esm2cjs

Command line utility to compile a JS project from ES modules to CommonJS. This is handy when building hybrid ESM/CommonJS npm packages.

Built on top of the blazing fast esbuild and supports all modern JS features.

Install

Either globally:

# using npm
npm i -g @alcalzone/esm2cjs
# using yarn
yarn add --global @alcalzone/esm2cjs

Or locally as a devDependency:

# using npm
npm i -D @alcalzone/esm2cjs
# using yarn
yarn add --dev @alcalzone/esm2cjs

Usage

Using the binary outside of package.json scripts requires global installation.

esm2cjs --in path/to/input --out path/to/output [options]

Detailed help is shown on the command line using

esm2cjs --help

Example for a TypeScript project

1. Configure tsconfig.json to write ESM output into build/esm:

   // tsconfig.json
   {
   	// ...
   	"compilerOption": {
   		// ...
   		"outDir": "build/esm",
   		"module": "ES2020" // or some of the new options in TS 4.5 "node12", "nodenext"
   	}
   }

2. Add a postbuild script that transforms the ESM output to CommonJS:

   // package.json
   {
   	// ...
   	"scripts": {
   		// ...
   		"postbuild": "esm2cjs --in build/esm --out build/cjs -l error"
   	}
   }

3. Set up the exports field in package.json. Note that the syntax for types might need additional changes when TypeScript 4.5 lands.

   // package.json
   {
   	"main": "build/cjs/index.js",
   	"module": "build/esm/index.js",
   	"exports": {
   		".": {
   			"import": "./build/esm/index.js",
   			"require": "./build/cjs/index.js"
   		},
   		// This is necessary to require/import `your-library/package.json`
   		"./package.json": "./package.json",
   		// additional subpath exports go here, e.g. your-library/tools
   		"./tools": {
   			"import": "./build/esm/tools.js",
   			"require": "./build/cjs/tools.js"
   		},
   		// or everything in the source root:
   		"./*": {
   			"import": "./build/esm/*.js",
   			"require": "./build/cjs/*.js"
   		}
   	},
   	// ...
   }

Example for a JavaScript project without compilation

This assumes your ESM modules are located in lib/ and the cjs output goes to dist/cjs/.

1. Add a build script that transforms the ESM output to CommonJS:

   // package.json
   {
   	// ...
   	"scripts": {
   		// ...
   		"build": "esm2cjs --in lib --out dist/cjs -l error"
   	}
   }

2. Set up the exports field in package.json.

   // package.json
   {
   	"main": "dist/cjs/index.js",
   	"module": "lib/index.js",
   	"exports": {
   		".": {
   			"import": "./lib/index.js",
   			"require": "./dist/cjs/index.js"
   		},
   		// This is necessary to require/import `your-library/package.json`
   		"./package.json": "./package.json",
   		// additional subpath exports go here, e.g. `your-library/tools`
   		"./tools": {
   			"import": "./lib/tools.js",
   			"require": "./dist/cjs/tools.js"
   		},
   		// or everything in the lib folder:
   		"./*": {
   			"import": "./lib/*.js",
   			"require": "./dist/cjs/*.js"
   		}
   	}
   	// ...
   }

Performance

Test case 1: zwave-js

| No. of input files | total size | time taken | | ------------------ | ---------- | ---------- | | 141 | 987 KB | 390 ms |

Changelog

1.4.2 (2025-10-08)

Dependency updates

1.4.1 (2024-11-20)

• Set esbuild's keepNames option to true by default

1.4.0 (2024-11-11)

• Support inheriting and rewriting the imports field in package.json

1.3.0 (2024-11-06)

• Support specifying sideEffects in the generated package.json as a hint for bundlers

1.2.3 (2024-11-04)

• Update default target to node18
• Shim import.meta.url by default
• Update documentation for package.json setup

1.1.2 (2022-08-17)

Dependency updates

1.1.1 (2021-10-18)

Dependency updates

1.1.0 (2021-06-19)

Support specifying output target and platform

1.0.0 (2021-06-16)

Initial release