TypeScript Advanced typing recipes

recipes.ts

type Wrapper<T> = { wrapped: T }; 

type MapObject<T extends Record<keyof Object, any>> = {
  [K in keyof T]: Wrapper<T[K]>;
};
  
type MapTuple<T extends [...any[]]> = {
  [I in keyof T]: Wrapper<T[I]>;
} & { length: T["length"] };
  
type MapObject<T extends Record<keyof Object>;
               
export type Flatten<
  Arr extends ReadonlyArray<unknown>,
  Result extends ReadonlyArray<unknown> = []
> =
  // if Arr is empty -> return Result
  Arr extends readonly []
    ? Result
    : // if Arr is not empty - destruct it
    Arr extends readonly [infer Head, ...infer Tail]
    ? // check if Head is an Array
      Head extends ReadonlyArray<any>
      ? // if it is -> call Reducer with flat Head and Tail
        Flatten<readonly [...Head, ...Tail], Result>
      : // otherwise call Reducer with Head without flattening
        Flatten<Tail, readonly [...Result, Head]>
    : never;