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Bun's fast native bundler for JavaScript, TypeScript, JSX, and more
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| name | Bun Bundler |
| description | Bun's fast native bundler for JavaScript, TypeScript, JSX, and more |
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.
await Bun.build({ entrypoints, outdir })bun build <entry> --outdir ./out--watch for incremental rebuilds--target browser|bun|node--format esm|cjs|iife (experimental for cjs/iife)It's fast. The numbers below represent performance on esbuild's three.js benchmark.
The bundler is a key piece of infrastructure in the JavaScript ecosystem. As a brief overview of why bundling is so important:
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.getServerSideProps or Remix loaders), and server components.Let's jump into the bundler API.
The Bun bundler is not intended to replace tsc for typechecking or generating type declarations.
Let's build our first bundle. You have the following two files, which implement a simple client-side rendered React app.
```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();
```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>;
}
Here, index.tsx is the "entrypoint" to our application. Commonly, this will be a script that performs some side effect, like starting a server orโin this caseโinitializing 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:
```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
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:
.
โโโ index.tsx
โโโ Component.tsx
โโโ out
โโโ 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,
),
);
Like the runtime and test runner, the bundler supports watch mode natively.
bun build ./index.tsx --outdir ./out --watch
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<br/>import pkg from "./package.json";<br/>pkg.name; // => "my-package"<br/> |
.jsonc | JSON with comments. Files are parsed and inlined into the bundle as a JavaScript object.js<br/>import config from "./config.jsonc";<br/>config.name; // => "my-config"<br/> |
.toml | TOML files are parsed and inlined into the bundle as a JavaScript object.js<br/>import config from "./bunfig.toml";<br/>config.logLevel; // => "debug"<br/> |
.yaml .yml | YAML files are parsed and inlined into the bundle as a JavaScript object.js<br/>import config from "./config.yaml";<br/>config.name; // => "my-app"<br/> |
.txt | The contents of the text file are read and inlined into the bundle as a string.js<br/>import contents from "./file.txt";<br/>console.log(contents); // => "Hello, world!"<br/> |
.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. |
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.
// bundled output
var logo = "./logo-a7305bdef.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.
The behavior described in this table can be overridden or extended with plugins. Refer to the Bundler > Loaders page for complete documentation.
Required
An array of paths corresponding to the entrypoints of our application. One bundle will be generated for each entrypoint.
```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[] } ``` ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.ts ```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.
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.
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",
});
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.
The directory where output files will be written.
```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[] } ``` ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} 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.
The intended execution environment for the bundle.
```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 }) ``` ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.ts --outdir ./out --target browser ```Depending on the target, Bun will apply different module resolution rules and optimizations.
**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. 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.
Specifies the module format to be used in the generated bundles.
Bun defaults to "esm", and provides experimental support for "cjs" and "iife".
This is the default format, which supports ES Module syntax including top-level await, import.meta, and more.
To use ES Module syntax in browsers, set format to "esm" and make sure your <script type="module"> tag has type="module" set.
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).
TODO: document IIFE once we support globalNames.
jsxConfigure 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
Automatic runtime example (uses importSource):
# JSX configuration is handled via bunfig.toml or tsconfig.json
bun build ./app.tsx --outdir ./out
Whether to enable code splitting.
```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 }) ``` ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} 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";
export const shared = "shared";
To bundle entry-a.ts and entry-b.ts with code-splitting enabled:
Running this build will result in the following files:
.
โโโ entry-a.tsx
โโโ entry-b.tsx
โโโ shared.tsx
โโโ out
โโโ entry-a.js
โโโ entry-b.js
โโโ 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.
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.
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.
Injects environment variables into the bundled output by converting process.env.FOO references to string literals containing the actual environment variable values.
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");
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_*",
})
```
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);
Disables environment variable injection entirely.
Specifies the type of sourcemap to generate.
```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' }) ``` ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --sourcemap linked ```| 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<br/>// <bundled code here><br/><br/>//# sourceMappingURL=bundle.js.map<br/> |
"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<br/>// <generated bundle code><br/><br/>//# debugId=<DEBUG ID><br/> |
"inline" | A sourcemap is generated and appended to the end of the generated bundle as a base64 payload.js<br/>// <bundled code here><br/><br/>//# sourceMappingURL=data:application/json;base64,<encoded sourcemap here><br/> |
The associated *.js.map sourcemap will be a JSON file containing an equivalent debugId property.
