How to test ESM in TS projects?

[hyperknot]

Hi, I've been trying everything, following the advice on https://kulshekhar.github.io/ts-jest/docs/guides/esm-support but I still couldn't make ESM in TS work.

How would you test a .ts file which starts like this?

import fetch from 'node-fetch'

I'm confused about the 6 presets as well. tsc runs perfectly with this tsconfig.json:

{
  "compilerOptions": {
    "noImplicitReturns": true,
    "noUnusedLocals": true,
    "outDir": "lib",
    "sourceMap": true,
    "strict": true,
    "allowSyntheticDefaultImports": true,
    "moduleResolution": "node",
    "target": "es2020",
  },
  "compileOnSave": true,
  "include": ["src"]
}

What direction would you recommend me going into? It's not clear to me if I even need ES modules or not? I mean for testing I could just compile with "es5" target, could I not?

[hyperknot]

I found this example in the meanwhile:
https://github.com/kulshekhar/ts-jest/tree/main/examples/ts-only

Do I understand right that ESM is totally optional and it's not needed altogether? I mean even if the source files contain ES imports the following option:

{
  "compilerOptions": {
    "module": "CommonJS",
    "target": "ES2015"
  }
}

Will make sure everything works with the default preset?

[hyperknot]

OK, I think I made a kind of minimal reproducible case:

1. Take the ts-only example.
2. Add the following 2 lines:

import fetch from 'node-fetch'
export const f = fetch

3. Install node-fetch.

That's it. It reproduces all the problems I'm encountering. Can you tell me what config should I use for this use-case?

[hyperknot]

I managed to solve almost everything, except for one library: formulajs

I can import formulajs@3.0.0 via node, but not via ts-jest.

I'm trying this one, as per the homepage:

import * as formulajs from '@formulajs/formulajs'

The 3.0.0 release is super new, just came out today, so if there is something what the author should do differently I think we can give feedback to them. I don't have the knowledge about ESM library internals to tell them, but if you could have a look at what's wrong with import * as formulajs from '@formulajs/formulajs' then I'd be happy to communicate it to the author.