How to make an npm package that supports both cjs and esm?

Modularity is a commonplace issue, and packaging tools are emerging one after another.

So, how to use these packaging tools to create npm packages that support both cjs and esm.

This article does not involve concepts, it is some packaged tests.

demo repo: https://github.com/FrankKai/npm-bundle-demo

You can clone it and build the test locally.

• tsc
• rollup
• webpack
• esbuild

---

tsc

• tsconfig.json
• tsconfig-esm.json
• package.json

cjs

tsconfig.json

 {
  "compilerOptions": {
    "target": "ES2015",
    "module": "commonjs",
    "outDir": "./dist/cjs",
    "esModuleInterop": true,
    "moduleResolution": "node"
  }
}

esm

tsconfig-esm.json

 {
  "extends": "./tsconfig.json",

  "compilerOptions": {
    "target": "es2015",
    "module": "es2015",
    "outDir": "./dist/esm",
    "moduleResolution": "node"
  }
}

package.json

 {
  "main": "./dist/cjs/index.js",
  "module": "./dist/esm/index.js",
  "scripts": {
    "build": "rm -rf dist && tsc -p tsconfig.json && tsc -p tsconfig-esm.json"
  },
}

---

rollup

• rollup.config.js
• package.json

rollup.config.js

 export default [
  {
    input: "src/index.js",
    output: [
      { file: "dist/index.cjs.js", format: "cjs" },
      { file: "dist/index.esm.js", format: "es" },
    ],
  },
];

package.json

 {
  "main": "dist/index.cjs.js",
  "module": "dist/index.esm.js",
  "scripts": {
    "build": "rollup -c",
  },
}

---

webpack

• webpack.config.js
• package.json

webpack.config.js

 const path = require("path");

module.exports = {
  mode: 'none',
  entry: {
    "index.cjs": {
      import: './src/index.js',
      library: {
        type: 'commonjs2',
      },

    },
    "index.esm": {
      import: './src/index.js',
      library: {
        type: 'module',
      },
    },
  },
  output: {
    path: path.resolve(__dirname, 'dist'),
    filename: "[name].js",
    clean: true,
  },
  experiments: {
    outputModule: true
  }
};

package.json

 {
  "main": "dist/index.cjs.js",
  "module": "dist/index.esm.js",
  "scripts": {
    "build": "webpack",
  },
  "devDependencies": {
    "webpack": "^5.38.1",
    "webpack-cli": "^4.7.2"
  }
}

---

esbuild

• package.json

   {
  "main": "dist/index.cjs.js",
  "module": "dist/index.esm.js",
  "scripts": {
    "esbuild:cjs": "esbuild ./src/index.js --bundle --outfile=dist/index.cjs.js --format=cjs",
    "esbuild:esm": "esbuild ./src/index.js --bundle --outfile=dist/index.esm.js --format=esm",
    "build": "npm run esbuild:cjs && npm run esbuild:esm"
  },
  "devDependencies": {
    "esbuild": "^0.14.49"
  },
  }