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Sep 11, 2022



bun is a new:

  • JavaScript runtime with Web APIs like fetch, WebSocket, and several more built-in. bun embeds JavaScriptCore, which tends to be faster and more memory efficient than more popular engines like V8 (though harder to embed)
  • JavaScript/TypeScript/JSX transpiler
  • JavaScript & CSS bundler
  • Task runner for package.json scripts
  • npm-compatible package manager

All in one fast & easy-to-use tool. Instead of 1,000 node_modules for development, you only need bun.

bun is experimental software. Join bun’s Discord for help and have a look at things that don’t work yet.

Today, bun's primary focus is bun.js: bun's JavaScript runtime.


Native: (macOS x64 & Silicon, Linux x64, Windows Subsystem for Linux)

curl -fsSL | bash

Docker: (Linux x64)

docker pull jarredsumner/bun:edge
docker run --rm --init --ulimit memlock=-1:-1 jarredsumner/bun:edge

If using Linux, kernel version 5.6 or higher is strongly recommended, but the minimum is 5.1.

Table of Contents

Using bun.js - a new JavaScript runtime environment

bun.js focuses on performance, developer experience and compatibility with the JavaScript ecosystem.

// http.ts
export default {
  port: 3000,
  fetch(request: Request) {
    return new Response("Hello World");

// bun ./http.ts
Requests per second OS CPU bun version
260,000 macOS Apple Silicon M1 Max 0.0.76
160,000 Linux AMD Ryzen 5 3600 6-Core 2.2ghz 0.0.76
Measured with http_load_test by running:
./http_load_test  20 3000

bun.js prefers Web API compatibility instead of designing new APIs when possible. bun.js also implements some Node.js APIs.

  • TypeScript & JSX support is built-in, powered by Bun's JavaScript transpiler
  • ESM & CommonJS modules are supported (internally, bun.js uses ESM)
  • Many npm packages "just work" with bun.js (when they use few/no node APIs)
  • tsconfig.json "paths" is natively supported, along with "exports" in package.json
  • fs, path, and process from Node are partially implemented
  • Web APIs like fetch, Response, URL and more are built-in
  • HTMLRewriter makes it easy to transform HTML in bun.js
  • Starts 4x faster than Node (try it yourself)
  • .env files automatically load into process.env and Bun.env
  • top level await

The runtime uses JavaScriptCore, the JavaScript engine powering WebKit and Safari. Some web APIs like Headers and URL directly use Safari's implementation.

cat clone that runs 2x faster than GNU cat for large files on Linux

// cat.js
import { resolve } from "path";
import { write, stdout, file, argv } from "bun";

const path = resolve(;

await write(
  // stdout is a Blob
  // file(path) returns a Blob -

// bun ./cat.js ./path-to-file

Server-side render React:

// requires Bun v0.1.0 or later
// react-ssr.tsx
import { renderToReadableStream } from "react-dom/server";

const dt = new Intl.DateTimeFormat();

export default {
  port: 3000,
  async fetch(request: Request) {
    return new Response(
      await renderToReadableStream(
            <title>Hello World</title>
            <h1>Hello from React!</h1>
            <p>The date is {dt.format(new Date())}</p>

// bun react-ssr.tsx

There are some more examples in the examples folder.

PRs adding more examples are very welcome!

Types for bun.js (editor autocomplete)

The best docs right now are the TypeScript types in the bun-types npm package. A docs site is coming soon.

To get autocomplete for bun.js types in your editor,

  1. Install the bun-types npm package:
# yarn/npm/pnpm work too, "bun-types" is an ordinary npm package
bun add bun-types
  1. Add this to your tsconfig.json or jsconfig.json:
  "compilerOptions": {
    "lib": ["ESNext"],
    "module": "esnext",
    "target": "esnext",
    "moduleResolution": "node",
    // "bun-types" is the important part
    "types": ["bun-types"]

You can also view the types here.

To contribute to the types, head over to oven-sh/bun-types.

Fast paths for Web APIs

bun.js has fast paths for common use cases that make Web APIs live up to the performance demands of servers and CLIs.

Bun.file(path) returns a Blob that represents a lazily-loaded file.

When you pass a file blob to Bun.write, Bun automatically uses a faster system call:

const blob = Bun.file("input.txt");
await Bun.write("output.txt", blob);

On Linux, this uses the copy_file_range syscall and on macOS, this becomes clonefile (or fcopyfile).

Bun.write also supports Response objects. It automatically converts to a Blob.

// Eventually, this will stream the response to disk but today it buffers
await Bun.write("index.html", await fetch(""));

Using bun as a package manager

On Linux, bun install tends to install packages 20x - 100x faster than npm install. On macOS, it’s more like 4x - 80x.

To install packages from package.json:

bun install

To add or remove packages from package.json:

bun remove react
bun add preact
For Linux users: bun install needs Linux Kernel 5.6 or higher to work well

The minimum Linux Kernel version is 5.1. If you're on Linux kernel 5.1 - 5.5, bun install should still work, but HTTP requests will be slow due to a lack of support for io_uring's connect() operation.

If you're using Ubuntu 20.04, here's how to install a newer kernel:

# If this returns a version >= 5.6, you don't need to do anything
uname -r

# Install the official Ubuntu hardware enablement kernel
sudo apt install --install-recommends linux-generic-hwe-20.04

Using bun as a task runner

Instead of waiting 170ms for your npm client to start for each task, you wait 6ms for bun.

To use bun as a task runner, run bun run instead of npm run.

# Instead of "npm run clean"
bun run clean

# This also works
bun clean

Assuming a package.json with a "clean" command in "scripts":

  "name": "myapp",
  "scripts": {
    "clean": "rm -rf dist out node_modules"

Creating a Discord bot with Bun

Application Commands

Application commands are native ways to interact with apps in the Discord client. There are 3 types of commands accessible in different interfaces: the chat input, a message's context menu (top-right menu or right-clicking in a message), and a user's context menu (right-clicking on a user).

To get started you can use the interactions template:

bun create discord-interactions my-interactions-bot
cd my-interactions-bot

If you don't have a Discord bot/application yet, you can create one here (

Invite bot to your server by visiting<your_application_id>&scope=bot%20applications.commands

Afterwards you will need to get your bot's token, public key, and application id from the application page and put them into .env.example file

Then you can run the http server that will handle your interactions:

bun install
mv .env.example .env

bun run.js # listening on port 1337

Discord does not accept an insecure HTTP server, so you will need to provide an SSL certificate or put the interactions server behind a secure reverse proxy. For development, you can use ngrok/cloudflare tunnel to expose local ports as secure URL.

Using bun with Next.js

To create a new Next.js app with bun:

bun create next ./app
cd app
bun dev # start dev server

To use an existing Next.js app with bun:

bun add bun-framework-next
echo "framework = 'next'" > bunfig.toml
bun bun # bundle dependencies
bun dev # start dev server

Many of Next.js’ features are supported, but not all.

Here’s what doesn’t work yet:

  • getStaticPaths
  • same-origin fetch inside of getStaticProps or getServerSideProps
  • locales, zones, assetPrefix (workaround: change --origin \"http://localhost:3000/assetPrefixInhere\")
  • next/image is polyfilled to a regular <img src> tag.
  • proxy and anything else in next.config.js
  • API routes, middleware (middleware is easier to support, though! Similar SSR API)
  • styled-jsx (technically not Next.js, but often used with it)
  • React Server Components

When using Next.js, bun automatically reads configuration from .env.local, .env.development and .env (in that order). process.env.NEXT_PUBLIC_ and process.env.NEXT_ automatically are replaced via --define.

Currently, any time you import new dependencies from node_modules, you will need to re-run bun bun --use next. This will eventually be automatic.

Using bun with single-page apps

In your project folder root (where package.json is):

bun bun ./entry-point-1.js ./entry-point-2.jsx

By default, bun will look for any HTML files in the public directory and serve that. For browsers navigating to the page, the .html file extension is optional in the URL, and index.html will automatically rewrite for the directory.

Here are examples of routing from public/ and how they’re matched:

Dev Server URL File Path
/dir public/dir/index.html
/ public/index.html
/index public/index.html
/hi public/hi.html
/file public/file.html
/font/Inter.woff2 public/font/Inter.woff2
/hello public/index.html

If public/index.html exists, it becomes the default page instead of a 404 page, unless that pathname has a file extension.

Using bun with Create React App

To create a new React app:

bun create react ./app
cd app
bun dev # start dev server

To use an existing React app:

# To enable React Fast Refresh, ensure it is installed
bun add -d react-refresh

# Generate a bundle for your entry point(s)
bun bun ./src/index.js # jsx, tsx, ts also work. can be multiple files

# Start the dev server
bun dev

From there, bun relies on the filesystem for mapping dev server paths to source files. All URL paths are relative to the project root (where package.json is located).

Here are examples of routing source code file paths:

Dev Server URL File Path (relative to cwd)
/src/components/Button.tsx src/components/Button.tsx
/src/index.tsx src/index.tsx
/pages/index.js pages/index.js

You do not need to include file extensions in import paths. CommonJS-style import paths without the file extension work.

You can override the public directory by passing --public-dir="path-to-folder".

If no directory is specified and ./public/ doesn’t exist, bun will try ./static/. If ./static/ does not exist, but won’t serve from a public directory. If you pass --public-dir=./ bun will serve from the current directory, but it will check the current directory last instead of first.

Using bun with TypeScript

Transpiling TypeScript with Bun

TypeScript just works. There’s nothing to configure and nothing extra to install. If you import a .ts or .tsx file, bun will transpile it into JavaScript. bun also transpiles node_modules containing .ts or .tsx files. This is powered by bun’s TypeScript transpiler, so it’s fast.

bun also reads tsconfig.json, including baseUrl and paths.

Adding Type Definitions

To get TypeScript working with the global API, add bun-types to your project:

bun add -d bun-types

And to the types field in your tsconfig.json:

  "compilerOptions": {
    "types": ["bun-types"]

Not implemented yet

bun is a project with an incredibly large scope and is still in its early days.

You can see Bun's Roadmap, but here are some additional things that are planned:

Feature In
Web Streams with Fetch API bun.js
Web Streams with HTMLRewriter bun.js
WebSocket Server bun.js
Package hoisting that matches npm behavior bun install
Source Maps (unbundled is supported) JS Bundler
Source Maps CSS
JavaScript Minifier JS Transpiler
CSS Minifier CSS
CSS Parser (it only bundles) CSS
Tree-shaking JavaScript
Tree-shaking CSS
TypeScript Decorators TS Transpiler
@jsxPragma comments JS Transpiler
Sharing .bun files bun
Dates & timestamps TOML parser
Hash components for Fast Refresh JSX Transpiler
JS Transpiler == JavaScript Transpiler
TS Transpiler == TypeScript Transpiler
Package manager == bun install
bun.js == bun’s JavaScriptCore integration that executes JavaScript. Similar to how Node.js & Deno embed V8.

Limitations & intended usage

Today, bun is mostly focused on bun.js: the JavaScript runtime.

While you could use bun's bundler & transpiler separately to build for browsers or node, bun doesn't have a minifier or support tree-shaking yet. For production browser builds, you probably should use a tool like esbuild or swc.

