谷神星搜索

find the index of each x in the grid
for each x
  check the next three letters in a straight path in each of the eight directions
  if the path ends up spelling xmas
    add one to a running total

let grid = input.split('\n').map(line => line.split(''))

grid = grid.map(line => ['.','.','.',...line,'.','.','.'])
grid = [
  new array(grid[0].length).fill('.'),
  new array(grid[0].length).fill('.'),
  new array(grid[0].length).fill('.'),
  ...grid,
  new array(grid[0].length).fill('.'),
  new array(grid[0].length).fill('.'),
  new array(grid[0].length).fill('.')
]

................
................
................
...mmmsxxmasm...
...msamxmsmsa...
...amxsxmaamm...
...msamasmsmx...
...xmasamxamm...
...xxammxxama...
...smsmsasxss...
...saxamasaaa...
...mammmxmmmm...
...mxmxaxmasx...
................
................
................

let xs = []
for (let row = 0; row < grid.length; row++) {
  for (let col = 0; col < grid[0].length; col++) {
    if (grid[row][col] == "x") {
      xs.push([row, col])
    }
  }
}

let dirs = [
  [-1,-1],
  [-1,0],
  [-1,1],
  [0,-1],
  [0,1],
  [1,-1],
  [1,0],
  [1,1]
]

for each x
  for each direction
    create an array that starts with x
    do 3 times
      move one cell in this direction
      add the value of that cell to the array
    check whether the concatenation of all four values is "xmas"
      if it is, increment a tally

xs.reduce((total, coord) => {
  dirs.foreach((dir) => {
    let [row, col] = coord;
    let [y, x] = dir;
    let word = ["x"];
    for (let i = 0; i < 3; i++) {
      row += y;
      col += x;
      word.push(grid[row][col]);
    }
    if (word.join("") == "xmas") {
      total++;
    }
  });
  return total;
}, 0);

m s   m m   s m   s s
 a     a     a     a
m s   s s   s m   m m

MSSM
MMSS
SMMS
SSMM