Bundler

Bun's fast native bundler for JavaScript, TypeScript, JSX, and more

export const name_0 = undefined

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

At a Glance

JS API: await Bun.build({ entrypoints, outdir })
CLI: bun build <entry> --outdir ./out
Watch: --watch for incremental rebuilds
Targets: --target browser|bun|node
Formats: --format esm|cjs|iife (experimental for cjs/iife)

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

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.
Full-stack Applications. Bun's bundler can handle both server and client code in a single command, enabling optimized production builds and single-file executables. With build-time HTML imports, you can bundle your entire application — frontend assets and backend server — into a single deployable unit.

Let's jump into the bundler API.

<Note>The Bun bundler is not intended to replace tsc for typechecking or generating type declarations.</Note>

Basic example

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

<CodeGroup> ```tsx index.tsx icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} import * as ReactDOM from "react-dom/client"; import { Component } from "./Component";

const root = ReactDOM.createRoot(document.getElementById("root")!); root.render(<Component message="Sup!" />);


```tsx Component.tsx icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}}
export function Component(props: { message: string }) {
  return <h1>{props.message}</h1>;
}

</CodeGroup>

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:

<CodeGroup> ```ts build.ts icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ["./index.tsx"], outdir: "./out", }); ```

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

</CodeGroup>

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,
  ),
);

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. js import pkg from "./package.json"; pkg.name; // => "my-package"  | | .jsonc | JSON with comments. Files are parsed and inlined into the bundle as a JavaScript object. js import config from "./config.jsonc"; config.name; // => "my-config"  | | .toml | TOML files are parsed and inlined into the bundle as a JavaScript object. js import config from "./bunfig.toml"; config.logLevel; // => "debug"  | | .yaml .yml | YAML files are parsed and inlined into the bundle as a JavaScript object. js import config from "./config.yaml"; config.name; // => "my-app"  | | .txt | The contents of the text file are read and inlined into the bundle as a string. js import contents from "./file.txt"; console.log(contents); // => "Hello, world!"  | | .html | HTML files are processed and any referenced assets (scripts, stylesheets, images) are bundled. | | .css | CSS files are bundled together into a single .css file in the output directory. | | .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.

<CodeGroup> ```ts Input icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} // bundle entrypoint import logo from "./logo.svg"; console.log(logo); ```

// bundled output
var logo = "./logo-a7305bdef.svg";
console.log(logo);

</CodeGroup>

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

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

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

<Badge>Required</Badge>

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} const result = await Bun.build({ entrypoints: ["./index.ts"], }); // => { success: boolean, outputs: BuildArtifact[], logs: BuildMessage[] } ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.ts ``` </Tab> </Tabs>

files

A map of file paths to their contents for in-memory bundling. This allows you to bundle virtual files that don't exist on disk, or override the contents of files that do exist. This option is only available in the JavaScript API.

File contents can be provided as a string, Blob, TypedArray, or ArrayBuffer.

Bundle entirely from memory

You can bundle code without any files on disk by providing all sources via files:

const result = await Bun.build({
  entrypoints: ["/app/index.ts"],
  files: {
    "/app/index.ts": `
      import { greet } from "./greet.ts";
      console.log(greet("World"));
    `,
    "/app/greet.ts": `
      export function greet(name: string) {
        return "Hello, " + name + "!";
      }
    `,
  },
});

const output = await result.outputs[0].text();
console.log(output);

When all entrypoints are in the files map, the current working directory is used as the root.

Override files on disk

In-memory files take priority over files on disk. This lets you override specific files while keeping the rest of your codebase unchanged:

// Assume ./src/config.ts exists on disk with development settings
await Bun.build({
  entrypoints: ["./src/index.ts"],
  files: {
    // Override config.ts with production values
    "./src/config.ts": `
      export const API_URL = "https://api.production.com";
      export const DEBUG = false;
    `,
  },
  outdir: "./dist",
});

Mix disk and virtual files

Real files on disk can import virtual files, and virtual files can import real files:

// ./src/index.ts exists on disk and imports "./generated.ts"
await Bun.build({
  entrypoints: ["./src/index.ts"],
  files: {
    // Provide a virtual file that index.ts imports
    "./src/generated.ts": `
      export const BUILD_ID = "${crypto.randomUUID()}";
      export const BUILD_TIME = ${Date.now()};
    `,
  },
  outdir: "./dist",
});

This is useful for code generation, injecting build-time constants, or testing with mock modules.

outdir

The directory where output files will be written.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} const result = await Bun.build({ entrypoints: ['./index.ts'], outdir: './out' }); // => { success: boolean, outputs: BuildArtifact[], logs: BuildMessage[] } ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.ts --outdir ./out ``` </Tab> </Tabs>

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.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.ts'], outdir: './out', target: 'browser', // default }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.ts --outdir ./out --target browser ``` </Tab> </Tabs>

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

<Card title="browser" icon="globe"> **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. </Card> <Card title="bun" icon="server"> 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. This is the target to use for building full-stack applications with build-time HTML imports, where both server and client code are bundled together.

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. </Card>

<Card title="node" icon="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. </Card>

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.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', format: "esm", }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --format esm ``` </Tab> </Tabs>

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).

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', format: "cjs", }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --format cjs ``` </Tab> </Tabs>

format: "iife" - IIFE

TODO: document IIFE once we support globalNames.

`jsx`

Configure JSX transform behavior. Allows fine-grained control over how JSX is compiled.

Classic runtime example (uses factory and fragment):

<CodeGroup> ```ts index.ts icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ["./app.tsx"], outdir: "./out", jsx: { factory: "h", fragment: "Fragment", runtime: "classic", }, }); ```

# JSX configuration is handled via bunfig.toml or tsconfig.json
bun build ./app.tsx --outdir ./out

</CodeGroup>

Automatic runtime example (uses importSource):

<CodeGroup> ```ts index.ts icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ["./app.tsx"], outdir: "./out", jsx: { importSource: "preact", runtime: "automatic", }, }); ```

# JSX configuration is handled via bunfig.toml or tsconfig.json
bun build ./app.tsx --outdir ./out

</CodeGroup>

splitting

Whether to enable code splitting.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', splitting: false, // default }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --splitting ``` </Tab> </Tabs>

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:

<CodeGroup> ```ts entry-a.ts icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} import { shared } from "./shared.ts"; ```

import { shared } from "./shared.ts";

export const shared = "shared";

</CodeGroup>

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./entry-a.ts', './entry-b.ts'], outdir: './out', splitting: true, }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./entry-a.ts ./entry-b.ts --outdir ./out --splitting ``` </Tab> </Tabs>

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.

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

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.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', env: "inline", }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --env inline ``` </Tab> </Tabs>

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.

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

</Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --env ACME_PUBLIC_* ``` </Tab> </Tabs>

For example, given the following environment variables:

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

And source code:

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.

sourcemap

Specifies the type of sourcemap to generate.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', sourcemap: 'linked', // default 'none' }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --sourcemap linked ``` </Tab> </Tabs>

| Value | Description | | ------------ | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | "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. js // <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. js // <generated bundle code> //# debugId=<DEBUG ID>  | | "inline" | A sourcemap is generated and appended to the end of the generated bundle as a base64 payload. js // <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.

<Note>When targeting bun, identifiers will be minified by default.</Note>

To enable all minification options:

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', minify: true, // default false }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --minify ``` </Tab> </Tabs>

To granularly enable certain minifications:

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', minify: { whitespace: true, identifiers: true, syntax: true, }, }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --minify-whitespace --minify-identifiers --minify-syntax ``` </Tab> </Tabs>

external

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', external: ["lodash", "react"], // default: [] }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --external lodash --external react ``` </Tab> </Tabs>

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:

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:

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', external: ['zod'], }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --external zod ``` </Tab> </Tabs>

The generated bundle will look something like this:

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 *:

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', external: ['*'], }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --external '*' ``` </Tab> </Tabs>

packages

Control whether 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.

naming

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', naming: "[dir]/[name].[ext]", // default }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --entry-naming "[dir]/[name].[ext]" ``` </Tab> </Tabs>

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:

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', naming: 'files/[dir]/[name]-[hash].[ext]', }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --entry-naming 'files/[dir]/[name]-[hash].[ext]' ``` </Tab> </Tabs>

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.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', naming: { // default values entry: '[dir]/[name].[ext]', chunk: '[name]-[hash].[ext]', asset: '[name]-[hash].[ext]', }, }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out \ --entry-naming '[dir]/[name].[ext]' \ --chunk-naming '[name]-[hash].[ext]' \ --asset-naming '[name]-[hash].[ext]' ``` </Tab> </Tabs>

root

The root directory of the project.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./pages/a.tsx', './pages/b.tsx'], outdir: './out', root: '.', }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./pages/a.tsx ./pages/b.tsx --outdir ./out --root . ``` </Tab> </Tabs>

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:

<Tabs> <Tab title="JavaScript"> ```js theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./pages/index.tsx', './pages/settings.tsx'], outdir: './out', }) ``` </Tab> <Tab title="CLI"> ```bash theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./pages/index.tsx ./pages/settings.tsx --outdir ./out ``` </Tab> </Tabs>

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:

<Tabs> <Tab title="JavaScript"> ```js theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./pages/index.tsx', './pages/settings.tsx'], outdir: './out', root: '.', }) ``` </Tab> <Tab title="CLI"> ```bash theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./pages/index.tsx ./pages/settings.tsx --outdir ./out --root . ``` </Tab> </Tabs>

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:

<CodeGroup> ```ts Input icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} import logo from "./logo.svg"; console.log(logo); ```

var logo = "./logo-a7305bdef.svg";
console.log(logo);

</CodeGroup>

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', publicPath: 'https://cdn.example.com/', // default is undefined }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --public-path 'https://cdn.example.com/' ``` </Tab> </Tabs>

The output file would now look something like this.

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.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', define: { STRING: JSON.stringify("value"), "nested.boolean": "true", }, }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --define STRING='"value"' --define nested.boolean=true ``` </Tab> </Tabs>

loader

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', loader: { ".png": "dataurl", ".txt": "file", }, }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --loader .png:dataurl --loader .txt:file ``` </Tab> </Tabs>

banner

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.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', banner: '"use client";' }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --banner '"use client";' ``` </Tab> </Tabs>

footer

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.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', footer: '// built with love in SF' }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --footer '// built with love in SF' ``` </Tab> </Tabs>

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.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', drop: ["console", "debugger", "anyIdentifier.or.propertyAccess"], }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --drop console --drop debugger ``` </Tab> </Tabs>

features

Enable compile-time feature flags for dead-code elimination. This provides a way to conditionally include or exclude code paths at bundle time using import { feature } from "bun:bundle".

import { feature } from "bun:bundle";

if (feature("PREMIUM")) {
  // Only included when PREMIUM flag is enabled
  initPremiumFeatures();
}

if (feature("DEBUG")) {
  // Only included when DEBUG flag is enabled
  console.log("Debug mode");
}

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./app.ts'], outdir: './out', features: ["PREMIUM"], // PREMIUM=true, DEBUG=false }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./app.ts --outdir ./out --feature PREMIUM ``` </Tab> </Tabs>

The feature() function is replaced with true or false at bundle time. Combined with minification, unreachable code is eliminated:

import { feature } from "bun:bundle";
const mode = feature("PREMIUM") ? "premium" : "free";

var mode = "premium";

var mode = "free";

Key behaviors:

feature() requires a string literal argument — dynamic values are not supported
The bun:bundle import is completely removed from the output
Works with bun build, bun run, and bun test
Multiple flags can be enabled: --feature FLAG_A --feature FLAG_B
For type safety, augment the Registry interface to restrict feature() to known flags (see below)

Use cases:

Platform-specific code (feature("SERVER") vs feature("CLIENT"))
Environment-based features (feature("DEVELOPMENT"))
Gradual feature rollouts
A/B testing variants
Paid tier features

Type safety: By default, feature() accepts any string. To get autocomplete and catch typos at compile time, create an env.d.ts file (or add to an existing .d.ts) and augment the Registry interface:

declare module "bun:bundle" {
  interface Registry {
    features: "DEBUG" | "PREMIUM" | "BETA_FEATURES";
  }
}

Ensure the file is included in your tsconfig.json (e.g., "include": ["src", "env.d.ts"]). Now feature() only accepts those flags, and invalid strings like feature("TYPO") become type errors.

metafile

Generate metadata about the build in a structured format. The metafile contains information about all input files, output files, their sizes, imports, and exports. This is useful for:

Bundle analysis: Understand what's contributing to bundle size
Visualization: Feed into tools like esbuild's bundle analyzer or other visualization tools
Dependency tracking: See the full import graph of your application
CI integration: Track bundle size changes over time

if (result.metafile) {
  // Analyze inputs
  for (const [path, meta] of Object.entries(result.metafile.inputs)) {
    console.log(`${path}: ${meta.bytes} bytes`);
  }

  // Analyze outputs
  for (const [path, meta] of Object.entries(result.metafile.outputs)) {
    console.log(`${path}: ${meta.bytes} bytes`);
  }

  // Save for external analysis tools
  await Bun.write('./dist/meta.json', JSON.stringify(result.metafile));
}
```

</Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./src/index.ts --outdir ./dist --metafile ./dist/meta.json ``` </Tab> </Tabs>

Markdown metafile

Use --metafile-md to generate a markdown metafile, which is LLM-friendly and easy to read in the terminal:

bun build ./src/index.ts --outdir ./dist --metafile-md ./dist/meta.md

Both --metafile and --metafile-md can be used together:

bun build ./src/index.ts --outdir ./dist --metafile ./dist/meta.json --metafile-md ./dist/meta.md

`metafile` option formats

In the JavaScript API, metafile accepts several forms:

// Boolean — include metafile in the result object
await Bun.build({
  entrypoints: ["./src/index.ts"],
  outdir: "./dist",
  metafile: true,
});

// String — write JSON metafile to a specific path
await Bun.build({
  entrypoints: ["./src/index.ts"],
  outdir: "./dist",
  metafile: "./dist/meta.json",
});

// Object — specify separate paths for JSON and markdown output
await Bun.build({
  entrypoints: ["./src/index.ts"],
  outdir: "./dist",
  metafile: {
    json: "./dist/meta.json",
    markdown: "./dist/meta.md",
  },
});

The metafile structure contains:

interface BuildMetafile {
  inputs: {
    [path: string]: {
      bytes: number;
      imports: Array<{
        path: string;
        kind: ImportKind;
        original?: string; // Original specifier before resolution
        external?: boolean;
      }>;
      format?: "esm" | "cjs" | "json" | "css";
    };
  };
  outputs: {
    [path: string]: {
      bytes: number;
      inputs: {
        [path: string]: { bytesInOutput: number };
      };
      imports: Array<{ path: string; kind: ImportKind }>;
      exports: string[];
      entryPoint?: string;
      cssBundle?: string; // Associated CSS file for JS entry points
    };
  };
}

Outputs

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

interface BuildOutput {
  outputs: BuildArtifact[];
  success: boolean;
  logs: Array<object>; // see docs for details
  metafile?: BuildMetafile; // only when metafile: true
}

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:

| Property | Description | | ----------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------ | | 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.

