[Feature]: Lift `moduleFileExtensions` option validation.

🚀 Feature Proposal

Do not validate that moduleFileExtensions must include "js". We could change it to not empty or even allow empty array values to result in not finding any test files. Perhaps with a warning, example below.

Motivation

As we are moving towards supporting ESM #9430, users writing test entirely as esModules (or transpiled/formed to) would find it confusing that they must include a "js" extension.

Example

// jest.config.mjs
export default {
  moduleFileExtensions: [
    "mjs",
  ],
  testRegex: "test\.mjs",
  transform: {},
};

As of v27.4.0 will get this error

● Validation Error:

  moduleFileExtensions must include 'js':
  but instead received:
    ["mjs"]
  Please change your configuration to include 'js'.

  Configuration Documentation:
  https://jestjs.io/docs/configuration

The script failed with exit code 1

Pitch

Well, this is clearly a jest validation rule.

The existing no tests found message

No tests found, exiting with code 1
Run with `--passWithNoTests` to exit with code 0
No files found in /path/to/root/test/dir.
Make sure Jest's configuration does not exclude this directory.
To set up Jest, make sure a package.json file exists.
Jest Documentation: https://jestjs.io/docs/configuration
Pattern:  - 0 matches
The script failed with exit code 1

Particularly, the line

Make sure Jest's configuration does not exclude this directory.

Could be extened to

Make sure Jest's configuration does not exclude this directory and extension of test files.

Or perhaps

Make sure Jest's configuration includes this directory and extension of test files.

[ahnpnl]

I know what you want to say, it’s just my opinion. It’s not like I don’t get what you meant:)

[demsking]

How to mock fs/promises with ESM?