bun-build

name	bun-build
description	Create optimized production bundles with Bun's native bundler. Use when building applications for production, optimizing bundle sizes, setting up multi-environment builds, or replacing webpack/esbuild/rollup.
compatibility	Requires Bun 1.0+
allowed-tools	["Bash","Write","Read"]
metadata	{"author":"dale","category":"bun-runtime","tags":["bun","build","bundler","production","optimization"]}

Bun Production Build Configuration

Set up production builds using Bun's native bundler - fast, optimized bundle creation without webpack or esbuild.

Quick Reference

For detailed patterns, see:

• Build Targets: targets.md - Browser, Node.js, library, CLI configurations
• Optimization: optimization.md - Tree shaking, code splitting, analysis
• Plugins: plugins.md - Custom loaders and transformations

Core Workflow

1. Check Prerequisites

# Verify Bun installation
bun --version

# Check project structure
ls -la package.json src/

2. Determine Build Requirements

Ask the user about their build needs:

• Application Type: Frontend SPA, Node.js backend, CLI tool, or library
• Target Platform: Browser, Node.js, Bun runtime, or Cloudflare Workers
• Output Format: ESM (modern), CommonJS (legacy), or both

3. Create Basic Build Script

Create build.ts in project root:

#!/usr/bin/env bun

const result = await Bun.build({
  entrypoints: ['./src/index.ts'],
  outdir: './dist',
  target: 'browser', // or 'node', 'bun'
  format: 'esm', // or 'cjs', 'iife'
  minify: true,
  splitting: true,
  sourcemap: 'external',
});

if (!result.success) {
  console.error('Build failed');
  for (const message of result.logs) {
    console.error(message);
  }
  process.exit(1);
}

console.log('✅ Build successful');
console.log(`📦 ${result.outputs.length} files generated`);

// Show bundle sizes
for (const output of result.outputs) {
  const size = (output.size / 1024).toFixed(2);
  console.log(`  ${output.path} - ${size} KB`);
}

4. Configure for Target Platform

For Browser/Frontend:

await Bun.build({
  entrypoints: ['./src/index.tsx'],
  outdir: './dist',
  target: 'browser',
  format: 'esm',
  minify: true,
  splitting: true,
  define: {
    'process.env.NODE_ENV': '"production"',
  },
  loader: {
    '.png': 'file',
    '.svg': 'dataurl',
    '.css': 'css',
  },
});

For Node.js Backend:

await Bun.build({
  entrypoints: ['./src/server.ts'],
  outdir: './dist',
  target: 'node',
  format: 'esm',
  minify: true,
  external: ['*'], // Don't bundle node_modules
});

For libraries, CLI tools, and other targets, see targets.md.

5. Add Production Optimizations

await Bun.build({
  entrypoints: ['./src/index.ts'],
  outdir: './dist',
  target: 'browser',

  // Maximum minification
  minify: {
    whitespace: true,
    identifiers: true,
    syntax: true,
  },

  // Code splitting for optimal caching
  splitting: true,

  // Content hashing for cache busting
  naming: {
    entry: '[dir]/[name].[hash].[ext]',
    chunk: 'chunks/[name].[hash].[ext]',
    asset: 'assets/[name].[hash].[ext]',
  },

  // Source maps for debugging
  sourcemap: 'external',
});

For advanced optimizations (tree shaking, bundle analysis, size limits), see optimization.md.

6. Environment-Specific Builds

Create build-env.ts:

#!/usr/bin/env bun

const env = process.env.NODE_ENV || 'development';

const configs = {
  development: {
    minify: false,
    sourcemap: 'inline',
    define: {
      'process.env.NODE_ENV': '"development"',
      'process.env.API_URL': '"http://localhost:3000"',
    },
  },
  production: {
    minify: true,
    sourcemap: 'external',
    define: {
      'process.env.NODE_ENV': '"production"',
      'process.env.API_URL': '"https://api.example.com"',
    },
  },
};

const config = configs[env as keyof typeof configs];

const result = await Bun.build({
  entrypoints: ['./src/index.ts'],
  outdir: './dist',
  target: 'browser',
  format: 'esm',
  splitting: true,
  ...config,
});