<CodeGroup> ```ts build.ts icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} // build.ts const build = await Bun.build({ /* */ });

const artifact = build.outputs[0]; console.log(artifact);


```bash Shell output theme={"theme":{"light":"github-light","dark":"dracula"}}
bun run build.ts

BuildArtifact (entry-point) {
  path: "./index.js",
  loader: "tsx",
  kind: "entry-point",
  hash: "824a039620219640",
  Blob (74756 bytes) {
    type: "text/javascript;charset=utf-8"
  },
  sourcemap: BuildArtifact (sourcemap) {
    path: "./index.js.map",
    loader: "file",
    kind: "sourcemap",
    hash: "e7178cda3e72e301",
    Blob (24765 bytes) {
      type: "application/json;charset=utf-8"
    },
    sourcemap: null
  }
}

</CodeGroup>

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. Requires "target": "bun" and is dependent on a matching version of Bun.

CommonJS: Works with or without compile: true. Generates a .jsc file alongside each entrypoint.
ESM: Requires compile: true. Bytecode and module metadata are embedded in the standalone executable.

Without an explicit format, bytecode defaults to CommonJS.

// ESM bytecode (requires compile)
await Bun.build({
  entrypoints: ["./index.tsx"],
  outfile: "./mycli",
  bytecode: true,
  format: "esm",
  compile: true,
})
```

</Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} # CommonJS bytecode bun build ./index.tsx --outdir ./out --bytecode

