/bun-bundler

name: bun-bundler
description: This skill should be used when the user asks about "bun build", "Bun.build", "bundling with Bun", "code splitting", "tree shaking", "minification", "sourcemaps", "bundle optimization", "esbuild alternative", "building for production", "bundling TypeScript", "bundling for browser", "bundling for Node", or JavaScript/TypeScript bundling with Bun.
metadata:
  version: "1.0.0"
license: MIT

Bun Bundler

Bun's bundler is a fast JavaScript/TypeScript bundler built on the same engine as Bun's runtime. It's an esbuild-compatible alternative with native performance.

Quick Start

CLI

# Basic bundle
bun build ./src/index.ts --outdir ./dist

# Production build
bun build ./src/index.ts --outdir ./dist --minify

# Multiple entry points
bun build ./src/index.ts ./src/worker.ts --outdir ./dist

JavaScript API

// Since Bun 1.2, Bun.build REJECTS on failure (throws).
// Wrap in try/catch to handle errors; pass { throw: false } to restore the
// old resolve-with-{ success, logs } contract if you prefer that style.
try {
  const result = await Bun.build({
    entrypoints: ["./src/index.ts"],
    outdir: "./dist",
  });
  console.log(`Built ${result.outputs.length} files`);
} catch (err) {
  console.error("Build failed:", err);
  process.exit(1);
}

Bun.build Options

await Bun.build({
  // Entry points (required)
  entrypoints: ["./src/index.ts"],

  // Output directory
  outdir: "./dist",

  // Target environment
  target: "browser",  // "browser" | "bun" | "node"

  // Output format
  format: "esm",  // "esm" | "cjs" | "iife"

  // Minification
  minify: true,  // or { whitespace: true, identifiers: true, syntax: true }

  // Code splitting
  splitting: true,

  // Source maps
  sourcemap: "external",  // "none" | "inline" | "external" | "linked"

  // Naming patterns
  naming: {
    entry: "[dir]/[name].[ext]",
    chunk: "[name]-[hash].[ext]",
    asset: "[name]-[hash].[ext]",
  },

  // Define globals
  define: {
    "process.env.NODE_ENV": JSON.stringify("production"),
  },

  // External packages
  external: ["react", "react-dom"],

  // Loaders
  loader: {
    ".svg": "text",
    ".png": "file",
  },

  // Plugins
  plugins: [myPlugin],

  // Root directory
  root: "./src",

  // Public path for assets
  publicPath: "/static/",
});

CLI Flags

bun build <entrypoints> [flags]

| Flag | Description | |------|-------------| | `--outdir` | Output directory | | `--outfile` | Output single file | | `--target` | `browser`, `bun`, `node` | | `--format` | `esm`, `cjs`, `iife` | | `--minify` | Enable minification | | `--minify-whitespace` | Minify whitespace only | | `--minify-identifiers` | Minify identifiers only | | `--minify-syntax` | Minify syntax only | | `--splitting` | Enable code splitting | | `--sourcemap` | `none`, `inline`, `external`, `linked` | | `--external` | Mark packages as external | | `--define` | Define compile-time constants | | `--loader` | Custom loaders for extensions | | `--public-path` | Public path for assets | | `--root` | Root directory | | `--entry-naming` | Entry point naming pattern | | `--chunk-naming` | Chunk naming pattern | | `--asset-naming` | Asset naming pattern |

Target Environments

Browser (default)

await Bun.build({
  entrypoints: ["./src/index.ts"],
  target: "browser",
  outdir: "./dist",
});

Bun Runtime

await Bun.build({
  entrypoints: ["./src/server.ts"],
  target: "bun",
  outdir: "./dist",
});

Node.js

await Bun.build({
  entrypoints: ["./src/server.ts"],
  target: "node",
  outdir: "./dist",
});

Code Splitting

await Bun.build({
  entrypoints: ["./src/index.ts", "./src/admin.ts"],
  splitting: true,
  outdir: "./dist",
});

Shared dependencies are extracted into separate chunks automatically.

Loaders

| Loader | Extensions | Output | |--------|------------|--------| | `js` | `.js`, `.mjs`, `.cjs` | JavaScript | | `jsx` | `.jsx` | JavaScript | | `ts` | `.ts`, `.mts`, `.cts` | JavaScript | | `tsx` | `.tsx` | JavaScript | | `json` | `.json` | JavaScript | | `toml` | `.toml` | JavaScript | | `text` | - | String export | | `file` | - | File path export | | `base64` | - | Base64 string | | `dataurl` | - | Data URL | | `css` | `.css` | CSS file |

Custom loaders:

await Bun.build({
  entrypoints: ["./src/index.ts"],
  loader: {
    ".svg": "text",
    ".png": "file",
    ".woff2": "file",
  },
});

Plugins

const myPlugin = {
  name: "my-plugin",
  setup(build) {
    // Resolve hook
    build.onResolve({ filter: /\.special$/ }, (args) => {
      return { path: args.path, namespace: "special" };
    });

    // Load hook
    build.onLoad({ filter: /.*/, namespace: "special" }, (args) => {
      return {
        contents: `export default "special"`,
        loader: "js",
      };
    });
  },
};

await Bun.build({
  entrypoints: ["./src/index.ts"],
  plugins: [myPlugin],
});

Build Output

// Bun 1.2+: Bun.build rejects on failure. Use try/catch to surface build
// errors (or pass { throw: false } and keep the legacy { success, logs } shape).
try {
  const result = await Bun.build({
    entrypoints: ["./src/index.ts"],
    outdir: "./dist",
  });

  // Access outputs
  for (const output of result.outputs) {
    console.log(output.path);   // File path
    console.log(output.kind);   // "entry-point" | "chunk" | "asset"
    console.log(output.hash);   // Content hash
    console.log(output.loader); // Loader used

    // Read content
    const text = await output.text();
  }
} catch (err) {
  console.error("Build failed:", err);
  process.exit(1);
}

Common Patterns

Production Build

await Bun.build({
  entrypoints: ["./src/index.ts"],
  outdir: "./dist",
  target: "browser",
  minify: true,
  sourcemap: "external",
  splitting: true,
  define: {
    "process.env.NODE_ENV