Include a custom resolver in Jest preset

[SimenB]

Introduction

Metro resolves a custom field in package.json called react-native. This is not supported by Jest's default resolver - just like e.g. module isn't. This means different modules are resolved when running tests than when metro is bundling code.

Details

The Jest preset in react-native could ship a resolver which uses e.g. packageFilter to prioritize react-native over main etc. (there's an example for modules in the docs, but I guess react-native is more like browser and needs more logic - using metro-resolver somehow probably makes the most sense).

While the long term solution to this issue is probably exports and passing a react-native condition, Metro (currently) doesn't support exports and regardless, that'll leave all existent modules in the dust.

Note that this field seems to be undocumented? Sorta mentioned here, but not really 😅

Like with #509, I'd be happy to help review and/or write the code if this is accepted.

Discussion points

Different resolution of module leads to more different code than necessary running in tests vs production code, making them more brittle and less useful. Providing a custom resolver in the preset could help alleviate this.

[thymikee]

Metro team has plans to support exports. cc @motiz88 @huntie

[SimenB]

Yeah, I linked to the comment where the team stated it was on the roadmap. Regardless, that'd still leave 100% of existing modules behind, so I think support for react-native should be added

[NickGerleman]

Jest's default resolver is also missing some of the smarts for out-of-tree platforms (so the wrong platform's implementation could be included). I remember adding a custom resolver in the RNW repo for this a while back, but IIRC @tido64 did something better as part of the rnx-kit efforts which should be publicly usable.

[kelset]

Maybe you are thinking of this? https://github.com/microsoft/rnx-kit/tree/main/packages/typescript-react-native-resolver

[tido64]

I think @NickGerleman is referring to https://github.com/microsoft/rnx-kit/tree/main/packages/jest-preset. It contains logic for picking out the correct platform if within an out-of-tree platform, and provides affordances for specifying platforms otherwise.