Bun

Bun.build

Bun's fast native bundler is now in beta. It can be used via the bun build CLI command or the Bun.build() JavaScript API.

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './build',
});
CLI
bun build ./index.tsx --outdir ./build

It's fast. The numbers below represent performance on esbuild's three.js benchmark.

Bundling 10 copies of three.js from scratch, with sourcemaps and minification

Why bundle?

The bundler is a key piece of infrastructure in the JavaScript ecosystem. As a brief overview of why bundling is so important:

  • Reducing HTTP requests. A single package in node_modules may consist of hundreds of files, and large applications may have dozens of such dependencies. Loading each of these files with a separate HTTP request becomes untenable very quickly, so bundlers are used to convert our application source code into a smaller number of self-contained "bundles" that can be loaded with a single request.
  • Code transforms. Modern apps are commonly built with languages or tools like TypeScript, JSX, and CSS modules, all of which must be converted into plain JavaScript and CSS before they can be consumed by a browser. The bundler is the natural place to configure these transformations.
  • Framework features. Frameworks rely on bundler plugins & code transformations to implement common patterns like file-system routing, client-server code co-location (think getServerSideProps or Remix loaders), and server components.

Let's jump into the bundler API.

Note that the Bun bundler is not intended to replace tsc for typechecking or generating type declarations.

Basic example

Let's build our first bundle. You have the following two files, which implement a simple client-side rendered React app.

./index.tsx
./Component.tsx
./index.tsx
import * as ReactDOM from 'react-dom/client';
import {Component} from "./Component"

const root = ReactDOM.createRoot(document.getElementById('root')!);
root.render(<Component message="Sup!" />)
./Component.tsx
export function Component(props: {message: string}) {
  return <p>{props.message}</p>
}

Here, index.tsx is the "entrypoint" to our application. Commonly, this will be a script that performs some side effect, like starting a server or—in this case—initializing a React root. Because we're using TypeScript & JSX, we need to bundle our code before it can be sent to the browser.

To create our bundle:

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
})
CLI
bun build ./index.tsx --outdir ./out

For each file specified in entrypoints, Bun will generate a new bundle. This bundle will be written to disk in the ./out directory (as resolved from the current working directory). After running the build, the file system looks like this:

.
├── index.tsx
├── Component.tsx
└── out
    └── index.js

The contents of out/index.js will look something like this:

out/index.js
// ...
// ~20k lines of code
// including the contents of `react-dom/client` and all its dependencies
// this is where the $jsxDEV and $createRoot functions are defined


// Component.tsx
function Component(props) {
  return $jsxDEV("p", {
    children: props.message
  }, undefined, false, undefined, this);
}

// index.tsx
var rootNode = document.getElementById("root");
var root = $createRoot(rootNode);
root.render($jsxDEV(Component, {
  message: "Sup!"
}, undefined, false, undefined, this));

Tutorial: Run this file in your browser

Watch mode

Like the runtime and test runner, the bundler supports watch mode natively.

bun build ./index.tsx --outdir ./out --watch

Content types

Like the Bun runtime, the bundler supports an array of file types out of the box. The following table breaks down the bundler's set of standard "loaders". Refer to Bundler > File types for full documentation.

ExtensionsDetails
.js .jsx, .cjs .mjs .mts .cts .ts .tsxUses Bun's built-in transpiler to parse the file and transpile TypeScript/JSX syntax to vanilla JavaScript. The bundler executes a set of default transforms, including dead code elimination, tree shaking, and environment variable inlining. At the moment Bun does not attempt to down-convert syntax; if you use recently ECMAScript syntax, that will be reflected in the bundled code.

.json

JSON files are parsed and inlined into the bundle as a JavaScript object.

import pkg from "./package.json";
pkg.name; // => "my-package"

.toml

TOML files are parsed and inlined into the bundle as a JavaScript object.

import config from "./bunfig.toml";
config.logLevel; // => "debug"

.txt

The contents of the text file are read and inlined into the bundle as a string.

import contents from "./file.txt";
console.log(contents); // => "Hello, world!"
.node .wasmThese files are supported by the Bun runtime, but during bundling they are treated as assets.

Assets

If the bundler encounters an import with an unrecognized extension, it treats the imported file as an external file. The referenced file is copied as-is into outdir, and the import is resolved as a path to the file.

Input
Output
Input
// bundle entrypoint
import logo from "./logo.svg";
console.log(logo);
Output
// bundled output
var logo = "./logo-ab237dfe.svg";
console.log(logo);

The exact behavior of the file loader is also impacted by naming and publicPath.

