tsconfig-to-dual-package

A Node.js dual package tool for TypeScript.

You can support CommonJS and ESModules in one package via this tool. This tool add package.json which is { "type": "module" } or { "type": "commonjs" } based on tsconfig's module and outDir option.

You can use this tool with tsc command.

$ tsc -p . && tsc -p ./tsconfig.cjs.json && tsconfig-to-dual-package # add "{ourDir}/package.json"

Install

Install with npm:

npm install tsconfig-to-dual-package --save-dev

Requirements: This tool depended on typescript package for parsing tsconfig.json file. It means that You need to install typescript as devDependencies in your project.

• PeerDependency:
  • typescript: * (any version)
• Node.js v16.17.0+

Usage

Usage
  $ tsconfig-to-dual-package [Option] <tsconfig.json>

Options
  --cwd                 [String] current working directory. Default: process.cwd()
  --debug               [Boolean] Enable debug output
  --help                [Boolean] show help

Examples
  # Find tsconfig*.json in cwd and convert to dual package
  $ tsconfig-to-dual-package
  # Convert specified tsconfig.json to dual package
  $ tsconfig-to-dual-package ./config/tsconfig.esm.json ./config/tsconfig.cjs.json

How it works

This tool adds package.json to tsconfig's outDir for dual package. Each generated package.json has type field that is commonjs or module.

You can see example repository in following:

• tsconfig-to-dual-package-example

For example, This project package.json is following:

{
  "name": "my-package",
  "version": "1.0.0",
  "type": "module",
  "main": "./lib/index.js",
  "types": "./lib/index.d.ts",
  "module": "./module/index.js",
  // Note: Normally same .js extension can not be used as dual package
  //      but this tool add custom `package.json` to each outDir(=lib/, module/) and resolve it.
  "exports": {
    ".": {
      "import": {
        "types": "./module/index.d.ts",
        "default": "./module/index.js"
      },
      "require": {
        "types": "./lib/index.d.ts",
        "default": "./lib/index.js"
      },
      "default": "./lib/index.js"
    }
  }
}

And, This project has tsconfig.json and tsconfig.cjs.json:

tsconfig.json: for ES Module

{
  "compilerOptions": {
    "module": "ESNext",
    "moduleResolution": "NodeNext",
    "esModuleInterop": true,
    "newLine": "LF",
    "outDir": "./module/",  // <= Output ESM to `module` directory
    "target": "ES2018",
    "strict": true,
  },
  "include": [
    "**/*"
  ]
}

tsconfig.cjs.json: for CommonJS

{
  "extends": "./tsconfig.json",
  "compilerOptions": {
    "module": "CommonJS",
    "moduleResolution": "Node",
    "outDir": "./cjs/" // <= Output CommonJS to `cjs` directory
  },
  "include": [
    "**/*"
  ]
}

Then, You can run tsconfig-to-dual-package after you compile both CommonJS and ES Module with following command:

{
  "scripts": {
    "build": "tsc -p ./tsconfig.json && tsc -p ./tsconfig.cjs.json && tsconfig-to-dual-package",
  }
}

tsconfig-to-dual-package command adds package.json to module and cjs directory.

As a result, you can publish both CommonJS and ESModule in a single package. It is called dual package.

- package.json          // { "type": "module" }
- index.ts              // Node.js treat this as ESModule
- tsconfig.json         // output to `module` directory
- tsconfig.cjs.json     // output to `cjs` directory
- cjs/
    - package.json      // { "type": "commonjs" }
    - index.js          // Node.js treat it as CommonJS module
- module/
    - package.json      // { "type": "module" }
    - index.js          // Node.js treat it as ESModule

For more details, please see Dual CommonJS/ES module packages in Node.js official document.

• Example: tsconfig-to-dual-package-example
  • Distribution files: https://www.npmjs.com/package/@azu/tsconfig-to-dual-package-example?activeTab=explore

Limitation

This tool copy almost fields from package.json to generated {outDir}/package.json. However, it does not copy main, module, exports, types fields because it points invalid file path. It defined in OMIT_FIELDS constant.

