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{% callout %} Note — Available in the Bun v0.6.0 nightly. Run bun upgrade --canary to try it out. {% /callout %}

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.

{% codetabs %}

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

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

{% /codetabs %}

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.

Basic example

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

{% codetabs %}

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>
}

{% /codetabs %}

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:

{% codetabs %}

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

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

{% /codetabs %}

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 the out/index.js file looks 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));

{% details summary="Tutorial: Run this file in your browser" %} We can load this file in the browser to see our app in action. Create an index.html file in the out directory:

$ touch out/index.html

Then paste the following contents into it:

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

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

$ bunx serve out

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

{% /details %}

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.

{% table %}

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
JSON files are parsed and inlined into the bundle as a JavaScript object.
```
import pkg from "./package.json";
pkg.name; // => "my-package"
```

.toml

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

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

.txt

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

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

{% /table %}

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.

{% codetabs %}

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

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

var logo = "./logo-ab237dfe.svg";
console.log(logo);

{% /codetabs %}

{% callout %} The exact behavior of the file loader is also impacted by naming and publicPath. {% /callout %}

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

Plugins

The behavior described in this table can be overridden 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.

{% codetabs %}

const result = await Bun.build({
  entrypoints: ['./index.ts']
}); // => Promise

await result;
// => { outputs: Array<{ path: string; result: Blob }> }

$ bun build --entrypoints ./index.ts
# the bundle will be printed to stdout
# <bundled code>

{% /codetabs %}

`outdir`

The directory where output files will be written.

{% codetabs %}

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

result;
// => { outputs: Array<{ path: string; result: BunFile }> }

$ bun build --entrypoints ./index.ts --outdir ./out
# the bundle will be printed to stdout
# ...

{% /codetabs %}

When outdir is specified:

The JavaScript API will write the generated bundles to the appropriate location in outdir. The result of the Bun.build() call will contain BunFile instances corresponding to the new files.

const result = await Bun.build({
  /* ... */
});
// => { outputs: Array<{ path: string; result: BunFile }> }

for (const { path, result } of result.outputs) {
  console.log(`Wrote file: ${path}`);
}

The CLI will print a summary of the written files. The bundled code will not be written to stdout.

`target`

The intended execution environment for the bundle.

{% codetabs %}

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

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

{% /codetabs %}

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

{% table %}

browser
Default. For generating bundles that are intended for execution by a browser. Prioritizes the "browser" export condition when resolving imports. An error will be thrown if any Node.js or Bun built-ins are imported or used, e.g. node:fs or Bun.serve.

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

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

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

{% /table %}

`splitting`

Whether to enable code splitting.

{% codetabs %}

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

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

{% /codetabs %}

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:

{% codetabs %}

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

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

export const shared = 'shared';

{% /codetabs %}

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

{% codetabs %}

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

{% /codetabs %}

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.

{% codetabs %}

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

n/a

{% /codetabs %}

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

`manifest`

Whether to return a build manifest in the result of Bun.build.

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

console.log(result.manifest);

{% details summary="Manifest structure" %}

The manifest has the following form:

export type BuildManifest = {
  inputs: {
    [path: string]: {
      output: {
        path: string;
      };
      imports: {
        path: string;
        kind: ImportKind;
        external?: boolean;
      }[];
    };
  };
  outputs: {
    [path: string]: {
      type: "chunk" | "entry-point" | "asset";
      inputs: { path: string }[];
      imports: {
        path: string;
        kind: ImportKind;
        external?: boolean;
        asset?: boolean;
      }[];
      exports: string[];
    };
  };
};

export type ImportKind =
  | "entry-point"
  | "import-statement"
  | "require-call"
  | "dynamic-import"
  | "require-resolve"
  | "import-rule"
  | "url-token";

{% /details %}

By design, the manifest is a simple JSON object that can easily be serialized or written to disk. It is also compatible with esbuild's metafile format.

`sourcemap`

Specifies the type of sourcemap to generate.

{% codetabs %}

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

$ bun build ./index.tsx --outdir ./out --sourcemap=inline

{% /codetabs %}

{% table %}

"none"
Default. No sourcemap is generated.

"inline"
A sourcemap is generated and appended to the end of the generated bundle as a base64 payload inside a //# sourceMappingURL= comment.

"external"
A separate *.js.map file is created alongside each *.js bundle.

{% /table %}

`minify`

Whether to enable minification. Default false.

{% callout %} When targeting bun, identifiers will be minified by default. {% /callout %}

To enable all minification options:

{% codetabs %}

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

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

{% /codetabs %}

To granularly enable certain minifications:

{% codetabs %}

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

{% /codetabs %}

`external`

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

{% codetabs %}

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

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

{% /codetabs %}

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:

{% codetabs %}

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

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

{% /codetabs %}

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));

`naming`

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

{% codetabs %}

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

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

{% /codetabs %}

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:

{% table %}

Token
[name]
[ext]
[hash]
[dir]

./index.tsx
index
js
a1b2c3d4
"" (empty string)

./nested/entry.ts
entry
js
c3d4e5f6
"nested"

{% /table %}

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

{% codetabs %}

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]

{% /codetabs %}

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.

{% codetabs %}

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]"

{% /codetabs %}

`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:

{% codetabs %}

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);

{% /codetabs %}

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

{% codetabs %}

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

n/a

{% /codetabs %}

The output file would now look something like this.

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

Reference

await Bun.build({
  entrypoints: string[]; // list of file path
  outdir?: string; // default to in-memory build
  target?: "browser" | "bun" | "node"; // default: "browser"
  splitting?: boolean; // default true
  plugins?: BunPlugin[];
  manifest?: boolean; // default false
  external?: string[];
  naming?: string | {
    entry?: string; // default '[dir]/[name].[ext]'
    chunk?: string; // default '[name]-[hash].[ext]'
    asset?: string; // default '[name]-[hash].[ext]'
  };
  publicPath?: string; // e.g. http://mydomain.com/
  minify?: boolean | { // default false
    identifiers?: boolean;
    whitespace?: boolean;
    syntax?: boolean;
  };
});

23 KiB Raw Blame History

Why bundle?

Basic example

Content types

Assets

Plugins

API

entrypoints

outdir

target

splitting

plugins

manifest

sourcemap

minify

external

naming

publicPath

Reference

23 KiB

Raw Blame History

`entrypoints`

`outdir`

`target`

`splitting`

`plugins`

`manifest`

`sourcemap`

`minify`

`external`

`naming`

`publicPath`