Longer-term, bun intends to replace Node.js, Webpack, Babel, yarn, and PostCSS (in production).

Upcoming breaking changes

  • Bun's CLI flags will change to better support bun as a JavaScript runtime. They were chosen when bun was just a frontend development tool.
  • Bun's bundling format will change to accommodate production browser bundles and on-demand production bundling



bunfig.toml is bun's configuration file.

It lets you load configuration from a file instead of passing flags to the CLI each time. The config file is loaded before CLI arguments are parsed, which means CLI arguments can override them.

Here is an example:

# Set a default framework to use
# By default, bun will look for an npm package like `bun-framework-${framework}`, followed by `${framework}`
framework = "next"
logLevel = "debug"

# publicDir = "public"
# external = ["jquery"]

# Remap any import like this:
#     import {graphql} from 'react-relay';
# To:
#     import {graphql} from 'macro:bun-macro-relay';
react-relay = { "graphql" = "bun-macro-relay" }

saveTo = "node_modules.bun"
# Don't need this if `framework` is set, but showing it here as an example anyway
entryPoints = ["./app/index.ts"]

# If you're bundling packages that do not actually live in a `node_modules` folder or do not have the full package name in the file path, you can pass this to bundle them anyway
"@bigapp/design-system" = true

# Change the default port from 3000 to 5000
# Also inherited by Bun.serve
port = 5000

# Replace any usage of "process.env.bagel" with the string `lox`.
# The values are parsed as JSON, except single-quoted strings are supported and `'undefined'` becomes `undefined` in JS.
# This will probably change in a future release to be just regular TOML instead. It is a holdover from the CLI argument parsing.
"process.env.bagel" = "'lox'"

# When loading a .bagel file, run the JS parser
".bagel" = "js"

# When navigating to a blob: or src: link, open the file in your editor
# If not, it tries $EDITOR or $VISUAL
# If that still fails, it will try Visual Studio Code, then Sublime Text, then a few others
# This is used by Bun.openInEditor()
editor = "code"

# List of editors:
# - "subl", "sublime"
# - "vscode", "code"
# - "textmate", "mate"
# - "idea"
# - "webstorm"
# - "nvim", "neovim"
# - "vim","vi"
# - "emacs"
# - "atom"
# If you pass it a file path, it will open with the file path instead
# It will recognize non-GUI editors, but I don't think it will work yet

TODO: list each property name


A loader determines how to map imports & file extensions to transforms and output.

Currently, bun implements the following loaders:

Input Loader Output
.js JSX + JavaScript .js
.jsx JSX + JavaScript .js
.ts TypeScript + JavaScript .js
.tsx TypeScript + JSX + JavaScript .js
.mjs JavaScript .js
.cjs JavaScript .js
.mts TypeScript .js
.cts TypeScript .js
.toml TOML .js
.css CSS .css
.env Env N/A
.* file string

Everything else is treated as file. file replaces the import with a URL (or a path).

You can configure which loaders map to which extensions by passing --loaders to bun. For example:

bun --loader=.js:js

This will disable JSX transforms for .js files.

CSS in JS (bun dev only)

When importing CSS in JavaScript-like loaders, CSS is treated special.

By default, bun will transform a statement like this:

import "../styles/global.css";
When platform is browser
  new CustomEvent("onimportcss", {
    detail: "http://localhost:3000/styles/globals.css",

An event handler for turning that into a <link> is automatically registered when HMR is enabled. That event handler can be turned off either in a framework’s package.json or by setting globalThis["Bun_disableCSSImports"] = true; in client-side code. Additionally, you can get a list of every .css file imported this way via globalThis["__BUN"].allImportedStyles.

When platform is bun
//@import url("http://localhost:3000/styles/globals.css");

Additionally, bun exposes an API for SSR/SSG that returns a flat list of URLs to css files imported. That function is Bun.getImportedStyles().

// This specifically is for "framework" in package.json when loaded via `bun dev`
// This API needs to be changed somewhat to work more generally with Bun.js
// Initially, you could only use bun.js through `bun dev`
// and this API was created at that time
addEventListener("fetch", async (event: FetchEvent) => {
  var route = Bun.match(event);
  const App = await import("pages/_app");

  // This returns all .css files that were imported in the line above.
  // It’s recursive, so any file that imports a CSS file will be included.
  const appStylesheets = bun.getImportedStyles();

  // of code

This is useful for preventing flash of unstyled content.

CSS Loader

bun bundles .css files imported via @import into a single file. It doesn’t autoprefix or minify CSS today. Multiple .css files imported in one JavaScript file will not be bundled into one file. You’ll have to import those from a .css file.

This input:

@import url("./hi.css");
@import url("./hello.css");
@import url("./yo.css");


/* hi.css */
/* ...contents of hi.css */
/* hello.css */
/* ...contents of hello.css */
/* yo.css */
/* ...contents of yo.css */

CSS runtime

To support hot CSS reloading, bun inserts @supports annotations into CSS that tag which files a stylesheet is composed of. Browsers ignore this, so it doesn’t impact styles.

By default, bun’s runtime code automatically listens to onimportcss and will insert the event.detail into a <link rel="stylesheet" href={${event.detail}}> if there is no existing link tag with that stylesheet. That’s how bun’s equivalent of style-loader works.


Warning This will soon have breaking changes. It was designed when Bun was mostly a dev server and not a JavaScript runtime.

Frameworks preconfigure bun to enable developers to use bun with their existing tooling.

Frameworks are configured via the framework object in the package.json of the framework (not in the application’s package.json):

Here is an example:

  "name": "bun-framework-next",
  "version": "0.0.0-18",
  "description": "",
  "framework": {
    "displayName": "Next.js",
    "static": "public",
    "assetPrefix": "_next/",
    "router": {
      "dir": ["pages", "src/pages"],
      "extensions": [".js", ".ts", ".tsx", ".jsx"]
    "css": "onimportcss",
    "development": {
      "client": "client.development.tsx",
      "fallback": "fallback.development.tsx",
      "server": "server.development.tsx",
      "css": "onimportcss",
      "define": {
        "client": {
          ".env": "NEXT_PUBLIC_",
          "defaults": {
            "process.env.__NEXT_TRAILING_SLASH": "false",
            "process.env.NODE_ENV": "\"development\"",
            "process.env.__NEXT_ROUTER_BASEPATH": "''",
            "process.env.__NEXT_SCROLL_RESTORATION": "false",
            "process.env.__NEXT_I18N_SUPPORT": "false",
            "process.env.__NEXT_HAS_REWRITES": "false",
            "process.env.__NEXT_ANALYTICS_ID": "null",
            "process.env.__NEXT_OPTIMIZE_CSS": "false",
            "process.env.__NEXT_CROSS_ORIGIN": "''",
            "process.env.__NEXT_STRICT_MODE": "false",
            "process.env.__NEXT_IMAGE_OPTS": "null"
        "server": {
          ".env": "NEXT_",
          "defaults": {
            "process.env.__NEXT_TRAILING_SLASH": "false",
            "process.env.__NEXT_OPTIMIZE_FONTS": "false",
            "process.env.NODE_ENV": "\"development\"",
            "process.env.__NEXT_OPTIMIZE_IMAGES": "false",
            "process.env.__NEXT_OPTIMIZE_CSS": "false",
            "process.env.__NEXT_ROUTER_BASEPATH": "''",
            "process.env.__NEXT_SCROLL_RESTORATION": "false",
            "process.env.__NEXT_I18N_SUPPORT": "false",
            "process.env.__NEXT_HAS_REWRITES": "false",
            "process.env.__NEXT_ANALYTICS_ID": "null",
            "process.env.__NEXT_CROSS_ORIGIN": "''",
            "process.env.__NEXT_STRICT_MODE": "false",
            "process.env.__NEXT_IMAGE_OPTS": "null",
            "global": "globalThis",
            "window": "undefined"

Here are type definitions:

type Framework = Environment & {
  // This changes what’s printed in the console on load
  displayName?: string;

  // This allows a prefix to be added (and ignored) to requests.
  // Useful for integrating an existing framework that expects internal routes to have a prefix
  // e.g. "_next"
  assetPrefix?: string;

  development?: Environment;
  production?: Environment;

  // The directory used for serving unmodified assets like fonts and images
  // Defaults to "public" if exists, else "static", else disabled.
  static?: string;

  // "onimportcss" disables the automatic "onimportcss" feature
  // If the framework does routing, you may want to handle CSS manually
  // "facade" removes CSS imports from JavaScript files,
  //    and replaces an imported object with a proxy that mimics CSS module support without doing any class renaming.
  css?: "onimportcss" | "facade";

  // bun’s filesystem router
  router?: Router;

type Define = {
  // By passing ".env", bun will automatically load .env.local, .env.development, and .env if exists in the project root
  //    (in addition to the processes’ environment variables)
  // When "*", all environment variables will be automatically injected into the JavaScript loader
  // When a string like "NEXT_PUBLIC_", only environment variables starting with that prefix will be injected

  ".env": string | "*";

  // These environment variables will be injected into the JavaScript loader
  // These are the equivalent of Webpack’s resolve.alias and esbuild’s --define.
  // Values are parsed as JSON, so they must be valid JSON. The only exception is '' is a valid string, to simplify writing stringified JSON in JSON.
  // If not set, `process.env.NODE_ENV` will be transformed into "development".
  defaults: Record<string, string>;

type Environment = {
  // This is a wrapper for the client-side entry point for a route.
  // This allows frameworks to run initialization code on pages.
  client: string;
  // This is a wrapper for the server-side entry point for a route.
  // This allows frameworks to run initialization code on pages.
  server: string;
  // This runs when "server" code fails to load due to an exception.
  fallback: string;

  // This is how environment variables and .env is configured.
  define?: Define;

// bun’s filesystem router
// Currently, bun supports pages by either an absolute match or a parameter match.
// pages/index.tsx will be executed on navigation to "/" and "/index"
// pages/posts/[id].tsx will be executed on navigation to "/posts/123"
// Routes & parameters are automatically passed to `fallback` and `server`.
type Router = {
  // This determines the folder to look for pages
  dir: string[];

  // These are the allowed file extensions for pages.
  extensions?: string[];

To use a framework, you pass bun bun --use package-name.

Your framework’s package.json name should start with bun-framework-. This is so that people can type something like bun bun --use next and it will check bun-framework-next first. This is similar to how Babel plugins tend to start with babel-plugin-.

For developing frameworks, you can also do bun bun --use ./relative-path-to-framework.

If you’re interested in adding a framework integration, please reach out. There’s a lot here, and it’s not entirely documented yet.


bun not running on an M1 (or Apple Silicon)

If you see a message like this

[1] 28447 killed bun create next ./test

It most likely means you’re running bun’s x64 version on Apple Silicon. This happens if bun is running via Rosetta. Rosetta is unable to emulate AVX2 instructions, which bun indirectly uses.

The fix is to ensure you installed a version of bun built for Apple Silicon.

error: Unexpected

If you see an error like this:


It usually means the max number of open file descriptors is being explicitly set to a low number. By default, bun requests the max number of file descriptors available (which on macOS, is something like 32,000). But, if you previously ran into ulimit issues with, e.g., Chokidar, someone on The Internet may have advised you to run ulimit -n 8096.

