default moduleFileExtensions order is problematic for typescript projects

[adamborowski]

Hi. I think that the default order of extensions in moduleFileExtensions can be very problematic if you have typescript files in a project.
https://jestjs.io/docs/en/configuration#modulefileextensions-arraystring

As doc says, the lookup of module will have the order as set in this array.

This default order defines thet json files have precedence over ts files.

Why this can be problematic? Because sometimes you have both json and ts/js files of the same name.
For example if you use react-intl, you have messages.json next to messages.ts

This order breaks tests as file may import { messages } from '[...]/i18n/messages';

As the result, the json file is loaded, not messages.ts. This causes improper module to be loaded during tests, causing all tests that use translations to fail.

I recommend to reorder extensions in default setup.

moduleFileExtensions: ['ts', 'tsx', 'js', 'jsx', 'json']

I think this will bring less problems than just mentioning

We recommend placing the extensions most commonly used in your project on the left, so if you are using TypeScript, you may want to consider moving "ts" and/or "tsx" to the beginning of the array.

[jeysal]

The order of moduleFileExtensions can have performance implications, if we have lots of misses when checking for the first few possible extensions. Hence our recommendation in the docs.
For this reason it's probably at the very least too early to move TS in front of JS. But I would be down with moving JSON behind TS, as there are usually few JSON modules. If you want to send a PR changing that, I think we can accept it, although only for the next major version since it could be breaking.

[jackneer]

May I help this? thanks!

[SimenB]

Of course! 👍

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