Research: ES Modules vs CommonJS

Motivation

We've decided on using JavaScript config files. But this presents the problem whether to support both CommonJS and ES Modules, and how that can be implemented and tested.

Comparison

	CommonJS	ES Modules
ecosystem trends	older and more widely supported	Node 13+ and some newer packages
import/export syntax	require() and module.exports =	import and export, also dynamic import()
explicit file extensions	.cjs/.cts	.mjs/.mts
implicit file extensions (.js)	if "type" not specified or set to "commonjs" in package.json	if "type": "module" in package.json
package.json's "exports" (separate entry points)	"require": "<path/to/cjs/script>"	"import": "<path/to/esm/script>"
interoperability	ESM->CJS: must use async import()	CJS->ESM: import ... from './foo.cjs'

References

• CommonJS (cjs) and Modules (esm): Import compatibility
• TypeScript: ECMAScript Modules in Node.js
• Jest: ECMAScript Modules

Build tools

• some bundlers support multiple formats (tsup, Rollup)
  • Nx has a Rollup plugin (@nx/rollup) 😍

Testing

• Jest runs as CommonJS, support for ESM experimental
• Vitest runs as ESM (also an Nx plugin 😍)