[Typescript] 33. Medium Flatten

In this challenge, you would need to write a type that takes an array and emitted the flatten array type.

For example:

type flatten = Flatten<[1, 2, [3, 4], [[[5]]]]> // [1, 2, 3, 4, 5]

 

/* _____________ Your Code Here _____________ */

type Flatten<T extends any[]> = T extends [infer First, ...infer RT] 
    ? First extends any[]
      ? [...Flatten<First>, ...Flatten<RT>]
      : [First, ...Flatten<RT>]
    : [];

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

type cases = [
  Expect<Equal<Flatten<[]>, []>>,
  Expect<Equal<Flatten<[1, 2, 3, 4]>, [1, 2, 3, 4]>>,
  Expect<Equal<Flatten<[1, [2]]>, [1, 2]>>,
  Expect<Equal<Flatten<[1, 2, [3, 4], [[[5]]]]>, [1, 2, 3, 4, 5]>>,
  Expect<Equal<Flatten<[{ foo: 'bar'; 2: 10 }, 'foobar']>, [{ foo: 'bar'; 2: 10 }, 'foobar']>>,
]