Clarify behavior of transformIgnorePatterns when including multiple patterns.

[AndrewSouthpaw]

Summary

I recently bumped into an mystery working with transformIgnorePatterns, where I had two negative lookaheads for node_modules, like so:

{
  "transformIgnorePatterns": [
    "node_modules/(?!(foo)/)",
    "node_modules/(?!(bar)/)"
  ]
}

As I came to realize, this meant that both node_modules/foo and node_modules/bar folders were not transformed, because they would always match in the other pattern. I missed the significance of the line:

If the file path matches any of the patterns, it will not be transformed.

Maybe this is completely evident to other people, but it does seem like an easy detail to miss while working with it.

I figured I'd share these suggestions, and if the team thinks it's a worthwhile addition, I can add the changes in the other doc versions.

Test plan

Checked running website locally.

[codecov-commenter]

Codecov Report

Merging #11796 (35e6dc9) into master (98f10e6) will not change coverage.
The diff coverage is n/a.

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  Files         312      312           
  Lines       16366    16366           
  Branches     4746     4746           
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  Hits        11300    11300           
  Misses       5039     5039           
  Partials       27       27           

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[sigveio]

Hi @AndrewSouthpaw! Seems like a good addition to clarify this considering that not everyone knows how Jest treats this under the hood, or are experienced with RegExp. 🙂

Here's a few things I spotted looking over your PR.

[AndrewSouthpaw]

@sigveio thanks for the thoughtful and thorough feedback! I'm glad you think the additions are useful.

I've pushed up some changes, let me know if they address your concerns. Once it looks good, I'll copy the changes to the versioned docs as well.

[sigveio]

@sigveio thanks for the thoughtful and thorough feedback! I'm glad you think the additions are useful.

You are welcome! 😌

I've pushed up some changes, let me know if they address your concerns.

Thanks - I've added some follow-up comments.

With regards to the patterns; what Jest does internally is essentially this after loading the config:

https://github.com/facebook/jest/blob/98f10e698ae986c19fef2d8117be2341bcfb8f7f/packages/jest-config/src/normalize.ts#L325-L327

To substitute in <rootDir> for those using that in their patterns. And then in the transformer:

https://github.com/facebook/jest/blob/98f10e698ae986c19fef2d8117be2341bcfb8f7f/packages/jest-transform/src/ScriptTransformer.ts#L967

To construct a combined and appropriately escaped pattern in the form of a RegExp object.

So if you'd like to verify your patterns, you could do the same: run it through the RegExp() constructor and console.log() it to get the string. And use a testing tool like this to try it out: https://regex101.com/r/JsLIDM/1

(Note that I added the flags gm in the tool there just to allow multiple matches across multiple lines for the purpose of that demonstration. Lines where node_modules is colored = what Jest would ignore for transformations.)

[AndrewSouthpaw]

@sigveio Ready for another review! I copied the changes over, but the actual tweaks can be seen in 5b13a37.

[SimenB]

this is awesome, thank you!! Since this was opened I've added a 27.2 version - would you mind merging in main and updating that as well? 🙂

[AndrewSouthpaw]

Done! Thanks for the heads up @SimenB!

[github-actions]

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