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dual-packaging

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example of how dual packaging works in Node

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# Node "exports", "module" fields explanation

Node doesn't allow using `.js` extension for both `esm` and `commonjs` files in the same project when importing files _from_ that project. This is why this `exports` field is **BAD**:

```json
{
  "exports": {
    ".": {
      "import": "./index.esm.js",
      "require": "./index.cjs.js"
    }
  }
}
```

Node also doesn't support "module" field by its resolution algorithm (this is bundlers convention that wasn't officially adopted by Node), so it is highly encouraged to use "exports" field alongside "module" field for the older bundlers. In the examples bellow only "exports" field is used.

## Prerequisite

We have 4 packages:

- **test-cjs** is a commonjs package that has incorrect `exports` field for "import" files (has `.js` extension)
- **test-esm** is a esm pckage that has incorrect `exports` field for "require" files (has `.js` extension)
- **test-mixed** is commonjs package that has correct `exports` field for "import" and "require" files (has `.mjs` and `.cjs` extensions)
- **test-folder** is a commonjs package that has correct `exports` field for "import" and "require" files (has `.js` extension, but `esm` is inside `/esm` folder with near `package.json` having `"type": "module"`)

## Examples

> Run `pnpm install` before running examples

When running this package, you will get an error, depending on its `type` field:

- run `node cjs/test-esm.js` to get `require() of ES modules is not supported` error (cjs `requires` cjs-like file inside esm package)

- which means you can only use `import` syntax with this module in Node
  - example: when running `node esm/test-esm.mjs`, you will not get an error (esm `imports` esm)

- run `node esm/test-cjs.mjs` to get `The requested module 'test-cjs' is a CommonJS module` error (esm `imports` esm-like file inside cjs package)

- which means you can only use `require` syntax with this module in Node
  - example: when running `node cjs/test-cjs.js`, you will not get an error (cjs `requires` cjs)

- if you run `node cjs/test-mixed.js` or `node esm/test-mixed.mjs` you will see no errors because it has correct `exports` field.
- if you run `node cjs/test-folder.js` or `node esm/test-folder.mjs` you will see no errors because it has correct `exports` field, and `esm` file has a `package.json` near it with `"type": "module"`.

To fix this, bundle your files with appropriate extensions:

1. If your package doesn't have `"type"` in your `package.json`, use `.cjs`/`.js` extensions, when files have `commonjs` syntax, and `.mjs` extension for files with `esm` syntax

```jsonc
{
  "exports": {
    ".": {
      "import": "./index.mjs",
      "require": "./index.js" // or "./index.cjs"
    }
  }
}
```

2. If your package has `"type": "commonjs"` in your `package.json`, use `.cjs`/`.js` extensions, when files have `commonjs` syntax, and `.mjs` extension for files with `esm` syntax

```jsonc
{
  "type": "commonjs",
  "exports": {
    ".": {
      "import": "./index.mjs",
      "require": "./index.js" // or "./index.cjs"
    }
  }
}
```

3. If your package has `"type": "module"` in your `package.json`, use `.cjs` extension, when files have `commonjs` syntax, and `.mjs`/`.js` extension for files with `esm` syntax

```jsonc
{
  "type": "module",
  "exports": {
    ".": {
      "import": "./index.js", // or "./index.mjs"
      "require": "./index.cjs"
    }
  }
}
```

4. If your bundler doesn't allow mixing extensions, you can build `esm` files inside `/esm` folder and put a `package.json` with `"type": "module"` inside.

`package.json` in the root:

```jsonc
{
  "exports": {
    ".": {
      "import": "./esm/index.js",
      "require": "./index.js"
    }
  }
}
```

`package.json` in `/esm`:

```jsonc
{
  "type": "module"
}
```

> Warning: These examples work with Webpack/Rollup/Vite and other bundler's pipelines because they don't _run_ these files, they only read and analyze them, but they will **FAIL** when run inside Node by tools like Vitest, SSR or manually.

To build you project correctly, use one of these configs (you can see this in `examples` folder):

## rollup.config.js

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

## webpack.config.js

> TODO

## vite.config.js

```js
export default {
  build: {
    lib: {
      entry: path.resolve(__dirname, "src/index.js"),
      name: "MyName",
      fileName: (format) => `index.${format == "es" ? "mjs" : "js"}`,
    },
  },
};
```

## tsup.config.js

```js
export default {
  entry: ["src/index.js"],
  format: ["esm", "cjs"],
};
```

## using tsc

This will create `dist` folder with commonjs files, and `esm` folder inside with esm files.

You will need two `tsconfig.json` configs:

```jsonc
// tsconfig.cjs.json
{
  "extends": "./tsconfig.json",
  "compilerOptions": {
    "module": "commonjs",
    "outDir": "dist",
    "target": "es2015"
  }
}
```

```jsonc
// tsconfig.esm.json
{
  "extends": "./tsconfig.json",
  "compilerOptions": {
    "module": "esnext",
    "outDir": "dist/esm",
    "target": "esnext"
  }
}
```

Run these command to generate `dist`:

```bash
$ tsc -p tsconfig.cjs.json
$ tsc -p tsconfig.esm.json
```

Then you need to put `package.json` with `"type": "module"` inside `dist/esm` folder. You can do it in several ways, but the easiest would be running this command:

```bash
$ echo >dist/esm/package.json "{\"type\":\"module\"}"
```

# See also

- [Dual CommonJS/ES module packages in official Node.js documentation](https://nodejs.org/api/packages.html#dual-commonjses-module-packages)
- [Publish ESM and CJS in a single package](https://antfu.me/posts/publish-esm-and-cjs) by [Anthony Fu](https://github.com/antfu)
- [How to Create a Hybrid NPM Module for ESM and CommonJS](https://www.sensedeep.com/blog/posts/2021/how-to-create-single-source-npm-module.html) on SenseDeep
- [publint](https://publint.bjornlu.com/) tool to check if package is published right, by [Bjorn Lu](https://bjornlu.com/)

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