esm-interop-lab

Tests how different tools handle import thunk from 'redux-thunk' where redux-thunk@2 ships both a CJS build (main: lib/index.js, has __esModule: true) and a native ESM build (module: es/index.js, export default thunk).

Setup

npm install

Results

Bundlers

Script	Resolves via	is function?	how
npm run inspect	require() directly	-	raw shape: {__esModule:true, default:fn}
npm run node	main (Node ignores module)	❌	no interop
npm run babel	main via require()	✅	_interopRequireDefault at call site
npm run esbuild:module	module (native ESM)	✅	no interop needed - real ESM default
npm run esbuild:main	main (CJS + __esModule)	❌	no interop
npm run webpack:module	module (native ESM)	✅	no interop needed - real ESM default
npm run webpack:main	main (CJS + __esModule)	❌	no interop
npm run typescript:cjs	main via require()	✅	__importDefault at call site
npm run typescript:esm	main (NodeNext, no helpers)	❌	no interop helpers emitted

Jest

Script	Mode	Resolves via	is function?
npm run jest:cjs	CJS (babel-jest)	main	✅
npm run jest:esm	ESM (--experimental-vm-modules)	main	❌
npm run jest:esm:module	ESM (--experimental-vm-modules)	module	💥

Why jest:esm:module fails

The resolver returns node_modules/redux-thunk/es/index.js via the module field, but Jest's shouldLoadAsEsm() checks the file extension (.js) and the package's "type" field to decide CJS vs ESM. redux-thunk@2 has neither .mjs nor "type": "module", so Jest treats it as CJS and cjs-module-lexer chokes on the export statement.

Run everything

npm run all