inop

Interoperability

Fast SWC-powered build tool that compiles TypeScript/JavaScript to dual ESM/CommonJS packages. Solve the dual package hazard and ship modern packages with ease.

Why inop?

Publishing JavaScript packages that work in both CommonJS and ESM environments is complicated:

• Dual Package Hazard: Risk of loading the same module twice in different formats
• Extension Hell: Managing .js, .cjs, .mjs extensions correctly
• Import Rewriting: Adjusting import paths for each module system
• Package.json Exports: Complex configuration for dual module support

inop handles all of this automatically - just write TypeScript, get both formats.

Installation

npm install --save-dev inop
# or
pnpm add -D inop
# or
yarn add -D inop

Quick Start

Transform your TypeScript source to dual modules:

npx inop src build

This creates both ESM (.js) and CommonJS (.cjs) versions of your code with proper source maps.

Example

Given this source structure:

src/
├── index.ts
├── utils.ts
└── constants.ts

Running npx inop src build produces:

build/
├── index.js        # ESM version
├── index.cjs       # CommonJS version  
├── index.js.map    # Source map for ESM
├── index.cjs.map   # Source map for CommonJS
├── utils.js
├── utils.cjs
├── constants.js
└── constants.cjs

Common Use Cases

Publishing npm packages

Use the -p flag to automatically configure package.json for dual module publishing:

npx inop src build -p

This updates your package.json with:

{
  "type": "module",
  "main": "build/index.cjs",       // CommonJS entry
  "module": "build/index.js",       // ESM entry
  "types": "build/index.d.ts",      // TypeScript types
  "exports": {
    "require": "./build/index.cjs", // Node.js require()
    "import": "./build/index.js"    // ESM import
  },
  "files": ["build", "src"]         // Include source for source maps
}

Monorepo packages

Exclude test files and mocks when building:

npx inop src build -i __tests__ -i __mocks__

Custom extensions

Use different file extensions for your output:

# Use .mjs for ESM and .js for CommonJS
npx inop src dist --esm-ext .mjs --commonjs-ext .js

# Generate only ESM
npx inop src dist --skip-commonjs

# Generate only CommonJS  
npx inop src dist --skip-esm

Optimized packages

Create a standalone package with only used dependencies:

npx inop src dist -p --copy

This creates a dist/package.json with only the dependencies actually imported in your code.

TypeScript Configuration

Since SWC doesn't generate type declarations, combine inop with TypeScript:

# Build modules and generate types
npx inop src build && tsc --declaration --emitDeclarationOnly

Or in your package.json:

{
  "scripts": {
    "build": "inop src build && tsc --declaration --emitDeclarationOnly"
  }
}

Recommended tsconfig.json

{
  "compilerOptions": {
    "target": "ESNext",
    "module": "NodeNext",
    "declaration": true,
    "esModuleInterop": true,
    "moduleResolution": "NodeNext",
    "rootDir": "src",
    "outDir": "build"
  }
}

Jest Configuration

For testing with Jest and SWC:

export default {
  coverageProvider: 'v8',
  extensionsToTreatAsEsm: ['.ts'],
  moduleNameMapper: {
    '^(\\.{1,2}/.*)\\.js$': '$1',  // Handle .js extensions in imports
  },
  transform: {
    '^.+\\.ts$': '@swc/jest',      // Use SWC for fast transpilation
  },
};

CLI Reference

npx inop <source> <build> [options]

Arguments

• source - Source directory containing TypeScript/JavaScript files
• build - Output directory for compiled modules

Options

• -p, --package - Update package.json with dual module configuration
• -c, --copy - Copy package.json to build directory with optimized dependencies
• -i, --ignore [patterns...] - Ignore file patterns (e.g., __tests__, *.spec.ts)
• -t, --type <type> - Source file type: js or ts (default: ts)
• -m, --match <pattern> - Override file matching pattern (default: **/*.ts)
• -s, --swcrc <path> - Custom .swcrc config path (default: .swcrc)
• --commonjs-ext <ext> - CommonJS file extension (default: .cjs)
• --esm-ext <ext> - ESM file extension (default: .js)
• --skip-commonjs - Generate only ESM output
• --skip-esm - Generate only CommonJS output
• -V, --version - Show version number
• -h, --help - Show help

Troubleshooting

Import paths not working?

Ensure your TypeScript uses .js extensions in imports:

// ✅ Correct - will be transformed appropriately
import { utils } from './utils.js';

// ❌ Wrong - missing extension
import { utils } from './utils';

Types not generated?

inop focuses on fast transpilation. Run TypeScript separately for declarations:

tsc --declaration --emitDeclarationOnly

Source maps not resolving?

Make sure source files are included in your npm package by using the -p flag or manually adding them to files in package.json.

License

License The MIT License Copyright (c) 2023-2024 Ivan Zakharchanka