Bun.build – Bundler

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

We can load this file in the browser to see our app in action. Create an index.html file in the out directory:

touch out/index.html

Then paste the following contents into it:

<html>
  <body>
    <div id="root"></div>
    <script type="module" src="/index.js"></script>
  </body>
</html>

Then spin up a static file server serving the out directory:

bunx serve out

Visit http://localhost:5000 to see your bundled app in action.

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.

Extensions	Details
`.js` `.jsx`, `.cjs` `.mjs` `.mts` `.cts` `.ts` `.tsx`	Uses 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 and tree shaking. 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` `.wasm`	These 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.

browser Default. 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.

`browser`	Default. 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"`. When using `target: "bun"` and `format: "cjs"` together, the `// @bun @bun-cjs` pragma is added and the CommonJS wrapper function is not compatible with Node.js.
`node`	For 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.

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".

When using target: "bun" and format: "cjs" together, the // @bun @bun-cjs pragma is added and the CommonJS wrapper function is not compatible with Node.js.

node For 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.

Bun defaults to "esm", and provides experimental support for "cjs" and "iife".

`format: "esm"` - ES Module

This is the default format, which supports ES Module syntax including top-level await, import.meta, and more.

JavaScript

CLI

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

CLI
bun build ./index.tsx --outdir ./out --format esm

To use ES Module syntax in browsers, set format to "esm" and make sure your <script type="module"> tag has type="module" set.

`format: "cjs"` - CommonJS

To build a CommonJS module, set format to "cjs". When choosing "cjs", the default target changes from "browser" (esm) to "node" (cjs). CommonJS modules transpiled with format: "cjs", target: "node" can be executed in both Bun and Node.js (assuming the APIs in use are supported by both).

JavaScript

CLI

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

CLI
bun build ./index.tsx --outdir ./out --format cjs

`format: "iife"` - IIFE

TODO: document IIFE once we support globalNames.

`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.

`env`

Controls how environment variables are handled during bundling. Internally, this uses define to inject environment variables into the bundle, but makes it easier to specify the environment variables to inject.

`env: "inline"`

Injects environment variables into the bundled output by converting process.env.FOO references to string literals containing the actual environment variable values.

JavaScript

CLI

JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  env: "inline",
})

CLI
FOO=bar BAZ=123 bun build ./index.tsx --outdir ./out --env inline

For the input below:

input.js
console.log(process.env.FOO);
console.log(process.env.BAZ);

The generated bundle will contain the following code:

output.js
console.log("bar");
console.log("123");

`env: "PUBLIC_*"` (prefix)

Inlines environment variables matching the given prefix (the part before the * character), replacing process.env.FOO with the actual environment variable value. This is useful for selectively inlining environment variables for things like public-facing URLs or client-side tokens, without worrying about injecting private credentials into output bundles.

JavaScript

CLI

JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',

  // Inline all env vars that start with "ACME_PUBLIC_"
  env: "ACME_PUBLIC_*",
})

CLI
FOO=bar BAZ=123 ACME_PUBLIC_URL=https://acme.com bun build ./index.tsx --outdir ./out --env 'ACME_PUBLIC_*'

For example, given the following environment variables:

FOO=bar BAZ=123 ACME_PUBLIC_URL=https://acme.com

And source code:

index.tsx
console.log(process.env.FOO);
console.log(process.env.ACME_PUBLIC_URL);
console.log(process.env.BAZ);

The generated bundle will contain the following code:

console.log(process.env.FOO);
console.log("https://acme.com");
console.log(process.env.BAZ);

`env: "disable"`

Disables environment variable injection entirely.

For example, given the following environment variables:

FOO=bar BAZ=123 ACME_PUBLIC_URL=https://acme.com

And source code:

index.tsx
console.log(process.env.FOO);
console.log(process.env.ACME_PUBLIC_URL);
console.log(process.env.BAZ);

The generated bundle will contain the following code:

console.log(process.env.FOO);
console.log(process.env.BAZ);

`sourcemap`

Specifies the type of sourcemap to generate.