Refer to the Bundler > Loaders page for more complete documentation on the file loader.

Plugins

The behavior described in this table can be overridden or extended with plugins. Refer to the Bundler > Loaders page for complete documentation.

API

entrypoints

Required. An array of paths corresponding to the entrypoints of our application. One bundle will be generated for each entrypoint.

JavaScript
CLI
JavaScript
const result = await Bun.build({
  entrypoints: ["./index.ts"],
});
// => { success: boolean, outputs: BuildArtifact[], logs: BuildMessage[] }
CLI
bun build --entrypoints ./index.ts
# the bundle will be printed to stdout
# <bundled code>

outdir

The directory where output files will be written.

JavaScript
CLI
JavaScript
const result = await Bun.build({
  entrypoints: ['./index.ts'],
  outdir: './out'
});
// => { success: boolean, outputs: BuildArtifact[], logs: BuildMessage[] }
CLI
bun build --entrypoints ./index.ts --outdir ./out
# a summary of bundled files will be printed to stdout

If outdir is not passed to the JavaScript API, bundled code will not be written to disk. Bundled files are returned in an array of BuildArtifact objects. These objects are Blobs with extra properties; see Outputs for complete documentation.

const result = await Bun.build({
  entrypoints: ["./index.ts"],
});

for (const res of result.outputs) {
  // Can be consumed as blobs
  await res.text();

  // Bun will set Content-Type and Etag headers
  new Response(res);

  // Can be written manually, but you should use `outdir` in this case.
  Bun.write(path.join("out", res.path), res);
}

When outdir is set, the path property on a BuildArtifact will be the absolute path to where it was written to.

target

The intended execution environment for the bundle.

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.ts'],
  outdir: './out',
  target: 'browser', // default
})
CLI
bun build --entrypoints ./index.ts --outdir ./out --target browser

Depending on the target, Bun will apply different module resolution rules and optimizations.

browserDefault. For generating bundles that are intended for execution by a browser. Prioritizes the "browser" export condition when resolving imports. Importing any built-in modules, like node:events or node:path will work, but calling some functions, like fs.readFile will not work.

bun

For generating bundles that are intended to be run by the Bun runtime. In many cases, it isn't necessary to bundle server-side code; you can directly execute the source code without modification. However, bundling your server code can reduce startup times and improve running performance.

All bundles generated with target: "bun" are marked with a special // @bun pragma, which indicates to the Bun runtime that there's no need to re-transpile the file before execution.

If any entrypoints contains a Bun shebang (#!/usr/bin/env bun) the bundler will default to target: "bun" instead of "browser".

nodeFor generating bundles that are intended to be run by Node.js. Prioritizes the "node" export condition when resolving imports, and outputs .mjs. In the future, this will automatically polyfill the Bun global and other built-in bun:* modules, though this is not yet implemented.

format

Specifies the module format to be used in the generated bundles.

Currently the bundler only supports one module format: "esm". Support for "cjs" and "iife" are planned.

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  format: "esm",
})
CLI
bun build ./index.tsx --outdir ./out --format esm

splitting

Whether to enable code splitting.

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  splitting: false, // default
})
CLI
bun build ./index.tsx --outdir ./out --splitting

When true, the bundler will enable code splitting. When multiple entrypoints both import the same file, module, or set of files/modules, it's often useful to split the shared code into a separate bundle. This shared bundle is known as a chunk. Consider the following files:

entry-a.ts
entry-b.ts
shared.ts
entry-a.ts
import { shared } from './shared.ts';
entry-b.ts
import { shared } from './shared.ts';
shared.ts
export const shared = 'shared';

To bundle entry-a.ts and entry-b.ts with code-splitting enabled:

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./entry-a.ts', './entry-b.ts'],
  outdir: './out',
  splitting: true,
})
CLI
bun build ./entry-a.ts ./entry-b.ts --outdir ./out --splitting

Running this build will result in the following files:

.
├── entry-a.tsx
├── entry-b.tsx
├── shared.tsx
└── out
    ├── entry-a.js
    ├── entry-b.js
    └── chunk-2fce6291bf86559d.js

The generated chunk-2fce6291bf86559d.js file contains the shared code. To avoid collisions, the file name automatically includes a content hash by default. This can be customized with naming.

plugins

A list of plugins to use during bundling.

