Bun Bundler Skill | Agent Skills

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

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

if (!result.success) {
  console.error("Build failed:", result.logs);
}

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

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

// Check success
if (!result.success) {
  for (const log of result.logs) {
    console.error(log);
  }
  process.exit(1);
}

// 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();
}

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": JSON.stringify("production"),
  },
});

Library Build

await Bun.build({
  entrypoints: ["./src/index.ts"],
  outdir: "./dist",
  target: "bun",
  format: "esm",
  external: ["*"],  // Externalize all dependencies
  sourcemap: "external",
});

Build Script

// build.ts
const result = await Bun.build({
  entrypoints: ["./src/index.ts"],
  outdir: "./dist",
  minify: process.env.NODE_ENV === "production",
});

if (!result.success) {
  console.error("Build failed");
  process.exit(1);
}

console.log(`Built ${result.outputs.length} files`);

Run: bun run build.ts

Common Errors

| Error | Cause | Fix | |-------|-------|-----| | Could not resolve | Missing import | Install package or fix path | | No matching export | Named export missing | Check export name | | Unexpected token | Syntax error | Fix source code | | Target not supported | Invalid target | Use browser, bun, or node |

When to Load References

Load references/options.md when:

Need complete option reference
Configuring advanced features

Load references/plugins.md when:

Writing custom plugins
Understanding plugin API

Load references/macros.md when:

Using compile-time macros
Build-time code generation

Agent Skills: Bun Bundler

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