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📦 Zero-configuration bundler for tiny modules.

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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
  • Support for ESnext & async/await (via Babel & async-to-promises)
  • Produces tiny, optimized code for all inputs
  • Supports multiple entry modules (
    , 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

  "name": "foo",                     // your package name
  "type": "module",
  "source": "src/foo.js",            // your source code
  "exports": "./dist/foo.modern.js", // where to generate the modern bundle (see below)
  "main": "./dist/foo.cjs",          // where to generate the CommonJS 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

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 95% of web browsers without needing to be transpiled. Specifically, it uses Babel's "bugfixes" mode (previously known as preset-modules) to target the set of browsers that support
 - 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 plain 

Take the following source code for example:

// 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

make-dom.modern.js (117b) make-dom.module.js (194b)
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) {
            return n.append.apply(n, e), Object.assign(n, t);
    } catch (e) {
        return Promise.reject(e);

This is enabled by default. All you have to do is add an

field to your
    "main": "./dist/foo.umd.js", // legacy UMD output (for Node & CDN use)
    "module": "./dist/foo.module.js", // legacy ES Modules output (for bundlers)
    "exports": "./dist/foo.modern.js", // modern ES2017 output
    "scripts": {
        "build": "microbundle src/foo.js"


field can also be an object for packages with multiple entry modules:
    "name": "foo",
    "exports": {
        ".": "./dist/foo.modern.js", // import "foo" (the default)
        "./lite": "./dist/lite.modern.js", // import "foo/lite"
        "./full": "./dist/full.modern.js" // import "foo/full"
    "scripts": {
        "build": "microbundle src/*.js" // build foo.js, lite.js and full.js

📦 Usage & Configuration

Microbundle includes two commands -

(the default) and
. Neither require any options, but you can tailor things to suit your needs a bit if you like.
  • microbundle
    – bundles your code once and exits. (alias:
    microbundle build
  • microbundle watch
    – bundles your code, then re-bundles when files change.

ℹ️ Microbundle automatically determines which dependencies to inline into bundles based on your


Read more about How Microbundle decides which dependencies to bundle, including some example configurations.

Specifying filenames in package.json

Unless overridden via the command line, microbundle uses the

property in your
to determine which of your JavaScript files to start bundling from (your "entry module"). The filenames and paths for generated bundles in each format are defined by the
properties in your
  "source": "src/index.js",            // input
  "main": "dist/foo.js",               // CommonJS output bundle
  "umd:main": "dist/foo.umd.js",       // UMD output bundle
  "module": "dist/foo.m.js",           // ES Modules output bundle 
  "exports": {
    "require": "./dist/foo.js",        // CommonJS output bundle
    "default": "./dist/foo.modern.js", // Modern ES Modules output bundle
  "types": "dist/foo.d.ts"             // TypeScript typings directory

When deciding which bundle to use, Node.js 12+ and webpack 5+ will prefer the

property, while older Node.js releases use the
property, and other bundlers prefer the
field. For more information about the meaning of the different properties, refer to the Node.js documentation.

For UMD builds, microbundle will use a camelCase version of the

field in your
as export name. Alternatively, this can be explicitly by adding an
key in your
, or passing the
command line argument.

Usage with

Node.js 12.16+ adds a new "ES Module package", which can be enabled by adding

to your package.json. This property changes the default source type of
files to be ES Modules instead of CommonJS. When using
, the file extension for CommonJS bundles generated by Microbundle must be changed to
  "type": "module",
  "module": "dist/foo.js",  // ES Module bundle
  "main": "dist/foo.cjs",   // CommonJS bundle

Additional Configuration Options

Config also can be overridded by the

property in your

  "main": "src/index.ts",          // this would be used in the dev environment (e.g. Jest)
  "publishConfig": {
    "source": "src/index.js",      // input
    "main": "dist/my-library.js",  // output
  "scripts": {
    "build": "microbundle"

Building a single bundle with 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

Using with TypeScript

Just point the input to a

file through either the cli or the
key in your
and you’re done.

Microbundle will generally respect your TypeScript config defined in a

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"
"target": "ESNext"
in your

To ensure Microbundle does not process extraneous files, by default it only includes your entry point. If you want to include other files for compilation, such as ambient declarations, make sure to add either "files" or "include" into your


If you're using TypeScript with CSS Modules, you will want to set

"include": ["node_modules/microbundle/index.d.ts"]
in your
to tell TypeScript how to handle your CSS Module imports.

CSS and CSS Modules

Importing CSS files is supported via

import "./foo.css"
. By default, generated CSS output is written to disk. The
--css inline
command line option will inline generated CSS into your bundles as a string, returning the CSS string from the import:
// with the default external CSS:
import './foo.css'; // generates a minified .css file in the output directory

// with microbundle --css inline: import css from './foo.css'; console.log(css); // the generated minified stylesheet

CSS Modules: CSS files with names ending in

are treated as a CSS Modules. To instead treat imported
files as modules, run Microbundle with
--css-modules true
. To disable CSS Modules for your project, pass
--css-modules false

The default scope name for CSS Modules is

in watch mode, and
for production builds. This can be customized by passing the command line argument
--css-modules "[name]_[hash:base64:7]"
, using these fields and naming conventions.

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

Building Module Workers

Microbundle is able to detect and bundle Module Workers when generating bundles in the

formats. To use this feature, instantiate your Web Worker as follows:
worker = new Worker(new URL('./worker.js', import.meta.url), { type: 'module' });
// or simply:
worker = new Worker('./worker.js', { type: 'module' });

... then add the

flag to your build command:
microbundle --workers

For more information see @surma/rollup-plugin-off-main-thread.

Mangling Properties

To achieve the smallest possible bundle size, libraries often wish to rename internal object properties or class members to smaller names - transforming

. Microbundle doesn't do this by default, however it can be enabled by creating a
file (or a
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.

Defining build-time constants


option can be used to inject or replace build-time constants when bundling. In addition to injecting string or number constants, prefixing the define name with
allows injecting JavaScript expressions.

| Build command | Source code | Output | | -------------------------------------------- | ---------------------- | ----------------------- | |

microbundle --define VERSION=2
| |
microbundle --define API_KEY='abc123'
| |
microbundle --define @assign=Object.assign
assign(a, b)
Object.assign(a, b)

All CLI Options

    $ microbundle  [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

Options -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,esm,cjs,umd or iife) (default modern,esm,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 (use @key=exp to replace an expression) --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 --generateTypes Whether or not to generate types, if types or typings is set in package.json then it will default to be true --css Where to output CSS: "inline" or "external" (default: "external") --css-modules Configures .css to be treated as modules (default: null) --workers Bundle module workers - see (default false) -h, --help Displays this message

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

🛣 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


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