pkgroll

Write your code in ESM & TypeScript and bundle it to get ESM, CommonJS, and type declaration outputs with a single command!

Features

• Zero config Configuration automatically inferred from package.json
• ESM/CJS Detection Uses .mjs/.cjs extension or package.json#type to infer module type
• Export & Import maps Uses export and import maps to configure endpoints and aliases
• TypeScript friendly Transform TS and emit .d.ts files (Supports .cts/.mts too!)
• Node.js ESM ⇄ CJS friendly Preserves named exports from ESM to CJS
• Fast Transformations powered by esbuild

Install

npm install --save-dev pkgroll

Quick setup

1. Setup your project with source files in src and output in dist (configurable).

2. Specify output files in package.json:

   {
       "name": "my-package",
   
       // Set "module" or "commonjs" (https://nodejs.org/api/packages.html#type)
       // "type": "module",
   
       // Define the output files
       "main": "./dist/file.js",
       "module": "./dist/file.mjs",
       "types": "./dist/file.d.ts",
   
       // Define output files for Node.js export maps (https://nodejs.org/api/packages.html#exports)
       "exports": {
           "require": "./dist/file.js",
           "import": "./dist/file.mjs",
           "types": "./dist/file.d.ts"
       },
   
       // bin files will be compiled to be executable with the Node.js hashbang
       "bin": "./dist/cli.js",
   
       // (Optional) Add a build script referencing `pkgroll`
       "scripts": {
           "build": "pkgroll"
       }
   
       // ...
   }

   Paths that start with ./dist/ are automatically mapped to files in the ./src/ directory.

3. Package roll!

   npm run build # or npx pkgroll

Usage

Entry-points

Pkgroll parses package entry-points from package.json by reading properties main, module, types, and exports.

The paths in ./dist are mapped to paths in ./src (configurable with --src and --dist flags) to determine bundle entry-points.

Output formats

Pkgroll detects the format for each entry-point based on the file extension or the package.json property it's placed in, using the same lookup logic as Node.js.

Auto-detect infers the type by extension or package.json#type:

Dependency bundling & externalization

Packages to externalize are detected by reading dependency types in package.json. Only dependencies listed in devDependencies are bundled in.

When generating type declarations (.d.ts files), this also bundles and tree-shakes type dependencies declared in devDependencies as well.

// package.json
{
    // ...

    "peerDependencies": {
        // Externalized
    },
    "dependencies": {
        // Externalized
    },
    "optionalDependencies": {
        // Externalized
    },
    "devDependencies": {
        // Bundled
    },
}

Aliases

Aliases can be configured in the import map, defined in package.json#imports.

For native Node.js import mapping, all entries must be prefixed with # to indicate an internal subpath import. Pkgroll takes advantage of this behavior to define entries that are not prefixed with # as an alias.

Native Node.js import mapping supports conditional imports (eg. resolving different paths for Node.js and browser), but Pkgroll does not.

⚠️ Aliases are not supported in type declaration generation. If you need type support, do not use aliases.

{
    // ...

    "imports": {
        // Mapping '~utils' to './src/utils'
        "~utils": "./src/utils",

        // Native Node.js import mapping (can't reference ./src)
        "#internal-package": "./vendors/package/index.js",
    }
}

Target

Pkgroll uses esbuild to handle TypeScript and JavaScript transformation and minification.

The target specifies the environments the output should support. Depending on how new the target is, it can generate less code using newer syntax. Read more about it in the esbuild docs.

By default, the target is set to the version of Node.js used. It can be overwritten with the --target flag:

pkgroll --target es2020 --target node14.18.0

It will also automatically detect and include the target specified in tsconfig.json#compilerOptions.

Strip node: protocol

Node.js builtin modules can be prefixed with the node: protocol for explicitness:

import fs from 'node:fs/promises';

This is a new feature and may not work in older versions of Node.js. While you can opt out of using it, your dependencies may still be using it (example package using node:: path-exists).

Pass in a Node.js target that that doesn't support it to strip the node: protocol from imports:

pkgroll --target node12.19

ESM ⇄ CJS interoperability

Node.js ESM offers interoperability with CommonJS via static analysis. However, not all bundlers compile ESM to CJS syntax in a way that is statically analyzable.

Because pkgroll uses Rollup, it's able to produce CJS modules that are minimal and interoperable with Node.js ESM.

This means you can technically output in CommonJS to get ESM and CommonJS support.

require() in ESM

Sometimes it's useful to use require() or require.resolve() in ESM. This can be seamlessly compiled to CommonJS. But when compiling to ESM, Node.js will error because require doesn't exist in the module scope.

When compiling to ESM, Pkgroll detects require() usages and shims it with createRequire(import.meta.url).

Minification

Pass in the --minify flag to minify assets.

pkgroll --minify

Watch mode

Run the bundler in watch mode during development:

pkgroll --watch

FAQ

Why bundle with Rollup?

Rollup has the best tree-shaking performance, outputs simpler code, and produces seamless CommonJS and ESM formats (minimal interop code). Notably, CJS outputs generated by Rollup supports named exports so it can be parsed by Node.js ESM. TypeScript & minification transformations are handled by esbuild for speed.

Why bundle Node.js packages?

• ESM and CommonJS outputs

  As the Node.js ecosystem migrates to ESM, there will be both ESM and CommonJS users. A bundler helps accommodate both distribution types.

• Dependency bundling yields smaller and faster installation.

  Tree-shaking only pulls in used code from dependencies, preventing unused code and unnecessary files (eg. README.md, package.json, etc.) from getting downloaded.

  Removing dependencies also eliminates dependency tree traversal, which is one of the biggest bottlenecks.

• Inadvertent breaking changes

  Dependencies can introduce breaking changes due to a discrepancy in environment support criteria, by accident, or in rare circumstances, maliciously.

  Compiling dependencies will make sure new syntax & features are downgraded to support the same environments. And also prevent any unexpected changes from sneaking in during installation.

• Type dependencies must be declared in the dependencies object in package.json for it to be resolved by the consumer.

  This can be unintuitive because types are a development enhancement and also adds installation bloat. Bundling filters out unused types and allows type dependencies to be declared in devDependencies.

• Minification strips dead-code, comments, white-space, and shortens variable names.