That advice unfortunately lowers the hard limit to 8096. This can be a problem in large repositories or projects with lots of dependencies. Chokidar (and other watchers) don’t seem to call setrlimit, which means they’re reliant on the (much lower) soft limit.

To fix this issue:

  1. Remove any scripts that call ulimit -n and restart your shell.
  2. Try again, and if the error still occurs, try setting ulimit -n to an absurdly high number, such as ulimit -n 2147483646
  3. Try again, and if that still doesn’t fix it, open an issue

Unzip is required

Unzip is required to install bun on Linux. You can use one of the following commands to install unzip:

Debian / Ubuntu / Mint

sudo apt install unzip

RedHat / CentOS / Fedora

sudo dnf install unzip

Arch / Manjaro

sudo pacman -S unzip


sudo zypper install unzip

bun install is stuck

Please run bun install --verbose 2> logs.txt and send them to me in bun's discord. If you're on Linux, it would also be helpful if you run sudo perf trace bun install --silent and attach the logs.


bun install

bun install is a fast package manager & npm client.

bun install can be configured via bunfig.toml, environment variables, and CLI flags.

Configuring bun install with bunfig.toml

bunfig.toml is searched for in the following paths on bun install, bun remove, and bun add:

  1. $XDG_CONFIG_HOME/.bunfig.toml or $HOME/.bunfig.toml
  2. ./bunfig.toml

If both are found, the results are merged together.

Configuring with bunfig.toml is optional. bun tries to be zero configuration in general, but that's not always possible.

# Using scoped packages with bun install

# Scope name      The value can be a URL string or an object
"@mybigcompany" = { token = "123456", url = "" }
# URL is optional and fallsback to the default registry

# The "@" in the scope is optional
mybigcompany2 = { token = "123456" }

# Environment variables can be referenced as a string that starts with $ and it will be replaced
mybigcompany3 = { token = "$npm_config_token" }

# Setting username and password turns it into a Basic Auth header by taking base64("username:password")
mybigcompany4 = { username = "myusername", password = "$npm_config_password", url = "" }
# You can set username and password in the registry URL. This is the same as above.
mybigcompany5 = ""

# You can set a token for a registry URL:
mybigcompany6 = "https://:$"

# Default registry
# can be a URL string or an object
registry = ""
# as an object
#registry = { url = "", token = "123456" }

# Install for production? This is the equivalent to the "--production" CLI argument
production = false

# Don't actually install
dryRun = true

# Install optionalDependencies (default: true)
optional = true

# Install local devDependencies (default: true)
dev = true

# Install peerDependencies (default: false)
peer = false

# When using `bun install -g`, install packages here
globalDir = "~/.bun/install/global"

# When using `bun install -g`, link package bins here
globalBinDir = "~/.bun/bin"

# cache-related configuration
# The directory to use for the cache
dir = "~/.bun/install/cache"

# Don't load from the global cache.
# Note: bun may still write to node_modules/.cache
disable = false

# Always resolve the latest versions from the registry
disableManifest = false

# Lockfile-related configuration

# Print a yarn v1 lockfile
# Note: it does not load the lockfile, it just converts bun.lockb into a yarn.lock
print = "yarn"

# Path to read bun.lockb from
path = "bun.lockb"

# Path to save bun.lockb to
savePath = "bun.lockb"

# Save the lockfile to disk
save = true

If it's easier to read as TypeScript types:

export interface Root {
  install: Install;

export interface Install {
  scopes: Scopes;
  registry: Registry;
  production: boolean;
  dryRun: boolean;
  optional: boolean;
  dev: boolean;
  peer: boolean;
  globalDir: string;
  globalBinDir: string;
  cache: Cache;
  lockfile: Lockfile;
  logLevel: "debug" | "error" | "warn";

type Registry =
  | string
  | {
      url?: string;
      token?: string;
      username?: string;
      password?: string;

type Scopes = Record<string, Registry>;

export interface Cache {
  dir: string;
  disable: boolean;
  disableManifest: boolean;

export interface Lockfile {
  print?: "yarn";
  path: string;
  savePath: string;
  save: boolean;

Configuring with environment variables

Environment variables have a higher priority than bunfig.toml.

Name Description
BUN_CONFIG_REGISTRY Set an npm registry (default:
BUN_CONFIG_TOKEN Set an auth token (currently does nothing)
BUN_CONFIG_LOCKFILE_SAVE_PATH File path to save the lockfile to (default: bun.lockb)
BUN_CONFIG_YARN_LOCKFILE Save a Yarn v1-style yarn.lock
BUN_CONFIG_LINK_NATIVE_BINS Point bin in package.json to a platform-specific dependency
BUN_CONFIG_SKIP_INSTALL_PACKAGES Don’t install any packages

bun always tries to use the fastest available installation method for the target platform. On macOS, that’s clonefile and on Linux, that’s hardlink. You can change which installation method is used with the --backend flag. When unavailable or on error, clonefile and hardlink fallsback to a platform-specific implementation of copying files.

bun stores installed packages from npm in ~/.bun/install/cache/${name}@${version}. Note that if the semver version has a build or a pre tag, it is replaced with a hash of that value instead. This is to reduce the chances of errors from long file paths, but unfortunately complicates figuring out where a package was installed on disk.

When the node_modules folder exists, before installing, bun checks if the "name" and "version" in package/package.json in the expected node_modules folder matches the expected name and version. This is how it determines whether it should install. It uses a custom JSON parser which stops parsing as soon as it finds "name" and "version".

When a bun.lockb doesn’t exist or package.json has changed dependencies, tarballs are downloaded & extracted eagerly while resolving.

When a bun.lockb exists and package.json hasn’t changed, bun downloads missing dependencies lazily. If the package with a matching name & version already exists in the expected location within node_modules, bun won’t attempt to download the tarball.

Platform-specific dependencies?

bun stores normalized cpu and os values from npm in the lockfile, along with the resolved packages. It skips downloading, extracting, and installing packages disabled for the current target at runtime. This means the lockfile won’t change between platforms/architectures even if the packages ultimately installed do change.

Peer dependencies?

Peer dependencies are handled similarly to yarn. bun install does not automatically install peer dependencies and will try to choose an existing dependency.


bun.lockb is bun’s binary lockfile format.

Why is it binary?

In a word: Performance. bun’s lockfile saves & loads incredibly quickly, and saves a lot more data than what is typically inside lockfiles.

How do I inspect it?

For now, the easiest thing is to run bun install -y. That prints a Yarn v1-style yarn.lock file.

What does the lockfile store?

Packages, metadata for those packages, the hoisted install order, dependencies for each package, what packages those dependencies resolved to, an integrity hash (if available), what each package was resolved to and which version (or equivalent).

Why is it fast?

It uses linear arrays for all data. Packages are referenced by an auto-incrementing integer ID or a hash of the package name. Strings longer than 8 characters are de-duplicated. Prior to saving on disk, the lockfile is garbage-collected & made deterministic by walking the package tree and cloning the packages in dependency order.


To delete the cache:

rm -rf ~/.bun/install/cache

Platform-specific backends

bun install uses different system calls to install dependencies depending on the platform. This is a performance optimization. You can force a specific backend with the --backend flag.

hardlink is the default backend on Linux. Benchmarking showed it to be the fastest on Linux.

rm -rf node_modules
bun install --backend hardlink

clonefile is the default backend on macOS. Benchmarking showed it to be the fastest on macOS. It is only available on macOS.

rm -rf node_modules
bun install --backend clonefile

clonefile_each_dir is similar to clonefile, except it clones each file individually per directory. It is only available on macOS and tends to perform slower than clonefile. Unlike clonefile, this does not recursively clone subdirectories in one system call.

rm -rf node_modules
bun install --backend clonefile_each_dir

copyfile is the fallback used when any of the above fail, and is the slowest. on macOS, it uses fcopyfile() and on linux it uses copy_file_range().

rm -rf node_modules
bun install --backend copyfile

symlink is typically only used for file: dependencies (and eventually link:) internally. To prevent infinite loops, it skips symlinking the node_modules folder.

If you install with --backend=symlink, Node.js won't resolve node_modules of dependencies unless each dependency has it's own node_modules folder or you pass --preserve-symlinks to node. See Node.js documentation on --preserve-symlinks.

rm -rf node_modules
bun install --backend symlink

node --preserve-symlinks ./my-file.js

bun's runtime does not currently expose an equivalent of --preserve-symlinks, though the code for it does exist.

npm registry metadata

bun uses a binary format for caching NPM registry responses. This loads much faster than JSON and tends to be smaller on disk. You will see these files in ~/.bun/install/cache/*.npm. The filename pattern is ${hash(packageName)}.npm. It’s a hash so that extra directories don’t need to be created for scoped packages.

bun’s usage of Cache-Control ignores Age. This improves performance, but means bun may be about 5 minutes out of date to receive the latest package version metadata from npm.

bun run

bun run is a fast package.json script runner. Instead of waiting 170ms for your npm client to start every time, you wait 6ms for bun.

By default, bun run prints the script that will be invoked:

bun run clean
$ rm -rf node_modules/.cache dist

You can disable that with --silent

bun run --silent clean

bun run ${script-name} runs the equivalent of npm run script-name. For example, bun run dev runs the dev script in package.json, which may sometimes spin up non-bun processes.

bun run ${javascript-file.js} will run it with bun, as long as the file doesn't have a node shebang.

To print a list of scripts, bun run without additional args:

# This command
bun run

# Prints this
hello-create-react-app scripts:

bun run start
react-scripts start

bun run build
react-scripts build

bun run test
react-scripts test

bun run eject
react-scripts eject

4 scripts

bun run automatically loads environment variables from .env into the shell/task. .env files are loaded with the same priority as the rest of bun, so that means:

  1. .env.local is first
  2. if ($NODE_ENV === "production") .env.production else .env.development
  3. .env

If something is unexpected there, you can run bun run env to get a list of environment variables.

The default shell it uses is bash, but if that’s not found, it tries sh and if still not found, it tries zsh. This is not configurable right now, but if you care, file an issue.

bun run automatically adds any parent node_modules/.bin to $PATH and if no scripts match, it will load that binary instead. That means you can run executables from packages, too.

# If you use Relay
bun run relay-compiler

# You can also do this, but:
# - It will only lookup packages in `node_modules/.bin` instead of `$PATH`
# - It will start bun’s dev server if the script name doesn’t exist (`bun` starts the dev server by default)
bun relay-compiler

To pass additional flags through to the task or executable, there are two ways:

# Explicit: include "--" and anything after will be added. This is the recommended way because it is more reliable.
bun run relay-compiler -- -–help

# Implicit: if you do not include "--", anything *after* the script name will be passed through
# bun flags are parsed first, which means e.g. `bun run relay-compiler --help` will print bun’s help instead of relay-compiler’s help.
bun run relay-compiler --schema foo.graphql

bun run supports lifecycle hooks like post${task} and pre{task}. If they exist, they will run, matching the behavior of npm clients. If the pre${task} fails, the next task will not be run. There is currently no flag to skip these lifecycle tasks if they exist, if you want that file an issue.

bun create

bun create is a fast way to create a new project from a template.

