Zero-configuration bundler for tiny JS libs, powered by Rollup.

Usage no npm install needed!

<script type="module">
  import akinoxsolutionsMicrobundle from 'https://cdn.skypack.dev/@akinoxsolutions/microbundle';



Microbundle npm travis

The zero-configuration bundler for tiny modules, powered by Rollup.

Guide → SetupFormatsModern ModeUsage & ConfigurationAll Options

✨ Features

  • One dependency to bundle your library using only a package.json
  • Support for ESnext & async/await (via Babel & async-to-promises)
  • Produces tiny, optimized code for all inputs
  • Supports multiple entry modules (cli.js + index.js, etc)
  • Creates multiple output formats for each entry (CJS, UMD & ESM)
  • 0 configuration TypeScript support
  • Built-in Terser compression & gzipped bundle size tracking

🔧 Installation & Setup

1️⃣ Install by running: npm i -D microbundle

2️⃣ Set up your package.json:

  "name": "foo",                   // your package name
  "source": "src/foo.js",          // your source code
  "main": "dist/foo.js",           // where to generate the CommonJS/Node bundle
  "module": "dist/foo.module.js",  // where to generate the ESM bundle
  "unpkg": "dist/foo.umd.js",      // where to generate the UMD bundle (also aliased as "umd:main")
  "scripts": {
    "build": "microbundle",        // compiles "source" to "main"/"module"/"unpkg"
    "dev": "microbundle watch"     // re-build when source files change

3️⃣ Try it out by running npm run build.

💽 Output Formats

Microbundle produces esm, cjs, umd bundles with your code compiled to syntax that works pretty much everywhere. While it's possible to customize the browser or Node versions you wish to support using a browserslist configuration, the default setting is optimal and strongly recommended.

🤖 Modern Mode

In addition to the above formats, Microbundle also outputs a modern bundle specially designed to work in all modern browsers. This bundle preserves most modern JS features when compiling your code, but ensures the result runs in 90% of web browsers without needing to be transpiled. Specifically, it uses preset-modules to target the set of browsers that support <script type="module"> - that allows syntax like async/await, tagged templates, arrow functions, destructured and rest parameters, etc. The result is generally smaller and faster to execute than the esm bundle:

// Our source, "src/make-dom.js":
export default async function makeDom(tag, props, children) {
    let el = document.createElement(tag);
    el.append(...(await children));
    return Object.assign(el, props);

Compiling the above using Microbundle produces the following modern and esm bundles:

make-dom.modern.js (123b) make-dom.module.js (166b)
export default async function(e, t, a) {
    let n = document.createElement(e);
    n.append(...(await a));
    return Object.assign(n, t);
export default function(e, t, r) {
    try {
        var n = document.createElement(e);
        return Promise.resolve(r).then(function(e) {
            n.append.apply(n, e);
            return Object.assign(n, t);
    } catch (e) {
        return Promise.reject(e);

This is enabled by default - all you have to do is add the field to your package.json.

💁‍♂️ How to point to modern code in a package.json is being discussed. You might use the "module" field.
  "main": "dist/foo.umd.js",              // legacy UMD bundle (for Node & CDN use)
  "module": "dist/foo.modern.module.js",  // modern ES2017 bundle
  "scripts": {
    "build": "microbundle src/foo.js -f modern,umd"

📦 Usage & Configuration

Microbundle includes two commands - build (the default) and watch. Neither require any options, but you can tailor things to suit your needs a bit if you like.

microbundle / microbundle build

Unless overridden via the command line, microbundle uses the source property in your package.json to locate the input file, and the main property for the output:

  "source": "src/index.js",      // input
  "main": "dist/my-library.js",  // output
  "scripts": {
    "build": "microbundle"

For UMD builds, microbundle will use a camelCase version of the name field in your package.json as export name. This can be customized using an "amdName" key in your package.json or the --name command line argument.

microbundle watch

Acts just like microbundle build, but watches your source files and rebuilds on any change.

Using with TypeScript

Just point the input to a .ts file through either the cli or the source key in your package.json and you’re done.

Microbundle will generally respect your TypeScript config defined in a tsconfig.json file with notable exceptions being the "target" and "module" settings. To ensure your TypeScript configuration matches the configuration that Microbundle uses internally it's strongly recommended that you set "module": "ESNext" and "target": "ESNext" in your tsconfig.json.

Using CSS Modules

By default any css file imported as .module.css, will be treated as a css-module. If you wish to treat all .css imports as a module, specify the cli flag --css-modules true. If you wish to disable all css-module behaviours set the flag to false.

