flatten.ts

/* flatten 함수 구현하기. */
const arr1 = [{ a: 1 }, 2, [3, 4]];
const arr2 = [1, 2, [3, [4, [5, 6], 7], [8, 9]]];

type ArgumentArrayType<T> = (T | T[])[];
type RecursiveArrayType<T> = T | RecursiveArrayType<T>[];
type RecursiveArgumentArrayType<T> = (
  | RecursiveArrayType<T>
  | RecursiveArrayType<T>[]
)[];

/* 1차로 중첩된 배열만 flatten */
const flatten = <T>(array: ArgumentArrayType<T>) => {
  return array.reduce((acc: T[], cur: T | T[]) => acc.concat(cur), []);
};

console.log(flatten(arr1)); // [1, 2, 3, 4]

/* 무한히 중첩될 수 있는 배열 flatten */
const flattenRecursive = <T>(array: RecursiveArgumentArrayType<T>) => {
  return array.reduce(
    (
      acc: RecursiveArrayType<T>[],
      cur: RecursiveArrayType<T> | RecursiveArrayType<T>[]
    ) => {
      if (Array.isArray(cur)) {
        return acc.concat(flattenRecursive(cur));
      } else {
        return acc.concat(cur);
      }
    },
    []
  );
};

console.log(flattenRecursive(arr2)); // [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]