019.Flatten

写一个接受数组的类型，并且返回扁平化的数组类型

例如：

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

答案

// 答案 1
type Flatten<T extends readonly any[]> = T extends [infer F, ...infer R]
  ? F extends readonly any[]
    ? [...Flatten<F>, ...Flatten<R>]
    : [F, ...Flatten<R>]
  : T
 
// 答案 2
type Flatten<T extends readonly any[]> = T extends [infer L, ...infer R]
  ? [...(L extends readonly any[] ? Flatten<L> : [L]), ...Flatten<R>]
  : []
 
// 答案 3
type Flatten<T extends readonly any[], Res extends readonly any[] = []> = T extends [infer L, ...infer R]
  ? L extends readonly any[]
    ? Flatten<[...L, ...R], Res>
    : Flatten<R, [...Res, L]>
  : Res