00459-medium-flatten.ts

// ============= Test Cases =============
import type { Equal, Expect } from './test-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']>>,
]

// ============= Your Code Here =============
type Flatten<T extends readonly unknown[]> =
  T extends [infer First, ...infer Rest]
    ? First extends readonly unknown[] ? [...Flatten<First>, ...Flatten<Rest>] : [First, ...Flatten<Rest>]
    : []