Add exports field to package.json in order to support moduleResolution node12/nodenext

🚀 Feature request

Current Behavior

When using "moduleResolution": "NodeNext", in your tsconfig it expects an "exports": ... field in package.json in order to resolve the package.

Otherwise: Cannot find module 'fp-ts/Either' or its corresponding type declarations.ts(2307)

Desired Behavior

Importing to work.

Suggested Solution

Shove this in package.json:

  "exports": {
    ".": {
      "import": {
        "types": "./es6/index.d.ts",
        "default": "./es6/index.js"
      },
      "require": {
        "types": "./lib/index.d.ts",
        "default": "./lib/index.js"
      }
    },
    "./*": {
      "import": {
        "types": "./es6/*.d.ts",
        "default": "./es6/*.js"
      },
      "require": {
        "types": "./lib/*.d.ts",
        "default": "./lib/*.js"
      }
    },
    "./es6/*": {
      "import": {
        "types": "./es6/*.d.ts",
        "default": "./es6/*.js"
      }
    },
    "./lib/*": {
      "require": {
        "types": "./lib/*.d.ts",
        "default": "./lib/*.js"
      }
    }
  },

Tested it locally by putting that above "main" like in the example as seen in the TS 4.7 link below, seems to do the job. The "./es6/*" or "./lib/*" parts are only needed for backwards-compatibility with the old import paths.

Who does this impact? Who is this for?

People wanting to use fp-ts with the new ECMAScript Module Support added with TypeScript 4.7.

Describe alternatives you’ve considered

Not using the new ECMAScript Module support feature. 🤷‍♂️ Most NPM packages aren’t updated to support this yet.

Additional context

TS 4.7 link: https://devblogs.microsoft.com/typescript/announcing-typescript-4-7/#esm-nodejs Node.js link: https://nodejs.org/api/packages.html#package-entry-points

Your environment

Software	Version(s)
fp-ts	2.12.1
TypeScript	4.7.3

[andreavaccari]

The above suggestion is mostly correct, but there’s a caveat with the file extensions. Depending on the the value of the type field in package.json, Node will treat .js files either as CJS or ESM, while it will always treat .cjs file as the former and .mjs files as the latter.

To make Node happy we have to either:

1. Rename ./es6/index.js to ./es6/index.mjs and ./es6/*.js to ./es6/*.mjs
2. Add type: module and rename ./lib/index.js to ./lib/index.cjs and ./lib/*.js to ./lib/*.cjs
3. Add a package.json inside ./es6 with type: module (and for good measure do the equivalent in ./lib)

There’s also the option of dropping CJS completely, adding type: module in package.json, and living a happier life.

[gcanti]

going to release 2.13.0-rc.3 with the fix asap

done