Support require(esm) in Node.js to avoid dual package hazard

[fabon-f]

(This proposal is same to fullcalendar/temporal-polyfill#62)

// in Node.js REPL
const { Temporal: Temporal1 } = await import('@js-temporal/polyfill')
const { Temporal: Temporal2 } = require('@js-temporal/polyfill')
Temporal2.Now.instant() instanceof Temporal2.Instant // true
Temporal2.Now.instant() instanceof Temporal1.Instant // false!!!

Temporal object exported from ESM and CJS is actually different, so identifying the object type by instanceof can cause the bug. (real world example with Luxon: 11ty/eleventy#3674)

Dual package hazard can be minimized by adding module-sync export condition. If Node.js supports require(esm), require and import returns identical module object from ESM, so instanceof works fine.

The module-sync condition is simply ignored in older versions of Node.js (no downside).

https://nodejs.org/docs/latest-v22.x/api/packages.html#conditional-exports

package.json should be like below. (perhaps types in module-sync is unnecessary, because TypeScript ignores module-sync export condition)

{
	"exports": {
		".": [
			{
				// add this
				"module-sync": {
					"types": "./index.d.ts",
					"default": "./dist/index.esm.js"
				},
				"import": {
					"types": "./index.d.ts",
					"default": "./dist/index.esm.js"
				},
				"require": {
					"types": "./index.d.cts",
					"default": "./dist/index.cjs"
				},
				"default": "./dist/index.cjs"
			},
			"./dist/index.cjs"
		]
	}
}

Note that Node.js v23, v22, and v20 already support require(esm). Node.js v18 will reach EOL in next month. Therefore, most users (except few users using legacy Node.js) will benefit from this change.

[fabon-f]

I noticed that module-sync is originally intended to be used as below, which resolves dual package hazard also in legacy Node.js (nodejs/node#54648). With this setup, both import and require loads CommonJS version in Node.js v18 or below (even legacy Node.js can import CommonJS module via interop feature). This package doesn't use export default, so interop shouldn't be confusing.

{
  "type": "module",
  "exports": {
    "node": {
      // load ESM in modern Node.js  (both `import` and `require`)
      "module-sync": "./index.js",
      // load CJS in legacy Node.js (both `import` and `require`)
      "default": "./dist/index.cjs"
    },
    // other than Node.js (ESM)
    "default": "./index.js"
  }
}

But I'm not sure where to specify d.ts file in this setup.