Module systems

A barrel can only re-export files it is able to load. Barrelify works out which those are from the barrel's own extension, and the "type" of the nearest package.json, the same way Node and TypeScript's nodenext resolution do.

Extensions

Every export uses the extension TypeScript compiles the file to:

Source file	Exported as
foo.ts	./foo.js
Foo.tsx	./Foo.js
foo.cts	./foo.cjs
foo.mts	./foo.mjs

.tsx compiles to .js, as with TypeScript's react-jsx output. If foo.ts and foo.tsx both exist, ./foo.js is exported once.

Which files are exported

A .ts or .tsx file is ESM when the nearest package.json sets "type": "module", and CommonJS otherwise. .mts is always ESM, and .cts always CommonJS. ESM can import CommonJS, but CommonJS can't require ESM, so:

Barrel	"type": "module"	"type": "commonjs" or unset
index.ts, index.tsx	.ts, .tsx, .cts, .mts	.ts, .tsx, .cts
index.mts	.ts, .tsx, .cts, .mts	.ts, .tsx, .cts, .mts
index.cts	.cts	.ts, .tsx, .cts

For example, the same files in two packages:

├─┬ esm
│ ├── package.json // { "type": "module" }
│ ├── commonjs.cts
│ ├── esm.ts
│ ├── module.mts
│ └── index.ts     // AUTO-BARREL
└─┬ cjs
  ├── package.json // { "type": "commonjs" }
  ├── cjs.ts
  ├── commonjs.cts
  ├── module.mts
  └── index.ts     // AUTO-BARREL

esm/index.ts

// AUTO-BARREL

export * from './commonjs.cjs';
export * from './esm.js';
export * from './module.mjs';

cjs/index.ts

// AUTO-BARREL

export * from './cjs.js';
export * from './commonjs.cjs';

cjs/index.ts compiles to CommonJS, which can't load module.mts, so it is left out. Rename the barrel to index.mts to export it too.

The nearest package.json

Barrelify finds the nearest package.json with Node's own lookup (module.findPackageJSON), so a nested package.json with its own "type" changes the module system of the barrels below it. A malformed nearest package.json is an error, as it would be for Node.