# ESM bytecode (requires --compile)
bun build ./index.tsx --outfile ./mycli --bytecode --format=esm --compile
```

</Tab> </Tabs>

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";
  /**
   * JSX configuration object for controlling JSX transform behavior
   */
  jsx?: {
    runtime?: "automatic" | "classic";
    importSource?: string;
    factory?: string;
    fragment?: string;
    sideEffects?: boolean;
    development?: boolean;
  };
  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" | 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.
   *
   * - CommonJS: works with or without `compile: true`
   * - ESM: requires `compile: true`
   *
   * Without an explicit `format`, defaults to CommonJS.
   *
   * 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`, returns a {@link BuildOutput} with `{success: false}`
   *
   * @default true
   */
  throw?: boolean;

  /**
   * Custom tsconfig.json file path to use for path resolution.
   * Equivalent to `--tsconfig-override` in the CLI.
   */
  tsconfig?: string;

  outdir?: string;
}

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"
  | "css"
  | "json"
  | "jsonc"
  | "toml"
  | "yaml"
  | "text"
  | "file"
  | "napi"
  | "wasm"
  | "html";

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 <entry points>

General Configuration

<ParamField path="--production" type="boolean"> Set <code>NODE\_ENV=production</code> and enable minification </ParamField> <ParamField path="--bytecode" type="boolean"> Use a bytecode cache when compiling </ParamField> <ParamField path="--target" type="string" default="browser"> Intended execution environment for the bundle. One of <code>browser</code>, <code>bun</code>, or <code>node</code> </ParamField> <ParamField path="--conditions" type="string"> Pass custom resolution conditions </ParamField> <ParamField path="--env" type="string" default="disable"> Inline environment variables into the bundle as <code>process.env.\${name_0}</code>. To inline variables matching a prefix, use a glob like <code>FOO\_PUBLIC\_\*</code> </ParamField>

Output & File Handling

<ParamField path="--outdir" type="string" default="dist"> Output directory (used when building multiple entry points) </ParamField> <ParamField path="--outfile" type="string"> Write output to a specific file </ParamField> <ParamField path="--sourcemap" type="string" default="none"> Generate source maps. One of <code>linked</code>, <code>inline</code>, <code>external</code>, or <code>none</code> </ParamField> <ParamField path="--banner" type="string"> Add a banner to the output (e.g. <code>"use client"</code> for React Server Components) </ParamField> <ParamField path="--footer" type="string"> Add a footer to the output (e.g. <code>// built with bun!</code>) </ParamField> <ParamField path="--format" type="string" default="esm"> Module format of the output bundle. One of <code>esm</code>, <code>cjs</code>, or <code>iife</code>. Defaults to{" "} <code>cjs</code> when <code>--bytecode</code> is used. </ParamField>

File Naming

<ParamField path="--entry-naming" type="string" default="[dir]/[name].[ext]"> Customize entry point filenames </ParamField> <ParamField path="--chunk-naming" type="string" default="[name]-[hash].[ext]"> Customize chunk filenames </ParamField> <ParamField path="--asset-naming" type="string" default="[name]-[hash].[ext]"> Customize asset filenames </ParamField>

Bundling Options

<ParamField path="--root" type="string"> Root directory used when bundling multiple entry points </ParamField> <ParamField path="--splitting" type="boolean"> Enable code splitting for shared modules </ParamField> <ParamField path="--public-path" type="string"> Prefix to be added to import paths in bundled code </ParamField> <ParamField path="--external" type="string"> Exclude modules from the bundle (supports wildcards). Alias: <code>-e</code> </ParamField> <ParamField path="--packages" type="string" default="bundle"> How to treat dependencies: <code>external</code> or <code>bundle</code> </ParamField> <ParamField path="--no-bundle" type="boolean"> Transpile only — do not bundle </ParamField> <ParamField path="--css-chunking" type="boolean"> Chunk CSS files together to reduce duplication (only when multiple entry points import CSS) </ParamField>

Minification & Optimization

<ParamField path="--emit-dce-annotations" type="boolean" default="true"> Re-emit Dead Code Elimination annotations. Disabled when <code>--minify-whitespace</code> is used </ParamField> <ParamField path="--minify" type="boolean"> Enable all minification options </ParamField> <ParamField path="--minify-syntax" type="boolean"> Minify syntax and inline constants </ParamField> <ParamField path="--minify-whitespace" type="boolean"> Minify whitespace </ParamField> <ParamField path="--minify-identifiers" type="boolean"> Minify variable and function identifiers </ParamField> <ParamField path="--keep-names" type="boolean"> Preserve original function and class names when minifying </ParamField>

Development Features

<ParamField path="--watch" type="boolean"> Rebuild automatically when files change </ParamField> <ParamField path="--no-clear-screen" type="boolean"> Don’t clear the terminal when rebuilding with <code>--watch</code> </ParamField> <ParamField path="--react-fast-refresh" type="boolean"> Enable React Fast Refresh transform (for development testing) </ParamField>

Standalone Executables

<ParamField path="--compile" type="boolean"> Generate a standalone Bun executable containing the bundle. Implies <code>--production</code> </ParamField> <ParamField path="--compile-exec-argv" type="string"> Prepend arguments to the standalone executable’s <code>execArgv</code> </ParamField>

Windows Executable Details

<ParamField path="--windows-hide-console" type="boolean"> Prevent a console window from opening when running a compiled Windows executable </ParamField> <ParamField path="--windows-icon" type="string"> Set an icon for the Windows executable </ParamField> <ParamField path="--windows-title" type="string"> Set the Windows executable product name </ParamField> <ParamField path="--windows-publisher" type="string"> Set the Windows executable company name </ParamField> <ParamField path="--windows-version" type="string"> Set the Windows executable version (e.g. <code>1.2.3.4</code>) </ParamField> <ParamField path="--windows-description" type="string"> Set the Windows executable description </ParamField> <ParamField path="--windows-copyright" type="string"> Set the Windows executable copyright notice </ParamField>

Experimental & App Building

<ParamField path="--app" type="boolean"> (EXPERIMENTAL) Build a web app for production using Bun Bake </ParamField> <ParamField path="--server-components" type="boolean"> (EXPERIMENTAL) Enable React Server Components </ParamField> <ParamField path="--debug-dump-server-files" type="boolean"> When <code>--app</code> is set, dump all server files to disk even for static builds </ParamField> <ParamField path="--debug-no-minify" type="boolean"> When <code>--app</code> is set, disable all minification </ParamField>

Bundler

Bun's fast native bundler for JavaScript, TypeScript, JSX, and more

export const name_0 = undefined

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

At a Glance

JS API: await Bun.build({ entrypoints, outdir })
CLI: bun build <entry> --outdir ./out
Watch: --watch for incremental rebuilds
Targets: --target browser|bun|node
Formats: --format esm|cjs|iife (experimental for cjs/iife)

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

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.
Full-stack Applications. Bun's bundler can handle both server and client code in a single command, enabling optimized production builds and single-file executables. With build-time HTML imports, you can bundle your entire application — frontend assets and backend server — into a single deployable unit.

Let's jump into the bundler API.

<Note>The Bun bundler is not intended to replace tsc for typechecking or generating type declarations.</Note>

Basic example

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

const root = ReactDOM.createRoot(document.getElementById("root")!); root.render(<Component message="Sup!" />);


```tsx Component.tsx icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}}
export function Component(props: { message: string }) {
  return <h1>{props.message}</h1>;
}

</CodeGroup>

To create our bundle:

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

</CodeGroup>

.
├── 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,
  ),
);

Watch mode

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

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

Content types

Assets

// bundled output
var logo = "./logo-a7305bdef.svg";
console.log(logo);

</CodeGroup>

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

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

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

<Badge>Required</Badge>

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

files

File contents can be provided as a string, Blob, TypedArray, or ArrayBuffer.

Bundle entirely from memory

You can bundle code without any files on disk by providing all sources via files:

const result = await Bun.build({
  entrypoints: ["/app/index.ts"],
  files: {
    "/app/index.ts": `
      import { greet } from "./greet.ts";
      console.log(greet("World"));
    `,
    "/app/greet.ts": `
      export function greet(name: string) {
        return "Hello, " + name + "!";
      }
    `,
  },
});

const output = await result.outputs[0].text();
console.log(output);

When all entrypoints are in the files map, the current working directory is used as the root.

