题目
中文
实现一个通用First<T>，它接受一个数组T并返回它的第一个元素的类型。
例如：
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
英文
Implement a generic First<T> that takes an Array T and returns it's first element's type.
For example:
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
答案
type First<T extends any[]> = T extends { length: infer U } ? U extends 0 ? never : T[0] : never;