JavaScript

CLI

JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  sourcemap: 'linked', // default 'none'
})

CLI
bun build ./index.tsx --outdir ./out --sourcemap=linked

`"none"`	Default. No sourcemap is generated.
`"linked"`	A separate `.js.map` file is created alongside each `.js` bundle using a `//# sourceMappingURL` comment to link the two. Requires `--outdir` to be set. The base URL of this can be customized with `--public-path`. `// <bundled code here> //# sourceMappingURL=bundle.js.map`
`"external"`	A separate `.js.map` file is created alongside each `.js` bundle without inserting a `//# sourceMappingURL` comment.

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>

"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>
```
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 '*'

`packages`

Control whatever package dependencies are included to bundle or not. Possible values: bundle (default), external. Bun treats any import which path do not start with ., .. or / as package.

JavaScript

CLI

JavaScript
await Bun.build({
  entrypoints: ['./index.ts'],
  packages: 'external',
})

CLI
bun build ./index.ts --packages 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 project root to the parent directory of the source file.

For example:

Token	`[name]`	`[ext]`	`[hash]`	`[dir]`
`./index.tsx`	`index`	`js`	`a1b2c3d4`	`""` (empty string)
`./nested/entry.ts`	`entry`	`js`	`c3d4e5f6`	`"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

A banner to be added to the final bundle, this can be a directive like "use client" for react or a comment block such as a license for the code.

JavaScript

CLI

JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  banner: '"use client";'
})

CLI
bun build ./index.tsx --outdir ./out --banner "\"use client\";"

A footer to be added to the final bundle, this can be something like a comment block for a license or just a fun easter egg.

JavaScript

CLI

JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  footer: '// built with love in SF'
})

CLI
bun build ./index.tsx --outdir ./out --footer="// built with love in SF"

`drop`

Remove function calls from a bundle. For example, --drop=console will remove all calls to console.log. Arguments to calls will also be removed, regardless of if those arguments may have side effects. Dropping debugger will remove all debugger statements.

JavaScript

CLI

JavaScript
await Bun.build({
  entrypoints: ['./index.tsx'],
  outdir: './out',
  drop: ["console", "debugger", "anyIdentifier.or.propertyAccess"],
})

CLI
bun build ./index.tsx --outdir ./out --drop=console --drop=debugger --drop=anyIdentifier.or.propertyAccess

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.bytes(); // => Uint8Array
  await output.text(); // string
}

Each artifact also contains the following properties:

`kind`	What kind of build output this file is. A build generates bundled entrypoints, code-split "chunks", sourcemaps, bytecode, and copied assets (like images).
`path`	Absolute path to the file on disk
`loader`	The loader was used to interpret the file. See Bundler > Loaders to see how Bun maps file extensions to the appropriate built-in loader.
`hash`	The hash of the file contents. Always defined for assets.
`sourcemap`	The 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
}

Bytecode

The bytecode: boolean option can be used to generate bytecode for any JavaScript/TypeScript entrypoints. This can greatly improve startup times for large applications. Only supported for "cjs" format, only supports "target": "bun" and dependent on a matching version of Bun. This adds a corresponding .jsc file for each entrypoint.

JavaScript

CLI

JavaScript
await Bun.build({
  entrypoints: ["./index.tsx"],
  outdir: "./out",
  bytecode: true,
})

CLI
bun build ./index.tsx --outdir ./out --bytecode

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

On failure, Bun.build returns a rejected promise with an AggregateError. This can be logged to the console for pretty printing of the error list, or programmatically read with a try/catch block.

try {
  const result = await Bun.build({
    entrypoints: ["./index.tsx"],
    outdir: "./out",
  });
} catch (e) {