Override files on disk

In-memory files take priority over files on disk. This lets you override specific files while keeping the rest of your codebase unchanged:

// Assume ./src/config.ts exists on disk with development settings
await Bun.build({
  entrypoints: ["./src/index.ts"],
  files: {
    // Override config.ts with production values
    "./src/config.ts": `
      export const API_URL = "https://api.production.com";
      export const DEBUG = false;
    `,
  },
  outdir: "./dist",
});

Mix disk and virtual files

Real files on disk can import virtual files, and virtual files can import real files:

// ./src/index.ts exists on disk and imports "./generated.ts"
await Bun.build({
  entrypoints: ["./src/index.ts"],
  files: {
    // Provide a virtual file that index.ts imports
    "./src/generated.ts": `
      export const BUILD_ID = "${crypto.randomUUID()}";
      export const BUILD_TIME = ${Date.now()};
    `,
  },
  outdir: "./dist",
});

This is useful for code generation, injecting build-time constants, or testing with mock modules.

outdir

The directory where output files will be written.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} const result = await Bun.build({ entrypoints: ['./index.ts'], outdir: './out' }); // => { success: boolean, outputs: BuildArtifact[], logs: BuildMessage[] } ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.ts --outdir ./out ``` </Tab> </Tabs>

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.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.ts'], outdir: './out', target: 'browser', // default }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.ts --outdir ./out --target browser ``` </Tab> </Tabs>

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

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. </Card>

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.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', format: "esm", }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --format esm ``` </Tab> </Tabs>

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', format: "cjs", }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --format cjs ``` </Tab> </Tabs>

format: "iife" - IIFE

TODO: document IIFE once we support globalNames.

`jsx`

Configure JSX transform behavior. Allows fine-grained control over how JSX is compiled.

Classic runtime example (uses factory and fragment):

# JSX configuration is handled via bunfig.toml or tsconfig.json
bun build ./app.tsx --outdir ./out

</CodeGroup>

Automatic runtime example (uses importSource):

<CodeGroup> ```ts index.ts icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ["./app.tsx"], outdir: "./out", jsx: { importSource: "preact", runtime: "automatic", }, }); ```

# JSX configuration is handled via bunfig.toml or tsconfig.json
bun build ./app.tsx --outdir ./out

</CodeGroup>

splitting

Whether to enable code splitting.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', splitting: false, // default }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --splitting ``` </Tab> </Tabs>

import { shared } from "./shared.ts";

export const shared = "shared";

</CodeGroup>

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./entry-a.ts', './entry-b.ts'], outdir: './out', splitting: true, }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./entry-a.ts ./entry-b.ts --outdir ./out --splitting ``` </Tab> </Tabs>

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.

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

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

env

env: "inline"

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', env: "inline", }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --env inline ``` </Tab> </Tabs>

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)

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

</Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --env ACME_PUBLIC_* ``` </Tab> </Tabs>

For example, given the following environment variables:

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

And source code:

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.

sourcemap

Specifies the type of sourcemap to generate.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', sourcemap: 'linked', // default 'none' }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --sourcemap linked ``` </Tab> </Tabs>

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

minify

Whether to enable minification. Default false.

<Note>When targeting bun, identifiers will be minified by default.</Note>

To enable all minification options:

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', minify: true, // default false }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --minify ``` </Tab> </Tabs>

To granularly enable certain minifications:

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', minify: { whitespace: true, identifiers: true, syntax: true, }, }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --minify-whitespace --minify-identifiers --minify-syntax ``` </Tab> </Tabs>

external

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', external: ["lodash", "react"], // default: [] }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --external lodash --external react ``` </Tab> </Tabs>

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:

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:

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', external: ['zod'], }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --external zod ``` </Tab> </Tabs>

The generated bundle will look something like this:

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 *:

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', external: ['*'], }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --external '*' ``` </Tab> </Tabs>

packages

Control whether 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.

naming

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', naming: "[dir]/[name].[ext]", // default }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --entry-naming "[dir]/[name].[ext]" ``` </Tab> </Tabs>

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

[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:

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', naming: 'files/[dir]/[name]-[hash].[ext]', }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --entry-naming 'files/[dir]/[name]-[hash].[ext]' ``` </Tab> </Tabs>

This build would result in the following file structure:

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', naming: { // default values entry: '[dir]/[name].[ext]', chunk: '[name]-[hash].[ext]', asset: '[name]-[hash].[ext]', }, }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out \ --entry-naming '[dir]/[name].[ext]' \ --chunk-naming '[name]-[hash].[ext]' \ --asset-naming '[name]-[hash].[ext]' ``` </Tab> </Tabs>

root

The root directory of the project.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./pages/a.tsx', './pages/b.tsx'], outdir: './out', root: '.', }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./pages/a.tsx ./pages/b.tsx --outdir ./out --root . ``` </Tab> </Tabs>

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:

This would result in a file structure like this:

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

This behavior can be overridden by specifying the root option:

<Tabs> <Tab title="JavaScript"> ```js theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./pages/index.tsx', './pages/settings.tsx'], outdir: './out', root: '.', }) ``` </Tab> <Tab title="CLI"> ```bash theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./pages/index.tsx ./pages/settings.tsx --outdir ./out --root . ``` </Tab> </Tabs>

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.

