tslib should follow standards for ESM/CJS detection

tslib’s package.json has a few issues RE detecting whether it’s CJS or ESM. Due to lack of standards support, it has to be special-cased in a number of places, eg jest-preset-angular. Workaround like this are needed in any projects using jest, typescript, and tslib.

The following changes to the package.json would fix things - happy to provide a PR if it will be considered:

1. There is no "type": "module" field, which tells node code that .js files support ESM
2. The ESM tslib (currently "tslib.es6.js") should have an .mjs file extension
3. The CJS tslib (currently `“tslib.js”) should have a .cjs file extension.

To be clear, just the type field would help, but I think it would be preferable to complete all 3 at once.

[johncrim]

Hi @wmertens - there are 2 fixes I’ve used:

1. Explicitly link tslib.es6.js in your jest.config.js moduleNameMapper section, eg:

 ...
  moduleNameMapper: {
    tslib: 'tslib/tslib.es6.js',
  }
  ...

This is hard-coded in jest-preset-angular b/c the ability to resolve tslib ES6 in jest is broken (thus this issue).

1. Create a copy of tslib.es6.js and rename it to tslib.mjs - jest (and node) always treat .mjs files as ESM. Then put it in a place, or update webpack config, so that it’s linked instead of node_modules/tslib.

[wmertens]

@johncrim when I try option 1. it complains that node_modules isn’t transpiled, and I’d prefer to keep it that way, unless there’s a way to tell jest to only transpile tslib?

Option 2 is a variation of the package changes I made, so basically the same maintenance burden.