Used by

• eventmit
  • Work on CJS: https://github.com/azu/events-to-async/pull/4
  • Work on ESM: https://github.com/azu/eventmit-module-env
  • Work on Deno: https://github.com/azu/eventmit-deno-env
  • Work on Browser: https://codesandbox.io/s/determined-poitras-yll61f?file=/index.html
• safe-marked
  • Migraion PR: feat: Support CJS and ESM as dual package by azu · Pull Request #58 · azu/safe-marked

Motivation

• TypeScript disallow to change file extension of generated files from .ts by Design
  • Feature Request: allow change file extension of generated files from .ts · Issue #49462 · microsoft/TypeScript
  • allow voluntary .ts suffix for import paths · Issue #37582 · microsoft/TypeScript
  • bug(esm): TypeScript is not an ECMAScript superset post-ES2015 · Issue #50501 · microsoft/TypeScript
• Node.js require separate .mjs and .cjs if you need to get dual package in one package

As a result, TypeScript and Node.js ESM support is conflicting. It is hard that you can support dual package with same .js extension.

Of course, you can use tsc-multi or Packemon to support dual packages. However, These are build tools. I want to use TypeScript compiler(tsc) directly.

tsconfig-to-dual-package do not touch TypeScript compiler(tsc) process. It just put package.json({ "type": "module" } or "{ "type": "commonjs" }) to outDir for each tsconfig.json after tsc compile source codes.

@aduh95 describe the mechanism in https://github.com/nodejs/node/issues/34515#issuecomment-664209714

For reference, the library-package/package.json contains:

{
	"name": "library-package",
	"version": "1.0.0",
	"main": "./index-cjs.js",
	"exports": {
		"import": "./index-esm.js",
		"require": "./index-cjs.js"
	},
	"type": "module"
}

Setting "type": "module" makes Node.js interpret all .js files as ESM, including index-cjs.js. When you remove it, all .js files will be interpreted as CJS, including index-esm.js. If you want to support both with .js extension, you should create two subfolders:

$ mkdir ./cjs ./esm
$ echo '{"type":"commonjs"}' > cjs/package.json
$ echo '{"type":"module"}' > esm/package.json
$ git mv index-cjs.js cjs/index.js
$ git mv index-esm.js esm/index.js

And then have your package exports point to those subfolders:

{
	"name": "library-package",
	"version": "1.0.0",
	"main": "./cjs/index.js",
	"exports": {
		"import": "./esm/index.js",
		"require": "./cjs/index.js"
	},
	"type": "module"
}

Also, Node.js documentation describe this behavior as follows

The nearest parent package.json is defined as the first package.json found when searching in the current folder, that folder's parent, and so on up until a node_modules folder or the volume root is reached.

// package.json
{
  "type": "module"
}

# In same folder as preceding package.json
node my-app.js # Runs as ES module

If the nearest parent package.json lacks a "type" field, or contains "type": "commonjs", .js files are treated as CommonJS. If the volume root is reached and no package.json is found, .js files are treated as CommonJS.

-- https://nodejs.org/api/packages.html#type

Pros and Cons

Pros

• You can use TypeScript compiler(tsc) directly
  • No additional bundler, transpiler, build tool

Cons

• You need to run tsconfig-to-dual-package after tsc compile
• This tool copy package.json to outDir. This approach may affect path finding for package.json like read-pkg-up
• Dual package hazard - I recommend that you should not use this approach for stateful package.
  • For example, a singleton and instanceof check for user-input may cause unexpected behavior.
  • This Dual package has a risk of loading double(require and import load separate resources).
  • Very large package may want to prevent loading double package. For example, a large dictionary included package.
• Dual package is hard to use some API like __diranme, __filename without transpiler
  • Normally, you can use import.meta.url and new URL(..., import.meta.url) to get __dirname and __filename in ESM.
  • On the other hands, import.meta.url is disallowed syntax in CJS
  • import.meta is not defined in CJS
  • __diraname is not defined in ESM
  • As a result, it is hard to use __dirname and __filename in dual package.
  • Some package get these via Error stack trace
    • bevry/filedirname: Fetch the current file and directory path, no matter your environment (Deno, Node.js, Web Browsers, ESM, CJS)
    • fwh1990/this-file: Create dynamic __dirname, __filename and require method for both ESM and CJS
    • JumpLink/cross-dirname: Node.js + Gjs + Deno module that returns the current script dirname. Similar to __dirname but also works in CommonJs and ES modules.
  • 🆘 If you know a solution about this problem, please send pull request!

