A simple and lightweight tool for preparing the publish of NPM packages.

npm, publish, prepare, release, library, tooling, rollup, babel, bundling, esm, jsx, npm-publish, react, typescript
npm install preppy@10.10.0



An incredibly simple and lightweight tool for preparing packaging for the publishing process.

Demo of Preppy

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Build Status Unix Build Status Windows Code Coverage Dependencies

🥁 Features:

  • Rock solid infrastructure. Builds on well maintained Acorn, Babel and Rollup under the hood.
  • Supports multiple entries (cli, client, server, library, ...) - even multiple binary entries.
  • Creates multiple output formats (ESM, CommonJS, UMD, ...)
  • Exports TypeScript definitions (respects types definition in package.json).
  • Lazy JSX support (powered by a custom Rollup plugin) to keep JSX intact while bundling. Major benefit for e.g. UI components: This moves decision over JSX debug capabilities or optimization settings into the application space.
  • Rebases assets to the bundled output destination. Say hello to images, web fonts, and more. It also supports assets references in CSS/SCSS.
  • Includes a watch mode for live development. Very useful for developing libraries.
  • Supports auto-executing binaries. This is super useful when dealing with development web servers for example.
  • Offers builds by compressing bundles with Terser as needed (for files with .min in their name).
  • Prints out generated file sizes of all bundles.
  • Injects common env-variables into the build (BUNDLE_{NAME|VERSION|TARGET}). Also NODE_ENV for all UMD builds.
  • Executes Babel with an environment based on NODE_ENV but with additional data from the target (e.g. node, lib or cli) and the output format (e.g. esm, cjs)
  • Supports JSON out of the box and inlines the serialized content into the bundle.

🔧 Installation:

For Preppy itself installation is done by executing one command.

$ npm install -D preppy

Dependending on your transpiling needs you need Babel with the requires presets/plugins. This is nothing extra to install typically as you might have these things in-place already. Example:

$ npm install -D @babel/plugin-transform-runtime @babel/preset-env @babel/preset-typescript @babel/core
$ npm install @babel/runtime corejs

Configure Babel

As transpiling happens via Babel you have to install the Babel Core, Plugins and Presets on your own. You also need to use a standard Babel Configuration inside your package.

Example babel.config.js (has to be CommonJS unfortunately):

module.exports = (api) => {
  const env = api.env()
  const caller = api.caller((inst) => (inst && inst.name) || "any")

  const isBundler = caller === "rollup-plugin-babel"
  const isCli = caller === "@babel/node"
  const isTest = (/\b(test)\b/).exec(env)
  const modules = (isTest && !isBundler) || isCli ? "commonjs" : false
  const isUmd = (/\b(umd)\b/).exec(env)

  return {
    sourceMaps: true,
    plugins: [
      isUmd ? null : [
        "@babel/transform-runtime", {
          corejs: 3
    presets: [
          useBuiltIns: "usage",
          corejs: 3,
          loose: true,
          allExtensions: true,
          isTSX: true

Note: env gets a lot more depth when working with Preppy. It's actually set to this: ${env}-${target}-${format} e.g. "development-browser-esm". This gives you more control e.g. more setting up targets for Browserslist.

Note: Leave out the "@babel/typescript" when you do not need TypeScript transpiling.

Note: Please disable transform-runtime for all UMD builds as UMD is better working when only clean named imports are kept external.

Installing Babel Dependencies

📦 Usage:

To keep things simple and reduce to number of dependencies, Preppy uses your local Babel configuration to transpile your code. You have to make sure that all required Babel mechanics, presets and plugins are installed locally to your project.

Preppy also support extracting TypeScript types into .d.ts files to make them usable by users of your libraries. The generated code is still transpiled by Babel. The standard typescript CLI is used for extracting the types.

Input Files

These are the typical entry points looked up for by Preppy:

  • src/index.{js|jsx|ts|tsx}

We made the experience that this works pretty fine for most projects. If you have the need for more input files, please report back to us.

Output Targets

Preppy produces exports of your sources depending on the entries of your packages package.json. It supports building for ESM, CommonJS and UMD. Just add the relevant entries to the package configuration.