At the time of writing, bun create react app runs ~11x faster on my local computer than yarn create react-app app. bun create currently does no caching (though your npm client does)


Create a new Next.js project:

bun create next ./app

Create a new React project:

bun create react ./app

Create from a GitHub repo:

bun create ahfarmer/calculator ./app

To see a list of examples, run:

bun create


bun create github-user/repo-name destination
bun create local-example-or-remote-example destination
bun create /absolute/path/to-template-folder destination
bun create destination
bun create destination

Note: you don’t need bun create to use bun. You don’t need any configuration at all. This command exists to make it a little easier.

Local templates

If you have your own boilerplate you prefer using, copy it into $HOME/.bun-create/my-boilerplate-name.

Before checking bun’s examples folder, bun create checks for a local folder matching the input in:

  • $HOME/.bun-create/
  • $(pwd)/.bun-create/

If a folder exists in any of those folders with the input, bun will use that instead of a remote template.

To create a local template, run:

mkdir -p $HOME/.bun-create/new-template-name
echo '{"name":"new-template-name"}' > $HOME/.bun-create/new-template-name/package.json

This lets you run:

bun create new-template-name ./app

Now your new template should appear when you run:

bun create

Warning: unlike with remote templates, bun will delete the entire destination folder if it already exists.


Flag Description
--npm Use npm for tasks & install
--yarn Use yarn for tasks & install
--pnpm Use pnpm for tasks & install
--force Overwrite existing files
--no-install Skip installing node_modules & tasks
--no-git Don’t initialize a git repository
--open Start & open in-browser after finish
Environment Variables Description
GITHUB_API_DOMAIN If you’re using a GitHub enterprise or a proxy, you can change what the endpoint requests to GitHub go
GITHUB_API_TOKEN This lets bun create work with private repositories or if you get rate-limited

By default, bun create will cancel if there are existing files it would overwrite and it's a remote template. You can pass --force to disable this behavior.

Publishing a new template

Clone this repository and a new folder in examples/ with your new template. The package.json must have a name that starts with @bun-examples/. Do not worry about publishing it, that will happen automatically after the PR is merged.

Make sure to include a .gitignore that includes node_modules so that node_modules aren’t checked in to git when people download the template.

Testing your new template

To test your new template, add it as a local template or pass the absolute path.

bun create /path/to/my/new/template destination-dir

Warning: This will always delete everything in destination-dir.


The bun-create section of package.json is automatically removed from the package.json on disk. This lets you add create-only steps without waiting for an extra package to install.

There are currently two options:

  • postinstall
  • preinstall

They can be an array of strings or one string. An array of steps will be executed in order.

Here is an example:

  "name": "@bun-examples/next",
  "version": "0.0.31",
  "main": "index.js",
  "dependencies": {
    "next": "11.1.2",
    "react": "^17.0.2",
    "react-dom": "^17.0.2",
    "react-is": "^17.0.2"
  "devDependencies": {
    "@types/react": "^17.0.19",
    "bun-framework-next": "^0.0.0-21",
    "typescript": "^4.3.5"
  "bun-create": {
    "postinstall": ["bun bun --use next"]

By default, all commands run inside the environment exposed by the auto-detected npm client. This incurs a significant performance penalty, something like 150ms spent waiting for the npm client to start on each invocation.

Any command that starts with "bun " will be run without npm, relying on the first bun binary in $PATH.

How bun create works

When you run bun create ${template} ${destination}, here’s what happens:

IF remote template

  1. GET${template}/latest and parse it

  2. GET${template}/-/${template}-${latestVersion}.tgz

  3. Decompress & extract ${template}-${latestVersion}.tgz into ${destination}

    • If there are files that would overwrite, warn and exit unless --force is passed

IF GitHub repo

  1. Download the tarball from GitHub’s API

  2. Decompress & extract into ${destination}

    • If there are files that would overwrite, warn and exit unless --force is passed

ELSE IF local template

  1. Open local template folder

  2. Delete destination directory recursively

  3. Copy files recursively using the fastest system calls available (on macOS fcopyfile and Linux, copy_file_range). Do not copy or traverse into node_modules folder if exists (this alone makes it faster than cp)

  4. Parse the package.json (again!), update name to be ${basename(destination)}, remove the bun-create section from the package.json and save the updated package.json to disk.

    • IF Next.js is detected, add bun-framework-next to the list of dependencies
    • IF Create React App is detected, add the entry point in /src/index.{js,jsx,ts,tsx} to public/index.html
    • IF Relay is detected, add bun-macro-relay so that Relay works
  5. Auto-detect the npm client, preferring pnpm, yarn (v1), and lastly npm

  6. Run any tasks defined in "bun-create": { "preinstall" } with the npm client

  7. Run ${npmClient} install unless --no-install is passed OR no dependencies are in package.json

  8. Run any tasks defined in "bun-create": { "preinstall" } with the npm client

  9. Run git init; git add -A .; git commit -am "Initial Commit";

    • Rename gitignore to .gitignore. NPM automatically removes .gitignore files from appearing in packages.
    • If there are dependencies, this runs in a separate thread concurrently while node_modules are being installed
    • Using libgit2 if available was tested and performed 3x slower in microbenchmarks
  10. Done

misctools/publish-examples.js publishes all examples to npm.

bun bun

Run bun bun ./path-to.js to generate a node_modules.bun file containing all imported dependencies (recursively).

Why bundle?

  • For browsers, loading entire apps without bundling dependencies is typically slow. With a fast bundler & transpiler, the bottleneck eventually becomes the web browser’s ability to run many network requests concurrently. There are many workarounds for this. <link rel="modulepreload">, HTTP/3, etc., but none are more effective than bundling. If you have reproducible evidence to the contrary, feel free to submit an issue. It would be better if bundling wasn’t necessary.
  • On the server, bundling reduces the number of filesystem lookups to load JavaScript. While filesystem lookups are faster than HTTP requests, there’s still overhead.

What is .bun?

Note: This format may change soon

The .bun file contains:

  • all the bundled source code
  • all the bundled source code metadata
  • project metadata & configuration

Here are some of the questions .bun files answer:

  • when I import react/index.js, where in the .bun is the code for that? (not resolving, just the code)
  • what modules of a package are used?
  • what framework is used? (e.g., Next.js)
  • where is the routes directory?
  • how big is each imported dependency?
  • what is the hash of the bundle’s contents? (for etags)
  • what is the name & version of every npm package exported in this bundle?
  • what modules from which packages are used in this project? ("project" is defined as all the entry points used to generate the .bun)

All in one file.

It’s a little like a build cache, but designed for reuse across builds.

Position-independent code

From a design perspective, the most important part of the .bun format is how code is organized. Each module is exported by a hash like this:

// preact/dist/preact.module.js
export var $eb6819b = $$m({
  "preact/dist/preact.module.js": (module, exports) => {
    var n, l, u, i, t, o, r, f, e = {}, c = [], s = /acit|ex(?:s|g|n|p|$)|rph|grid|ows|mnc|ntw|ine[ch]|zoo|^ord|itera/i;
    // ... rest of code

This makes bundled modules position-independent. In theory, one could import only the exact modules in-use without reparsing code and without generating a new bundle. One bundle can dynamically become many bundles comprising only the modules in use on the webpage. Thanks to the metadata with the byte offsets, a web server can send each module to browsers zero-copy using sendfile. bun itself is not quite this smart yet, but these optimizations would be useful in production and potentially very useful for React Server Components.

To see the schema inside, have a look at JavascriptBundleContainer. You can find JavaScript bindings to read the metadata in src/api/schema.js. This is not really an API yet. It’s missing the part where it gets the binary data from the bottom of the file. Someday, I want this to be usable by other tools too.

Where is the code?

.bun files are marked as executable.

To print out the code, run ./node_modules.bun in your terminal or run bun ./path-to-node_modules.bun.

Here is a copy-pastable example:

./node_modules.bun > node_modules.js

This works because every .bun file starts with this:

#!/usr/bin/env bun

To deploy to production with bun, you’ll want to get the code from the .bun file and stick that somewhere your web server can find it (or if you’re using Vercel or a Rails app, in a public folder).

Note that .bun is a binary file format, so just opening it in VSCode or vim might render strangely.


By default, bun bun only bundles external dependencies that are imported or required in either app code or another external dependency. An "external dependency" is defined as, "A JavaScript-like file that has /node_modules/ in the resolved file path and a corresponding package.json".

To force bun to bundle packages which are not located in a node_modules folder (i.e., the final, resolved path following all symlinks), add a bun section to the root project’s package.json with alwaysBundle set to an array of package names to always bundle. Here’s an example:

  "name": "my-package-name-in-here",
  "bun": {
    "alwaysBundle": ["@mybigcompany/my-workspace-package"]

Bundled dependencies are not eligible for Hot Module Reloading. The code is served to browsers & bun.js verbatim. But, in the future, it may be sectioned off into only parts of the bundle being used. That’s possible in the current version of the .bun file (so long as you know which files are necessary), but it’s not implemented yet. Longer-term, it will include all import and export of each module inside.

What is the module ID hash?

The $eb6819b hash used here:

export var $eb6819b = $$m({

Is generated like this:

  1. Murmur3 32-bit hash of This is the hash uniquely identifying the npm package.
  2. Wyhash 64 of the package.hash + package_path. package_path means "relative to the root of the npm package, where is the module imported?". For example, if you imported react/jsx-dev-runtime.js, the package_path is jsx-dev-runtime.js. react-dom/cjs/react-dom.development.js would be cjs/react-dom.development.js
  3. Truncate the hash generated above to a u32

The implementation details of this module ID hash will vary between versions of bun. The important part is the metadata contains the module IDs, the package paths, and the package hashes, so it shouldn’t really matter in practice if other tooling wants to make use of any of this.

bun upgrade

To upgrade bun, run bun upgrade.

It automatically downloads the latest version of bun and overwrites the currently-running version.

This works by checking the latest version of bun in bun-releases-for-updater and unzipping it using the system-provided unzip library (so that Gatekeeper works on macOS)

If for any reason you run into issues, you can also use the curl install script:

curl | bash

It will still work when bun is already installed.

bun is distributed as a single binary file, so you can also do this manually:

  • Download the latest version of bun for your platform in bun-releases-for-updater (darwin == macOS)
  • Unzip the folder
  • Move the bun binary to ~/.bun/bin (or anywhere)

Canary builds

Canary builds are generated on every commit.

To install a canary build of bun, run:

bun upgrade --canary

This flag is not persistent (though that might change in the future). If you want to always run the canary build of bun, set the BUN_CANARY environment variable to 1 in your shell's startup script.

This will download the release zip from

To revert to the latest published version of bun, run:

bun upgrade

bun init

bun init is a quick way to start a blank project with Bun. It guesses with sane defaults and is non-destructive when run multiple times.


It creates:

  • a package.json file with a name that defaults to the current directory name
  • a tsconfig.json file or a jsconfig.json file, depending if the entry point is a TypeScript file or not
  • an entry point which defaults to index.ts unless any of index.{tsx, jsx, js, mts, mjs} exist or the package.json specifies a module or main field
  • a file

If you pass -y or --yes, it will assume you want to continue without asking questions.

At the end, it runs bun install to install bun-types.

Added in Bun v0.1.7.

