Bun's fast native bundler is now in beta. It can be used via the bun build
CLI command or the Bun.build()
JavaScript API.
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.
Let's jump into the bundler API.
Note that the Bun bundler is not intended to replace tsc
for typechecking or generating type declarations.
Basic example
Let's build our first bundle. You have the following two files, which implement a simple client-side rendered React app.
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 <p>{props.message}</p>
}
Here, index.tsx
is the "entrypoint" to our application. Commonly, this will be a script that performs some side effect, like starting a server or—in this case—initializing a React root. Because we're using TypeScript & JSX, we need to bundle our code before it can be sent to the browser.
To create our bundle:
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:
// ...
// ~20k lines of code
// including the contents of `react-dom/client` and all its dependencies
// this is where the $jsxDEV and $createRoot functions are defined
// Component.tsx
function Component(props) {
return $jsxDEV("p", {
children: props.message
}, undefined, false, undefined, this);
}
// index.tsx
var rootNode = document.getElementById("root");
var root = $createRoot(rootNode);
root.render($jsxDEV(Component, {
message: "Sup!"
}, undefined, false, undefined, this));
Tutorial: Run this file in your browser
Watch mode
Like the runtime and test runner, the bundler supports watch mode natively.
bun build ./index.tsx --outdir ./out --watch
Content types
Like the Bun runtime, the bundler supports an array of file types out of the box. The following table breaks down the bundler's set of standard "loaders". Refer to Bundler > File types for full documentation.
Extensions | Details |
---|---|
.js .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, tree shaking, and environment variable inlining. At the moment Bun does not attempt to down-convert syntax; if you use recently ECMAScript syntax, that will be reflected in the bundled code. |
| JSON files are parsed and inlined into the bundle as a JavaScript object.
|
| TOML files are parsed and inlined into the bundle as a JavaScript object.
|
| The contents of the text file are read and inlined into the bundle as a string.
|
.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.
// bundle entrypoint
import logo from "./logo.svg";
console.log(logo);
// bundled output
var logo = "./logo-ab237dfe.svg";
console.log(logo);
The exact behavior of the file loader is also impacted by naming
and publicPath
.
Refer to the Bundler > Loaders page for more complete documentation on the file loader.
Plugins
The behavior described in this table can be overridden or extended with plugins. Refer to the Bundler > Loaders page for complete documentation.
API
entrypoints
Required. An array of paths corresponding to the entrypoints of our application. One bundle will be generated for each entrypoint.
const result = await Bun.build({
entrypoints: ["./index.ts"],
});
// => { success: boolean, outputs: BuildArtifact[], logs: BuildMessage[] }
bun build --entrypoints ./index.ts
# the bundle will be printed to stdout
# <bundled code>
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 --entrypoints ./index.ts --outdir ./out
# a summary of bundled files will be printed to stdout
If outdir
is not passed to the JavaScript API, bundled code will not be written to disk. Bundled files are returned in an array of BuildArtifact
objects. These objects are Blobs with extra properties; see Outputs for complete documentation.
const result = await Bun.build({
entrypoints: ["./index.ts"],
});
for (const result of result.outputs) {
// Can be consumed as blobs
await result.text();
// Bun will set Content-Type and Etag headers
new Response(result);
// Can be written manually, but you should use `outdir` in this case.
Bun.write(path.join("out", result.path), result);
}
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 --entrypoints ./index.ts --outdir ./out --target browser
Depending on the target, Bun will apply different module resolution rules and optimizations.
browser | Default. For generating bundles that are intended for execution by a browser. Prioritizes the "browser" export condition when resolving imports. Importing any built-in modules, like node:events or node:path will work, but calling some functions, like fs.readFile will not work. |
| For generating bundles that are intended to be run by the Bun runtime. In many cases, it isn't necessary to bundle server-side code; you can directly execute the source code without modification. However, bundling your server code can reduce startup times and improve running performance. All bundles generated with If any entrypoints contains a Bun shebang ( |
node | For generating bundles that are intended to be run by Node.js. Prioritizes the "node" export condition when resolving imports, and outputs .mjs . In the future, this will automatically polyfill the Bun global and other built-in bun:* modules, though this is not yet implemented. |
format
Specifies the module format to be used in the generated bundles.