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  plugins: [/* ... */],
})
CLI
n/a

Bun implements a universal plugin system for both Bun's runtime and bundler. Refer to the plugin documentation for complete documentation.

sourcemap

Specifies the type of sourcemap to generate.

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  sourcemap: "external", // default "none"
})
CLI
bun build ./index.tsx --outdir ./out --sourcemap=external
"none"Default. No sourcemap is generated.

"inline"

A sourcemap is generated and appended to the end of the generated bundle as a base64 payload.

// <bundled code here>

//# sourceMappingURL=data:application/json;base64,<encoded sourcemap here>
"external"A separate *.js.map file is created alongside each *.js bundle.

Generated bundles contain a debug id that can be used to associate a bundle with its corresponding sourcemap. This debugId is added as a comment at the bottom of the file.

// <generated bundle code>

//# debugId=<DEBUG ID>

The associated *.js.map sourcemap will be a JSON file containing an equivalent debugId property.

minify

Whether to enable minification. Default false.

When targeting bun, identifiers will be minified by default.

To enable all minification options:

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  minify: true, // default false
})
CLI
bun build ./index.tsx --outdir ./out --minify

To granularly enable certain minifications:

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  minify: {
    whitespace: true,
    identifiers: true,
    syntax: true,
  },
})
CLI
bun build ./index.tsx --outdir ./out --minify-whitespace --minify-identifiers --minify-syntax

external

A list of import paths to consider external. Defaults to [].

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  external: ["lodash", "react"], // default: []
})
CLI
bun build ./index.tsx --outdir ./out --external lodash --external react

An external import is one that will not be included in the final bundle. Instead, the import statement will be left as-is, to be resolved at runtime.

For instance, consider the following entrypoint file:

index.tsx
import _ from "lodash";
import {z} from "zod";

const value = z.string().parse("Hello world!")
console.log(_.upperCase(value));

Normally, bundling index.tsx would generate a bundle containing the entire source code of the "zod" package. If instead, we want to leave the import statement as-is, we can mark it as external:

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  external: ['zod'],
})
CLI
bun build ./index.tsx --outdir ./out --external zod

The generated bundle will look something like this:

out/index.js
import {z} from "zod";

// ...
// the contents of the "lodash" package
// including the `_.upperCase` function

var value = z.string().parse("Hello world!")
console.log(_.upperCase(value));

To mark all imports as external, use the wildcard *:

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  external: ['*'],
})
CLI
bun build ./index.tsx --outdir ./out --external '*'

naming

Customizes the generated file names. Defaults to ./[dir]/[name].[ext].

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  naming: "[dir]/[name].[ext]", // default
})
CLI
bun build ./index.tsx --outdir ./out --entry-naming [dir]/[name].[ext]

By default, the names of the generated bundles are based on the name of the associated entrypoint.

.
├── index.tsx
└── out
    └── index.js

With multiple entrypoints, the generated file hierarchy will reflect the directory structure of the entrypoints.

.
├── index.tsx
└── nested
    └── index.tsx
└── out
    ├── index.js
    └── nested
        └── index.js

The names and locations of the generated files can be customized with the naming field. This field accepts a template string that is used to generate the filenames for all bundles corresponding to entrypoints. where the following tokens are replaced with their corresponding values:

  • [name] - The name of the entrypoint file, without the extension.
  • [ext] - The extension of the generated bundle.
  • [hash] - A hash of the bundle contents.
  • [dir] - The relative path from the build root to the parent directory of the file.

For example:

Token[name][ext][hash][dir]
./index.tsxindexjsa1b2c3d4"" (empty string)
./nested/entry.tsentryjsc3d4e5f6"nested"

We can combine these tokens to create a template string. For instance, to include the hash in the generated bundle names:

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  naming: 'files/[dir]/[name]-[hash].[ext]',
})
CLI
bun build ./index.tsx --outdir ./out --entry-naming [name]-[hash].[ext]

This build would result in the following file structure:

.
├── index.tsx
└── out
    └── files
        └── index-a1b2c3d4.js

When a string is provided for the naming field, it is used only for bundles that correspond to entrypoints. The names of chunks and copied assets are not affected. Using the JavaScript API, separate template strings can be specified for each type of generated file.

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  naming: {
    // default values
    entry: '[dir]/[name].[ext]',
    chunk: '[name]-[hash].[ext]',
    asset: '[name]-[hash].[ext]',
  },
})
CLI
bun build ./index.tsx --outdir ./out --entry-naming "[dir]/[name].[ext]" --chunk-naming "[name]-[hash].[ext]" --asset-naming "[name]-[hash].[ext]"

root

The root directory of the project.