How is bun init different than bun create?

bun init is for blank projects. bun create applies templates.

bun completions

This command installs completions for zsh and/or fish. It runs automatically on every bun upgrade and on install. It reads from $SHELL to determine which shell to install for. It tries several common shell completion directories for your shell and OS.

If you want to copy the completions manually, run bun completions > path-to-file. If you know the completions directory to install them to, run bun completions /path/to/directory.

Loader API

Bun v0.1.11 introduces custom loaders.

  • import and require .svelte, .vue, .yaml, .scss, .less and other file extensions that Bun doesn't implement a builtin loader for
  • Dynamically generate ESM & CJS modules

YAML loader via js-yaml

This is an "object" loader. object loaders let you return a JS object that Bun converts to an ESM & CJS module.

Plugin implementation (my-yaml-plugin.js)

import { plugin } from "bun";

  name: "YAML",

  setup(builder) {
    const { load } = require("js-yaml");
    const { readFileSync } = require("fs");
    // Run this function on any import that ends with .yaml or .yml
    builder.onLoad({ filter: /\.(yaml|yml)$/ }, (args) => {
      // Read the YAML file from disk
      const text = readFileSync(args.path, "utf8");

      // parse the YAML file with js-yaml
      const exports = load(text);

      return {
        // Copy the keys and values from the parsed YAML file into the ESM module namespace object

        // we're returning an object
        loader: "object",

Plugin usage:

import "./my-yaml-plugin.js";
import { hello } from "./myfile.yaml";

console.log(hello); // "world"

Svelte loader using svelte/compiler

This is a "js" loader, which lets you return a JS string or ArrayBufferView that Bun converts to an ESM & CJS module.

Plugin implementation (myplugin.js)

import { plugin } from "bun";

await plugin({
  name: "svelte loader",
  async setup(builder) {
    const { compile } = await import("svelte/compiler");
    const { readFileSync } = await import("fs");

    // Register a loader for .svelte files
    builder.onLoad({ filter: /\.svelte$/ }, ({ path }) => ({
      // Run the Svelte compiler on the import path
      contents: compile(readFileSync(path, "utf8"), {
        filename: path,
        generate: "ssr",

      // Set the loader to "js"
      // This runs it through Bun's transpiler
      loader: "js",

Note: in a production implementation, you'd want to cache the compiled output and include additional error handling.

Plugin usage:

import "./myplugin.js";
import MySvelteComponent from "./component.svelte";


Loader API Reference

Bun's loader API interface is loosely based on esbuild. Some esbuild plugins "just work" in Bun.


import { plugin } from "bun";
import { renderToStaticMarkup } from "react-dom/server";

// it's the esbuild plugin, but it works using Bun's transpiler.
import mdx from "@mdx-js/esbuild";


import Foo from "./bar.mdx";
console.log(renderToStaticMarkup(<Foo />));

At the core of the loader API are filter and namespace. filter is a RegExp matched against import paths. namespace is a prefix inserted into the import path (unlike esbuild, Bun inserts the prefix into transpiled output). For example, if you have a loader with a filter of \.yaml$ and a namespace of yaml:, then the import path ./myfile.yaml will be transformed to yaml:./myfile.yaml.

plugin function

At the top-level, a plugin function exported from "bun" expects a "name" string and a "setup" function that takes a builder object.

For plugins to automatically activate, the plugin function must be from an import statement like this:

import { plugin } from "bun";

// This automatically activates on import
  name: "my plugin",
  setup(builder) {},

/* Bun.plugin() does not automatically activate. */

Inside the setup function, you can:

  • register loaders using builder.onLoad()
  • register resolvers using builder.onResolve()

Internally, Bun's transpiler automatically turns plugin() calls into separate files (at most 1 per file). This lets loaders activate before the rest of your application runs with zero configuration.

builder.onLoad({ filter, namespace?: "optional-namespace" }, callback)

builder.onLoad() registers a loader for a matching filter RegExp and namespace string.

The callback function is called with an args object that contains the following properties:

  • path: the path of the file being loaded

For now, that's the only property. More will likely be added in the future.

Loader types

There are different types of loaders:

  • "js", "jsx", "ts", "tsx": these loaders run the source text through Bun's transpiler
  • "json", "toml": these loaders run the source text through Bun's built-in parsers
  • "object": this loader inserts a new ECMAScript Module into the ECMAScript Module registry by copying all the keys and values from the "exports" object into the Module Namespace Object

The callback function expects a return value that contains contents and loader properties, unless the loader is "object".

"contents" is the source code. It can be a string or an ArrayBufferView.

"loader" is the loader type. It can be "js", "jsx", "ts", "tsx", "json", "toml", or "object".

If "loader" is "object", the callback function expects a "exports" object instead of "contents". The keys and values will be copied onto the ESM module namespace object.

"object" loaders are useful when the return value is parsed into an object, like when parsing YAML, JSON, or other data formats. Most loader APIs force you to stringify values and parse again. This loader lets you skip that step, which improves performance and is a little easier sometimes.

Bun.serve - fast HTTP server

For a hello world HTTP server that writes "bun!", Bun.serve serves about 2.5x more requests per second than node.js on Linux:

Requests per second Runtime
~64,000 Node 16
~160,000 Bun

Bigger is better



  fetch(req: Request) {
    return new Response(`bun!`);
  port: 3000,


  .createServer((req, res) => res.end("bun!"))


Two ways to start an HTTP server with bun.js:

  1. export default an object with a fetch function

If the file used to start bun has a default export with a fetch function, it will start the HTTP server.

// hi.js
export default {
  fetch(req) {
    return new Response("HI!");

// bun ./hi.js

fetch receives a Request object and must return either a Response or a Promise<Response>. In a future version, it might have additional arguments for things like cookies.

  1. Bun.serve starts the HTTP server explicitly
  fetch(req) {
    return new Response("HI!");

Error handling

For error handling, you get an error function.

If development: true and error is not defined or doesn't return a Response, you will get an exception page with a stack trace:


It will hopefully make it easier to debug issues with bun until bun gets debugger support. This error page is based on what bun dev does.

If the error function returns a Response, it will be served instead

  fetch(req) {
    throw new Error("woops!");
  error(error: Error) {
    return new Response("Uh oh!!\n" + error.toString(), { status: 500 });

If the error function itself throws and development is false, a generic 500 page will be shown

To stop the server, call server.stop():

const server = Bun.serve({
  fetch() {
    return new Response("HI!");


The interface for Bun.serve is based on what Cloudflare Workers does.

Bun.write – optimizing I/O

Bun.write lets you write, copy or pipe files automatically using the fastest system calls compatible with the input and platform.

interface Bun {
    destination: string | number | FileBlob,
    input: string | FileBlob | Blob | ArrayBufferView
  ): Promise<number>;
Output Input System Call Platform
file file copy_file_range Linux
file pipe sendfile Linux
pipe pipe splice Linux
terminal file sendfile Linux
terminal terminal sendfile Linux
socket file or pipe sendfile (if http, not https) Linux
file (path, doesn't exist) file (path) clonefile macOS
file file fcopyfile macOS
file Blob or string write macOS
file Blob or string write Linux

All this complexity is handled by a single function.

// Write "Hello World" to output.txt
await Bun.write("output.txt", "Hello World");
// log a file to stdout
await Bun.write(Bun.stdout, Bun.file("input.txt"));
// write the HTTP response body to disk
await Bun.write("index.html", await fetch(""));
// this does the same thing
await Bun.write(Bun.file("index.html"), await fetch(""));
// copy input.txt to output.txt
await Bun.write("output.txt", Bun.file("input.txt"));

bun:sqlite (SQLite3 module)

bun:sqlite is a high-performance built-in SQLite3 module for bun.js.

  • Simple, synchronous API (synchronous is faster)
  • Transactions
  • Binding named & positional parameters
  • Prepared statements
  • Automatic type conversions (BLOB becomes Uint8Array)
  • toString() prints as SQL


# there's nothing to install
# bun:sqlite is built-in to bun.js


import { Database } from "bun:sqlite";

const db = new Database("mydb.sqlite");
);"INSERT INTO foo (greeting) VALUES (?)", "Welcome to bun!");"INSERT INTO foo (greeting) VALUES (?)", "Hello World!");

// get the first row
db.query("SELECT * FROM foo").get();
// { id: 1, greeting: "Welcome to bun!" }

// get all rows
db.query("SELECT * FROM foo").all();
// [
//   { id: 1, greeting: "Welcome to bun!" },
//   { id: 2, greeting: "Hello World!" },
// ]

// get all rows matching a condition
db.query("SELECT * FROM foo WHERE greeting = ?").all("Welcome to bun!");
// [
//   { id: 1, greeting: "Welcome to bun!" },
// ]

// get first row matching a named condition
db.query("SELECT * FROM foo WHERE greeting = $greeting").get({
  $greeting: "Welcome to bun!",
// [
//   { id: 1, greeting: "Welcome to bun!" },
// ]

bun:sqlite Benchmark

Database: Northwind Traders.

This benchmark can be run from ./bench/sqlite.

Here are results from an M1 Pro (64GB) on macOS 12.3.1.


Library Runtime ms/iter
bun:sqlite3 Bun 0.0.83 14.31 (1x)
better-sqlite3 Node 18.0.0 40.81 (2.8x slower) Deno 1.21.2 125.96 (8.9x slower)

SELECT * FROM "Product"

Library Runtime us/iter
bun:sqlite3 Bun 0.0.83 33.85 (1x)
better-sqlite3 Node 18.0.0 121.09 (3.5x slower) Deno 1.21.2 187.64 (8.9x slower)

SELECT * FROM "OrderDetail"

Library Runtime ms/iter
bun:sqlite3 Bun 0.0.83 146.92 (1x)
better-sqlite3 Node 18.0.0 875.73 (5.9x slower) Deno 1.21.2 541.15 (3.6x slower)

In screenshot form (which has a different sorting order)


Getting started with bun:sqlite

bun:sqlite's API is loosely based on better-sqlite3, though the implementation is different.

bun:sqlite has two classes:

  • class Database
  • class Statement


Calling new Database(filename) opens or creates the SQLite database.

      filename: string,
        | number
        | {
             * Open the database as read-only (no write operations, no create).
             * Equivalent to {@link constants.SQLITE_OPEN_READONLY}
            readonly?: boolean;
             * Allow creating a new database
             * Equivalent to {@link constants.SQLITE_OPEN_CREATE}
            create?: boolean;
             * Open the database as read-write
             * Equivalent to {@link constants.SQLITE_OPEN_READWRITE}
            readwrite?: boolean;

To open or create a SQLite3 database:

import { Database } from "bun:sqlite";

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

Open an in-memory database:

import { Database } from "bun:sqlite";

// all of these do the same thing
var db = new Database(":memory:");
var db = new Database();
var db = new Database("");

Open read-write and throw if the database doesn't exist:

import { Database } from "bun:sqlite";
const db = new Database("mydb.sqlite", { readwrite: true });

Open read-only and throw if the database doesn't exist:

import { Database } from "bun:sqlite";
const db = new Database("mydb.sqlite", { readonly: true });

Open read-write, don't throw if new file:

import { Database } from "bun:sqlite";
const db = new Database("mydb.sqlite", { readonly: true, create: true });

Open a database from a Uint8Array:

import { Database } from "bun:sqlite";
import { readFileSync } from "fs";

// unlike passing a filepath, this will not persist any changes to disk
// it will be read-write but not persistent
const db = new Database(readFileSync("mydb.sqlite"));

Close a database:

var db = new Database();

Note: close() is called automatically when the database is garbage collected. It is safe to call multiple times but has no effect after the first.


query(sql) creates a Statement for the given SQL and caches it, but does not execute it.

class Database {
  query(sql: string): Statement;

query returns a Statement object.

