fix(pick): return Partial<Pick<T, K>> for non-tuple key arrays

[Seojooyeon-creat]

Closes #1623

Summary

• pick currently returns Pick<T, K> regardless of whether keys is a tuple or a variable-length array. This is incorrect when K is a union type: Pick<T, 'a' | 'b'> implies both a and b are always present, but the runtime value may only contain one of them.
• This PR fixes the return type using a conditional type that distinguishes between tuple and variable-length array inputs.

Changes

src/object/pick.ts

Changed the signature from:

pick<T extends Record<string, any>, K extends keyof T>(obj: T, keys: readonly K[]): Pick<T, K>

to:

pick<T extends Record<string, any>, Keys extends ReadonlyArray<keyof T>>(
  obj: T,
  keys: readonly [...Keys]
): number extends Keys['length'] ? Partial<Pick<T, Keys[number]>> : Pick<T, Keys[number]>

• readonly [...Keys] forces TypeScript to infer tuple types for array literals like ['a', 'b']
• number extends Keys['length'] distinguishes tuple (length is a specific literal) from variable-length array (length is number)
• Result: literal tuple → Pick<T, K>, variable-length array → Partial<Pick<T, K>>

src/object/pick.spec.ts

Added two type-level tests using expectTypeOf:

• Literal tuple ['a', 'b'] → Pick<T, 'a' | 'b'> (all keys guaranteed)
• Variable array ('a' | 'b')[] → Partial<Pick<T, 'a' | 'b'>> (only present keys)

docs/ (en, ko, ja, zh_hans)

Updated parameter and return type descriptions to reflect the new behavior.

Context

A similar fix was previously applied to omit (#1498) using overloads and then reverted (#1595). This PR takes a different approach — a single signature with a conditional return type — which avoids the added complexity of overloads while correctly narrowing the type based on tuple vs. array input.

Benchmark

Runtime logic is unchanged. Performance is maintained:

es-toolkit/pick    19,473,619 hz  (4.40x faster than lodash)

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Project	Deployment	Actions	Updated (UTC)
es-toolkit	Ready	Preview, Comment	Apr 18, 2026 4:34am