Add ES2024/ES2025 polyfills with fixes for reduce bug, groupBy prototype safety, and iterator resilience

[Copilot]

• Create initial polyfills (Object.groupBy, Map.groupBy, Promise.try, Iterator helpers)
• Fix getIteratorPrototype() – use Array iterator chain instead of new GeneratorFunction(...) to avoid CSP eval violation
• Fix iteratorFrom() – box primitives with Object(obj) so strings and other primitive iterables are accepted
• Expand isPolyfilled() – check all 11 helper methods (filter, take, drop, flatMap, reduce, toArray, forEach, some, every, find, and map), not just map
• Add Iterator.from('abc') string test
• All 60 tests pass, lint clean

Original prompt

Create a pull request in github/browser-support to apply recommended improvements to PR #104 ("Add ES2024 and ES2025 polyfills for Safari 17.0 compatibility").

Goals:

1. Fix potential Iterator.prototype.reduce polyfill bug:

   • In src/iterator-helpers.ts, the overload implementation currently checks if (arguments.length < 2) to determine whether an initialValue was provided.
   • This is incorrect because the function will always receive at least 2 arguments (reducer is always present in a valid call), so reduce(fn) may be treated like reduce(fn, initialValue) and skip the "use first iterator value as accumulator" behavior.
   • Update the logic to correctly detect when initialValue is not passed and implement expected behavior:
     • If initialValue is omitted: take first iterator value as accumulator; throw TypeError for empty iterator.
     • If initialValue is provided (even if undefined is intentionally provided): use it.

2. Make Object.groupBy polyfill more robust against prototype pollution / inherited keys:

   • In src/object-groupby.ts, result is currently {} and membership checks are performed with key in result.
   • Improve by using a null-prototype object and/or own-property checks so that keys like __proto__ don’t behave unexpectedly.
   • Ensure typing remains reasonable (may require casting) and behavior remains consistent.

3. Iterator helpers resilience:

   • Consider the case where getIteratorPrototype() returns null.
   • Update apply() behavior so that chaining still works for iterators produced by helpers when possible.
   • At minimum, ensure the polyfill doesn’t silently fail to install helper methods when IteratorPrototype can’t be discovered; document or implement a fallback.

4. Update/expand tests to cover the fixes:

   • Add/adjust tests in test/iterator-helpers.js to ensure reduce(fn) without an explicit initial value uses the first element as accumulator and uses correct indices.
   • Add a test for reduce(fn, undefined) to ensure passing an explicit initial value works as expected.
   • Add tests in test/object-groupby.js for keys like "__proto__" to verify results are correct and safe.

5. Keep within repository conventions:

   • Follow existing pattern: each polyfill exports isSupported, isPolyfilled, apply.
   • Keep ESM import style consistent.
   • Run formatting/lint expectations.

Context:

• The original PR adds polyfills for ES2024/ES2025 for Safari 17.0: Object.groupBy, Map.groupBy, Promise.try, Iterator helpers and Iterator.from.
• PR Add ES2024 and ES2025 polyfills for Safari 17.0 compatibility #104 is currently open; this PR should build on main and can be merged independently (or used as follow-up).

Reference PR:

• Add ES2024 and ES2025 polyfills for Safari 17.0 compatibility #104

This pull request was created from Copilot chat.

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[Copilot]

Pull request overview

Adds ES2024/ES2025 polyfills (Object.groupBy, Map.groupBy, Promise.try, Iterator helpers/Iterator.from) with Safari-compatibility fixes, plus a corresponding test suite, and wires these polyfills into the library’s exported polyfills set.

Changes:

• Introduces new polyfill modules for Object.groupBy, Map.groupBy, Promise.try, and Iterator helpers (including a reduce initialValue detection fix + chaining fallback).
• Adds test coverage for the new polyfills, including reduce() edge cases and __proto__ handling for Object.groupBy.
• Exports/registers the new polyfills in src/index.ts and updates the lockfile.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/iterator-helpers.ts	Implements iterator helper polyfills, Iterator.from, and reduce fix; adds fallback prototype behavior
src/object-groupby.ts	Adds Object.groupBy polyfill using null-prototype result + own-property checks
src/map-groupby.ts	Adds Map.groupBy polyfill
src/promise-try.ts	Adds Promise.try polyfill
src/index.ts	Registers new polyfills in the exported polyfills object
test/iterator-helpers.js	Adds iterator helper tests, including reduce() edge cases and chaining
test/object-groupby.js	Adds Object.groupBy tests including __proto__ safety
test/map-groupby.js	Adds Map.groupBy tests
test/promise-try.js	Adds Promise.try tests
package-lock.json	Lockfile metadata changes (adds "peer": true flags in several entries)

Comments suppressed due to low confidence (1)

src/iterator-helpers.ts:314

• isSupported() treats globalThis.Iterator as supported only when typeof IteratorConstructor === 'object' and only checks 'from' in IteratorConstructor. If a native Iterator exists as a function/constructor (or from exists but is non-callable), isSupported() can return false/true incorrectly and impact the library-wide isSupported() signal. Consider allowing function-or-object and verifying typeof IteratorConstructor.from === 'function' (similar to other polyfills in this repo).

  const IteratorConstructor = (globalThis as typeof globalThis & {Iterator?: {from?: unknown}}).Iterator
  return (
    IteratorPrototype !== null &&
    'map' in IteratorPrototype &&
    'filter' in IteratorPrototype &&
    'take' in IteratorPrototype &&
    'drop' in IteratorPrototype &&
    'flatMap' in IteratorPrototype &&
    'reduce' in IteratorPrototype &&
    'toArray' in IteratorPrototype &&
    'forEach' in IteratorPrototype &&
    'some' in IteratorPrototype &&
    'every' in IteratorPrototype &&
    'find' in IteratorPrototype &&
    IteratorConstructor !== undefined &&
    typeof IteratorConstructor === 'object' &&
    IteratorConstructor !== null &&
    'from' in IteratorConstructor
  )

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