It performs the same operation as Database.prototype.prepare, except:

  • query caches the prepared statement in the Database object
  • query doesn't bind parameters

This intended to make it easier for bun:sqlite to be fast by default. Calling .prepare compiles a SQLite query, which can take some time, so it's better to cache those a little.

You can bind parameters on any call to a statement.

import { Database } from "bun:sqlite";

// generate some data
var db = new Database();
);"INSERT INTO foo (greeting) VALUES ($greeting)", {
  $greeting: "Welcome to bun",

// get the query
const stmt = db.query("SELECT * FROM foo WHERE greeting = ?");

// run the query
stmt.all("Welcome to bun!");
stmt.get("Welcome to bun!");"Welcome to bun!");


prepare(sql) creates a Statement for the given SQL, but does not execute it.

Unlike query(), this does not cache the compiled query.

import { Database } from "bun:sqlite";

// generate some data
var db = new Database();

// compile the prepared statement
const stmt = db.prepare("SELECT * FROM foo WHERE bar = ?");

// run the prepared statement

Internally, this calls sqlite3_prepare_v3.

Database.prototype.exec &

exec is for one-off executing a query which does not need to return anything. run is an alias.

class Database {
  // exec is an alias for run
  exec(sql: string, ...params: ParamsType): void;
  run(sql: string, ...params: ParamsType): void;

This is useful for things like

Creating a table:

import { Database } from "bun:sqlite";

var db = new Database();

Inserting one row:

import { Database } from "bun:sqlite";

var db = new Database();

// insert one row
db.exec("INSERT INTO foo (greeting) VALUES ($greeting)", {
  $greeting: "Welcome to bun",

For queries which aren't intended to be run multiple times, it should be faster to use exec() than prepare() or query() because it doesn't create a Statement object.

Internally, this function calls sqlite3_prepare, sqlite3_step, and sqlite3_finalize.


Creates a function that always runs inside a transaction. When the function is invoked, it will begin a new transaction. When the function returns, the transaction will be committed. If an exception is thrown, the transaction will be rolled back (and the exception will propagate as usual).

// setup
import { Database } from "bun:sqlite";
const db =":memory:");

const insert = db.prepare("INSERT INTO cats (name, age) VALUES ($name, $age)");
const insertMany = db.transaction((cats) => {
  for (const cat of cats);

  { $name: "Joey", $age: 2 },
  { $name: "Sally", $age: 4 },
  { $name: "Junior", $age: 1 },

Transaction functions can be called from inside other transaction functions. When doing so, the inner transaction becomes a savepoint.

// setup
import { Database } from "bun:sqlite";
const db =":memory:");
const newExpense = db.prepare(
  "INSERT INTO expenses (note, dollars) VALUES (?, ?)"
const insert = db.prepare("INSERT INTO cats (name, age) VALUES ($name, $age)");
const insertMany = db.transaction((cats) => {
  for (const cat of cats);

const adopt = db.transaction((cats) => {"adoption fees", 20);
  insertMany(cats); // nested transaction

  { $name: "Joey", $age: 2 },
  { $name: "Sally", $age: 4 },
  { $name: "Junior", $age: 1 },

Transactions also come with deferred, immediate, and exclusive versions.

insertMany(cats); // uses "BEGIN"
insertMany.deferred(cats); // uses "BEGIN DEFERRED"
insertMany.immediate(cats); // uses "BEGIN IMMEDIATE"
insertMany.exclusive(cats); // uses "BEGIN EXCLUSIVE"

Any arguments passed to the transaction function will be forwarded to the wrapped function, and any values returned from the wrapped function will be returned from the transaction function. The wrapped function will also have access to the same binding as the transaction function.

bun:sqlite's transaction implementation is based on better-sqlite3 (along with this section of the docs), so thanks to Joshua Wise and better-sqlite3 contributors.


SQLite has a built-in way to serialize and deserialize databases to and from memory.

bun:sqlite fully supports it:

var db = new Database();

// write some data
);"INSERT INTO foo VALUES (?)", "Welcome to bun!");"INSERT INTO foo VALUES (?)", "Hello World!");

const copy = db.serialize();
// => Uint8Array

const db2 = new Database(copy);
db2.query("SELECT * FROM foo").all();
// => [
//   { id: 1, greeting: "Welcome to bun!" },
//   { id: 2, greeting: "Hello World!" },
// ]

db.serialize() returns a Uint8Array of the database.

Internally, it calls sqlite3_serialize.


bun:sqlite supports SQLite extensions.

To load a SQLite extension, call Database.prototype.loadExtension(name):

import { Database } from "bun:sqlite";

var db = new Database();


If you're on macOS, you will need to first use a custom SQLite install (you can install with homebrew). By default, bun uses Apple's proprietary build of SQLite because it benchmarks about 50% faster. However, they disabled extension support, so you will need to have a custom build of SQLite to use extensions on macOS.

import { Database } from "bun:sqlite";

// on macOS, this must be run before any other calls to `Database`
// if called on linux, it will return true and do nothing
// on linux it will still check that a string was passed

var db = new Database();


To install sqlite with homebrew:

brew install sqlite


Statement is a prepared statement. Use it to run queries that get results.


You can bind parameters on any call to a statement. Named parameters and positional parameters are supported. Bound parameters are remembered between calls and reset the next time you pass parameters to bind.

import { Database } from "bun:sqlite";

// setup
var db = new Database();
);"INSERT INTO foo VALUES (?)", "Welcome to bun!");"INSERT INTO foo VALUES (?)", "Hello World!");

// Statement object
var statement = db.query("SELECT * FROM foo");

// returns all the rows

// returns the first row

// runs the query, without returning anything;


Calling all() on a Statement instance runs the query and returns the rows as an array of objects.

import { Database } from "bun:sqlite";

// setup
var db = new Database();
);"INSERT INTO foo (greeting, count) VALUES (?, ?)", "Welcome to bun!", 2);"INSERT INTO foo (greeting, count) VALUES (?, ?)", "Hello World!", 0);
  "INSERT INTO foo (greeting, count) VALUES (?, ?)",
  "Welcome to bun!!!!",

// Statement object
var statement = db.query("SELECT * FROM foo WHERE count = ?");

// return all the query results, binding 2 to the count parameter
// => [
//   { id: 1, greeting: "Welcome to bun!", count: 2 },
//   { id: 3, greeting: "Welcome to bun!!!!", count: 2 },
// ]

Internally, this calls sqlite3_reset and repeatedly calls sqlite3_step until it returns SQLITE_DONE.


Calling values() on a Statement instance runs the query and returns the rows as an array of arrays.

import { Database } from "bun:sqlite";

// setup
var db = new Database();
);"INSERT INTO foo (greeting, count) VALUES (?, ?)", "Welcome to bun!", 2);"INSERT INTO foo (greeting, count) VALUES (?, ?)", "Hello World!", 0);
  "INSERT INTO foo (greeting, count) VALUES (?, ?)",
  "Welcome to bun!!!!",

// Statement object
var statement = db.query("SELECT * FROM foo WHERE count = ?");

// return all the query results as an array of arrays, binding 2 to "count"
// => [
//   [ 1, "Welcome to bun!", 2 ],
//   [ 3, "Welcome to bun!!!!", 2 ],
// ]

// Statement object, but with named parameters
var statement = db.query("SELECT * FROM foo WHERE count = $count");

// return all the query results as an array of arrays, binding 2 to "count"
statement.values({ $count: 2 });
// => [
//   [ 1, "Welcome to bun!", 2 ],
//   [ 3, "Welcome to bun!!!!", 2 ],
// ]

Internally, this calls sqlite3_reset and repeatedly calls sqlite3_step until it returns SQLITE_DONE.


Calling get() on a Statement instance runs the query and returns the first result as an object.

import { Database } from "bun:sqlite";

// setup
var db = new Database();
);"INSERT INTO foo (greeting, count) VALUES (?, ?)", "Welcome to bun!", 2);"INSERT INTO foo (greeting, count) VALUES (?, ?)", "Hello World!", 0);
  "INSERT INTO foo (greeting, count) VALUES (?, ?)",
  "Welcome to bun!!!!",

// Statement object
var statement = db.query("SELECT * FROM foo WHERE count = ?");

// return the first row as an object, binding 2 to the count parameter
// => { id: 1, greeting: "Welcome to bun!", count: 2 }

// Statement object, but with named parameters
var statement = db.query("SELECT * FROM foo WHERE count = $count");

// return the first row as an object, binding 2 to the count parameter
statement.get({ $count: 2 });
// => { id: 1, greeting: "Welcome to bun!", count: 2 }

Internally, this calls sqlite3_reset and calls sqlite3_step once. Stepping through all the rows is not necessary when you only want the first row.

Calling run() on a Statement instance runs the query and returns nothing.

This is useful if you want to repeatedly run a query, but don't care about the results.

import { Database } from "bun:sqlite";

// setup
var db = new Database();
);"INSERT INTO foo (greeting, count) VALUES (?, ?)", "Welcome to bun!", 2);"INSERT INTO foo (greeting, count) VALUES (?, ?)", "Hello World!", 0);
  "INSERT INTO foo (greeting, count) VALUES (?, ?)",
  "Welcome to bun!!!!",

// Statement object (TODO: use a better example query)
var statement = db.query("SELECT * FROM foo");

// run the query, returning nothing;

Internally, this calls sqlite3_reset and calls sqlite3_step once. Stepping through all the rows is not necessary when you don't care about the results.


This method finalizes the statement, freeing any resources associated with it.

After a statement has been finalized, it cannot be used for any further queries. Any attempt to run the statement will throw an error. Calling it multiple times will have no effect.

It is a good idea to finalize a statement when you are done with it, but the garbage collector will do it for you if you don't.

import { Database } from "bun:sqlite";

// setup
var db = new Database();
);"INSERT INTO foo (greeting, count) VALUES (?, ?)", "Welcome to bun!", 2);"INSERT INTO foo (greeting, count) VALUES (?, ?)", "Hello World!", 0);
  "INSERT INTO foo (greeting, count) VALUES (?, ?)",
  "Welcome to bun!!!!",

// Statement object
var statement = db.query("SELECT * FROM foo WHERE count = ?");


// this will throw;


Calling toString() on a Statement instance prints the expanded SQL query. This is useful for debugging.

import { Database } from "bun:sqlite";

// setup
var db = new Database();
);"INSERT INTO foo (greeting, count) VALUES (?, ?)", "Welcome to bun!", 2);"INSERT INTO foo (greeting, count) VALUES (?, ?)", "Hello World!", 0);
  "INSERT INTO foo (greeting, count) VALUES (?, ?)",
  "Welcome to bun!!!!",

// Statement object
const statement = db.query("SELECT * FROM foo WHERE count = ?");

// => "SELECT * FROM foo WHERE count = NULL"; // bind the param

// => "SELECT * FROM foo WHERE count = 2"

Internally, this calls sqlite3_expanded_sql.