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:

<CodeGroup> ```ts Input icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} import logo from "./logo.svg"; console.log(logo); ```

var logo = "./logo-a7305bdef.svg";
console.log(logo);

</CodeGroup>

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', publicPath: 'https://cdn.example.com/', // default is undefined }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --public-path 'https://cdn.example.com/' ``` </Tab> </Tabs>

The output file would now look something like this.

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.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', define: { STRING: JSON.stringify("value"), "nested.boolean": "true", }, }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --define STRING='"value"' --define nested.boolean=true ``` </Tab> </Tabs>

loader

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

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', loader: { ".png": "dataurl", ".txt": "file", }, }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --loader .png:dataurl --loader .txt:file ``` </Tab> </Tabs>

banner

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.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', banner: '"use client";' }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --banner '"use client";' ``` </Tab> </Tabs>

footer

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.

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', footer: '// built with love in SF' }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --footer '// built with love in SF' ``` </Tab> </Tabs>

drop

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./index.tsx'], outdir: './out', drop: ["console", "debugger", "anyIdentifier.or.propertyAccess"], }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --drop console --drop debugger ``` </Tab> </Tabs>

features

Enable compile-time feature flags for dead-code elimination. This provides a way to conditionally include or exclude code paths at bundle time using import { feature } from "bun:bundle".

import { feature } from "bun:bundle";

if (feature("PREMIUM")) {
  // Only included when PREMIUM flag is enabled
  initPremiumFeatures();
}

if (feature("DEBUG")) {
  // Only included when DEBUG flag is enabled
  console.log("Debug mode");
}

<Tabs> <Tab title="JavaScript"> ```ts title="build.ts" icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} await Bun.build({ entrypoints: ['./app.ts'], outdir: './out', features: ["PREMIUM"], // PREMIUM=true, DEBUG=false }) ``` </Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./app.ts --outdir ./out --feature PREMIUM ``` </Tab> </Tabs>

The feature() function is replaced with true or false at bundle time. Combined with minification, unreachable code is eliminated:

import { feature } from "bun:bundle";
const mode = feature("PREMIUM") ? "premium" : "free";

var mode = "premium";

var mode = "free";

Key behaviors:

feature() requires a string literal argument — dynamic values are not supported
The bun:bundle import is completely removed from the output
Works with bun build, bun run, and bun test
Multiple flags can be enabled: --feature FLAG_A --feature FLAG_B
For type safety, augment the Registry interface to restrict feature() to known flags (see below)

Use cases:

Platform-specific code (feature("SERVER") vs feature("CLIENT"))
Environment-based features (feature("DEVELOPMENT"))
Gradual feature rollouts
A/B testing variants
Paid tier features

declare module "bun:bundle" {
  interface Registry {
    features: "DEBUG" | "PREMIUM" | "BETA_FEATURES";
  }
}

Ensure the file is included in your tsconfig.json (e.g., "include": ["src", "env.d.ts"]). Now feature() only accepts those flags, and invalid strings like feature("TYPO") become type errors.

metafile

Generate metadata about the build in a structured format. The metafile contains information about all input files, output files, their sizes, imports, and exports. This is useful for:

Bundle analysis: Understand what's contributing to bundle size
Visualization: Feed into tools like esbuild's bundle analyzer or other visualization tools
Dependency tracking: See the full import graph of your application
CI integration: Track bundle size changes over time

if (result.metafile) {
  // Analyze inputs
  for (const [path, meta] of Object.entries(result.metafile.inputs)) {
    console.log(`${path}: ${meta.bytes} bytes`);
  }

  // Analyze outputs
  for (const [path, meta] of Object.entries(result.metafile.outputs)) {
    console.log(`${path}: ${meta.bytes} bytes`);
  }