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

console.log(`✅ ${env} build successful`);

Run with:

NODE_ENV=production bun run build-env.ts

7. Update package.json

Add build scripts:

{
  "scripts": {
    "build": "bun run build.ts",
    "build:dev": "NODE_ENV=development bun run build-env.ts",
    "build:prod": "NODE_ENV=production bun run build-env.ts",
    "build:watch": "bun run build.ts --watch",
    "clean": "rm -rf dist"
  }
}

For libraries, also add:

{
  "type": "module",
  "main": "./dist/cjs/index.js",
  "module": "./dist/esm/index.js",
  "types": "./dist/esm/index.d.ts",
  "exports": {
    ".": {
      "import": "./dist/esm/index.js",
      "require": "./dist/cjs/index.js",
      "types": "./dist/esm/index.d.ts"
    }
  },
  "files": ["dist"]
}

8. Generate Type Declarations (Libraries)

For libraries, generate TypeScript declarations:

// build-lib-with-types.ts
import { $ } from 'bun';

// Build JavaScript
await Bun.build({
  entrypoints: ['./src/index.ts'],
  outdir: './dist',
  target: 'node',
  format: 'esm',
  minify: true,
});

// Generate type declarations
await $`bunx tsc --declaration --emitDeclarationOnly --outDir dist`;

console.log('✅ Built library with type declarations');

Build Options Reference

Target

• browser: For web applications (includes browser globals)
• node: For Node.js applications (assumes Node.js APIs)
• bun: For Bun runtime (optimized for Bun-specific features)

Format

• esm: ES Modules (modern, tree-shakeable) - Recommended
• cjs: CommonJS (legacy Node.js)
• iife: Immediately Invoked Function Expression (browser scripts)

Minification

minify: true                  // Basic minification
minify: {                     // Granular control
  whitespace: true,
  identifiers: true,
  syntax: true,
}

Source Maps

• external: Separate .map files (production)
• inline: Inline in bundle (development)
• none: No source maps

Verification

After building:

# 1. Check output directory
ls -lh dist/

# 2. Verify bundle size
du -sh dist/*

# 3. Test bundle
bun run dist/index.js

# 4. Check for errors
echo $?  # Should be 0

Common Build Patterns

Watch mode for development:

import { watch } from 'fs';

async function build() {
  await Bun.build({
    entrypoints: ['./src/index.ts'],
    outdir: './dist',
    minify: false,
  });
}

await build();

watch('./src', { recursive: true }, async (event, filename) => {
  if (filename?.endsWith('.ts')) {
    console.log(`Rebuilding...`);
    await build();
  }
});

Custom asset loaders:

loader: {
  '.png': 'file',     // Copy file, return path
  '.svg': 'dataurl',  // Inline as data URL
  '.txt': 'text',     // Inline as string
  '.json': 'json',    // Parse and inline
}

For custom plugins and advanced transformations, see plugins.md.

Troubleshooting

Build fails:

if (!result.success) {
  for (const log of result.logs) {
    console.error(log);
  }
}

Bundle too large: See optimization.md for:

• Bundle analysis
• Code splitting
• Tree shaking
• Size limits

Module not found: Check external configuration:

external: ['*']         // Exclude all node_modules
external: ['react']     // Exclude specific packages
external: []            // Bundle everything

Completion Checklist

• ✅ Build script created
• ✅ Target platform configured
• ✅ Minification enabled
• ✅ Source maps configured
• ✅ Environment-specific builds set up
• ✅ Package.json scripts added
• ✅ Build tested successfully
• ✅ Bundle size verified

Next Steps

After basic build setup:

1. Optimization: Add bundle analysis and size limits
2. CI/CD: Automate builds in your pipeline
3. Type Checking: Add pre-build type checking
4. Testing: Run tests before building
5. Deployment: Integrate with bun-deploy for containerization

For detailed implementations, see the reference files linked above.