Add fetch to global scope during Jest setup

[dstaley]

Summary

In the Jest documentation, the react-native preset in this repo is described as follows:

The preset is a node environment that mimics the environment of a React Native app.

However, while writing unit tests, I noticed that the Jest environment is missing a fetch implementation. According to the React Native docs, React Native includes a polyfill for fetch. Putting these two statements together, it follows that there should be an implementation of fetch present in the Jest environment while testing.

This PR adds a single line, jest.requireActual('../Libraries/Network/fetch'), to the Jest setup script. (As an alternative, I believe I could just use require('whatwg-fetch'), so if that's preferred please let me know and I'll update the PR.)

Changelog

[General] [Fixed] - Added fetch to global scope during Jest tests

Test Plan

I'm not sure if this change would require any updates to existing tests. If that's not the case, please let me know! As far as I can tell, there aren't any tests specific to the Jest preset.

[cpojer]

Thanks! Can you explain why you had to update whatwg-fetch? I'd prefer to just make the code change to the Jest setup in this PR if possible.

[dstaley]

Thanks! Can you explain why you had to update whatwg-fetch? I'd prefer to just make the code change to the Jest setup in this PR if possible.

v3 referenced self which is undefined in Node environments. This was fixed in v3.3, but I figured since all the tests passed with v3.5 that I might as well go all the way. There's more details about the changes here.

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[fabOnReact]

this is awesome thanks 🙏

[daniel-nagy]

It seems Libraries/Core/setUpXHR.js needs to be loaded for fetch to work properly. For example, fetch requires the AbortController and AbortSignal APIs.

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