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Phryxia/keyOfArrayLike.ts

Created June 28, 2024 05:46
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TypeScript snippet for extracting key type of array and tuple
Raw
keyOfArrayLike.ts
type KeyOfArrayLike<T> =
T extends [] ?
never
: T extends [unknown, ...infer R] ?
R['length'] | KeyOfArrayLike<R>
: T extends unknown[] ?
number
: never
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Phryxia commented Jun 28, 2024

This type is useful when you don't know actual T during inference of complex generic type.

KeyOfArrayLike<[number, number]> // 0 | 1
KeyOfArrayLike<number[]> // number
KeyOfArrayLike<[number, ...number[]]> // number

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