Possibility to write testEnvironment in esm

[kuka-radovan]

🚀 Feature Proposal

It would be awesome if there will be possibility to create testEnvironment as esm. It is actually possible only as cjs.

Motivation

I need to test API of my server written as esm. I want to start it in testEnvironment so I will need to import it.

Pitch

From Jest v. 25.4.0 there is support of esm, so I believe, this feature is meaningful from this perspective.

[SimenB]

Thanks for opening a separate issue! We can probably do something similar to what we do for importing config files, catching the error and check if it's ERR_REQUIRE_ESM: https://github.com/facebook/jest/blob/0e0eeed9d794fe0d6d7e84f555d9684f9e944221/packages/jest-config/src/readConfigFileAndSetRootDir.ts#L26-L42

My one concern is how this affects e.g. #8751. Probably not a huge issue - your transformer should just leave it alone. And I don't see a use case for both ESM and transpiling (people should just transpile to CJS in that case).

[kuka-radovan]

Your proposal seems reasonable. I don't see use case of ESM and code transpiling either.

[gilles-yvetot]

@SimenB Thanks for all the work on Jest! Especially the transition to ESM!

Do you think this feature could land in v27 ?

[SimenB]

This landed in #8751 and will be available in jest@27. You can test it via jest@next now

[SimenB]

Wait, no you mean native ESM! 😅

That's not done but should be quite straightforward if anybody wants to open a PR. Need to take the approach of #9573. Should probably attempt to transpile it first, then check the if an error thrown is ERR_REQUIRE_ESM and then use import

[gilles-yvetot]

Ahah yeah I meant native ESM. Let me take a look at the code and see if I am able to help

[gilles-yvetot]

@SimenB I have never contributed yet to to open source projects so would you mind pointing to the right file for that.
To my understanding it is this one ?

[SimenB]

Yep!

https://github.com/facebook/jest/blob/04a914a4c3e916b2eee3bc735b3dd4a1bd72169f/packages/jest-runner/src/runTest.ts#L107-L109

[gilles-yvetot]

Hey @SimenB, it took me a while to make this a priority. But here we are with my first open-source PR. I don't really know how to add test though

[gilles-yvetot]

@SimenB checking in again to see how I can help move this forward

[kuka-radovan]

@gilles-yvetot that's awesome someone has the same interest to introduce this feature. Unfortunately, I have no time to contribute, but I will appreciate, if I will be able to import and start my es6 server inside testEnvironment. 🤞 🤞 🤞

[gilles-yvetot]

@kuka-radovan thanks! It is actually my first open-source PR and I am waiting for approval or advise on how to make it better

[SimenB]

Fixed in #11033

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