fix: group prototype-named keys safely

[hiSandog]

Summary

• Accumulate groupBy buckets in a Map so keys like proto are handled as ordinary group names
• Convert the grouped entries back to the existing record return shape
• Add regression coverage for grouping under a prototype property name

Tests

• pnpm vitest run test/internal/group-by.spec.ts
• pnpm run ts-check
• git diff --check

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.91%. Comparing base (8b4e2b1) to head (ea5b9fd).

Additional details and impacted files

@@           Coverage Diff           @@
##             next    #3855   +/-   ##
=======================================
  Coverage   98.91%   98.91%           
=======================================
  Files         905      905           
  Lines        3146     3148    +2     
  Branches      581      581           
=======================================
+ Hits         3112     3114    +2     
  Misses         30       30           
  Partials        4        4           

Files with missing lines	Coverage Δ
src/internal/group-by.ts	100.00% <100.00%> (ø)

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[Copilot]

Pull request overview

This PR updates the internal groupBy helper to safely handle prototype-named keys (e.g. __proto__) by accumulating groups in a Map, and adds a regression test to ensure those keys are treated as ordinary group names.

Changes:

• Switched groupBy’s internal accumulator from a plain object to a Map to avoid special-case prototype keys during grouping.
• Converted grouped Map entries back into the existing Record<string, …> return shape via Object.fromEntries.
• Added a regression test covering grouping under __proto__.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/internal/group-by.ts	Uses a Map for safe bucket accumulation and converts back to an object on return.
test/internal/group-by.spec.ts	Adds regression coverage for grouping under prototype property names.

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[ST-DDT]

I'm not sure, whether I would want anyone to use __proto__ as a key.
Do you have a need for that?

What do the others think?

[xDivisionByZerox]

The method groupBy is an internal method that mimics the API of the official Object#groupBy function. We cannot use the native function due to it requiring at least Node v21 - we currently still support Node v20.

While you are correct that Object#groupBy does support the use case you are adding, I do not see any reason for us to add it to our code base as of right now. groupBy is [only used in 3 different places](https://github.com/search?q=repo%3Afaker-js%2Ffaker%20groupBy(&type=code) and none of those require safe prototype name grouping:

• faker/scripts/apidocs/processing/method.ts

  Line 101 in 8b4e2b1

  const groupedSignatures = groupBy(methods, (v) => v.getName());

• faker/scripts/apidocs/output/page-index.ts

  Lines 18 to 22 in 8b4e2b1

  	const pagesByCategory: Record<string, DefaultTheme.SidebarItem[]> = groupBy(
  	pages,
  	(page) => page.category ?? '',
  	({ title: text, camelTitle }) => ({ text, link: `/api/${camelTitle}.html` })
  	);

• faker/src/modules/word/filter-word-list-by-length.ts

  Line 17 in 8b4e2b1

  const wordsByLength = groupBy(wordList, (word) => word.length);

Furthermore, only the last of these cases is actually used during the runtime of Faker and it clearly maps a numeric value to the object keys - so no prototype names.

Because of the stated reasons, I would decline this PR.