Extend the module extension list in package.json

[srittau]

🚀 Feature Proposal

Unless I am overlooking something, it's currently only possible to override the module extension list in package.json, not extend it. The documentation actually gives an example how to extend it in jest.config.js.

Motivation

This came up in enzymejs/enzyme#2046. Replacing the default list is problematic, since it can break third-party package and the fix is not easily obvious. In this particular case, enzyme-react-adapter-16's update from version 1.10.0 to 1.11.0 added an import of a package.json file, without the file extension. This is reasonable, since .json is on node's default suffix list, but broke jest for many people.

While it is possible to extend the list in jest.config.js, using a declarative configuration in package.json is probably what most people prefer to do.

This potential problem should also be warned about in the description of the moduleFileExtensions option.

[jeysal]

We're planning a config rework coming up soon. I personally think the exact thing you're proposing might not become part of it and there's always a limit do what you can do declaratively in a reasonably simple way, but it looks like we should definitely have better warnings in place, so thanks for bringing this up!

[SimenB]

You can do import {defaults} from 'jest-config' if you use JS config, which is called out in the docs: https://jestjs.io/docs/en/configuration#defaults

I do think adding some docs calling out that replacing the extensions potentially can break integrations. Wanna send a PR to the docs?

Beyond that, as @jeysal mentions, we wanna rework our config (#7185). If anything, I think it makes sense to move towards more config in js and less in json since it's so inflexible. Adding a new option that just extends without replacing doesn't sound like something we want to do...

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