Typescript 通过使用删除 let 变量。map

Question

In my current code I do not want to change directly input b but instead I make a copy of it using the function which return bn .

I would like to know if there is a better way to do so, my using exclusively map on b , removing the let bn .

export const addToBoard = (t: TetroEnum) => (d: DirectionEnum) => (x: number) => (
  y: number
) => (b: Board) => {
  const tetro = getTetroFromPieces(t)(d);
  let bn = b.map(r => r.map(c => c)); // I use let so I can modify bn in tetro.forEach
  tetro.forEach((tR, tRx) => tR.forEach((tC, tCx) => (bn[tRx + y][tCx + x] = t)));
  return bn;
};