Whether to enable minification. Default false.
When targeting bun, identifiers will be minified by default.
To enable all minification options:
```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 }) ``` ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --minify ```To granularly enable certain minifications:
```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, }, }) ``` ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --minify-whitespace --minify-identifiers --minify-syntax ```A list of import paths to consider external. Defaults to [].
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:
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 *:
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.
Customizes the generated file names. Defaults to ./[dir]/[name].[ext].
By default, the names of the generated bundles are based on the name of the associated entrypoint.
.
โโโ index.tsx
โโโ out
โโโ index.js
With multiple entrypoints, the generated file hierarchy will reflect the directory structure of the entrypoints.
.
โโโ index.tsx
โโโ nested
โโโ index.tsx
โโโ out
โโโ index.js
โโโ nested
โโโ 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:
```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]', }) ``` ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --entry-naming 'files/[dir]/[name]-[hash].[ext]' ```This build would result in the following file structure:
.
โโโ index.tsx
โโโ out
โโโ files
โโโ 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.
The root directory of the project.
```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: '.', }) ``` ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} 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:
.
โโโ pages
โโโ index.tsx
โโโ settings.tsx
We can build both entrypoints in the pages directory:
This would result in a file structure like this:
.
โโโ pages
โโโ index.tsx
โโโ settings.tsx
โโโ out
โโโ index.js
โโโ 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:
By specifying . as root, the generated file structure will look like this:
.
โโโ pages
โโโ index.tsx
โโโ settings.tsx
โโโ out
โโโ pages
โโโ index.js
โโโ settings.js
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.
*.svg, the bundler defers to the file loader, which copies the file into outdir as is. The import is converted into a variablesplitting 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:
```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);
Setting publicPath will prefix all file paths with the specified value.
The output file would now look something like this.
var logo = "https://cdn.example.com/logo-a7305bdef.svg";
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.
```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", }, }) ``` ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --define STRING='"value"' --define nested.boolean=true ```A map of file extensions to built-in loader names. This can be used to quickly customize how certain files are loaded.
```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", }, }) ``` ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --loader .png:dataurl --loader .txt:file ```A banner to be added to the final bundle, this can be a directive like "use client" for react or a comment block such as a license for the code.
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.
```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' }) ``` ```bash terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}} bun build ./index.tsx --outdir ./out --footer '// built with love in SF' ```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.
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");
}
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 โ dynamic values are not supportedbun:bundle import is completely removed from the outputbun build, bun run, and bun test--feature FLAG_A --feature FLAG_BRegistry interface to restrict feature() to known flags (see below)Use cases:
feature("SERVER") vs feature("CLIENT"))feature("DEVELOPMENT"))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.
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:
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));
}
```
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 formatsIn the JavaScript API, metafile accepts several forms:
// Boolean โ include metafile in the result object
await Bun.build({
entrypoints: ["./src/index.ts"],
outdir: "./dist",
metafile: true,
});
// String โ write JSON metafile to a specific path
await Bun.build({
entrypoints: ["./src/index.ts"],
outdir: "./dist",
metafile: "./dist/meta.json",
});
// Object โ 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
};
};
}
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.
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
}
}
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.
compile: true. Generates a .jsc file alongside each entrypoint.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,
})
```
# ESM bytecode (requires --compile)
bun build ./index.tsx --outfile ./mycli --bytecode --format=esm --compile
```
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.
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);
}
}
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;
}
bun build <entry points>
NODE\_ENV=production and enable minification
Use a bytecode cache when compiling
Intended execution environment for the bundle. One of browser, bun, or node
Pass custom resolution conditions
Inline environment variables into the bundle as process.env.\${name_0}. To inline variables matching a
prefix, use a glob like FOO\_PUBLIC\_\*
linked, inline, external, or none
Add a banner to the output (e.g. "use client" for React Server Components)
Add a footer to the output (e.g. // built with bun!)
Module format of the output bundle. One of esm, cjs, or iife. Defaults to{" "}
cjs when --bytecode is used.
-e
How to treat dependencies: external or bundle
Transpile only โ do not bundle
Chunk CSS files together to reduce duplication (only when multiple entry points import CSS)
--minify-whitespace is used
Enable all minification options
Minify syntax and inline constants
Minify whitespace
Minify variable and function identifiers
Preserve original function and class names when minifying
--watch