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

Author

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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