Currently the bundler only supports one module format: "esm"
. Support for "cjs"
and "iife"
are planned.
await Bun.build({
entrypoints: ['./index.tsx'],
outdir: './out',
format: "esm",
})
bun build ./index.tsx --outdir ./out --format esm
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';
import { shared } from './shared.ts';
export const shared = 'shared';
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: [/* ... */],
})
n/a
Bun implements a universal plugin system for both Bun's runtime and bundler. Refer to the plugin documentation for complete documentation.
sourcemap
Specifies the type of sourcemap to generate.
await Bun.build({
entrypoints: ['./index.tsx'],
outdir: './out',
sourcemap: "external", // default "none"
})
bun build ./index.tsx --outdir ./out --sourcemap=external
"none" | Default. No sourcemap is generated. |
| A sourcemap is generated and appended to the end of the generated bundle as a base64 payload.
|
"external" | A separate *.js.map file is created alongside each *.js bundle. |
Generated bundles contain a debug id that can be used to associate a bundle with its corresponding sourcemap. This debugId
is added as a comment at the bottom of the file.
// <generated bundle code>
//# debugId=<DEBUG ID>
The associated *.js.map
sourcemap will be a JSON file containing an equivalent debugId
property.
minify
Whether to enable minification. Default false
.
When targeting bun
, identifiers will be minified by default.
To enable all minification options:
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 '*'
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 build root to the parent directory of the 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:
await Bun.build({
entrypoints: ['./index.tsx'],
outdir: './out',
naming: 'files/[dir]/[name]-[hash].[ext]',
})
bun build ./index.tsx --outdir ./out --entry-naming [name]-[hash].[ext]
This build would result in the following file structure:
.
├── index.tsx
└── out
└── files
└── index-a1b2c3d4.js
When a string
is provided for the naming
field, it is used only for bundles that correspond to entrypoints. The names of chunks and copied assets are not affected. Using the JavaScript API, separate template strings can be specified for each type of generated file.
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: '.',
})
n/a
If unspecified, it is computed to be the first common ancestor of all entrypoint files. Consider the following file structure:
.
└── pages
└── index.tsx
└── settings.tsx
We can build both entrypoints in the pages
directory:
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 thefile
loader, which copies the file intooutdir
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, theimport
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);
// logo.svg is copied into <outdir>
// and hash is added to the filename to prevent collisions
var logo = './logo-a7305bdef.svg';
console.log(logo);
Setting publicPath
will prefix all file paths with the specified value.
await Bun.build({
entrypoints: ['./index.tsx'],
outdir: './out',
publicPath: 'https://cdn.example.com/', // default is undefined
})
n/a
The output file would now look something like this.
var logo = './logo-a7305bdef.svg';
var logo = 'https://cdn.example.com/logo-a7305bdef.svg';
define
A map of global identifiers to be replaced at build time. Keys of this object are identifier names, and values are JSON strings that will be inlined.
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 file 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
Outputs
The Bun.build
function returns a Promise<BuildOutput>
, defined as:
interface BuildOutput {
outputs: BuildArtifact[];
success: boolean;
logs: Array<object>; // see docs for details
}
interface BuildArtifact extends Blob {
kind: "entry-point" | "chunk" | "asset" | "sourcemap";
path: string;
loader: Loader;
hash: string | null;
sourcemap: BuildArtifact | null;
}
The outputs
array contains all the files that were generated by the build. Each artifact implements the Blob
interface.
const build = await Bun.build({
/* */
});
for (const output of build.outputs) {
await output.arrayBuffer(); // => ArrayBuffer
await output.text(); // string
}
Each artifact also contains the following properties:
kind | What kind of build output this file is. A build generates bundled entrypoints, code-split "chunks", sourcemaps, and copied assets (like images). |
path | Absolute path to the file on disk |
loader | The loader was used to interpret the file. See Bundler > Loaders to see how Bun maps file extensions to the appropriate built-in loader. |
hash | The hash of the file contents. Always defined for assets. |
sourcemap | The sourcemap file corresponding to this file, if generated. Only defined for entrypoints and chunks. |
Similar to BunFile
, BuildArtifact
objects can be passed directly into new Response()
.
const build = await Bun.build({
/* */
});
const artifact = build.outputs[0];
// Content-Type header is automatically set
return new Response(artifact);
The Bun runtime implements special pretty-printing of BuildArtifact
object to make debugging easier.