  • CommonJS: main
  • EcmaScript Modules (ESM): module: New module standard for optimal tree shaking of bundlers.
  • Universal Module Definition (UMD): umd + unpkg for delivering a minified bundle to the CDN.

Basic Example:

  "name": "mypackage",
  "main": "lib/index.cjs.js",
  "module": "lib/index.esm.js",
  "unpkg": "lib/index.umd.min.js",

For exporting types with TypeScript you should add a types entry to your package.json as well:

  "name": "mypackage",
  "main": "lib/index.cjs.js",
  "module": "lib/index.esm.js",
  "unpkg": "lib/index.umd.min.js",
  "types": "lib/index.d.ts"

Binary Output(s)

Additionally Preppy is capable in generating for binary targets e.g. CLI tools. Not just one, but each of the one listed in the bin section of your package.json.

The following example generates a mycli binary which is generated from the matching source file.

Binaries are generally generated from one of these source files:

  • src/{name}.{js|ts}
  • src/cli/{name}.{js|ts}
  • src/cli.{js|ts}
  • src/cli/index.{js|ts}

Example Configuration:

  "name": "mycli",
  "bin": {
    "mycli": "bin/mycli"

Universal Output

Preppy also has some support for building universal libraries. While for most projects it's completely feasible to have one library for both NodeJS and browsers (or only for one of these), others might want (slightly) different packages to browsers and NodeJS.

The only difference here is that you have to define a new browser definition inside your package.json. Also the input fields differ from the one of the normal libraries or binaries:

  • src/client.{js|jsx|ts|tsx}
  • src/browser.{js|jsx|ts|tsx}
  • src/client/index.{js|jsx|ts|tsx}
  • src/browser/index.{js|jsx|ts|tsx}

For the NodeJS part you can use any of the following entries:

  • src/node.{js|jsx|ts|tsx}
  • src/server.{js|jsx|ts|tsx}
  • src/node/index.{js|jsx|ts|tsx}
  • src/server/index.{js|jsx|ts|tsx}

Example Configuration:

  "name": "mylib",
  "browser": "lib/browser.esm.js",
  "unpkg": "lib/browser.umd.js",
  "main": "lib/node.cjs.js",
  "module": "lib/node.esm.js"

Note: The bundle under browser is a ESM bundle which is ideally used by bundlers like Webpack or Parcel.

Note: When any of these files exists, we use prefer it over the normal library for the unpkg entry in package.json as well.

Environment Settings

Preppy injects these environment values into your code base:

  • process.env.BUNDLE_NAME: Extracted name field from package.json.
  • process.env.BUNDLE_VERSION: Extracted version field from package.json.
  • process.env.BUNDLE_TARGET: One of the supported targets. Either node, browser, lib or cli.
  • process.env.BUNDLE_ENV: Environment name. Typical values are development, production and test. Use this instead of NODE_ENV to inject the current value at bundle time.

These values are injected into your code base by replacing the original instance.

For UMD bundles we also replace:

  • process.env.NODE_ENV: Same as BUNDLE_ENV - mainly replaced as this is a pretty common standard field which can't be resolved in browsers.


  • It only works correctly when you use the whole identifier.
  • In verbose mode we are logging the environment settings configured.

Command Line Interface

Preppy comes with a binary which can be called from within your scripts section in the package.json file.

"scripts": {
  "prepare": "preppy"

There is also some amount of parameters you can use if the auto detection of your library does not work out correctly.

  $ preppy

  --entry-lib        Entry file for Library (universal) target [auto]
  --entry-browser    Entry file for Browser target [auto]
  --entry-node       Entry file for NodeJS target [auto]
  --entry-cli        Entry file for CLI target [auto]

  --root             The root folder of your project [auto]
  --output           Override output folder (and package.json entries) [auto]
  --watch            Keeps running and rebuilds on any change [false]
  --limit            Limits the current build scope to files matching [null]
  --exec             Executes the generated binary after creation [false]
  --notify           Enables notifications (useful when used in watch mode) [false]

  --no-sourcemap     Disables creation of a source map file during processing [false]

  -v, --verbose      Verbose output mode [false]
  -q, --quiet        Quiet output mode [false]


Apache License; Version 2.0, January 2004


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