Generated package.json should honour main and module fields

[terrymun]

Description

For now, it seems like the main and module fields of the generated package.json file are hardcoded to be the following format:

• [package-name].umd.js, and
• [package-name].esm.js

There are no ways to customise this, and it may cause issues especially when using the preserveModules: true option. When the entry file is set to be index.ts, for example, the built entry file will still be index.js even though the main/module fields refer to a file named after the package name + module type.

The package.json references may look like this:

{
  "name": "my-component-library",
  "version": "0.0.1",
  "main": "dist/index.js",
  "module": "dist/index.js",
  "typings": "dist/index.d.ts",
}

Yet, the generated package.json looks like this:

{
  "name": "my-component-library",
  "version": "0.0.1",
  "main": "./my-component-library.umd.js",
  "module": "./my-component-library.esm.js",
  "typings": "./index.d.ts",
}

while the file directory in the dist/ folder looks like:

dist/
├── index.js
└── index.d.ts

Motivation

This generally doesn't cause an issue since the consuming application, when encountering a non-existent main or module file, will simply fallback to look for index.js as a fail-safe behavior. However, we should still be allowed to customise the values of these fields.

There are also several use-cases on StackOverflow that highlights this need is not specifically relevant to me, but also a few others:

• nx - configure main/module for generated package.json
• @nrwl/web:package - output CJS

Suggested Implementation

Reconsider how the main and module fields are being written. Right now it is hardcoded to the package name: see lines here

nx/packages/web/src/executors/package/package.impl.ts

Lines 302 to 303 in 9d78629

	packageJson.main = entryFileTmpl.replace('<%= extension %>', 'umd');
	packageJson.module = entryFileTmpl.replace('<%= extension %>', 'esm');

Alternate Implementations

Allow us to define the main and module fields at the very least.

[jaysoo]

This is fixed in latest Nx version (14.1.9) where CJS vs ESM is handled better

[github-actions]

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