Reverse

In this medium-level challenge, you'll implement the type version of Array.reverse().

type a = Reverse<['a', 'b']> // ['b', 'a']
type b = Reverse<['a', 'b', 'c']> // ['c', 'b', 'a']

// Video Solution: https://youtube.com/watch?v=8a3_sxxtRnE

type cases = [
  Expect<Equal<Reverse<[]>, []>>,
  Expect<Equal<Reverse<['a', 'b']>, ['b', 'a']>>,
  Expect<Equal<Reverse<['a', 'b', 'c']>, ['c', 'b', 'a']>>,
]

type errors = [
  // @ts-expect-error
  Reverse<'string'>,
  // @ts-expect-error
  Reverse<{ key: 'value' }>,
]

Detailed Explanation

type Reverse<T extends unknown[]> = T extends [...infer Rest, infer Last]
  ? [Last, ...Reverse<Rest>]
  : T

How it works:

• First we constrain the type T to be an array.
• Then we use a conditional type to check if the tuple has members (T extends [...infer Rest, infer Last]), while capturing the rest of the tuple (infer Rest) and the last element (infer Last).
• If it has members, we return a new tuple with the last element (Last) and the rest of the tuple recursively passed into Reverse<Rest> ([Last, ...Reverse<Rest>]).
• If it doesn't have members, we return the original tuple (T).

This challenge helps you understand tuple manipulation and how to apply this concept in real-world scenarios.