我有一个2D数组的副本，它返回的阅读属性是undefined，但原始数组没有。有人知道这是为什么吗？我附上了我的代码。我想这可能与我复制数组的方式有关，但当我console.log它们时，它们返回的是相同的东西，只是一个填充了false，另一个填充了字符串数字。

/**
 * @param {character[][]} grid
 * @return {number}
 */
var numIslands = function(grid) {
  let MAX_X = grid.length;
  let MAX_Y = grid[0].length;
  let num_of_islands = 0;
  const visited = grid.map((row) => {
      return (
          row.map((square) => {
              return (
                  false
              )
          })
      )
  })
   console.log(grid);
   console.log(visited);

  for(let x = 0; x < MAX_X; x++) {
      for(let y = 0; y < MAX_Y; y++) {
          if(grid[x][y] === '1' && !visited[x][y]) {
              num_of_islands+= 1;
              DFS(grid, x, y, visited)
          }
      }
  } 
   return num_of_islands
};

function DFS(grid, x, y, visited) {
   if(x < 0 || x >= grid.length || y < 0 || y >= grid[0].length) {
       return;
   }
   if(grid[x][y] === '0' || visited[x][y]) {
       return;
   }
   
   grid[x][y] = '0';
   visited[x][y] = true;
   DFS(grid, x + 1, y);
   DFS(grid, x - 1, y); 
   DFS(grid, x, y + 1);
   DFS(grid, x, y - 1);
}
let grid = [["1","1","1","1","0"],["1","1","0","1","0"],["1","1","0","0","0"],["0","0","0","0","0"]];

console.log(numIslands(grid));