[@babel/traverse] Improve performance of mapped Visitor type

[awphi]

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Minimal repro of noticeable performance drain in this StackBlitz with this MR change applied as patch.

Before (prod):

After (this branch):

Note the ~100x drop in instantiations. I'm unsure what the root cause of this is so if anyone could share any wisdom on how/why tsc treats this different to Extract it'd be really appreciated!

[typescript-bot]

@awphi Thank you for submitting this PR! I see this is your first time submitting to DefinitelyTyped 👋 — I'm the local bot who will help you through the process of getting things through.

This is a live comment which I will keep updated.

1 package in this PR

• babel__traverse — on npm, on unpkg

Code Reviews

Because this is a widely-used package, a DT maintainer will need to review it before it can be merged.

You can test the changes of this PR in the Playground.

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• ✅ No merge conflicts
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This PR has been inactive for 13 days — please try to get reviewers!

---

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[typescript-bot]

Hey @awphi,

😒 Your PR doesn't modify any tests, so it's hard to know what's being fixed, and your changes might regress in the future. Please consider adding tests to cover the change you're making. Including tests allows this PR to be merged by yourself and the owners of this module.

This can potentially save days of time for you!

[typescript-bot]

🔔 @yortus @marvinhagemeister @rpetrich @mgroenhoff @dlgrit @ifiokjr @ExE-Boss @danez — please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

[danez]

That sounds amazing. I'm not versed enough why this happens.

Will check this in one of my projects.

[jakebailey]

I can think of a couple of reasons; one is just that you've switched (I think?) the mapped type to be homomorphic by specifically doing N in Node as N["type"]; this may actually end up being a behavior change just due to how these mappings work...

[awphi]

No obvious outward differences jumped out at me between the two kinds of mapped types at the bottom of this little playground (albeit it's hard to see the whole picture) besides the aforementioned speed. Can you think of what the downstream behavioural changes may be? If so I'm happy to investigate a bit further.

[jakebailey]

I'll be honest and say that I'm not an expert here; @weswigham would probably know how to better reason about this as I'm more or less parroting off of others 😄

[typescript-bot]

Re-ping @yortus, @marvinhagemeister, @rpetrich, @mgroenhoff, @dlgrit, @ifiokjr, @ExE-Boss, @danez:

This PR has been out for over a week, yet I haven't seen any reviews.

Could someone please give it some attention? Thanks!