Modules, Runtime & Tooling

ESM, CommonJS & Module Interop

ES modules are the JavaScript standard module system, while CommonJS is Node's older module format. Interop depends on runtime rules, package metadata, file extensions, compiler options, and bundler behavior.

ESM uses static import/export syntax
CommonJS uses runtime require/module.exports
Package type and file extension matter
Default import interop is a footgun
Libraries have harder compatibility choices than apps

Module systems
System	Syntax	Resolution/behavior
ESM	import/export	Standard, static, async-capable loading
CommonJS	require/module.exports	Node legacy, dynamic
Dual package	Exports ESM and CJS	Consumer compatibility, more complexity
Bundler output	Tool-generated	May hide runtime details until publishing

ESM vs CommonJS

// ESM
export function parse(input) {
  return JSON.parse(input)
}

import { parse } from "./parse.js"

// CommonJS
module.exports = { parse }
const { parse } = require("./parse")

Similar intent, different runtime/module-system rules.