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./pages/a.tsx', './pages/b.tsx'],
  outdir: './out',
  root: '.',
})
CLI
n/a

If unspecified, it is computed to be the first common ancestor of all entrypoint files. Consider the following file structure:

.
└── pages
  └── index.tsx
  └── settings.tsx

We can build both entrypoints in the pages directory:

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./pages/index.tsx', './pages/settings.tsx'],
  outdir: './out',
})
CLI
bun build ./pages/index.tsx ./pages/settings.tsx --outdir ./out

This would result in a file structure like this:

.
└── pages
  └── index.tsx
  └── settings.tsx
└── out
  └── index.js
  └── settings.js

Since the pages directory is the first common ancestor of the entrypoint files, it is considered the project root. This means that the generated bundles live at the top level of the out directory; there is no out/pages directory.

This behavior can be overridden by specifying the root option:

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./pages/index.tsx', './pages/settings.tsx'],
  outdir: './out',
  root: '.',
})
CLI
bun build ./pages/index.tsx ./pages/settings.tsx --outdir ./out --root .

By specifying . as root, the generated file structure will look like this:

.
└── pages
  └── index.tsx
  └── settings.tsx
└── out
  └── pages
    └── index.js
    └── settings.js

publicPath

A prefix to be appended to any import paths in bundled code.

In many cases, generated bundles will contain no import statements. After all, the goal of bundling is to combine all of the code into a single file. However there are a number of cases with the generated bundles will contain import statements.

  • Asset imports — When importing an unrecognized file type like *.svg, the bundler defers to the file loader, which copies the file into outdir as is. The import is converted into a variable
  • External modules — Files and modules can be marked as external, in which case they will not be included in the bundle. Instead, the import statement will be left in the final bundle.
  • Chunking. When splitting is enabled, the bundler may generate separate "chunk" files that represent code that is shared among multiple entrypoints.

In any of these cases, the final bundles may contain paths to other files. By default these imports are relative. Here is an example of a simple asset import:

Input
Output
Input
import logo from './logo.svg';
console.log(logo);
Output
// logo.svg is copied into <outdir>
// and hash is added to the filename to prevent collisions
var logo = './logo-a7305bdef.svg';
console.log(logo);

Setting publicPath will prefix all file paths with the specified value.

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  publicPath: 'https://cdn.example.com/', // default is undefined
})
CLI
bun build ./index.tsx --outdir ./out --public-path https://cdn.example.com/

The output file would now look something like this.

Output
var logo = './logo-a7305bdef.svg';
var logo = 'https://cdn.example.com/logo-a7305bdef.svg';

define

A map of global identifiers to be replaced at build time. Keys of this object are identifier names, and values are JSON strings that will be inlined.

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  define: {
    STRING: JSON.stringify("value"),
    "nested.boolean": "true",
  },
})
CLI
bun build ./index.tsx --outdir ./out --define 'STRING="value"' --define "nested.boolean=true"

loader

A map of file extensions to built-in loader names. This can be used to quickly customize how certain files are loaded.

JavaScript
CLI
JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  loader: {
    ".png": "dataurl",
    ".txt": "file",
  },
})
CLI
bun build ./index.tsx --outdir ./out --loader .png:dataurl --loader .txt:file

Outputs

The Bun.build function returns a Promise<BuildOutput>, defined as:

interface BuildOutput {
  outputs: BuildArtifact[];
  success: boolean;
  logs: Array<object>; // see docs for details
}

interface BuildArtifact extends Blob {
  kind: "entry-point" | "chunk" | "asset" | "sourcemap";
  path: string;
  loader: Loader;
  hash: string | null;
  sourcemap: BuildArtifact | null;
}

The outputs array contains all the files that were generated by the build. Each artifact implements the Blob interface.

const build = await Bun.build({
  /* */
});

for (const output of build.outputs) {
  await output.arrayBuffer(); // => ArrayBuffer
  await output.text(); // string
}

Each artifact also contains the following properties:

kindWhat kind of build output this file is. A build generates bundled entrypoints, code-split "chunks", sourcemaps, and copied assets (like images).
pathAbsolute path to the file on disk
loaderThe loader was used to interpret the file. See Bundler > Loaders to see how Bun maps file extensions to the appropriate built-in loader.
hashThe hash of the file contents. Always defined for assets.
sourcemapThe sourcemap file corresponding to this file, if generated. Only defined for entrypoints and chunks.

