Bundler

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)

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

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

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.

Basic example

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

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

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

export function Component(props: { message: string }) {
  return <h1>{props.message}</h1>;
}

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:

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

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

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

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

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

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

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

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

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.

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

import config from "./config.jsonc";
config.name; // => "my-config"

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

import config from "./config.yaml";
config.name; // => "my-app"

import contents from "./file.txt";
console.log(contents); // => "Hello, world!"

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

import config from "./config.jsonc";
config.name; // => "my-config"

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

import config from "./config.yaml";
config.name; // => "my-app"

import contents from "./file.txt";
console.log(contents); // => "Hello, world!"

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.

// bundle entrypoint
import logo from "./logo.svg";
console.log(logo);

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

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

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

const result = await Bun.build({
  entrypoints: ["./index.ts"],
});
// => { success: boolean, outputs: BuildArtifact[], logs: BuildMessage[] }

$ bun build ./index.ts

outdir

The directory where output files will be written.

const result = await Bun.build({
  entrypoints: ['./index.ts'],
  outdir: './out'
});
// => { success: boolean, outputs: BuildArtifact[], logs: BuildMessage[] }

$ bun build ./index.ts --outdir ./out

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.

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

$ bun build ./index.ts --outdir ./out --target browser

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

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.

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

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

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

format: "cjs" - CommonJS

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

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

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

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):

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

Automatic runtime example (uses importSource):

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

splitting

Whether to enable code splitting.

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

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

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

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

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

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

$ bun build ./entry-a.ts ./entry-b.ts --outdir ./out --splitting

Running this build will result in the following files:

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

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

plugins

A list of plugins to use during bundling.

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.

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

$ bun build ./index.tsx --outdir ./out --env inline

For the input below:

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

The generated bundle will contain the following code:

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

env: "PUBLIC_*" (prefix)

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

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

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

$ bun build ./index.tsx --outdir ./out --env ACME_PUBLIC_*

For example, given the following environment variables:

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

And source code:

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.

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

$ bun build ./index.tsx --outdir ./out --sourcemap linked

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

minify

Whether to enable minification. Default false.

To enable all minification options:

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

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

To granularly enable certain minifications:

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

$ bun build ./index.tsx --outdir ./out --minify-whitespace --minify-identifiers --minify-syntax

external

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

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

$ bun build ./index.tsx --outdir ./out --external lodash --external react

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

For instance, consider the following entrypoint file:

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:

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

$ bun build ./index.tsx --outdir ./out --external zod

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 *:

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

$ bun build ./index.tsx --outdir ./out --external '*'

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.

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

$ bun build ./index.ts --packages external

naming

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

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

$ bun build ./index.tsx --outdir ./out --entry-naming "[dir]/[name].[ext]"

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

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

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

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

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

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

For example:

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

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

$ bun build ./index.tsx --outdir ./out --entry-naming 'files/[dir]/[name]-[hash].[ext]'

This build would result in the following file structure:

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

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

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

$ bun build ./index.tsx --outdir ./out \
    --entry-naming '[dir]/[name].[ext]' \
    --chunk-naming '[name]-[hash].[ext]' \
    --asset-naming '[name]-[hash].[ext]'

root

The root directory of the project.

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

$ bun build ./pages/a.tsx ./pages/b.tsx --outdir ./out --root .

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:

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

$ bun build ./pages/index.tsx ./pages/settings.tsx --outdir ./out

This would result in a file structure like this:

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

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

This behavior can be overridden by specifying the root option:

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

$ bun build ./pages/index.tsx ./pages/settings.tsx --outdir ./out --root .

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

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

publicPath

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

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

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

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

import logo from './logo.svg';
console.log(logo);

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

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

$ bun build ./index.tsx --outdir ./out --public-path 'https://cdn.example.com/'

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.

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

$ bun build ./index.tsx --outdir ./out --define STRING='"value"' --define nested.boolean=true

loader

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

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

$ bun build ./index.tsx --outdir ./out --loader .png:dataurl --loader .txt:file

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.

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

$ bun build ./index.tsx --outdir ./out --banner '"use client";'

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.

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

$ bun build ./index.tsx --outdir ./out --footer '// built with love in SF'

drop

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

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

$ bun build ./index.tsx --outdir ./out --drop console --drop debugger

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");
}

await Bun.build({
  entrypoints: ['./app.ts'],
  outdir: './out',
  features: ["PREMIUM"],  // PREMIUM=true, DEBUG=false
})

$ bun build ./app.ts --outdir ./out --feature PREMIUM

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.

Outputs

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

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

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

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

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

for (const output of build.outputs) {
  await output.arrayBuffer(); // => ArrayBuffer
  await output.bytes(); // => Uint8Array
  await output.text(); // string
}

Each artifact also contains the following properties:

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

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

const artifact = build.outputs[0];

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

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

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

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

$ 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
  }
}

Bytecode

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

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

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

Executables

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

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

Refer to Bundler > Executables for complete documentation.

Logs and errors

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

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

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

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

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

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

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

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

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

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

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

Reference

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

interface BuildConfig {
  entrypoints: string[]; // list of file path
  outdir?: string; // output directory
  target?: Target; // default: "browser"
  /**
   * Output module format. Top-level await is only supported for `"esm"`.
   *
   * Can be:
   * - `"esm"`
   * - `"cjs"` (**experimental**)
   * - `"iife"` (**experimental**)
   *
   * @default "esm"
   */
  format?: "esm" | "cjs" | "iife";
  /**
   * 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.
   *
   * Bytecode is currently only supported for CommonJS (`format: "cjs"`).
   *
   * Must be `target: "bun"`
   * @default false
   */
  bytecode?: boolean;
  /**
   * Add a banner to the bundled code such as "use client";
   */
  banner?: string;
  /**
   * Add a footer to the bundled code such as a comment block like
   *
   * `// made with bun!`
   */
  footer?: string;

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

  /**
   * - When set to `true`, the returned promise rejects with an AggregateError when a build failure happens.
   * - When set to `false`, 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

Output & File Handling

File Naming

Bundling Options

Minification & Optimization

Development Features

Standalone Executables

Windows Executable Details

Experimental & App Building