FAQ

What should I do support dual package?

• Example repository: tsconfig-to-dual-package-example
• Pull Request: feat: support dual package by azu · Pull Request #2 · azu/tsconfig-to-dual-package-example
• Steps:
  • Install tsconfig-to-dual-package: npm install --save-dev tsconfig-to-dual-package
  • Add "type": "module" to package.json via npm pkg set type=module
  • Add tsconfig.json and tsconfig.cjs.json
  • Create tsconfig.json and set it to use module: "esnext"
  • Create tsconfig.cjs.json and set it to use module: "commonjs"
  • Add tsconfig-to-dual-package to build script
    • "build": "tsc -p ./tsconfig.json && tsc -p ./tsconfig.cjs.json && tsconfig-to-dual-package"
  • Add "main"/"types"(for backward compatibility)/"files"/"exports" fields to package.json
    • "files": ["lib/", "module/"] (lib/ = cjs, module/ = esm)
    • "main"/"types"/"exports"

    {
      "main": "./lib/index.js",
      "types": "./lib/index.d.ts",
      "exports": {
        ".": {
          "import": {
            "types": "./module/index.d.ts",
            "default": "./module/index.js"
          },
          "require": {
            "types": "./lib/index.d.ts",
            "default": "./lib/index.js"
          },
          "default": "./module/index.js"
        },
        "./package.json": "./package.json"
      }
    }
    

  • Check Check Check
    • Lint
      • npx publint is helpful
      • dependency-check@5 is useful
    • Test
      • use ts-node/esm instead of ts-node for testing
      • https://github.com/TypeStrong/ts-node#node-flags-and-other-tools
  • Publish!
    • npm publish
  • After Check!
    • publint
    • https://arethetypeswrong.github.io/
    • Load test via require/import

Is there a migration script?

It is not for everyone, but I wrote a migration script for TypeScript project.

• Migration Script: Convert TypeScript project to Node.js dual package
  • This script make almost migration automatic
  • Use npm pkg command for change package.json
  • Use tsconfig-to-dual-package to build dual package
  • Use eslint-cjs-to-esm to migrate source code to ESM from CJS
  • Use publint to check package.json

Example Result:

• refactor: migrate to dual package by azu · Pull Request #4 · azu/markdown-function

Should I migrate to dual package?

• If your package is a library, you should migrate to dual package if possible
  • Because dual package reduce interop issues between CJS and ESM
  • If your package is just logics, you can move to dual package
• If your package is a Command Line Tool(CLI), you not need to migrate to dual package
  • Because CLI is not loaded from require function
  • You can move to Pure ESM package

References

• Dual CommonJS/ES module packages
• Improve documentation on Dual Module Packages · Issue #34515 · nodejs/node
• TypeScript: Documentation - ECMAScript Modules in Node.js
  • Why types fields at first
  • Issue: package.json exports resolution uses fallback conditions, unlike Node · Issue #50762 · microsoft/TypeScript
• why is there a package.json export for package.json? · Issue #1 · tsmodule/tsmodule
  • Why add "./package.json": "./package.json"
  • Special treatment for package.json resolution and exports? · Issue #33460 · nodejs/node
• frehner/modern-guide-to-packaging-js-library: A guide to help ensure your JavaScript library is the most compatible, fast, and efficient library you can make.

Related

• publint: Lint your exports field in package.json
• eslint-cjs-to-esm: help you to migrate CJS to ESM
• isaacs/rimraf: A rm -rf util for nodejs: use same approach

Changelog

See Releases page.

Running tests

Install devDependencies and Run npm test:

npm test

Contributing

Pull requests and stars are always welcome.

For bugs and feature requests, please create an issue.

1. Fork it!
2. Create your feature branch: git checkout -b my-new-feature
3. Commit your changes: git commit -am 'Add some feature'
4. Push to the branch: git push origin my-new-feature
5. Submit a pull request :D

Author

• azu: GitHub, Twitter

License

MIT © azu