Bun

Bun is a new:

• JavaScript/TypeScript/JSX transpiler
• JavaScript & CSS bundler
• Development server with 60fps Hot Module Reloading (& WIP support for React Fast Refresh)
• JavaScript Runtime Environment (powered by JavaScriptCore, what WebKit/Safari uses)

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.

Install:

# Global install is recommended so bun appears in your $PATH
npm install -g bun-cli

Getting started

Using Bun with Next.js

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

npm install -D bun-framework-next
bun bun --use next
bun

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://localhsot:3000/assetPrefixInhere\")
• next/image is polyfilled to a regular <img src> tag.
• proxy and anything else in next.config.js
• API, catch-all & catch-all fallback routes. Dynamic routes are supported.

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
bun

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 use Bun with create-react-app, there are two changes you will need to make in public/index.html:

1. Replace %PUBLIC_URL% with /
2. Insert <script type="module" async src="/src/index.js"> just before </body>

These changes are (sadly) necessary until Bun supports parsing & transpiling HTML.

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

bun bun ./src/index.js
bun

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 works.

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

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.

Using Tailwind with Bun

Tailwind is a popular CSS utility framework. Currently, the easiest way to use Tailwind with Bun is through Tailwind's CLI. That means running both bun and tailwind, and importing the file tailwind's CLI outputs.

Tailwind's docs talk more about Tailwind's CLI usage, but the gist is you'll want to run this:

npx tailwindcss -i ./src/tailwind.css -o ./dist/tailwind.css --watch

From there, make sure to import the dist/tailwind.css file (or what you chose as the output).

Things that don't work yet

Bun is a project with incredibly large scope, and it's early days.

Feature	In
Symlinks	Resolver
Finish Fast Refresh	JSX Transpiler
Source Maps	JavaScript
Source Maps	CSS
Private Class Fields	JS Transpiler
Import Assertions	JS Transpiler
extends in tsconfig.json	TS Transpiler
jsx* in tsconfig.json	TS Transpiler
TypeScript Decorators	TS Transpiler
@jsxPragma comments	JS Transpiler
JSX source file name	JS Transpiler
Sharing .bun files	Bun
Finish fetch	Bun.js
setTimeout	Bun.js
bun run command	Bun.js

^{JS Transpiler == JavaScript Transpiler
TS Transpiler == TypeScript Transpiler
Bun.js == Bun's JavaScriptCore integration that executes JavaScript. Similar to how Node.js & Deno embed V8.}

Limitations & intended usage

Bun is great for building websites & webapps. For libraries, consider using Rollup or esbuild instead. Bun currently doesn't minify code and Bun's dead code elimination doesn't look beyond the current file.

Today, Bun is focused on:

• Development, not production
• Compatibility with existing frameworks & tooling

Ideally, most projects can use Bun with their existing tooling while making few changes to their codebase. That means using Bun in development, and continuing to use Webpack, esbuild, or another bundler in production. Using two bundlers might sound strange at first, but after all the production-only AST transforms, minification, and special development/production-only imported files...it's not far from the status quo.

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

Configuration

Loaders

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
.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

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:

globalThis.document?.dispatchEvent(
  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().

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();

  // ...rest 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");

Becomes:

/* 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.

Frameworks

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.

Credits

• While written in Zig instead of Go, Bun's JS transpiler, CSS lexer, and node module resolver source code is based off of @evanw's esbuild project. @evanw did a fantastic job with esbuild.

License

Bun itself is MIT-licensed.

However, JavaScriptCore (and WebKit) is LGPL-2 and Bun statically links it.

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: https://github.com/jarred-sumner/webkit. 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.

To successfully run zig build, you will need to install a patched version of Zig available here: https://github.com/jarred-sumner/zig/tree/jarred/zig-sloppy.

Bun also statically links these libraries:

• libicu, which can be found here: https://github.com/unicode-org/icu/blob/main/icu4c/LICENSE
• picohttp, which is dual-licensed under the Perl License or the MIT License
• mimalloc, which is MIT licensed

For compatibiltiy reasons, these NPM packages are embedded into Bun's binary and injected if imported.

• assert (MIT license)
• browserify-zlib (MIT license)
• buffer (MIT license)
• constants-browserify (MIT license)
• crypto-browserify (MIT license)
• domain-browser (MIT license)
• events (MIT license)
• https-browserify (MIT license)
• os-browserify (MIT license)
• path-browserify (MIT license)
• process (MIT license)
• punycode (MIT license)
• querystring-es3 (MIT license)
• stream-browserify (MIT license)
• stream-http (MIT license)
• string_decoder (MIT license)
• timers-browserify (MIT license)
• tty-browserify (MIT license)
• url (MIT license)
• util (MIT license)
• vm-browserify (MIT license)

Developing Bun

Estimated: 30-90 minutes :(

Compile Zig:

git clone https://github.com/jarred-sumner/zig
cd zig
git checkout jarred/zig-sloppy-with-small-structs
cmake . -DCMAKE_PREFIX_PATH=$(brew --prefix llvm) -DZIG_STATIC_LLVM=ON -DCMAKE_BUILD_TYPE=Release && make -j 16

You'll want to make sure zig is in $PATH. The zig binary wil be in the same folder as the newly-cloned zig repo. If you use fish, you can run fish_add_path (pwd).

In bun:

git submodule update --init --recursive --progress --depth=1
make vendor
zig build headers
make jsc-bindings-mac
zig build -Drelease-fast

Note that brew install zig won't work. Bun uses a build of Zig with a couple patches.

Additionally, you'll need cmake, npm and esbuild installed globally.