JavaScript type SQLite type
string TEXT
boolean INTEGER (1 or 0)
Uint8Array BLOB
Buffer BLOB
bigint INTEGER
null NULL

bun:ffi (Foreign Functions Interface)

bun:ffi lets you efficiently call native libraries from JavaScript. It works with languages that support the C ABI (Zig, Rust, C/C++, C#, Nim, Kotlin, etc).

This snippet prints sqlite3's version number:

import { dlopen, FFIType, suffix } from "bun:ffi";

// `suffix` is either "dylib", "so", or "dll" depending on the platform
// you don't have to use "suffix", it's just there for convenience
const path = `libsqlite3.${suffix}`;

const {
  symbols: {
    // sqlite3_libversion is the function we will call
} =
  // dlopen() expects:
  // 1. a library name or file path
  // 2. a map of symbols
  dlopen(path, {
    // `sqlite3_libversion` is a function that returns a string
    sqlite3_libversion: {
      // sqlite3_libversion takes no arguments
      args: [],
      // sqlite3_libversion returns a pointer to a string
      returns: FFIType.cstring,

console.log(`SQLite 3 version: ${sqlite3_libversion()}`);

Low-overhead FFI

3ns to go from JavaScript <> native code with bun:ffi (on my machine, an M1 Pro with 64GB of RAM)

  • 5x faster than napi (Node v17.7.1)
  • 100x faster than Deno v1.21.1

As measured in this simple benchmark

Why is bun:ffi fast?

Bun generates & just-in-time compiles C bindings that efficiently convert values between JavaScript types and native types.

To compile C, Bun embeds TinyCC, a small and fast C compiler.


With Zig:

// add.zig
pub export fn add(a: i32, b: i32) i32 {
  return a + b;

To compile:

zig build-lib add.zig -dynamic -OReleaseFast

Pass dlopen the path to the shared library and the list of symbols you want to import.

import { dlopen, FFIType, suffix } from "bun:ffi";

const path = `libadd.${suffix}`;

const lib = dlopen(path, {
  add: {
    args: [FFIType.i32, FFIType.i32],
    returns: FFIType.i32,

lib.symbols.add(1, 2);

With Rust:

pub extern "C" fn add(a: isize, b: isize) -> isize {
    a + b

To compile:

rustc --crate-type cdylib

Supported FFI types (FFIType)

FFIType C Type Aliases
cstring char*
ptr void* pointer, void*, char*
i8 int8_t int8_t
i16 int16_t int16_t
i32 int32_t int32_t, int
i64 int64_t int64_t
i64_fast int64_t
u8 uint8_t uint8_t
u16 uint16_t uint16_t
u32 uint32_t uint32_t
u64 uint64_t uint64_t
u64_fast uint64_t
f32 float float
f64 double double
bool bool
char char

Strings (CString)

JavaScript strings and C-like strings are different, and that complicates using strings with native libraries.

How are JavaScript strings and C strings different?

JavaScript strings:

  • UTF16 (2 bytes per letter) or potentially latin1, depending on the JavaScript engine & what characters are used
  • length stored separately
  • Immutable

C strings:

  • UTF8 (1 byte per letter), usually
  • The length is not stored. Instead, the string is null-terminated which means the length is the index of the first \0 it finds
  • Mutable

To help with that, bun:ffi exports CString which extends JavaScript's built-in String to support null-terminated strings and add a few extras:

class CString extends String {
   * Given a `ptr`, this will automatically search for the closing `\0` character and transcode from UTF-8 to UTF-16 if necessary.
  constructor(ptr: number, byteOffset?: number, byteLength?: number): string;

   * The ptr to the C string
   * This `CString` instance is a clone of the string, so it
   * is safe to continue using this instance after the `ptr` has been
   * freed.
  ptr: number;
  byteOffset?: number;
  byteLength?: number;

To convert from a null-terminated string pointer to a JavaScript string:

const myString = new CString(ptr);

To convert from a pointer with a known length to a JavaScript string:

const myString = new CString(ptr, 0, byteLength);

new CString clones the C string, so it is safe to continue using myString after ptr has been freed.


// this is safe because myString is a clone
Returning a string

When used in returns, FFIType.cstring coerces the pointer to a JavaScript string. When used in args, cstring is identical to ptr.

Function pointers (CFunction)

To call a function pointer from JavaScript, use CFunction

This is useful if using Node-API (napi) with Bun, and you've already loaded some symbols.

import { CFunction } from "bun:ffi";

var myNativeLibraryGetVersion = /* somehow, you got this pointer */

const getVersion = new CFunction({
  returns: "cstring",
  args: [],
  ptr: myNativeLibraryGetVersion,

If you have multiple function pointers, you can define them all at once with linkSymbols:

import { linkSymbols } from "bun:ffi";

// getVersionPtrs defined elsewhere
const [majorPtr, minorPtr, patchPtr] = getVersionPtrs();

const lib = linkSymbols({
  // Unlike with dlopen(), the names here can be whatever you want
  getMajor: {
    returns: "cstring",
    args: [],

    // Since this doesn't use dlsym(), you have to provide a valid ptr
    // That ptr could be a number or a bigint
    // An invalid pointer will crash your program.
    ptr: majorPtr,
  getMinor: {
    returns: "cstring",
    args: [],
    ptr: minorPtr,
  getPatch: {
    returns: "cstring",
    args: [],
    ptr: patchPtr,

const [major, minor, patch] = [


Bun represents pointers as a number in JavaScript.

How does a 64 bit pointer fit in a JavaScript number?

64-bit processors support up to 52 bits of addressable space.

JavaScript numbers support 53 bits of usable space, so that leaves us with about 11 bits of extra space.

Why not BigInt?

BigInt is slower. JavaScript engines allocate a separate BigInt which means they can't just fit in a regular javascript value.

If you pass a BigInt to a function, it will be converted to a number

To convert from a TypedArray to a pointer:

import { ptr } from "bun:ffi";
var myTypedArray = new Uint8Array(32);
const myPtr = ptr(myTypedArray);

To convert from a pointer to an ArrayBuffer:

import { ptr, toArrayBuffer } from "bun:ffi";
var myTypedArray = new Uint8Array(32);
const myPtr = ptr(myTypedArray);

// toTypedArray accepts a `byteOffset` and `byteLength`
// if `byteLength` is not provided, it is assumed to be a null-terminated pointer
myTypedArray = new Uint8Array(toArrayBuffer(myPtr, 0, 32), 0, 32);

To read data from a pointer

You have two options.

For long-lived pointers, a DataView is the fastest option:

import { toArrayBuffer } from "bun:ffi";
var myDataView = new DataView(toArrayBuffer(myPtr, 0, 32));

  myDataView.getUint8(0, true),
  myDataView.getUint8(1, true),
  myDataView.getUint8(2, true),
  myDataView.getUint8(3, true)

For short-lived pointers, read is the fastest option:

Available in Bun v0.1.12+

import { read } from "bun:ffi";

  // ptr, byteOffset
  read.u8(myPtr, 0),
  read.u8(myPtr, 1),
  read.u8(myPtr, 2),
  read.u8(myPtr, 3)

read behaves similarly to DataView, but it can be faster because it doesn't need to create a DataView or ArrayBuffer.

FFIType read function
ptr read.ptr
i8 read.i8
i16 read.i16
i32 read.i32
i64 read.i64
u8 read.u8
u16 read.u16
u32 read.u32
u64 read.u64
f32 read.f32
f64 read.f64

Memory management with pointers:

bun:ffi does not manage memory for you because it doesn't have the information necessary. You must free the memory when you're done with it.

From JavaScript:

If you want to track when a TypedArray is no longer in use from JavaScript, you can use a FinalizationRegistry.

From FFI (C, Rust, Zig, etc):

Available in Bun v0.1.8 and later.

If you want to track when a TypedArray is no longer in use from C or FFI, you can pass a callback and an optional context pointer to toArrayBuffer or toBuffer. This function is called at some point later, once the garbage collector frees the underlying ArrayBuffer JavaScript object.

The expected signature is the same as in JavaScriptCore's C API:

typedef void (*JSTypedArrayBytesDeallocator)(void *bytes, void *deallocatorContext);
import { toArrayBuffer } from "bun:ffi";

// with a deallocatorContext:


  // this is an optional pointer to a callback

  // this is a pointer to a function

// without a deallocatorContext:


  // this is a pointer to a function

Pointers & memory safety

Using raw pointers outside of FFI is extremely not recommended.

A future version of bun may add a CLI flag to disable bun:ffi (or potentially a separate build of bun).

Pointer alignment

If an API expects a pointer sized to something other than char or u8, make sure the typed array is also that size.

A u64* is not exactly the same as [8]u8* due to alignment

Passing a pointer

Where FFI functions expect a pointer, pass a TypedArray of equivalent size


import { dlopen, FFIType } from "bun:ffi";

const {
  symbols: { encode_png },
} = dlopen(myLibraryPath, {
  encode_png: {
    // FFIType's can be specified as strings too
    args: ["ptr", "u32", "u32"],
    returns: FFIType.ptr,

const pixels = new Uint8ClampedArray(128 * 128 * 4);
pixels.subarray(0, 32 * 32 * 2).fill(0);

const out = encode_png(
  // pixels will be passed as a pointer


The auto-generated wrapper converts the pointer to a TypedArray


If you don't want the automatic conversion or you want a pointer to a specific byte offset within the TypedArray, you can also directly get the pointer to the TypedArray:

import { dlopen, FFIType, ptr } from "bun:ffi";

const {
  symbols: { encode_png },
} = dlopen(myLibraryPath, {
  encode_png: {
    // FFIType's can be specified as strings too
    args: ["ptr", "u32", "u32"],
    returns: FFIType.ptr,

const pixels = new Uint8ClampedArray(128 * 128 * 4);

// this returns a number! not a BigInt!
const myPtr = ptr(pixels);

const out = encode_png(

  // dimensions:
Reading pointers
const out = encode_png(
  // pixels will be passed as a pointer

  // dimensions:

// assuming it is 0-terminated, it can be read like this:
var png = new Uint8Array(toArrayBuffer(out));

// save it to disk:
await Bun.write("out.png", png);
Not implemented yet

bun:ffi has a few more things planned but not implemented yet:

  • callback functions
  • async functions

Node-API (napi)

Bun.js implements 90% of the APIs available in Node-API (napi).

You can see the status of this here.

Loading Node-API modules in Bun.js works the same as in Node.js:

const napi = require("./my-node-module.node");

You can also use process.dlopen:

var mod = { exports: {} };
process.dlopen(mod, "./my-node-module.node");

As part of that work, Bun.js also polyfills the detect-libc package, which is used by many Node-API modules to detect which .node binding to require.

This implementation of Node-API is from scratch. It doesn't use any code from Node.js.