Similar to BunFile, BuildArtifact objects can be passed directly into new Response().

const build = await Bun.build({
  /* */
});

const artifact = build.outputs[0];

// Content-Type header is automatically set
return new Response(artifact);

The Bun runtime implements special pretty-printing of BuildArtifact object to make debugging easier.

Build script
Shell output
Build script
// build.ts
const build = await Bun.build({/* */});

const artifact = build.outputs[0];
console.log(artifact);
Shell output
bun run build.ts
BuildArtifact (entry-point) {
  path: "./index.js",
  loader: "tsx",
  kind: "entry-point",
  hash: "824a039620219640",
  Blob (114 bytes) {
    type: "text/javascript;charset=utf-8"
  },
  sourcemap: null
}

Executables

Bun supports "compiling" a JavaScript/TypeScript entrypoint into a standalone executable. This executable contains a copy of the Bun binary.

bun build ./cli.tsx --outfile mycli --compile
./mycli

Refer to Bundler > Executables for complete documentation.

Logs and errors

Bun.build only throws if invalid options are provided. Read the success property to determine if the build was successful; the logs property will contain additional details.

const result = await Bun.build({
  entrypoints: ["./index.tsx"],
  outdir: "./out",
});

if (!result.success) {
  console.error("Build failed");
  for (const message of result.logs) {
    // Bun will pretty print the message object
    console.error(message);
  }
}

Each message is either a BuildMessage or ResolveMessage object, which can be used to trace what problems happened in the build.

class BuildMessage {
  name: string;
  position?: Position;
  message: string;
  level: "error" | "warning" | "info" | "debug" | "verbose";
}

class ResolveMessage extends BuildMessage {
  code: string;
  referrer: string;
  specifier: string;
  importKind: ImportKind;
}

If you want to throw an error from a failed build, consider passing the logs to an AggregateError. If uncaught, Bun will pretty-print the contained messages nicely.

if (!result.success) {
  throw new AggregateError(result.logs, "Build failed");
}

Reference

interface Bun {
  build(options: BuildOptions): Promise<BuildOutput>;
}

interface BuildOptions {
  entrypoints: string[]; // required
  outdir?: string; // default: no write (in-memory only)
  format?: "esm"; // later: "cjs" | "iife"
  target?: "browser" | "bun" | "node"; // "browser"
  splitting?: boolean; // true
  plugins?: BunPlugin[]; // [] // See https://bun.sh/docs/bundler/plugins
  loader?: { [k in string]: Loader }; // See https://bun.sh/docs/bundler/loaders
  manifest?: boolean; // false
  external?: string[]; // []
  sourcemap?: "none" | "inline" | "external"; // "none"
  root?: string; // computed from entrypoints
  naming?:
    | string
    | {
        entry?: string; // '[dir]/[name].[ext]'
        chunk?: string; // '[name]-[hash].[ext]'
        asset?: string; // '[name]-[hash].[ext]'
      };
  publicPath?: string; // e.g. http://mydomain.com/
  minify?:
    | boolean // false
    | {
        identifiers?: boolean;
        whitespace?: boolean;
        syntax?: boolean;
      };
}

interface BuildOutput {
  outputs: BuildArtifact[];
  success: boolean;
  logs: Array<BuildMessage | ResolveMessage>;
}

interface BuildArtifact extends Blob {
  path: string;
  loader: Loader;
  hash?: string;
  kind: "entry-point" | "chunk" | "asset" | "sourcemap";
  sourcemap?: BuildArtifact;
}

type Loader =
  | "js"
  | "jsx"
  | "ts"
  | "tsx"
  | "json"
  | "toml"
  | "file"
  | "napi"
  | "wasm"
  | "text";

interface BuildOutput {
  outputs: BuildArtifact[];
  success: boolean;
  logs: Array<BuildMessage | ResolveMessage>;
}

declare class ResolveMessage {
  readonly name: "ResolveMessage";
  readonly position: Position | null;
  readonly code: string;
  readonly message: string;
  readonly referrer: string;
  readonly specifier: string;
  readonly importKind:
    | "entry_point"
    | "stmt"
    | "require"
    | "import"
    | "dynamic"
    | "require_resolve"
    | "at"
    | "at_conditional"
    | "url"
    | "internal";
  readonly level: "error" | "warning" | "info" | "debug" | "verbose";

  toString(): string;
}