5.2.0 • Public • Published

Universal JavaScript Build and Packaging

This is a toolchain for the next phase of JavaScript development. It enables developers to write JavaScript modules in ESM that work universally across many environments (Node.js import, Node.js require, Browser, Deno, etc).

You start by just writing a standard ESM module. You'll need to include "type": "module" in your package.json, as well as a "main" entry point or an export map.

npx ipjs build

This will build a package in the dist directory that has different versions of your files compiled for different environments and a new package.json file that exposes all of these files to the correct Node.js and compiler entry points.

That's it ;)

You can also compile your tests.

npx ipjs build --tests

Note: You'll need to use "named self imports", import mymodule from "mymodule", in your tests so that we can compile different CJS versions of those tests for Node.js and the browser.

You can publish to by either running npm publish in the dist directory or using:

npx ipjs publish


There's a few Node.js and ESM features you need to stick to using and a few you need to avoid. Some are because there just isn't a very good way to provide consistent representations and some are because we need some more explicit information about your library in order to detect the dependency tree and built it successfully.

Only use named exports in files in the package.json exports map

If you use default exports from files in the exports map:

  • CJS/JSDoc+Typescript users of your module will find that tsc fails to compile as it expects a the .default property to be present on anything required from your module
  • If they switch to .default to satisfy tsc, node will resolve cjs at runtime via the "require": entry from the exports map which does not have a .default so will fail
  • If they switch back to no .default, running bundled cjs in the browser will also fail as it will be supplied the esm version where .default is present after all

Only export individual files in export map (no directories or pattern matching)

Avoid instanceof

The exports map means ESM and CJS environments load the same paths from different files:

// package.json
  "name": "my-esm-project",
  "exports": {
    "my-class": {
      "import": "esm/src/my-class.js",
      "require": "cjs/src/my-class.js",
  // ...other fields

If a CJS file uses instanceof on an instance of a class loaded from and instantiated in an ESM file (or vice versa), the check will fail, even though there may only be one copy of that dependency in the tree - it's because they've been loaded from different files within that dependency.

For example, consider my-esm-project, built with ipjs and published as dual cjs/esm:

// is-my-class.cjs
const MyClass = require('my-esm-project/my-class')

module.exports = (obj) => {
  return obj instanceof MyClass
// main.js
import isMyClass from './is-my-class.cjs'
import MyClass from 'my-esm-project/my-class'

const obj = new MyClass()

console.info(isMyClass(obj)) // false - perhaps not what we were expecting

It's possible to cross the ESM/CJS boundary like this several times during code execution, sometimes even within the same project so it's recommended not to use instanceof.




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  • mikeal