459 - Flatten

[pokuwagata]

trafficstars

// your answers
type Flatten<S extends any[], T extends any[] = []> =  S extends [infer X, ...infer Y] ? 
  X extends any[] ?
   Flatten<[...X, ...Y], T> : Flatten<[...Y], [...T, X]> 
  : T

[Mario-Marion]

666

[ydaydayup]

type Shift<T extends any[]> = T extends [T[0], ...infer item] ? [...item] : []
type Flatten<T extends any[], R extends any[]=[]> = T extends [] ? R :
    (T[0] extends any[] ? Flatten<[...T[0], ...Shift<T>], R> : Flatten<Shift<T>, [...R, T[0]]>)

my answer is complex，yours is great, clever

[GBVFtou]

type Flatten<T extends any[], U extends any[] = [],> = T extends [infer A, ...infer B] ? A extends any[] ? Flatten<B, [...U, ...Flatten< A >]> : Flatten<B, [...U, A]> : U

[PKozyakSoftonix]

type Flatten<T extends any[]> = T extends [] ? [] : T extends [infer First, ...infer Rest] ? First extends any[] ? [...Flatten<First>, ...Flatten<Rest>] : [First, ...Flatten<Rest>] : never;

[Nikjee]

type Flatten<T extends any[],K extends any[] = []> = [T] extends [never] ? [] 
  : T extends [infer Value,...infer Rest] ?
    Value extends any[] ? 
      Flatten<[...Value,...Rest],K> :
      Flatten<Rest,[...K,Value]>
    : K

[ChibiBlasphem]

type Flatten<A extends unknown[]> = A extends [infer H, ...infer R]
  ? [...(H extends unknown[] ? Flatten<H> : [H]), ...Flatten<R>]
  : [];

[Qzhor]

// your answers
type Flatten<S extends any[], T extends any[] = []> =  S extends [infer X, ...infer Y] ? 
  X extends any[] ?
   Flatten<[...X, ...Y], T> : Flatten<[...Y], [...T, X]> 
  : T

[...Y] can change directly to Y

[ivannovazzi]

no need for two arguments

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

[w2xi]

one argument also works.

type Flatten<T extends any[]> = 
  T extends [infer F, ...infer R] 
    ? F extends any[]
        ? Flatten<[...F, ...R]>
        : [F, ...Flatten<R>]
    : T

[Zhang-Lu-Bin]

type Flatten<T extends any[],U extends any[] = []> = T extends [infer K,... infer Reset] ? K extends any[] ? Flatten<Reset,Flatten<K, U>>
: Flatten<Reset,[...U,K]> : U

[likepeach789]

Below is my answer, tested and it is working fine.

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

Anyone please help explain the answer which the writer provided, I didn't understand why there is an extra T there. Flatten<S extends any[], T extends any[] = []>

[lewton]

type Flatten<T extends unknown[], R extends unknown[] = []> = T extends [
  infer D,
  ...infer L,
]
  ? Flatten<L, D extends [...infer DD] ? Flatten<DD, R> : [...R, D]>
  : R;

[tanliyuan]

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

[linkinzhiyuan]

type Flatten<T extends any[]> = T extends [infer F, ...infer R] 
 ? F extends any[] 
   ? Flatten<[...F, ...R]> 
   : [F, ...Flatten<R>] 
 : T

[shx123qwe]

// your answers
type Flatten<S extends any[], T extends any[] = []> =  S extends [infer X, ...infer Y] ? 
  X extends any[] ?
   Flatten<[...X, ...Y], T> : Flatten<[...Y], [...T, X]> 
  : T

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

[michael-komeet]

type Flatten<T extends readonly any[]> = {[I in keyof T]: T[I] extends any[] ? Flatten<[...T, T[I]]> : T[I]}

[ernestpluta]

type Flatten<T extends any[], Acc extends any[] = []> = T extends [
    infer F,
    ...infer R,
]
    ? F extends any[]
        ? Flatten<[...R, ...F], Acc>
        : Flatten<R, [...Acc, F]>
    : Acc