TypeScript icon, indicating that this package has built-in type declarations

1.2.10 • Public • Published

Typescript ESM Packager

A collection of tools to build ESM packages with TypeScript.

npm (scoped) npm GitHub

Why would I need this?

If you're building a library that you want to publish to NPM, you'll probably want to publish it as an ESM module. And you'll probably want to write it in TypeScript because maintaining untyped JavaScript is a nightmare.

But this is a lot of work, and it's easy to get it wrong. This package is a collection of tools that will help you get it right.

Let's talk about what a Good Package looks like...

Entirely written as ESM modules

require/exports is dead. Long live import/export! ESM modules are supported in all modern JavaScript runtimes such as Node, Deno, and the browser.

This means that all of our files should have the .mjs extension, and the type declarations should have the .d.mts extension. It also means that we should be able to import our files using the import keyword, and we should be able to export our files using the export keyword.

Personally, I'm mildly-annoyed about the .mjs extension. But we're here and have other things to worry about, so let's move on...

You also don't want to be that developer who releases their package with busted type definitions, or forces everyone to use allowSyntheticDefaultImports in their tsconfig.json file.

You want to be the developer who releases a package that Just Works™️.

ESM everywhere. All the time.


If you're building a library, you want your users to be able to debug your library. This means shipping type-declarations and source maps.

And if you're working with an organization that has a specific style guide, it means shipping your source code in a format that matches that style guide. It's not fun trying to debug a library while your editor's linter lights up like a Christmas tree.

Compatible with Node's package.json exports field

The exports field in package.json allows you to specify which files in your package are accessible to users of your package. This is useful if you want to hide implementation details from your users.

Here's the bare minimum that you should have in your package.json file:

  "name": "my-awesome-package",
  // Tell Node that we're an ESM module.
  "type": "module",
  // Some tools still depend on the `main` field...
  "main": "./dist/mod.mjs",
  // Older versions of TypeScript still depend on the `types` field...
  "types": "./dist/mod.d.mts",
  "exports": {
    // Expose the `package.json` to make everyone's lives easier...
    "./package.json": "./package.json",
    // Expose `import { foo } from "my-awesome-package"`
    ".": {
      "import": "./dist/mod.mjs",
      "types": "./dist/mod.d.mts"
    // Let users import `my-awesome-package/mod` to get the specific file.
    "./mod": {
      "import": "./dist/mod.mjs",
      "types": "./dist/mod.d.mts"
    // Same as above, but with allowing users to access the `.mjs` extension.
    "./mod.mjs": {
      "import": "./dist/mod.mjs",
      "types": "./dist/mod.d.mts"

We use mod rather than index because Node does all sorts of magic to make index files work. We don't want that magic, to interfere with understanding exactly what's going on. If you use index files and later have to debug your package in the browser, it will be a nightmare.


A well formed NPM package should be able to be used in the browser with little to no effort. This means that we should be able to import our package using a <script type="module"> tag. No bundlers, no build steps, no nothing.

Some of Node's built in modules, such as fs, do not have browser equivalents. That's okay, but you should separate these exports into their own module so that your users can import them separately:

// file-loaders/node.mjs

// Prefix with `node:` to clue users in that this is a Node-specific module.
// TypeScript understands this syntax and will not complain about missing types.
import * as path from 'node:path'
import * as fs from 'node:fs/promises'

export function loadSomeFile() {
  // ...
// file-loaders/browser.mjs

export function loadSomeFile() {
    .then((response) => response.json())
    .then((data) => {
      // ...

Just make sure that you don't accidentally import the Node-specific module in the browser, or vice versa. You'll also need to update your package.json exports to exposes a mod.mjs file for each of these.


If you're building a library, your users shouldn't have to use a bundler to use your library.

However, if your users do want to use a bundler, this requires no extra effort on your part. As long as your distributed package is unbundled, your users can use a bundler to bundle your package.

Let your users bring a bundler if they want to take advantage of tree-shaking, minification, etc.


This is less important than the other points, but it's place of confusion for a lot of developers. Deno uses ESM modules, however it largely ignores the package.json config for a import-map approach. It's honestly a bit of a mess, but here's the important part:

import { foo } from 'https://deno.land/x/my-awesome-package/mod.mts'
import { someLocalFile } from './some-local-file.mts'
import { SomeReactComponent } from './some-local-file.tsx'

Deno expects fully qualified URLs for remote modules, and relative paths for local modules. It also expects the .mts extension for TypeScript files, and the .tsx extension for TypeScript JSX files.

This is where things get horrible.

Your project's tsconfig.json defines how TypeScript compiles your code. It does not define how TypeScript emits your code. The TypeScript team does not want to get into the business of rewriting import paths file extensions.

This means that your only option was to either omit file extensions on your imports and exports, or set module: "esnext" in your tsconfig.json file:

// Meanwhile, in mod.mts...
import { someLocalFile } from './some-local-file.mjs' // WTF???
import { SomeReactComponent } from './some-local-file' // Why no file extension?

The short-version of what's going on is that TypeScript will output exports exactly as written, but will only resolve them using the module setting in your tsconfig.json file. The JSX thing is a weird bug because there's no such thing as .mtsx or .mjsx files. And until TypeScript 5, it wasn't possible to include uncompiled file extensions in your TypeScript code.

You've just discovered why you might need this package. 🎉



yarn add @sister.software/typescript-esm-packager
# or
npm install --save @sister.software/typescript-esm-packager


Ironically, this package actually uses itself to rewrite its own source code. You can find a more detailed example in this repo's own build script.

Here's a quick example of how to use this package to rewrite '.mts' files to '.mjs' files:

import * as path from 'node:path'
import { fileURLToPath } from 'node:url'

import {
} from '@sister.software/typescript-esm-packager'

// ESM modules don't have __dirname, so we have to use import.meta.url...
const __dirname = path.dirname(fileURLToPath(import.meta.url))

const programConfig: SimpleProgramConfig = {
  // Load the tsconfig.json file...
  // This function is just a wrapper around TypeScript's `ts.readConfigFile` function...
  tsConfig: readParsedTSConfig(path.join(__dirname, 'tsconfig.json')),
  // Create a transformer that...
  transformer: new TSPathTransformer(),
  // Just for fun, we'll also format the output files with Prettier...
  writeFileCallback: await createPrettierWriteFileCallback(),

// Clear out any previous builds...
await cleanTSBuildDirectory(programConfig.tsConfig)

const watch = process.argv.includes('--watch')

if (watch) {
  // Create a program that watches for changes and re-emits the files...
} else {
  // Or, create a program that emits the files once...
  const program = createSimpleTSProgram(programConfig)


Your build directory should now contain .mjs files instead of .mts files!


@sister.software/typescript-esm-packager is licensed under the MIT License.

Inspiration for this project comes from Dropbox's deprecated ts-transformer-path-rewrite package 💕

Package Sidebar


npm i @sister.software/typescript-esm-packager

Weekly Downloads






Unpacked Size

58.9 kB

Total Files


Last publish


  • teffenellis