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fix(partition): accept any predicate return value like Array.prototype.filter - #1937

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Hprogram:fix/issue-1926-partition-unknown
Jul 24, 2026
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raon0211 merged 1 commit into
toss:mainfrom
Hprogram:fix/issue-1926-partition-unknown

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Summary

Closes #1926.

partition's fallback overload required the predicate to return a strict boolean, so common truthiness predicates like partition(values, i => i.good) (returning boolean | undefined) failed to compile — even though the implementation already branches on truthiness. This widens the fallback overload's return type from boolean to unknown, matching:

  • TypeScript's own Array.prototype.filter signature (predicate: (value, index, array) => unknown), and
  • es-toolkit/compat's partition, which already types its callback as (value: T) => unknown.

The type-guard overload (value is U) is unchanged, so narrowing behavior is identical. The change is source-compatible (it strictly accepts more programs) and has no runtime effect.

Changes

  • src/array/partition.ts: fallback overload + implementation signature => boolean → => unknown, with JSDoc wording adjusted to "truthy/falsy value"
  • src/array/partition.spec.ts: regression test using an optional-boolean property predicate (compile + runtime partitioning + expectTypeOf)
  • docs/**/array/partition.md (en / ko / ja / zh_hans): parameter type updated to => unknown

Scope note

The issue title mentions "(and other functions?)" — other predicate-style callbacks (dropWhile/dropRightWhile, takeWhile/takeRightWhile, remove, findKey, omitBy/pickBy) have the same strict boolean requirement. This PR is scoped to partition per the issue's primary ask; happy to follow up on the others if you'd like the same treatment.

Test results

  • yarn vitest run src/array/partition.spec.ts — 6 passed, including the new regression test
  • yarn tsc --noEmit
  • yarn eslint src/array/partition.ts src/array/partition.spec.ts
  • yarn prettier --check on all touched files

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Thanks!

@raon0211
raon0211 merged commit b854e15 into toss:main Jul 24, 2026
12 checks passed
racgoo pushed a commit to racgoo/es-toolkit that referenced this pull request Jul 28, 2026
gwennlbh pushed a commit to cigaleapp/cigale that referenced this pull request Aug 3, 2026
##### [v1.50.0](https://github.com/toss/es-toolkit/blob/HEAD/CHANGELOG.md#Version-v1500)

Released on July 24th, 2026.

- Added the `es-toolkit/types` entrypoint: a declaration-only module of compile-time
  type utilities TypeScript lacks natively — `ValueOf`, `Simplify`, `Writable`,
  `NonEmptyArray`, `DeepPartial`, and `DeepReadonly`. (\[[#1818](toss/es-toolkit#1818)])
- Added `flow` to `es-toolkit/fp`, a reusable data-last composition built on `pipe`. (\[[#1812](toss/es-toolkit#1812)])

The following bring `es-toolkit/compat` closer to Lodash. Behavior for the affected
edge cases now matches Lodash, so results may differ if you relied on the previous output.

- Fixed `compat/intersectionBy` to dedupe a single array (including by the iteratee)
  and to keep results consistent with Lodash. (\[[#1935](toss/es-toolkit#1935)])

- Fixed `compat/findIndex` and `compat/findLastIndex` to coerce a `NaN` `fromIndex`
  to `0` and to convert `fromIndex` to an integer. (\[[#1828](toss/es-toolkit#1828)], \[[#1834](toss/es-toolkit#1834)], \[[#1938](toss/es-toolkit#1938)])

- Fixed `compat/lastIndexOf` to coerce `fromIndex` like Lodash. (\[[#1832](toss/es-toolkit#1832)])

- Fixed `compat/every` to treat a falsy, non-nullish `doesMatch` as a `_.property`
  shorthand instead of identity. (\[[#1940](toss/es-toolkit#1940)])

- Fixed `compat/nth` to support string inputs. (\[[#1833](toss/es-toolkit#1833)])

- Fixed `compat/inRange` to not throw on a lone negative bound. (\[[#1835](toss/es-toolkit#1835)])

- Fixed `compat/toArray` to convert `Set`s to arrays like Lodash. (\[[#1840](toss/es-toolkit#1840)])

- Fixed `compat/values` to treat sparse array holes as `undefined`. (\[[#1894](toss/es-toolkit#1894)])

- Fixed `compat/maxBy` and `compat/minBy` to restore Lodash-compatible comparison. (\[[#1893](toss/es-toolkit#1893)])

- Fixed `compat/includes` to exclude the `length` property when matching array-like values. (\[[#1886](toss/es-toolkit#1886)])

- Fixed the case functions to split ordinal numbers like Lodash, and to count string
  size and padding by code points for multi-byte characters. (\[[#1836](toss/es-toolkit#1836)], \[[#1852](toss/es-toolkit#1852)], \[[#1853](toss/es-toolkit#1853)])

- Fixed `compat/unset` to not treat an own literal dotted key as a deep path. (\[[#1808](toss/es-toolkit#1808)])

- Moved `compat/flattenDepth`'s flattening logic out of `flatten`. (\[[#1847](toss/es-toolkit#1847)])

- Restored the standalone `eq` and `templateSettings` entry points. (\[[#1895](toss/es-toolkit#1895)])

- Fixed `deburr` to remove all combining diacritical marks, matching Lodash. (\[[#1807](toss/es-toolkit#1807)])

- Fixed `partition` to accept any predicate return value, like `Array.prototype.filter`. (\[[#1937](toss/es-toolkit#1937)])

- Fixed `has` and `hasIn` to prioritize nullish literal keys over paths. (\[[#1810](toss/es-toolkit#1810)])

- Fixed `invokeMap` to normalize a string path via `toPath` for `this` binding. (\[[#1814](toss/es-toolkit#1814)])

- Fixed `isDeepKey` to detect deep keys with stricter patterns. (\[[#1621](toss/es-toolkit#1621)])

- Fixed `unzipWith` to return an empty array instead of throwing on empty input. (\[[#1816](toss/es-toolkit#1816)])

- Improved documentation, JSDoc accuracy, and internal refactoring across `compat`
  and `fp`. (\[[#1817](toss/es-toolkit#1817)], \[[#1820](toss/es-toolkit#1820)], \[[#1822](toss/es-toolkit#1822)], \[[#1823](toss/es-toolkit#1823)], \[[#1825](toss/es-toolkit#1825)], \[[#1829](toss/es-toolkit#1829)], \[[#1830](toss/es-toolkit#1830)], \[[#1842](toss/es-toolkit#1842)],
  \[[#1845](toss/es-toolkit#1845)], \[[#1851](toss/es-toolkit#1851)], \[[#1858](toss/es-toolkit#1858)], \[[#1867](toss/es-toolkit#1867)], \[[#1879](toss/es-toolkit#1879)])

We sincerely thank [@Antoliny0919](https://github.com/Antoliny0919), [@raon0211](https://github.com/raon0211), [@dayongkr](https://github.com/dayongkr), [@spokodev](https://github.com/spokodev), [@sarathfrancis90](https://github.com/sarathfrancis90),
[@mayur-shenoy](https://github.com/mayur-shenoy), [@mahirhir](https://github.com/mahirhir), [@kojesung](https://github.com/kojesung), [@Hprogram](https://github.com/Hprogram), [@BangDori](https://github.com/BangDori), and everyone else who
contributed. We appreciate your great efforts!

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

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Predicate types for partition (and other functions?)

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