microbundle

library0.15.1jsnpmunverified

Microbundle is a zero-configuration bundler for small JavaScript libraries, currently at version 0.15.1. Powered by Rollup, it simplifies the build process by automatically generating optimized bundles in multiple formats: CommonJS (CJS), ECMAScript Modules (ESM), Universal Module Definition (UMD), and a 'modern' ESM bundle for contemporary browsers. It supports ESnext features, async/await, multiple entry points, TypeScript out-of-the-box, built-in Terser compression, and gzipped bundle size tracking. While not adhering to a strict release cadence, it receives frequent updates, often in the form of patch releases, indicating active maintenance. Key differentiators include its minimal setup (often just `package.json` configuration) and focus on producing tiny, performant outputs with modern browser targets by default.

devops

npm install microbundle

github ↗package ↗

INSTALL

IMPORT

SIG · MICROBUNDLE

microbundle

devopsjavascriptv0.15.1

Install

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Import

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Disk

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Pass rate

0/ 6

Env Coverage0 / 6

glibc

18–22

musl

18–22

Install & Compatibility

Where this runs

tested against v? · npm install

Install × environment matrix

Each cell = how many times install + import succeeded across repeated harness runs. Partial = flaky.
glibc = Debian/Ubuntu slim · musl = Alpine Linux

musl

node 18–226 runs

build_error

glibc

node 18–226 runs

build_error

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

microbundle

✓ /* Microbundle is a CLI tool. Use via package.json scripts or npx. */

✗ import microbundle from 'microbundle'; // or const microbundle = require('microbundle');

Microbundle is primarily a Command Line Interface (CLI) tool. It is not designed for programmatic import as a library. Its functionality is accessed via the `microbundle` command line utility, typically executed through `package.json` scripts like `npm run build` or directly with `npx microbundle`. Direct `import` or `require` of the package itself will not expose a callable API but rather execute the CLI logic.

build command

✓ npm run build // or npx microbundle

✗ import { build } from 'microbundle';

There is no `build` function exported by Microbundle for programmatic use. All bundling operations are configured and initiated via the command line interface, using options specified in `package.json` or as CLI flags. The recommended way to run it is through npm/yarn scripts or `npx`.

watch command

✓ npm run dev // (where 'dev' script is 'microbundle watch') // or npx microbundle watch

✗ import { watch } from 'microbundle';

Similar to the `build` command, there is no `watch` function exported. The `microbundle watch` command is part of the CLI for continuous rebuilding during development. Attempting to import a `watch` function will result in an undefined export or module resolution error.

Demonstrates a typical `package.json` setup for Microbundle, defining input/output paths for CJS, ESM, UMD, and modern bundles, along with example source code. Shows how to integrate Microbundle into `npm` scripts for building and watching files.

{ "name": "my-tiny-lib", "version": "1.0.0", "type": "module", "source": "src/index.js", "exports": { "require": "./dist/index.cjs", "default": "./dist/index.modern.js" }, "main": "./dist/index.cjs", "module": "./dist/index.module.js", "unpkg": "./dist/index.umd.js", "scripts": { "build": "microbundle", "dev": "microbundle watch" }, "devDependencies": { "microbundle": "^0.15.0" } } // src/index.js export const greet = (name) => `Hello, ${name}!`; export default function sum(a, b) { return a + b; } // To run: // 1. npm install // 2. npm run build // This will generate dist/index.cjs, dist/index.modern.js, dist/index.module.js, dist/index.umd.js

microbundle --version

Debug

Known issues

breakingStarting with v0.15.0, Microbundle now outputs ESM files with the `.mjs` extension when the `package.json` type is explicitly set to `"type": "module"` and the target is CJS, or if the package type is CJS and ESM is output.

fix

Update your import paths and `package.json` 'exports' fields to correctly reference `.mjs` files where applicable, especially when migrating projects or upgrading from older Microbundle versions. Ensure your build pipeline and consuming applications correctly resolve `.mjs` extensions.

affects: >=0.15.0

gotchaWhen bundling multiple entry points that reference different CSS assets, versions prior to 0.15.1 might incorrectly bundle only the CSS referenced by the first entry point, leading to missing styles in other bundles.

fix

Upgrade to Microbundle v0.15.1 or later to ensure all CSS assets across multiple entry points are correctly packaged.

affects: <0.15.1

gotchaTerser annotations (e.g., `/*@__NOINLINE__*/`) might have been incorrectly removed during the Babel pass in versions prior to v0.13.3, impacting optimization hints.

fix

Upgrade to Microbundle v0.13.3 or later to ensure Terser annotations are correctly preserved during the build process.

affects: <0.13.3

gotchaRelying solely on `main` and `module` fields in `package.json` without the `exports` field can lead to incorrect module resolution, especially in modern Node.js environments or when supporting different environments (ESM vs CJS).

fix

Always define the `exports` field in your `package.json` alongside `main` and `module` to ensure robust and explicit module resolution for various environments, as shown in the quickstart example.

affects: All versions

Errors

Common errors & fixes

Error: 'source' field missing from package.json

Microbundle requires a `source` field in `package.json` to identify the primary entry point(s) for your library.

fix

Add a `"source": "src/index.js"` (or your actual source file path) entry to your `package.json`.

SyntaxError: Cannot use import statement outside a module

This typically occurs when a CommonJS consumer tries to `require()` an ESM bundle or vice-versa, or when Node.js encounters an `import` statement in a file treated as CommonJS.

fix

Ensure your `package.json`'s `main`, `module`, and `exports` fields correctly point to the appropriate CJS and ESM bundles. For Node.js, ensure `type: "module"` is set for ESM-only packages, or use `.mjs`/`.cjs` extensions explicitly in `exports`.

TypeError: Cannot read properties of undefined (reading 'length') related to CSS bundling.

This error (or similar unexpected behavior with CSS) can occur in Microbundle versions prior to v0.15.1 when multiple entry files reference distinct CSS assets, as the bundler might fail to process all of them correctly.

fix

Update Microbundle to version `0.15.1` or newer. If the issue persists, verify your CSS import paths and ensure they are valid for bundling.

Upgrade

Version history

0.15.1latest on npm

Audit

Dependencies

No dependency data recorded yet.

Agent activity

2 hits · last 30 days

node

Resources

githubhttps://github.com/developit/microbundle ↗

packagehttps://www.npmjs.com/package/microbundle ↗