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What

  • A minimal example jest.config.ts
  • Tested works with edge ESM-only packages. Here is node-fetch as example.

Usage

Adapt below files to your project folder

  • jest.config.ts
  • config/tsconfig*json

If you accept --experimental-vm-modules flag, you can get full ESM jest, or I say hardcore ESM.

  • external ESM-only packages -> jest vm modules

Pro: No hardcode.

Con: Both Node.js and jest official document still state that as experimental.

$ export NODE_OPTIONS="$NODE_OPTIONS --experimental-vm-modules"
$ jest
$ env NODE_OPTIONS="$NODE_OPTIONS --experimental-vm-modules" jest

If you don't accept --experimental-vm-modules flag, you get ESM->CJS, or I say faking ESM. Hardcore ESM feature within test/config/ESMOnlyHard.spec.ts not work.

  • external ESM-only packages -> transformer(ts-jest) -> CJS -> jest vm

Pro: Stable

Con:

  • You need to know all ESM-only package name under recursive dependency tree.
  • Hard core ESM feature will fatal, or silent fail 

# modify `HARDCODE_ESMONLY` pattern jest.config.ts
$ jest

Tested Package Manager

[x] pnpm

[x] npm

[x] yarn

[x] yarn@berry

Test

$ git clone <repo> .
...

$ bash ./test.sh
...
ALL PASS! 🎉

Known not work EDGE case

Alarm, although I don't think there is real world use case, exclude I'm testing config.

Lazy to extract and reproduce, brief:

If you have

  • setted --experimental-vm-modules
  • jest config .projects length >= 2
  • jest config .projects include jsdom
  • using import('node-fetch') instead of import nf from 'node-fetch'

Last

Good luck, bro!