  // TypeScript does not allow annotations on the catch clause
  const error = e as AggregateError;
  console.error("Build Failed");

  // Example: Using the built-in formatter
  console.error(error);

  // Example: Serializing the failure as a JSON string.
  console.error(JSON.stringify(error, null, 2));
}

Most of the time, an explicit try/catch is not needed, as Bun will neatly print uncaught exceptions. It is enough to just use a top-level await on the Bun.build call.

Each item in error.errors is an instance of BuildMessage or ResolveMessage (subclasses of Error), containing detailed information for each error.

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;
}

On build success, the returned object contains a logs property, which contains bundler warnings and info messages.

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

if (result.logs.length > 0) {
  console.warn("Build succeeded with warnings:");
  for (const message of result.logs) {
    // Bun will pretty print the message object
    console.warn(message);
  }
}

Reference

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

interface BuildConfig {
  entrypoints: string[]; // list of file path
  outdir?: string; // output directory
  target?: Target; // default: "browser"
  /**
   * Output module format. Top-level await is only supported for `"esm"`.
   *
   * Can be:
   * - `"esm"`
   * - `"cjs"` (**experimental**)
   * - `"iife"` (**experimental**)
   *
   * @default "esm"
   */
  format?: "esm" | "cjs" | "iife";
  naming?:
    | string
    | {
        chunk?: string;
        entry?: string;
        asset?: string;
      };
  root?: string; // project root
  splitting?: boolean; // default true, enable code splitting
  plugins?: BunPlugin[];
  external?: string[];
  packages?: "bundle" | "external";
  publicPath?: string;
  define?: Record<string, string>;
  loader?: { [k in string]: Loader };
  sourcemap?: "none" | "linked" | "inline" | "external" | "linked" | boolean; // default: "none", true -> "inline"
  /**
   * package.json `exports` conditions used when resolving imports
   *
   * Equivalent to `--conditions` in `bun build` or `bun run`.
   *
   * https://nodejs.org/api/packages.html#exports
   */
  conditions?: Array<string> | string;

  /**
   * Controls how environment variables are handled during bundling.
   *
   * Can be one of:
   * - `"inline"`: Injects environment variables into the bundled output by converting `process.env.FOO`
   *   references to string literals containing the actual environment variable values
   * - `"disable"`: Disables environment variable injection entirely
   * - A string ending in `*`: Inlines environment variables that match the given prefix.
   *   For example, `"MY_PUBLIC_*"` will only include env vars starting with "MY_PUBLIC_"
   */
  env?: "inline" | "disable" | `${string}*`;
  minify?:
    | boolean
    | {
        whitespace?: boolean;
        syntax?: boolean;
        identifiers?: boolean;
      };
  /**
   * Ignore dead code elimination/tree-shaking annotations such as @__PURE__ and package.json
   * "sideEffects" fields. This should only be used as a temporary workaround for incorrect
   * annotations in libraries.
   */
  ignoreDCEAnnotations?: boolean;
  /**
   * Force emitting @__PURE__ annotations even if minify.whitespace is true.
   */
  emitDCEAnnotations?: boolean;

  /**
   * Generate bytecode for the output. This can dramatically improve cold
   * start times, but will make the final output larger and slightly increase
   * memory usage.
   *
   * Bytecode is currently only supported for CommonJS (`format: "cjs"`).
   *
   * Must be `target: "bun"`
   * @default false
   */
  bytecode?: boolean;
  /**
   * Add a banner to the bundled code such as "use client";
   */
  banner?: string;
  /**
   * Add a footer to the bundled code such as a comment block like
   *
   * `// made with bun!`
   */
  footer?: string;

  /**
   * Drop function calls to matching property accesses.
   */
  drop?: string[];

  /**
   * When set to `true`, the returned promise rejects with an AggregateError when a build failure happens.
   * When set to `false`, the `success` property of the returned object will be `false` when a build failure happens.
