Docs: setupFiles vs setupFilesAfterEnv

[thernstig]

I am writing this as an issue here as it might potentially be a request for improvements of the docs.

setupFiles and setupFilesAfterEnv has no clear indication as to why both are needed. My investigation seems to show that anything you do in setupFiles might as well equally well be done in setupFilesAfterEnv. setupFilesAfterEnv seems to be the more powerful option as it has more information available and can thus monkey-patch APIs.

[jeysal]

I imagine setupFiles could capture some globals before the environment modifies them, but I don't know about concrete use cases where only setupFiles works - @SimenB do you know any?

[silvenon]

One very contrived reason I could think of why setting globals in setupFiles is safer is that some library might define a global which conflicts with Jest's globals, so Jest will override it when setting up the framework. If you import this library in setupFilesAfterEnv, you couldn't bring the previous value back.

[thernstig]

I assume another reason could be performance?

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