// build.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 (114 bytes) {
type: "text/javascript;charset=utf-8"
},
sourcemap: null
}
Executables
Bun supports "compiling" a JavaScript/TypeScript entrypoint into a standalone executable. This executable contains a copy of the Bun binary.
bun build ./cli.tsx --outfile mycli --compile
./mycli
Refer to Bundler > Executables for complete documentation.
Logs and errors
Bun.build
only throws if invalid options are provided. Read the success
property to determine if the build was successful; the logs
property will contain additional details.
const result = await Bun.build({
entrypoints: ["./index.tsx"],
outdir: "./out",
});
if (!result.success) {
console.error("Build failed");
for (const message of result.logs) {
// Bun will pretty print the message object
console.error(message);
}
}
Each message is either a BuildMessage
or ResolveMessage
object, which can be used to trace what problems happened in the build.
class BuildMessage {
name: string;
position?: Position;
message: string;
level: "error" | "warning" | "info" | "debug" | "verbose";
}
class ResolveMessage extends BuildMessage {
code: string;
referrer: string;
specifier: string;
importKind: ImportKind;
}
If you want to throw an error from a failed build, consider passing the logs to an AggregateError
. If uncaught, Bun will pretty-print the contained messages nicely.
if (!result.success) {
throw new AggregateError(result.logs, "Build failed");
}
Reference
interface Bun {
build(options: BuildOptions): Promise<BuildOutput>;
}
interface BuildOptions {
entrypoints: string[]; // required
outdir?: string; // default: no write (in-memory only)
format?: "esm"; // later: "cjs" | "iife"
target?: "browser" | "bun" | "node"; // "browser"
splitting?: boolean; // true
plugins?: BunPlugin[]; // [] // See https://bun.sh/docs/bundler/plugins
loader?: { [k in string]: Loader }; // See https://bun.sh/docs/bundler/loaders
manifest?: boolean; // false
external?: string[]; // []
sourcemap?: "none" | "inline" | "external"; // "none"
root?: string; // computed from entrypoints
naming?:
| string
| {
entry?: string; // '[dir]/[name].[ext]'
chunk?: string; // '[name]-[hash].[ext]'
asset?: string; // '[name]-[hash].[ext]'
};
publicPath?: string; // e.g. http://mydomain.com/
minify?:
| boolean // false
| {
identifiers?: boolean;
whitespace?: boolean;
syntax?: boolean;
};
}
interface BuildOutput {
outputs: BuildArtifact[];
success: boolean;
logs: Array<BuildMessage | ResolveMessage>;
}
interface BuildArtifact extends Blob {
path: string;
loader: Loader;
hash?: string;
kind: "entry-point" | "chunk" | "asset" | "sourcemap";
sourcemap?: BuildArtifact;
}
type Loader =
| "js"
| "jsx"
| "ts"
| "tsx"
| "json"
| "toml"
| "file"
| "napi"
| "wasm"
| "text";
interface BuildOutput {
outputs: BuildArtifact[];
success: boolean;
logs: Array<BuildMessage | ResolveMessage>;
}
declare class ResolveMessage {
readonly name: "ResolveMessage";
readonly position: Position | null;
readonly code: string;
readonly message: string;
readonly referrer: string;
readonly specifier: string;
readonly importKind:
| "entry_point"
| "stmt"
| "require"
| "import"
| "dynamic"
| "require_resolve"
| "at"
| "at_conditional"
| "url"
| "internal";
readonly level: "error" | "warning" | "info" | "debug" | "verbose";
toString(): string;
}