14-first.ts

/**
 * 14 - First of Array
 *
 *
 * Implement a generic `First<T>` that takes an Array `T` and returns its first element's type.
 *
 * For example:
 *
 * ```ts
 * type arr1 = ['a', 'b', 'c']
 * type arr2 = [3, 2, 1]
 *
 * type head1 = First<arr1> // expected to be 'a'
 * type head2 = First<arr2> // expected to be 3
 * ```
 */

/* _____________ Your Code Here _____________ */

type First<T extends any[]> = T extends [infer A, ...infer Rest]
  ? A
  : never;

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<First<[3, 2, 1]>, 3>>,
  Expect<Equal<First<[() => 123, { a: string }]>, () => 123>>,
  Expect<Equal<First<[]>, never>>,
  Expect<Equal<First<[undefined]>, undefined>>,
]

type errors = [
  // @ts-expect-error
  First<'notArray'>,
  // @ts-expect-error
  First<{ 0: 'arrayLike' }>,
]