shard=M/N, --timings, --changed (diff-based), --retry, and jest.useFakeTimers().
• bun build: built-in React Compiler (--react-compiler), barrel-import optimization, tree-shaking through export * as and dynamic import(), --min-chunk-size, and module preloading.
• bun install: --linker=isolated global virtual store, --offline / --prefer-offline, bun add --catalog, nested overrides, and selfContained workspaces.
• New runtime APIs: Bun.Terminal (PTY), Bun.Archive (tar), Bun.JSON5/JSONL/JSONC/XML/TOML parsers, streaming Bun.write(path, response), WebSocket.pause()/resume(), crypto.argon2, and post-quantum ML-DSA/ML-KEM in crypto.subtle.

Release Highlights (1.3.14)

• Bun.Image: built-in image decoding, transforms, and encoding for common formats with no npm dependency or native addon build step.
• Test workflow: the 1.3.13 line improves dependency-aware filtering for changed-file test runs, which matters when you rely on partial local verification.
• Patch-line runtime work: 1.3.13-1.3.14 continues compatibility and performance work on top of the 1.3.12 WebView/cron/Markdown release line.

Release Highlights (1.3.12)

• Bun.WebView: native headless browser automation with WebKit on macOS and Chrome/Chromium via CDP on all platforms.
• Bun.cron() callback mode: in-process scheduler with no-overlap execution, UTC semantics, hot-reload cleanup, and Disposable job handles.
• Markdown in terminal: bun ./file.md and Bun.markdown.ansi() make terminal-native rendering a first-class workflow.
• Networking/runtime: UDP error/truncation handling, Node-compatible unix-socket lifecycle, proxy tunnel reuse, and Bun.serve() accept/perf improvements.

Essential Recipes

HTTP Server

Bun.serve({
  port: 3000,
  fetch(req) {
    const url = new URL(req.url);
    if (url.pathname === "/api/data") {
      return Response.json({ ok: true });
    }
    return new Response("Not Found", { status: 404 });
  },
});

File Operations

// Read
const content = await Bun.file("data.txt").text();

// Write
await Bun.write("output.txt", "Hello World");

// JSON
const config = await Bun.file("config.json").json();

SQLite

import { Database } from "bun:sqlite";

const db = new Database("app.db");
db.run("CREATE TABLE IF NOT EXISTS users (id INTEGER PRIMARY KEY, name TEXT)");

const insert = db.prepare("INSERT INTO users (name) VALUES (?)");
insert.run("Alice");

const users = db.query("SELECT * FROM users").all();

WebSocket Server

Bun.serve({
  fetch(req, server) {
    if (server.upgrade(req)) return;
    return new Response("Upgrade failed", { status: 400 });
  },
  websocket: {
    message(ws, message) {
      ws.send(`Echo: ${message}`);
    },
  },
});

Shell Commands

import { $ } from "bun";

// Simple command
const files = await $`ls -la`.text();

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

// Piping
await $`cat data.csv | grep "pattern" | wc -l`;

S3 Storage

import { s3 } from "bun";

// Upload
await s3.file("uploads/doc.pdf").write(data);

// Download
const content = await s3.file("uploads/doc.pdf").text();

// Presigned URL
const url = s3.presign("uploads/doc.pdf", { expiresIn: 3600 });

Redis

import { redis } from "bun";

await redis.set("key", "value");
const value = await redis.get("key");
await redis.expire("key", 3600);

Testing

import { expect, test, describe } from "bun:test";

describe("math", () => {
  test("2 + 2 = 4", () => {
    expect(2 + 2).toBe(4);
  });
});

Configuration (bunfig.toml)

[run]
watch = true

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

[test]
coverage = true

Environment Variables

# .env files loaded automatically
DATABASE_URL=postgres://localhost/mydb

// Access
Bun.env.DATABASE_URL;
process.env.DATABASE_URL;
import.meta.env.DATABASE_URL;

Links

• Documentation
• Releases
• GitHub
• Discord