Bun Bundler

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]

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

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": 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
// Bun 1.2+: Bun.build rejects on failure, so try/catch is the modern idiom.
try {
  const result = await Bun.build({
    entrypoints: ["./src/index.ts"],
    outdir: "./dist",
    minify: process.env.NODE_ENV === "production",
  });
  console.log(`Built ${result.outputs.length} files`);
} catch (err) {
  console.error("Build failed:", err);
  process.exit(1);
}

Run: bun run build.ts

Common Errors

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