Some implementation details

When requiring a *.node module, Bun's JavaScript transpiler transforms the require expression into a call to import.meta.require:

// this is the input

// this is the output

Bun doesn't currently support dynamic requires, but import.meta.require is an escape hatch for that. It uses a JavaScriptCore built-in function.


Bun.Transpiler lets you use Bun's transpiler from JavaScript (available in Bun.js)

type Loader = "jsx" | "js" | "ts" | "tsx";

interface TranspilerOptions {
  // Replace key with value. Value must be a JSON string.
  // @example
  // ```
  // { "process.env.NODE_ENV": "\"production\"" }
  // ```
  define: Record<string, string>,

  // What is the default loader used for this transpiler?
  loader: Loader,

  // What platform are we targeting? This may affect how import and/or require is used
  platform: "browser" | "bun" | "macro" | "node",

  // TSConfig.json file as stringified JSON or an object
  // Use this to set a custom JSX factory, fragment, or import source
  // For example, if you want to use Preact instead of React. Or if you want to use Emotion.
  tsconfig: string | TSConfig,

  // Replace imports with macros
  macros: MacroMap,

// This lets you use macros
interface MacroMap {
  // @example
  // ```
  // {
  //   "react-relay": {
  //     "graphql": "bun-macro-relay/bun-macro-relay.tsx"
  //   }
  // }
  // ```
  [packagePath: string]: {
    [importItemName: string]: string,

class Bun.Transpiler {
  constructor(options: TranspilerOptions)

  transform(code: string, loader?: Loader): Promise<string>
  transformSync(code: string, loader?: Loader): string

  scan(code: string): {exports: string[], imports: Import}
  scanImports(code: string): Import[]

type Import = {
  path: string,
  // import foo from 'bar'; in JavaScript
  | "import-statement"
  // require("foo") in JavaScript
  | "require-call"
  // require.resolve("foo") in JavaScript
  | "require-resolve"
  // Dynamic import() in JavaScript
  | "dynamic-import"
  // @import() in CSS
  | "import-rule"
  // url() in CSS
  | "url-token"
  // The import was injected by Bun
  | "internal"
  // Entry point
  // Probably won't see this one
  | "entry-point"

const transpiler = new Bun.Transpiler({ loader: "jsx" });


This lets you transpile JavaScript, TypeScript, TSX, and JSX using Bun's transpiler. It does not resolve modules.

It is synchronous and runs in the same thread as other JavaScript code.

const transpiler = new Bun.Transpiler({ loader: "jsx" });
import { __require as require } from "bun:wrap";
import * as JSX from "react/jsx-dev-runtime";
var jsx = require(JSX).jsxDEV;

export default jsx(
    children: "hi!",

If a macro is used, it will be run in the same thread as the transpiler, but in a separate event loop from the rest of your application. Currently, globals between macros and regular code are shared, which means it is possible (but not recommended) to share states between macros and regular code. Attempting to use AST nodes outside of a macro is undefined behavior.


This lets you transpile JavaScript, TypeScript, TSX, and JSX using Bun's transpiler. It does not resolve modules.

It is async and automatically runs in Bun's worker threadpool. That means, if you run it 100 times, it will run it across Math.floor($cpu_count * 0.8) threads without blocking the main JavaScript thread.

If code uses a macro, it will potentially spawn a new copy of Bun.js' JavaScript runtime environment in that new thread.

Unless you're transpiling many large files, you should probably use Bun.Transpiler.transformSync. The cost of the threadpool will often take longer than actually transpiling code.

const transpiler = new Bun.Transpiler({ loader: "jsx" });
await transpiler.transform("<div>hi!</div>");
import { __require as require } from "bun:wrap";
import * as JSX from "react/jsx-dev-runtime";
var jsx = require(JSX).jsxDEV;

export default jsx(
    children: "hi!",

You can also pass a Loader as a string

await transpiler.transform("<div>hi!</div>", "tsx");


This is a fast way to get a list of imports & exports used in a JavaScript/jsx or TypeScript/tsx file.

This function is synchronous.

const transpiler = new Bun.Transpiler({ loader: "ts" });

import React from 'react';
import Remix from 'remix';
import type {ReactNode} from 'react';

export const loader = () => import('./loader');
  "exports": [
  "imports": [
      "kind": "import-statement",
      "path": "react"
      "kind": "import-statement",
      "path": "remix"
      "kind": "dynamic-import",
      "path": "./loader"


This is a fast path for getting a list of imports used in a JavaScript/jsx or TypeScript/tsx file. It skips the visiting pass, which means it is faster but less accurate. You probably won't notice a difference between Bun.Transpiler.scan and Bun.Transpiler.scanImports often. You might notice it for very large files (megabytes).

This function is synchronous.

const transpiler = new Bun.Transpiler({ loader: "ts" });

import React from 'react';
import Remix from 'remix';
import type {ReactNode} from 'react';

export const loader = () => import('./loader');
    "kind": "import-statement",
    "path": "react"
    "kind": "import-statement",
    "path": "remix"
    "kind": "dynamic-import",
    "path": "./loader"

Environment variables

  • GOMAXPROCS: For bun bun, this sets the maximum number of threads to use. If you’re experiencing an issue with bun bun, try setting GOMAXPROCS=1 to force bun to run single-threaded
  • DISABLE_BUN_ANALYTICS=1 this disables bun’s analytics. bun records bundle timings (so we can answer with data, "is bun getting faster?") and feature usage (e.g., "are people actually using macros?"). The request body size is about 60 bytes, so it’s not a lot of data
  • TMPDIR: Before bun bun completes, it stores the new .bun in $TMPDIR. If unset, TMPDIR defaults to the platform-specific temporary directory (on Linux, /tmp and on macOS /private/tmp)


  • While written in Zig instead of Go, bun’s JS transpiler, CSS lexer, and node module resolver source code is based on @evanw’s esbuild project. Evan did a fantastic job with esbuild.
  • The idea for the name "bun" came from @kipply


bun itself is MIT-licensed.

However, JavaScriptCore (and WebKit) is LGPL-2 and bun statically links it. WebCore files from WebKit are also licensed under LGPL2.

Per LGPL2:

(1) If you statically link against an LGPL’d library, you must also provide your application in an object (not necessarily source) format, so that a user has the opportunity to modify the library and relink the application.

You can find the patched version of WebKit used by bun here: If you would like to relink bun with changes:

  • git submodule update --init --recursive
  • make jsc
  • zig build

This compiles JavaScriptCore, compiles bun’s .cpp bindings for JavaScriptCore (which are the object files using JavaScriptCore) and outputs a new bun binary with your changes.

bun also statically links these libraries:

For compatibility reasons, these NPM packages are embedded into bun’s binary and injected if imported.

Developing bun

Some links you should read about JavaScriptCore, the JavaScript engine Bun uses:

To get your development environment configured, expect it to take 30-90 minutes :(

VSCode Dev Container (Linux)

The VSCode Dev Container in this repository is the easiest way to get started. It comes with Zig, JavaScriptCore, Zig Language Server, vscode-zig, and more pre-installed on an instance of Ubuntu.

To develop on Linux, the following is required:

To get started, in the bun repository, locally run:

# devcontainer-build just sets the architecture so if you're on ARM64, it'll do the right thing.
make devcontainer-build

Next, open VS Code in the bun repository. To open the dev container, open the command palette (Ctrl + Shift + P) and run: Remote-Containers: Reopen in Container.

You will then need to clone the GitHub repository inside that container.

Inside the container, run this:

# First time setup
gh repo clone oven-sh/bun . -- --depth=1 --progress -j8

# update all submodules except webkit because webkit takes awhile and it's already compiled for you.
git -c submodule."src/bun.js/WebKit".update=none submodule update --init --recursive --depth=1 --progress

# Compile bun dependencies (zig is already compiled)
make devcontainer

# Build bun for development
make dev

# Run bun

It is very similar to my own development environment (except I use macOS)


Install LLVM 13 and homebrew dependencies:

brew install llvm@13 coreutils libtool cmake libiconv automake ninja gnu-sed pkg-config esbuild go rust

bun (& the version of Zig) need LLVM 13 and Clang 13 (clang is part of LLVM). Weird build & runtime errors will happen otherwise.

Make sure LLVM 13 is in your $PATH:

which clang-13

If it is not, you will have to run this to link it:

export PATH="$(brew --prefix llvm@13)/bin:$HOME/.bun-tools/zig:$PATH"
export LDFLAGS="$LDFLAGS -L$(brew --prefix llvm@13)/lib"
export CPPFLAGS="$CPPFLAGS -I$(brew --prefix llvm@13)/include"

On fish that looks like fish_add_path (brew --prefix llvm@13)/bin

Install Zig (macOS)

Note: you must use the same version of Zig used by Bun in oven-sh/zig. Installing zig from brew will not work. Installing the latest stable version of Zig won't work. If you don't use the same version Bun uses, you will get strange build errors and be sad because you put all this work into trying to get Bun to compile and it failed for weird reasons.

To install the zig binary:

# Custom path for the custom zig install
mkdir -p $HOME/.bun-tools

# Requires jq & grab latest binary
curl -o zig.tar.gz -sL$(uname -m).tar.gz

# This will extract to $HOME/.bun-tools/zig
tar -xvf zig.tar.gz -C $HOME/.bun-tools/
rm zig.tar.gz

# Make sure it gets trusted
# If you get an error 'No such xattr:', that means it's already trusted and you can continue
xattr -d $HOME/.bun-tools/zig/zig

Now you'll need to add Zig to your PATH.

Using zsh:

echo 'export PATH="$HOME/.bun-tools/zig:$PATH"' >> ~/.zshrc
source ~/.zshrc

Using fish:

# Add to PATH (fish)
fish_add_path $HOME/.bun-tools/zig

Using bash:

echo 'export PATH="$HOME/.bun-tools/zig:$PATH"' >> ~/.bash_profile
source ~/.bash_profile

The version of Zig used by Bun is not a fork, just a slightly older version. Zig is a new programming language and moves quickly.

Build bun (macOS)

If you're building on a macOS device, you'll need to have a valid Developer Certificate, or else the code signing step will fail. To check if you have one, open the Keychain Access app, go to the login profile and search for Apple Development. You should have at least one certificate with a name like Apple Development: (WDYABC123). If you don't have one, follow this guide to get one.

You can still work with the generated binary locally at packages/debug-bun-*/bun-debug even if the code signing fails.

In bun:

# If you omit --depth=1, `git submodule update` will take 17.5 minutes on 1gbps internet, mostly due to WebKit.
git submodule update --init --recursive --progress --depth=1 --checkout
make vendor identifier-cache bindings jsc dev

Verify it worked (macOS)

First ensure the node dependencies are installed

(cd test/snippets && npm i)
(cd test/scripts && npm i)


make test-dev-all

Troubleshooting (macOS)

If you see an error when compiling libarchive, run this:

brew install pkg-config

If you see an error about missing files on zig build obj, make sure you built the headers


Note: this is automatically installed on the devcontainer

You will want to install the fork of vscode-zig so you get a Run test and a Debug test button.

To do that:

curl -L > vscode-zig.vsix
code --install-extension vscode-zig.vsix

Troubleshooting (general)

If you encounter error: the build command failed with exit code 9 during the build process, this means you ran out of memory or swap. Bun currently needs about 22 GB of RAM to compile.