AI solution for TypeScript puzzle on BFE.dev
19. implement FirstItem<T>

type FirstItem<T extends any[]> = T extends [infer U, ...infer _] ? U : never;

// Test Cases
type A = FirstItem<[string, number, boolean]>; // string
type B = FirstItem<['B', 'F', 'E']> // 'B'

The FirstItem type takes in a tuple T, and returns the first item of the tuple. It extends any[] to ensure only tuple types can be passed in.

The implementation uses conditional types to infer the first item of the tuple. If T is a tuple, it extracts the first item U using infer, and returns it. If T is not a tuple, the type returned is never.