  // Save for external analysis tools
  await Bun.write('./dist/meta.json', JSON.stringify(result.metafile));
}
```

Markdown metafile

Use --metafile-md to generate a markdown metafile, which is LLM-friendly and easy to read in the terminal:

bun build ./src/index.ts --outdir ./dist --metafile-md ./dist/meta.md

Both --metafile and --metafile-md can be used together:

bun build ./src/index.ts --outdir ./dist --metafile ./dist/meta.json --metafile-md ./dist/meta.md

`metafile` option formats

In the JavaScript API, metafile accepts several forms:

// Boolean — include metafile in the result object
await Bun.build({
  entrypoints: ["./src/index.ts"],
  outdir: "./dist",
  metafile: true,
});

// String — write JSON metafile to a specific path
await Bun.build({
  entrypoints: ["./src/index.ts"],
  outdir: "./dist",
  metafile: "./dist/meta.json",
});

// Object — specify separate paths for JSON and markdown output
await Bun.build({
  entrypoints: ["./src/index.ts"],
  outdir: "./dist",
  metafile: {
    json: "./dist/meta.json",
    markdown: "./dist/meta.md",
  },
});

The metafile structure contains:

interface BuildMetafile {
  inputs: {
    [path: string]: {
      bytes: number;
      imports: Array<{
        path: string;
        kind: ImportKind;
        original?: string; // Original specifier before resolution
        external?: boolean;
      }>;
      format?: "esm" | "cjs" | "json" | "css";
    };
  };
  outputs: {
    [path: string]: {
      bytes: number;
      inputs: {
        [path: string]: { bytesInOutput: number };
      };
      imports: Array<{ path: string; kind: ImportKind }>;
      exports: string[];
      entryPoint?: string;
      cssBundle?: string; // Associated CSS file for JS entry points
    };
  };
}

Outputs

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

interface BuildOutput {
  outputs: BuildArtifact[];
  success: boolean;
  logs: Array<object>; // see docs for details
  metafile?: BuildMetafile; // only when metafile: true
}

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:

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.

<CodeGroup> ```ts build.ts icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}} // build.ts const build = await Bun.build({ /* */ });

const artifact = build.outputs[0]; console.log(artifact);


```bash Shell output theme={"theme":{"light":"github-light","dark":"dracula"}}
bun run build.ts

BuildArtifact (entry-point) {
  path: "./index.js",
  loader: "tsx",
  kind: "entry-point",
  hash: "824a039620219640",
  Blob (74756 bytes) {
    type: "text/javascript;charset=utf-8"
  },
  sourcemap: BuildArtifact (sourcemap) {
    path: "./index.js.map",
    loader: "file",
    kind: "sourcemap",
    hash: "e7178cda3e72e301",
    Blob (24765 bytes) {
      type: "application/json;charset=utf-8"
    },
    sourcemap: null
  }
}

</CodeGroup>

Bytecode

CommonJS: Works with or without compile: true. Generates a .jsc file alongside each entrypoint.
ESM: Requires compile: true. Bytecode and module metadata are embedded in the standalone executable.

Without an explicit format, bytecode defaults to CommonJS.

// ESM bytecode (requires compile)
await Bun.build({
  entrypoints: ["./index.tsx"],
  outfile: "./mycli",
  bytecode: true,
  format: "esm",
  compile: true,
})
```

</Tab> <Tab title="CLI"> ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} # CommonJS bytecode bun build ./index.tsx --outdir ./out --bytecode

# ESM bytecode (requires --compile)
bun build ./index.tsx --outfile ./mycli --bytecode --format=esm --compile
```

</Tab> </Tabs>

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";
  /**
   * JSX configuration object for controlling JSX transform behavior
   */
  jsx?: {
    runtime?: "automatic" | "classic";
    importSource?: string;
    factory?: string;
    fragment?: string;
    sideEffects?: boolean;
    development?: boolean;
  };
  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" | 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.
   *
   * - CommonJS: works with or without `compile: true`
   * - ESM: requires `compile: true`
   *
   * Without an explicit `format`, defaults to CommonJS.
   *
   * 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`, returns a {@link BuildOutput} with `{success: false}`
   *
   * @default true
   */
  throw?: boolean;

  /**
   * Custom tsconfig.json file path to use for path resolution.
   * Equivalent to `--tsconfig-override` in the CLI.
   */
  tsconfig?: string;

  outdir?: string;
}

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"
  | "css"
  | "json"
  | "jsonc"
  | "toml"
  | "yaml"
  | "text"
  | "file"
  | "napi"
  | "wasm"
  | "html";

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 <entry points>

Adoption

jarle/Bun Bundler

$ install --global

Security Scan Results

SKILL.md

Bundler

At a Glance

Why bundle?

Basic example

Watch mode

Content types

Assets

Plugins

API

entrypoints

files

Bundle entirely from memory

Override files on disk

Mix disk and virtual files

outdir

target

format

format: "esm" - ES Module

format: "cjs" - CommonJS

format: "iife" - IIFE

jsx

splitting

plugins

env

env: "inline"

env: "PUBLIC_*" (prefix)

env: "disable"

sourcemap

minify

external

packages

naming

root

publicPath

define

loader

banner

footer

drop

features

metafile

Markdown metafile

metafile option formats

Outputs

Bytecode

Executables

Logs and errors

Reference

CLI Usage

General Configuration

Output & File Handling

File Naming

Bundling Options

Minification & Optimization

Development Features

Standalone Executables

Windows Executable Details

Experimental & App Building

Related Skills

jarle/Bun TypeScript

jarle/Bun Writing tests

jarle/Bun Snapshots

jarle/Bun Runtime behavior

jarle/Bun Bundler

$ install --global

Security Scan Results

SKILL.md

Bundler

At a Glance

Why bundle?

Basic example

Watch mode

Content types

Assets

Plugins

`jsx`

`metafile` option formats

`jsx`

`metafile` option formats