   *
   * This defaults to `false` in Bun 1.1 and will change to `true` in Bun 1.2
   * as most usage of `Bun.build` forgets to check for errors.
   */
  throw?: boolean;
}

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

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

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;
}

CLI Usage

$bun build <entrypoints...>

Flags

Output Configuration

--outdir=<val>

Default to "dist" if multiple files

--outfile=<val>

Write to a file

--sourcemap=<val>

Build with sourcemaps - 'linked', 'inline', 'external', or 'none'

--banner=<val>

Add a banner to the bundled output such as "use client"; for a bundle being used with RSCs

--footer=<val>

Add a footer to the bundled output such as // built with bun!

--format=<val>

Specifies the module format to build to. Only "esm" is supported.

--root=<val>

Root directory used for multiple entry points

--public-path=<val>

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

Bundling Behavior

--compile

Generate a standalone Bun executable containing your bundled code

--bytecode

Use a bytecode cache

--watch

Automatically restart the process on file change

--no-clear-screen

Disable clearing the terminal screen on reload when --watch is enabled

--splitting

Enable code splitting

--no-bundle

Transpile file only, do not bundle

Target Environment

--target=<val>

The intended execution environment for the bundle. "browser", "bun" or "node"

--conditions=<val>

Pass custom conditions to resolve

External Dependencies

-e,--external=<val>

Exclude module from transpilation (can use * wildcards). ex: -e react

--packages=<val>

Add dependencies to bundle or keep them external. "external", "bundle" is supported. Defaults to "bundle".

File Naming

--entry-naming=<val>

Customize entry point filenames. Defaults to "[dir]/[name].[ext]"

--chunk-naming=<val>

Customize chunk filenames. Defaults to "[name]-[hash].[ext]"

--asset-naming=<val>

Customize asset filenames. Defaults to "[name]-[hash].[ext]"

Minification

--minify

Enable all minification flags

--minify-syntax

Minify syntax and inline data

--minify-whitespace

Minify whitespace

--minify-identifiers

Minify identifiers

--emit-dce-annotations

Re-emit DCE annotations in bundles. Enabled by default unless --minify-whitespace is passed.

CSS Handling

--css-chunking

Chunk CSS files together to reduce duplicated CSS loaded in a browser. Only has an effect when multiple entrypoints import CSS

Experimental Features

--react-fast-refresh

Enable React Fast Refresh transform (does not emit hot-module code, use this for testing)

--app

(EXPERIMENTAL) Build a web app for production using Bun Bake.

--server-components

(EXPERIMENTAL) Enable server components

Environment Variables

--env=<val>

Inline environment variables into the bundle as process.env.${name}. Defaults to 'disable'. To inline environment variables matching a prefix, use my prefix like 'FOO_PUBLIC_*'.

Windows-specific

--windows-hide-console

When using --compile targeting Windows, prevent a Command prompt from opening alongside the executable

--windows-icon=<val>

When using --compile targeting Windows, assign an executable icon

Examples

Frontend web apps:

bun build --outfile=bundle.js ./src/index.ts

bun build --minify --splitting --outdir=out ./index.jsx ./lib/worker.ts

Bundle code to be run in Bun (reduces server startup time)

bun build --target=bun --outfile=server.js ./server.ts

Creating a standalone executable (see https://bun.sh/docs/bundler/executables)

bun build --compile --outfile=my-app ./cli.ts

A full list of flags is available at https://bun.sh/docs/bundler

format: "esm" - ES Module

format: "cjs" - CommonJS

format: "iife" - IIFE

env: "inline"

env: "PUBLIC_*" (prefix)

env: "disable"

CLI Usage

Flags

Output Configuration

Bundling Behavior

Target Environment

External Dependencies

File Naming

Minification

CSS Handling

Experimental Features

Environment Variables

Windows-specific

Examples

`format: "esm"` - ES Module

`format: "cjs"` - CommonJS

`format: "iife"` - IIFE

`env: "inline"`

`env: "PUBLIC_*"` (prefix)

`env: "disable"`