The default scope name when css-modules is turned on will be, in watch mode _[name]__[local]__[hash:base64:5] and when you build _[hash:base64:5]. This can be overriden by specifying the flag, eg --css-modules "_something_[hash:base64:7]". Note: by setting this, it will be treated as a true, and thus, all .css imports will be scoped.

flag import is css module?
null import './my-file.css'; :x:
null import './my-file.module.css'; :white_check_mark:
false import './my-file.css'; :x:
false import './my-file.module.css'; :x:
true import './my-file.css'; :white_check_mark:
true import './my-file.module.css'; :white_check_mark:

Specifying builds in package.json

Microbundle uses the fields from your package.json to figure out where it should place each generated bundle:

  "main": "dist/foo.js",            // CommonJS bundle
  "umd:main": "dist/foo.umd.js",    // UMD bundle
  "module": "dist/foo.m.js",        // ES Modules bundle
  "esmodule": "dist/foo.modern.js", // Modern bundle
  "types": "dist/foo.d.ts"          // TypeScript typings directory

Building a single bundle with a fixed output name

By default Microbundle outputs multiple bundles, one bundle per format. A single bundle with a fixed output name can be built like this:

microbundle -i lib/main.js -o dist/bundle.js --no-pkg-main -f umd

Mangling Properties

To achieve the smallest possible bundle size, libraries often wish to rename internal object properties or class members to smaller names - transforming this._internalIdValue to this._i. Microbundle doesn't do this by default, however it can be enabled by creating a mangle.json file (or a "mangle" property in your package.json). Within that file, you can specify a regular expression pattern to control which properties should be mangled. For example: to mangle all property names beginning an underscore:

    "mangle": {
        "regex": "^_"

It's also possible to configure repeatable short names for each mangled property, so that every build of your library has the same output. See the wiki for a complete guide to property mangling in Microbundle.

All CLI Options

    $ microbundle <command> [options]

Available Commands
    build    Build once and exit
    watch    Rebuilds on any change

For more info, run any command with the `--help` flag
    $ microbundle build --help
    $ microbundle watch --help

    -v, --version      Displays current version
    -i, --entry        Entry module(s)
    -o, --output       Directory to place build files into
    -f, --format       Only build specified formats (any of modern,es,cjs,umd or iife) (default modern,es,cjs,umd)
    -w, --watch        Rebuilds on any change  (default false)
    --pkg-main         Outputs files analog to package.json main entries  (default true)
    --target           Specify your target environment (node or web)  (default web)
    --external         Specify external dependencies, or 'none' (default peerDependencies and dependencies in package.json)
    --globals          Specify globals dependencies, or 'none'
    --define           Replace constants with hard-coded values
    --alias            Map imports to different modules
    --compress         Compress output using Terser
    --no-compress      Disable output compressing
    --strict           Enforce undefined global context and add "use strict"
    --name             Specify name exposed in UMD and IIFE builds
    --cwd              Use an alternative working directory  (default .)
    --sourcemap        Generate source map  (default true)
    --raw              Show raw byte size  (default false)
    --jsx              A custom JSX pragma like React.createElement (default: h)
    --jsxImportSource  Specify the automatic import source for JSX like preact
    --tsconfig         Specify the path to a custom tsconfig.json
    --css-modules      Configures .css to be treated as modules (default: null)
    -h, --help         Displays this message

    $ microbundle build --globals react=React,jquery=$
    $ microbundle build --define API_KEY=1234
    $ microbundle build --alias react=preact
    $ microbundle watch --no-sourcemap # don't generate sourcemaps
    $ microbundle build --tsconfig tsconfig.build.json

🛣 Roadmap

Here's what's coming up for Microbundle:

🔨 Built with Microbundle

  • Preact Fast 3kB React alternative with the same modern API. Components & Virtual DOM.
  • Stockroom Offload your store management to a worker easily.
  • Microenvi Bundle, serve, and hot reload with one command.
  • Theme UI Build consistent, themeable React apps based on constraint-based design principles.
  • react-recomponent Reason-style reducer components for React using ES6 classes.
  • brazilian-utils Utils library for specific Brazilian businesses.
  • react-hooks-lib A set of reusable react hooks.
  • mdx-deck-live-code A library for mdx-deck to do live React and JS coding directly in slides.
  • react-router-ext An Extended react-router-dom with simple usage.
  • routex.js A dynamic routing library for Next.js.
  • hooked-form A lightweight form-management library for React.
  • goober Less than 1KB css-in-js alternative with a familiar API.
  • react-model The next generation state management library for React

🥂 License